Metallography and Microstructures of Heat-Resistant Alloys

ASM Handbook, Volume 9: Metallography and Microstructures Copyright © 2004 ASM International® G.F. Vander Voort, editor, p820–859 All rights reserved. DOI: 10.1361/asmhba0003737 www.asminternational.org

George F. Vander Voort and Gabriel M. Lucas, Buehler Ltd. Elena P. Manilova, Polzunov Central Boiler and Turbine Institute, St. Petersburg, Russia

HEAT-RESISTANT ALLOYS cover a wide nickel-base, and cobalt-base. The metallographic The procedures used to prepare metallo- range of chemical compositions, microstructural methods discussed also are suitable for preparing graphic specimens of cast or wrought heat-resis- constituents, and mechanical properties. This ar- both cast and wrought heat-resistant alloys; mi- tant grades are quite similar to those for iron- ticle summarizes metallographic techniques and crostructural constituents are quite similar ex- base alloys, especially stainless steels (see the microstructural constituents for three types of cept for obvious differences in homogeneity and Section “Metallographic Techniques” in this cast and wrought heat-resistant alloys: iron-base, porosity. Volume). Aspects particularly signiﬁcant to the

Table 1 Compositions of Fe-Cr-Ni heat-resistant casting alloys

Composition, % Grade Type C Mn, max Si, max P, max S, max Cr Ni Mo, max (a) HF 19 chromium, 9 nickel 0.20–0.40 2.00 2.00 0.04 0.04 18.0–23.0 8.0–12.0 0.50 HH 25 chromium, 12 nickel 0.20–0.50 2.00 2.00 0.04 0.04 24.0–28.0 11.0–14.0 0.50 HI 28 chromium, 15 nickel 0.20–0.50 2.00 2.00 0.04 0.04 26.0–30.0 14.0–18.0 0.50 HK 25 chromium, 20 nickel 0.20–0.60 2.00 2.00 0.04 0.04 24.0–28.0 18.0–22.0 0.50 HE 29 chromium, 9 nickel 0.20–0.50 2.00 2.00 0.04 0.04 26.0–30.0 8.0–11.0 0.50 HT 15 chromium, 35 nickel 0.35–0.75 2.00 2.50 0.04 0.04 17.0–21.0 33.0–37.0 0.50 HU 19 chromium, 39 nickel 0.35–0.75 2.00 2.50 0.04 0.04 17.0–21.0 37.0–41.0 0.50 HW 12 chromium, 60 nickel 0.35–0.75 2.00 2.50 0.04 0.04 10.0–14.0 58.0–62.0 0.50 HX 17 chromium, 66 nickel 0.35–0.75 2.00 2.50 0.04 0.04 10.0–14.0 58.0–62.0 0.50 HC 28 chromium 0.50 max 1.00 2.00 0.04 0.04 26.0–30.0 4.0 max 0.50 HD 28 chromium, 5 nickel 0.50 max 1.50 2.00 0.04 0.04 26.0–30.0 4.0–7.0 0.50 HL 29 chromium, 20 nickel 0.20–0.60 2.00 2.00 0.04 0.04 28.0–32.0 18.0–22.0 0.50 HN 20 chromium, 25 nickel 0.20–0.50 2.00 2.00 0.04 0.04 19.0–23.0 23.0–27.0 0.50 HP 26 chromium, 35 nickel 0.35–0.75 2.00 2.50 0.04 0.04 24.0–28.0 33.0–37.0 0.50 (a) Castings having a speciﬁed molybdenum range agreed on by the manufacturer and the purchaser may also be furnished under these speciﬁcations. Source: ASTM A 297

Table 2 Nominal compositions of nickel- and cobalt-base heat-resistant casting alloys

Composition, % Alloy C Cr Mo Nb Ti Al B Zr Fe W V Ta Re Hf Ni Co Nickel-base alloys Alloy 713 C 0.12 12.5 4.2 2 0.80 6.1 0.012 0.10 2.5 ...... bal ...... Ta 0.90 0.50 ...... 18.5 ...... bal ם Alloy 718 0.04 19 3.05 5.13 B-1900 0.10 8 6 0.1 max 1 6 0.015 0.08 0.35 0.1 . . . 4.3 ...... bal 10 Hastelloy B 0.10 0.6 28 ...... 5 . . . 0.30 ...... bal 2.5 max Hastelloy C 0.07 16 17 ...... 5 4 ...... bal 2.5 max IN-100 0.15 10 3 . . . 4.7 5.5 0.015 0.06 ...... 1 ...... bal 15 IN-738 0.17 16 1.75 0.9 3.4 3.4 0.01 0.10 0.50 max 2.6 . . . 1.75 ...... bal 8.5 MAR-M 246 0.15 9 2.5 . . . 1.5 5.5 0.015 0.05 . . . 10 . . . 1.5 ...... bal 10 TRW-NASA VI A 0.13 6.1 2 0.5 1 5.4 0.02 0.13 333 5.5 333 9 0.2 0.43 bal 7.5 U-700 0.15 max 15 5.2 . . . 3.5 4.25 0.05 max . . . 1 max ...... bal 18.5 Cobalt-base alloys Haynes 21 0.25 27 5 ...... 1 ...... 3 bal Haynes 31 0.50 25.5 ...... 0.01 . . . 2 7.5 ...... 10.5 bal Haynes 151 0.50 20 ...... 0.05 ...... 12.7 ...... bal MAR-M 302 0.85 21.5 ...... 0.005 0.20 . . . 10 . . . 9 ...... bal MAR-M 509 0.30 24 ...... 0.20 ...... 0.50 . . . 7 . . . 3.5 ...... 10 bal Wi-52 0.45 21 . . . 2 ...... 2 11 ...... 1.0 max bal Metallography and Microstructures of Heat-Resistant Alloys / 821 preparation of cast or wrought heat-resistant al- tenitic Fe-Ni-Cr heat-resistant alloys are the of ring patterns remains obscure. Examination loys are emphasized. Tables 1 to 3 list the nom- same as those recommended for cast and has revealed little difference between ring and inal compositions of Fe-Ni-Cr Alloy Casting In- wrought austenitic stainless steels (see the article matrix areas and no measurable influence on me- stitute H-series alloys and other iron-nickel, “Metallography and Microstructures of Stainless chanical properties. nickel-, and cobalt-base cast and wrought heat- Steels” in this Volume). Macroetchants for cast resistant alloys, respectively. and wrought iron-nickel-, nickel-, and cobalt- base heat-resistant alloys are given in Table 4. Specimen Preparation Features observed on the macroetched cast Macroetching disks depict the results of solidification. Near the Sectioning. Various sectioning devices have surface, the grain structure will be finer than been used with heat-resistant alloys. The usual Cast and wrought heat-resistant alloys are ex- elsewhere. Dendrites will be visible, with the pri- precautions regarding excessive heating should amined for macrostructure in the same manner mary axis in the solidification direction. Segre- be followed. For austenitic grades, which are as tool steels and stainless steels. Macroetching gation and shrinkage cavities may also be ob- sensitive to deformation, the more vigorous sec- of cast specimens is generally performed as a served. Porosity due to gas evolution is unlikely tioning techniques, such as band sawing or research tool to study the solidification charac- to be seen. Macroetch features on disks from power hacksawing, will introduce excessive dis- teristics; macroetching of wrought specimens consumable-electrode remelted superalloys are tortion or work hardening, depending on the al- can be performed as part of a research study but different from those observed in ingot cast steels. loy and heat treatment condition. Such methods is more commonly performed as a required test Unique macroetch features observed in these re- are suitable for initial sectioning of large pieces, by product specifications and is, therefore, a rou- melted alloys include freckles, radial segrega- but final cutting of specimens should be per- tine production test. To evaluate castings, the tion, and ring patterns (Ref. 1–7). Freckles are formed using abrasive cutoff machines with disk is generally removed so that the etched sur- dark-etching spots due to localized segregation abrasive blades designed for metallographic face (ground smooth before etching for best re- or to enrichment in carbides or Laves phase. work (blades designed for production cutting in- sults) is parallel to the solidification direction. They are detrimental to material quality. The mi- troduce too much damage and should not be used For wrought alloys, disks are cut (usually from crostructure of a freckle in Rene´41 is shown in for metallography). Heavy deformation intro- wrought billet samples) at representative loca- Fig. 1. Radial segregation appears as dark-etch- duced by sawing may be difficult, if not impos- tions, such as the top and bottom of ingots or ing elongated spots in a radial or spiral pattern. sible, to remove by subsequent grinding and pol- remelted (electroslag remelted or vacuum arc re- Ring patterns are concentric rings that etch ishing steps. Abrasive cutoff wheels used are melted) stock, and are ground before macroetch- lighter (usually) or darker than the matrix. They usually the consumable type. Coolant flow must ing. Macroetchants for the cast or wrought aus- are revealed only by macroetching. The nature be adequate and uniformly distributed to mini-

Table 3 Nominal compositions of wrought heat-resistant alloys

Composition, % Alloy C Fe Ni Co Cr Ti Mo Others Iron-nickel-base alloys A-286 (AISI 660) 0.05 bal 26.0 ... 15.0 2.15 1.25 0.3 V, 0.2 Al, 0.003 B Greek Ascoloy 0.18 bal 2.0 ... 13.0 ... 0.50 max 3.0 W Moly Ascoloy 0.12 bal 2.5 ... 12.0 ... 1.75 0.70 Mn, 0.32 V, 0.025 N Incoloy 800 0.05 bal 32.5 ... 21.0 0.38 ... 0.38 Al Incoloy 901 0.05 bal 42.7 ... 13.5 2.5 6.2 0.25 Al AISI 330 0.05 bal 36.0 ... 19.0 ...... Hastelloy X (AISI 680) 0.10 max 18.5 bal 1.5 22.0 ... 9.0 0.6 W Pyromet 31 0.04 bal 55.5 ... 22.7 2.5 2.0 1.5 Al, 1.1 Nb, 0.005 B Ta, 0.5 Al ם Alloy 718 0.04 18.5 bal ... 19.0 0.9 3.05 5.13 Nb Nickel-base alloys Astroloy 0.06 ... bal 17.0 15.0 3.5 5.25 4.0 Al, 0.03 B Hastelloy B ... 5.0 bal 2.5 max 1.0 max ... 28.0 ... Hastelloy C 0.07 5.0 bal 2.5 max 16.0 ...... 4.0 W Hastelloy C-276 0.02 5.5 bal 2.5 max 15.5 ... 16.0 3.75 W, 0.35 V Ta ם Hastelloy G30 0.03 max 15 bal 5.0 max 30 ... 5.0 1.0 Nb Hastelloy W 0.12 max 5.5 bal 2.5 5.0 ... 24.5 0.6 V Alloy 600 0.1 max 8.0 bal ... 15.5 ...... Alloy 617 0.07 ... bal 12.5 22 ... 9.0 1.0 Al Ta, 0.4 Al max ם Alloy 625 0.1 max 5.0 max bal 1.0 max 21.5 0.4 max 9.0 3.65 Nb Ta, 0.7 Al ם Alloy X-750 0.04 7.0 bal ... 15.5 2.5 ... 0.95 Nb ZMI-3U 0.08 ... bal 5.5 13 5.0 1.2 5.0 W, 3.0 Al, 0.015 B CNK7 0.08 ... bal 9.0 14.8 4.2 0.4 7.0 W, 0.01 B, 0.02 Ce Rene´41 0.09 ... bal 11.0 19.0 3.1 10.0 1.5 Al, 0.010 B max Rene´95 0.15 ... bal 8.0 14.0 2.5 3.5 3.5 Al, 3.5 W, 3.5 Nb U-520 0.05 ... bal 12.0 19.0 3.0 6.0 1.0 W, 2.0 Al, 0.005 B U-700 0.15 max 1.0 max bal 18.5 15.0 3.5 5.2 4.25 Al, 0.05 B U-710 0.07 ... bal 15.0 18.0 5.0 3.0 2.5 Al, 1.5 W, 0.02 B U-720 0.035 ... bal 14.7 18.0 5.0 3.0 1.25 W, 2.5 Al, 0.035 B Waspaloy 0.07 ... bal 13.5 19.5 3.0 4.3 1.4 Al, 0.07 Zr, 0.006 B Haynes HR-160 0.05 2.0 37.0 29.0 28.0 ...... 0.5 Mn Cobalt-base alloys Haynes 25 (L-605) 0.10 ... 10.0 bal 20.0 ...... 15.0 W, 1.5 Mn, 0.55 Si Haynes 188 0.10 1.5 22.0 bal 22.0 ...... 14.0 W, 0.08 La Stellite 6B 1.1 3.0 max 3.0 max bal 30.0 ...... 4.5 W S-816 0.38 4.0 20.0 bal 20.0 ... 4.0 4.0 W, 4.0 Nb MP35N 0.025 max 1.0 max 35.0 bal 20.0 1.0 max 10.0 0.010 B max 822 / Metallography and Microstructures of Nonferrous Alloys mize heat-induced damage. Sectioning of cast specimen is mounted using a compression- preserve that flatness through to the final step. A nickel alloys and all cobalt-base alloys requires molded thermosetting epoxy resin containing a medium-nap cloth can be used for the final pol- use of an abrasive blade that breaks down at a filler material, such as Epomet (Buchler Ltd.) ishing step without problems, unless shrinkage high rate, exposing fresh, sharp particles in the resin. This resin provides near-freedom from gaps are present between specimen and mount- cutting zone. These are the most difficult grades shrinkage gaps between specimen and mount ing material. When shrinkage gaps are present, to section of the heat-resistant alloys. that lead to edge rounding and bleed-out prob- use a polyurethane pad for the final step. Mounting. Many bulk specimens can be lems. Edge retention is much better when auto- When edge retention is not a primary require- ground and polished without mounting, although mated grinding-polishing devices are used. ment, specimens can be mounted using any of specimen identification is generally limited to Rigid grinding disks provide superb edge flat- the popular compression-mounting materials or stamp marks, such as a job number and a spec- ness, and napless, woven cloths or pressed pads castable resins. Most produce acceptable results; imen number. Some automatic grinding and polishing devices require a mounted specimen, usu- 1 1 Table 4 Macroetchants for wrought heat-resistant alloys ally 25, 32, or 38 mm (1, 1 ⁄4,or1⁄2 in.) in diameter; others do not. Mounting facilitates pol- Composition(a) Comments ishing of small or irregularly shaped specimens. 1. 1 part 30% H2O2, 2 parts HCl, 2 or 3 parts H2O Recommended for nickel-chromium alloys; use fresh solution; When it is necessary to view the microstruc- reveals grain structure; if the surface is stained, remove ture at the extreme edge of a specimen, it must with 50% aqueous HNO3; etch approximately 2 min 2. (a) 15 g (NH4)2S2O8 (ammonium persulfate) and 75 Lepito’s macroetch for general macrostructure and weldments; be mounted. Edge retention of nonmounted mL H2O mix a and b, add c; immerse 30–120 s at room temperature specimens is inferior, and examination above (b) 250 g FeCl3 and 100 mL HCl c) 30 mL HNO3) ן 100 magnification may be impossible. Opti- mal results are obtained when the edge of inter- 3. 200 g FeCl3, 200 mL HCl, H2O to 1000 mL For nickel-base superalloys; etch to 90 min at 100 C (212 F) 4. HCl saturated with FeCl3 For cobalt-base superalloys; add 5% HNO3 before use at est is plated with electroless or electrolytic nickel room temperature; clean surface by dipping in 50% before mounting (see the article “Mounting of aqueous HCl Specimens” in this Volume). However, excellent 5. (a) 21 mL H2SO4, 15 mL HCl, 21 mL HNO3,21mL For cobalt-base superalloys; etch 5 min in a, then 5 min in b results can be obtained without plating if the HF,22mLH2O (b) 40 mL 20% aqueous CuCl2 (copper chloride), 40 mL HCl, 20 mL HF 6. 50 mL saturated aqueous CuSO4 (copper sulfate) and For iron-nickel- and nickel-base alloys; swab or immerse, 50 mL HCl room temperature (a) Whenever water is specified, use distilled water.

Table 5 Preparation method for Fe-Ni-Cr- and nickel-base heat-resisting alloys

Load/ specimen Surface Abrasive/size N lbf Base speed, rpm/direction Time, min Waterproof paper (water cooled) 220–240-grit (P240–P280) SiC 27 6 240–300 Until plane Complementary or contra Silk cloth (PSA backing) 9 lm diamond with lubricant 27 6 120–150 5 Complementary or contra Polyester cloth 3 lm diamond with lubricant 27 6 120–150 4 Complementary or contra Polyester cloth 1 lm diamond with lubricant 27 6 120–150 3 Complementary or contra Synthetic rayon medium-nap cloth 0.05 lm alumina suspension 27 6 100–150 3 Complementary or contra Note: PSA, pressure-sensitive adhesive. Other surfaces can be substituted in step 1, as long as the abrasive size is similar (for example, a diamond Fig. 1 Microstructure of a freckle in as-rolled Rene´41 grinding disc or a rigid grinding disc could be used). In step 1, remove the damage from sectioning and get all of the specimens in the holder to a common plane. In steps 2 to 4, charge the cloth first with diamond in paste form. With a clean fingertip, press the paste into the cloth. Apply lubricant. (15 mL HCl, 10 mL acetic acid, 10 mL HNO3) During the cycle, periodically add diamond of the same size as a suspension to keep the cutting rate high. Step 4 is optional. Use it for the most difficult specimens. Use contra rotation if the specimen holder motor rotates at 100 rpm. At higher speeds, the suspensions, particularly the alumina, will be splattered all over the walls. Contra rotation is slightly more aggressive and gives better flatness. If the head rotates 100 rpm, the suspension will stay on the cloth better (in complementary rotation, the centrifugal forces shoot the suspensions off the platen and down the drain).

Table 6 Preparation method for cobalt-base heat-resisting alloys

Load/ specimen Surface Abrasive/size N lbf Base speed, rpm/direction Time, min Waterproof paper (water 220–320-grit (P240–P400) SiC 27 6 240–300 Until plane cooled) Complementary or contra Silk cloth (PSA backing) 9 lm diamond with lubricant 27 6 100–150 5 Complementary or contra Polyester cloth 3 lm diamond with lubricant 27 6 100–150 5 Complementary or contra Polyester cloth 1 lm diamond with lubricant 27 6 100–150 4 Complementary or contra Synthetic rayon medium- 0.05 lm colloidal silica or 27 6 80–120 3 nap cloth alumina suspension Complementary or contra Note: PSA, pressure-sensitive adhesive. It is vitally important to use the best possible abrasive cutoff blade, designed for metallography, when working Fig. 2 Example of flashing, a strange etching response with cobalt and its alloys. More aggressive surfaces are not recommended when preparing cobalt alloys (steps 1 and 2). Step 4 is not optional. Final that occurs frequently on nickel-base alloys final polishing is improved by attack polishing. Mix one part H2O2 (30% conc) with 5 parts of abrasive suspension. Avoid skin contact. For step 5, stop polished with colloidal silica and etched with reagents con- adding abrasive with 20 s left in the cycle. With 10 s left in the cycle, direct the water jet onto the cloth surface to wash both the specimen and the .ions (glyceregia etch of alloy 718) cloth. This simplifies cleaning מtaining Cl Metallography and Microstructures of Heat-Resistant Alloys / 823 each resin has advantages and disadvantages. is really good enough until the specimen has rpm. If the head operates at a higher speed, the The selection of a particular resin is often based been properly prepared. If the metallographer is abrasive may be thrown off the platen surface on familiarity or cost. simply asked to rate the grain size of a specimen onto the operator and the walls. When the head Grinding of specimens takes place by hand according to a chart method, then some degree rotates at 90 rpm, it stays on the platen surface or by use of automatic devices. Hand grinding of imperfection can be tolerated. However, in nicely during the cycle. When the head rotates should be avoided, if at all possible, because the failure analysis, research and many quality stud- in the same direction as the platen (complemen- specimens cannot be kept perfectly flat during ies, important information will be missed if the tary), the abrasive is thrown off the platen, into grinding. Grinding is most commonly performed specimen is not prepared properly so that the true the surrounding bowl, and down the drain due using water-cooled silicon carbide (SiC) paper at structure can be seen. to centrifugal force, regardless of the head speed. 240 to 300 rpm. Today, a variety of other grind- Although a wide range of cloths is available, Of course, the faster the platen and head speed, ing formats are available, although SiC remains low-nap or napless cloths are preferred for rough the greater the centrifugal force, and the faster very popular. Historically, the usual grit-size se- polishing, although canvas, which was quite the abrasive goes down the drain. quence was 120, 240, 320, 400, and 600 grit popular, provides economical durability. Today, A second diamond-polishing step, generally (P120, P280, P400, P600, and P1200 in the Fed- woven, napless cloths, such as natural silk or using 3 or 1 lm diamond, is usually performed eration of European Producers of Abrasives, or synthetic nylon or polyester, are recommended, in modern methods but was optional in the for- FEPA, scale); finer grits were occasionally used. because they provide good cutting rates, excel- mer “traditional” methods. A synthetic suede Today, preparation procedures employ only one lent surface flatness, and good surface finishes medium-nap cloth was commonly used, but grinding step before switching to diamond abra- for each abrasive size used. Synthetic, napless other cloths may also yield good results. This sives on a napless woven cloth or a pressed pad. chemo-textile pads are also used. Billiard cloth step is carried out in the same manner as the When proper metallographic abrasive blades and red felt have been used, but they can cause initial diamond polishing. After each diamond have been used to section the specimens, so that excessive relief, poor edge retention, pull-outs, polishing step, the specimen should be carefully cutting damage is minimal and the surface finish and other artifact problems, especially if the lu- cleaned to remove abrasive, extender oil, and is optimal, grinding should commence with the brication level is not correct. A lubricant/ex- polishing debris. Ultrasonic cleaning produces finest possible abrasive, such as 180-, 240-, or tender fluid, compatible with the diamond abra- excellent results but is not always required, es- 320-grit SiC (P180, P280 or P400 in the FEPA sive, should always be added to reduce friction pecially with wrought specimens. With a cast scale). The coarser the abrasive, the greater the and drag and to promote more efficient cutting. specimen that contains voids, it may be best to damage introduced by grinding. Thus, when cut- Wheel speeds for polishing are 100 to 150 rpm, ultrasonically clean after each preparation step. ting damage is minimal, a finer-size abrasive can in most work. Moderate pressure is applied. Today, medium-nap cloths are no longer rec- be used for the grinding step. This removes the During hand polishing, the specimen should ommended for use with diamond abrasives in damage from cutting while imparting less dam- be rotated counter to wheel rotation while it is most polishing routines. Instead, woven, napless age than a coarser grit size grinding abrasive. moved slowly from center to edge. Again, the cloths or pressed chemo-textile pads are pro- Moderately heavy pressure is used for hand specimen must be held firmly against the wheel moted, because flatness and edge retention are grinding. The specimen must be held flat against to avoid rocking. Polishing should continue until better. the paper. After each grinding, the specimen is grinding scratches are removed; 1 to 2 min is Final polishing, historically, involved one or rinsed, wiped clean, and rotated 45 to 90 before usually adequate. With an automated machine, more steps, depending on the need to remove all grinding with the next paper. Grinding should the specimen rotates in either the same direction scratches. For routine examination, polishing to proceed for approximately twice as long as or the opposite direction to the platen (comple- a1lm diamond finish may be adequate. When needed to remove all the scratches from the pre- mentary or contra rotation, respectively). Contra photomicroscopy is anticipated, additional steps vious step; 1 to 2 min per step is usually ade- rotation is slightly more aggressive. It is best are usually required. A wide range of final pol- quate. Automated grinding devices produce used when the specimen holder rotates at 90 ishing abrasives may be used. Alumina slurries omni-directional scratch patterns. Heat-resistant alloys are not susceptible to embedding of silicon carbide from the grinding paper, but if the Table 7 Electropolishing techniques for wrought heat-resistant alloys specimen contains cracks or pores, it may be ad- visable to clean ultrasonically after each step. Electrolyte composition Comments 2 For most specimens, a simple wash under run- 1. 37 mL H3PO4, 56 mL glycerol, 7 mL H2O For Inconel 625, use 1.2–1.6 A/cm2 (8–10 A/in. ); for Incoloy 800, use 3.1 A/cm2 (20 A/in.2); platinum cathode ning water will remove any loose abrasive or 2 2 2. 25 mL H3PO4,25mLHNO3,50mLH2O For Inconel 600 and X-750, use 17.8 A/cm (115 A/in. ), 5– grinding debris. Specimens should be rinsed in 10 s; platinum cathode alcohol (preferably ethanol) and dried with hot 3. 144 mL ethanol, 10 g AlCl3 (anhydrous aluminum For cobalt-base superalloys, use 23–25 V dc at room air. chloride), 45 g ZnCl2 (anhydrous zinc chloride), 16 mL temperature with successive 1 min periods N-butyl alcohol, 32 mL H2O Rough polishing, historically, often began 2 2 with6or3lm diamond abrasive, generally as a 4. 40 mL HClO4 (perchloric acid), 450 mL acetic acid, 15 For Nimonic alloys, use 15 V dc, 0.1 A/cm (0.65 A/in. ), mL H2O below 25 C (77 F) 2 2 paste, although aerosols or slurries were also 5. 10 mL HClO4 and 90 mL acetic acid For nickel-base alloys, use at 0.5–0.9 A/cm (3–6 A/in. ), 30 used. Diamond abrasives have largely replaced s for aged specimens, longer for solution-annealed ones; keep cool (10-15 C, or 50-60F); best results by polishing use of 5 lm alumina (Al2O3) for rough polishing, except where economic considerations do not in 5 s intervals 6. 70 mL methanol and 10 mL H2SO4 For nickel-base superalloys, use at 20–25 V dc, 0.3–0.8 A/ permit use of diamond abrasives. In modern pro- cm2 (2–5 A/in.2), room temperature; 10–15 s after a 600- Ј cedures, the grinding step is usually followed by grit finish or 5 s after a 0.3 lmAl2O3 finish; c slightly 9or6lm diamond on a napless, woven cloth or etched, carbides in relief cЈ pressed pad. This step is followed by one or two 7. 60 mL methanol, 10 mL H2SO4, 5 mL HCl Use same as No. 6; produces more etching of phase; if HNO3 is substituted for HCl, the surface will be smooth more diamond abrasive steps, depending on the without relief or attack. difficulty in preparing the alloy, or on the degree 8. 7 mL ethanol, 20 mL HClO4, 10 mL glycerol Use same as No. 6; mix carefully, keep cool; produces of perfection desired by the metallographer. Peo- smooth surfaces 9. 15 mL HCl and 85 mL methanol For nickel-base superalloys, use at 30–40 V dc, 0.3–1.2 A/ ple in production laboratories often believe that cm2 (2–8 A/in.2), at room temperature for 5–10 s after a they save time and money by preparing speci- 600-grit silicon carbide finish or 2–5 s after a 0.3 lmAl2O3 mens “just good enough” for their work. How- finish; produces strong carbide relief, etches cЈ; very good ever, one does not know that “just good enough” for SEM examination 824 / Metallography and Microstructures of Nonferrous Alloys are quite common, using 0.3 and/or 0.05 lm Final polishing is generally performed using a the cloth during polishing. Moderate pressure is Al2O3. Nearly all alumina abrasives are pro- synthetic suede medium-nap cloth or a polyure- used, and care must be taken to avoid rocking the duced by the calcination process, and agglom- thane pad at 100 to 150 rpm. During hand polish- specimen. A polishing time of 1 to 2 min is usu- erates are always present, even in the deagglom- ing, the specimen should be rotated counter to the ally adequate. After polishing, the specimen erated abrasives. However, alumina is now wheel rotation direction while it is moved from should be carefully cleaned and dried to avoid available in the 0.05 lm size, made by the sol- center to edge. The slurry is added periodically to staining, and the operator’s hands must be gel process where the particles are grown from a solution. This abrasive, called MasterPrep (Buehler Ltd.) alumina, produces superior re- Table 8 Microetchants for wrought heat-resistant alloys sults compared to ordinary aluminas and is free of the problems associated with the use of col- Composition Comments loidal silica. 1. 2 parts glycerol, 3 parts HCl, 1 part HNO3 Glyceregia; mix fresh, do not store; discard when solution is orange; Colloidal silica (Ref 1) produces excellent re- use by immersion or swabbing 5–60 s; very popular general etch for structure of iron- and nickel-base superalloys; cЈ in relief sults with these alloys but is more difficult to use. 2. 5 mL HF, 10 mL glycerol, 85 mL ethanol Electrolytic etch at 0.04–0.15 A/cm2 (0.25–1.0 A/in.2), 6–12 V dc; for The amorphous silica particles will crystallize if nickel-base alloys, cЈ in relief; stop etch when edges are brownish; the solution evaporates. This ruins the cloth. excellent etch for transmission electron microscopy (TEM) replica work Specimens must be carefully cleaned, because a 2 2 whitish film may be present that will alter etch 3. 12 mL H3PO4,47mLH2SO4, 41 mL HNO3 Electrolytic etch at 6 V dc, 0.12–0.15 A/cm (0.75–1.0 A/in. ), a few seconds; add to 100 mL H2O to slow etch; for nickel-base alloys; response. To clean the surface, stop adding abra- use under hood; mix H3PO4 and HNO3, then add H2SO4; stains sive with approximately 20 s left in the cycle matrix when cЈ is present; good for revealing segregation and for (using an automated polisher). With approxi- examining cЈ with TEM replicas; attacks Bakelite (Georgia-Pacific mately 10 s left in the cycle, direct the water jet Corp.); stop etch when edge of specimen is brownish 4. 30 mL lactic acid, 20 mL HCl, 10 mL HNO3 For nickel-base superalloys onto the cloth surface. The specimen and the 5. 5g CuCl2, 100 mL HCl, 100 mL ethanol Waterless Kalling’s reagent; immerse or swab to a few minutes; for cloth will be cleaned simultaneously. When the iron- and nickel-base superalloys machine stops, rinse the sample holder under 6. 10 g CuSO4, 50 mL HCl, 50 mL H2O(a) Marble’s reagent for iron-nickel- and cobalt-base superalloys; immerse or swab 5–60 s; a few drops of H2SO4 will increase etch activity; running water, scrub the surfaces with ethanol, reveals grain boundaries and second-phase particles and blow dry. However, even with this precau- 7. 5 mL H2SO4, 3 mL HNO3, 92 mL HCl For iron- and nickel-base alloys; add H2SO4 to HCl, stir, allow to cool, tion, strange etch results can occur. For example, add HNO3; discard when orange; swab 10–30 s; use under hood; do when swab etching with reagents such as gly- not store 8. 20 mL HNO3 and 60 mL HCl Aqua regia; for iron- and nickel-base superalloys; use under hood, do ceregia, it usually takes approximately 60 to 120 not store; immerse or swab 5–60 s; attacks r phase, outlines s to bring up etch detail and make the surface carbides, reveals grain boundaries Ј appear to be etched to the proper level by eye. 9. 50 mL HCl and 1–2 mL 30% H2O2 For nickel-base alloys; attacks c phase; immerse 10–15 s 2 2 However, after using colloidal silica, sometimes 10. 5 mL H2SO4, 8 g CrO3,85mLH3PO4 Electrolytic etch at 10 V dc, 0.2 A/cm (1.3 A/in. ), 5–30 s; reveals inhomogeneities in nickel-base alloys מwhen etching with reagents that contain Cl 11. 10 mL HNO3, 10 mL acetic acid, 15 mL HCl, Acetic glyceregia; use fresh, same precautions as glyceregia; used for ions, the surface immediately turns dull gray as 2–5 drops glycerol hard-to-etch solution-treated nickel-base alloys soon as the etchant touches the specimen (called 12. 15 mL HCl, 10 mL acetic acid, 10 mL HNO3 15-10-10 etch; use in same manner as glycerregia or acetic-glyceregia; “flashing”). This produces a heavy craze-crack use for more difficult to etch grades, such as alloy 625 13. (a) 33 mL HCl and 67 mL H2O Beraha’s tint etch for nickel-base alloys; add 0.6–1 g K2S2O5 appearance when viewed microscopically (Fig. (potassium metabisulfite) to 100 mL stock solution a; immerse (never 2). This false structure cannot be removed by swab) 60–150 s, slowly agitate; if colors are not developed, add 1– simply repeating the last preparation step. In- 1.5 g FeCl3 or 2–10 g NH4F-HF (ammonium bifluoride) to 100 mL stead, one must go back to approximately a 240- stock solution b; immerse 60–150 s, agitate gently; colors matrix grit (P280) SiC abrasive to remove the affected (b) 50 mL HCl and 50 mL H2O 14. 10 g K3Fe(CN)6 (potassium ferricyanide), 10 Murakami’s reagent; for iron- and nickel-base superalloys; use hot (75 layer. Interestingly, electrolytic etchants never g KOH, 100 mL H2O C, or 170 F) to darken a phase; use at room temperature to darken produce this problem. To avoid this problem, carbides; better results may be obtained if the specimen is first start the final step with colloidal silica, flush it etched in 50% aqueous HNO3 at 8 V dc; use under a hood 15. 10 g CrO3 and 100 mL H2O Electrolytic etch at 6 V dc, 10–30 s; for iron- and nickel-base off with water, and complete the cycle using the superalloys; r attacked, carbides outlined or attacked sol-gel alumina slurry. One can split the total 16. 80 mL H3PO4 and 10 mL H2O Electrolytic etch for nickel-base superalloys at 3 V dc (closed circuit), time equally between the two abrasives or use 0.11–0.12 A/cm2 (0.7–0.8 A/in.2), 7–9 s; if the surface is stained, any desired percentage of time with each, as long swab with the electrolyte; use fresh solution; used to determine the as approximately 10 s are dedicated at the end degree of carbide continuity at the grain boundaries 17. 25 g CrO3, 130 mL acetic acid, 7 mL H2O Electrolytic etch for nickel-base superalloys at 10 V dc (closed circuit) of the cycle to the sol-gel alumina abrasive. for 2 min; the current density will drop during the first 20 s; use Again, use the previously mentioned cleaning fresh; used to reveal prior grain boundaries procedure to clean both the cloth and specimen 18. 30 mL HCl, 7 mL H2O, 3 mL 30% H2O2 Popular etch for cobalt-base superalloys 19. 100 mL HCl and 5 mL 30% H2O2 Popular etch for cobalt-base superalloys; up to 20% H2O2 has been before the cycle ends, or use only the sol-gel used; mix fresh; immerse 1–10 s alumina suspension. 20. 5–10 mL HCl and 95–90 mL H2O Electrolytic etch for cobalt-base superalloys; use at 3 V dc, 1–5 s, Cleaning between steps is important, because carbon cathode contamination can occur. In many cases, the 21. 2 mL H2SO4 and 98 mL H2O Etch first with glyceregia to dissolve matrix uniformly, then etch electrolytically at 6–12 V dc, 0.12–0.15 A/cm2 (0.75–1.0 A/in.2) holder can be simply rinsed under running water until edge of specimen is brownish; good for TEM replica studies to remove the debris. However, it is often nec- 22. 5 mL HF, 10 mL glycerol, 10–50 mL ethanol, Etch first with glyceregia to dissolve matrix uniformly, then etch with Ј 2 essary to scrub the surfaces with ethanol to re- H2O to 100 mL total volume solution at left to dissolve c ; use at 6–12 V dc, 0.12–0.15 A/cm move adherent debris. If this falls down onto the (0.75–1.0 A/in.2) for less than 1 s; good for TEM replica work or cloth in the next step, the cloth will be contam- SEM examination 23. 100 mL water, 100 mL HCl, 100 mL HNO3, Molybdic acid etch; mix and let stand for 1 h minimum. Immerse for inated, and the results will be poor. The opera- 3 g molybdic acid several seconds. Etch can be stored after use. Excellent for as-cast tor’s hands must also be cleaned after each step, dendritic structures because contamination can result if this is not 24. 150 mL HCl, 50 mL lactic acid, 3 g oxalic Lucas’ reagent; use at 1–2 V dc for 10–20 s to reveal the structure of done. Cleanliness is important in metallography, acid iron-nickel-, nickel-, and cobalt-base superalloys and this extends to the entire laboratory. (a) When water is specified, use distilled water. Metallography and Microstructures of Heat-Resistant Alloys / 825 cleaned after each rough and final polishing step ture after etching. Tables 5 and 6 present auto- tron microscopy examination of second-phase to prevent contamination. mated procedures for preparing Fe-Ni-Cr-, particles in superalloys, because they stand Automatic polishing machines are quite useful nickel-, and cobalt-base heat-resisting alloys. above the matrix, and their shape can be easily for final polishing. A wide variety of devices is Mechanical polishing is sometimes followed assessed. However, such an image is unsuitable available. The time required using these units with a brief electropolish to remove any smeared for quantitative metallography unless the results ranges from a few minutes to approximately 30 or flowed metal without introducing preferential are corrected to account for the etch depth. Table min for vibratory polishing. Vibratory polishing attack of the second-phase constituents. Ex- 7 lists appropriate electropolishing solutions for will remove any minor damage that might be tended electropolishing should be avoided. This nickel- and cobalt-base heat-resistant alloys. present and will yield crisper images of the struc- procedure has been promoted for scanning elec- Electropolishing solutions for Fe-Ni-Cr austenitic alloys are the same as those used for wrought austenitic stainless steels. Etching. Some minor phases in heat-resisting alloys can be observed easily in the as-polished condition. Light relief can be introduced during final polishing to accentuate these particles by hand polishing the specimen using a figure-eight motion and light pressure for approximately 10 to 20 s on a stationary platen, using either alumina or colloidal silica. This makes the particles easier to see, even without etching. A brief electropolish can also be used for this purpose. If image analysis measurements are to be performed, relief must be minimized, or the stere- ological rules, unless corrected to account for the etch depth, are invalid, and measurement bias will result. The particles can be observed in bright-field illumination by color contrast, which is useful for phase identification. Carbides, nitrides, carbonitrides, and borides are readily observed without etching. Viewing with differential interference contrast illumination will bring out height differences between the particle and the matrix. Generally, nitrides show less relief than carbides relative to the matrix. The cast or wrought Fe-Ni-Cr heat-resistant alloys are basically austenitic stainless steels (the wrought alloys are usually called iron-nickel- base alloys even though they all contain more than 12% Cr). The techniques for etching and identifying phases in wrought austenitic stainless steels apply to these alloys (see the article “Met- allography and Microstructures of Stainless Steels” in this Volume). Preparation practices are identical. Glyceregia is one of the most prevalent re- Fig. 3 Cast dendritic structure of IN-738 revealed using (a) Kalling’s No. 2, (b) 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3, (c) the Lucas electrolytic reagent (2 V dc, 10 s), and (d) Beraha’s tint etch (50 mL HCl, 50 mL water, agents for revealing the general structure of Fe- 0.8 g K2S2O5,4gNH4F•HF, 1 g FeCl3 Ni-Cr- and nickel-base heat-resistant alloys. It

Fig. 4 Cast dendritic structure of MAR-M 247 revealed using (a) glyceregia, (b) the Lucas electrolytic reagent (2 V dc, 10 s), and (c) the molybdic acid reagent 826 / Metallography and Microstructures of Nonferrous Alloys should always be mixed fresh and discarded although second-phase particles will generally etch behavior (called ﬂashing) after polishing carefully when it turns orange. Glyceregia will be outlined. For etching solution-annealed with colloidal silica. As soon as etching begins, etch all the heat-resistant grades, except some of nickel-base alloys, the glycerol content is often the surface becomes dull. When viewed with the the high-cobalt-content superalloys. Glyceregia decreased, and the nitric acid (HNO3) content is microscope, a heavy craze-crack pattern is ob- will reveal grain and twin boundaries and second often increased. The standard composition is served that can be quite deep. The problem phases in the leaner alloys but will be less effec- ideal for solution-annealed and aged specimens, seems to be due to passivation effects. To tive, or ineffective, in revealing the grain struc- which are easier to etch. Specimens etched with counter this problem, follow the colloidal silica -ions may exhibit erratic polish with a short polish with alumina. Electro מture in the higher nickel and chromium alloys, reagents containing Cl lytic etchants do not have this problem, even after using colloidal silica. The mixture of hydrochloric acid, sulfuric acid, and nitric acid (HCl-H2SO4-HNO3) (95-5- 3) is also quite popular for these alloys and is used similarly. For grain size examination in aged specimens, etching with waterless Kalling’s reagent (Kalling’s number 2) or Marble’s reagent is quite common. These reagents can be made and stocked in reasonable quantities. Several electrolytic reagents are also commonly used. Color etchants, although not widely used for these alloys, can produce good results (see the article “Color Metallography” in this Volume). Table 8 lists some of the more commonly used reagents for etching Fe-Ni-Cr-, nickel-, and cobalt-base heat-resistant alloys. Additional information regarding the etching of wrought heat- resistant alloys can be found in Ref 1 and 8 to 15. The effects of 19 etchants on 11 Fe-Cr-Ni alloys containing 0.02 to 0.18% C have been documented (Ref 8). Vilella’s reagent proved superior for removing disturbed metal (using several etch and polish cycles) and for outlining r phase, carbide particles, and ferrite. Etching for 1 min at room temperature was recommended. Staining etchants form ﬁlms of reaction products on the surface of the specimen. These etchants are generally aqueous solutions of potassium hydroxide (KOH) or sodium hydroxide (NaOH) with an oxidizing agent added. Picrates, potassium perman- ganate (KMnO4), hydrogen peroxide (H2O2), and ferricyanides are used as oxidizing agents. These reagents are used to color carbides, delta ferrite, and sigma and chi phases. Murakami’s reagent, which contains NaOH with potassium ferricyanide [K Fe(CN) ] as the Fig. 5 Cast dendritic structure of Russian nickel-base alloy CNK7 revealed using (a) glyceregia, (b) Kalling’s No. 2, (c) 3 6 the Lucas electrolytic reagent (2 V dc, 10 s), and (d) Beraha’s tint etch (50 mL HCl, 50 mL water, 0.8 g K2S2O5, oxidizing agent, is a versatile staining etchant. 4gNH4F•HF, 1 g FeCl3 At least four variations of the original compo-

Fig. 6 Cast dendritic structure of MAR-M 509 cobalt-base superalloy revealed using (a) 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3, (b) the Lucas electrolytic reagent (2 V dc, 20 s), and (c) Beraha’s tint etch (50 mL HCl, 50 mL water, 0.8 g K2S2O5,4gNH4F•HF Metallography and Microstructures of Heat-Resistant Alloys / 827

Fig. 7 Alloy A-286 (AISI 660, 195 HV), solution annealed 2 h at 900 C (1650 F) and oil quenched. (a) View showing very fine austenite grain size. Glyceregia. Originalmagnification b) View showing area near the surface of the specimen with a duplex grain structure. Tint etch: 20 mL HCl, 100 mL H2O, 2.4 g NH4F•HF, and 0.8 g K2S2O5. Original) .ן100 ןc) View showing the very fine austenite matrix grains. Tint etch: 20 mL HCl, 100 mL H2O, 1 g NH4F•HF, 0.5 g K2S2O5. Original magnification 200) .ןmagnification 100

Fig. 8 Alloy A-286 (AISI 660, 357 HV), solution annealed 2 h at 900 C (1650 F), oil quenched, and held 16 h at 720 C (1325 F). (a) View showing very fine-grained structure similar to that shown in Fig. 7(a). Glyceregia. (b) View showing a region near the surface of the specimen with a duplex grain structure. Tint etched some as Fig. 7(b). (c) View ןshowing the very fine matrix grain structure. Tint etched same as Fig. 7(b). All original magnification 100

Fig. 9 Alloy A-286 (AISI 660, 150 HV), solution annealed 1 h at 980 C (1800 F) and oil quenched. (a) View showing a coarser grain structure than in Fig. 7 and 8 due to the higher solutionizing temperature. Glyceregia. (b) Tint etched using 20 mL HCl, 100 mL H2O,1gNH4F•HF, and 0.5 g K2S2O5. (c) Alloy A-286 (AISI 660, 318 HV), solution annealed ןh at 980 C (1800 F), oil quenched, aged 16 h at 720 C (1325 F), and air cooled. Glyceregia. All original magnification 100 1 828 / Metallography and Microstructures of Nonferrous Alloys sition have been reported in the literature. Mu- IN-738, a nickel-base cast alloy, etched with four different locations, etched with glyceregia versus rakami’s permits differentiation between several different etchants. The color etch (Fig. 3d), a Beraha-type color etch (20 mL HCl, 100 mL • types of carbide and sigma phase. Certain phases which is usually very sensitive to chemistry var- water, 0.5 g K2S2O5,1gNH4F HF). Figure 11 are colored only when it is used at room tem- iations, did the best job revealing the dendrites. shows the grain structure of solution annealed perature or when used boiling. Reliance on pro- Figure 4 shows the dendritic structure of cast and aged Pyromet 31, an iron-nickel superalloy duction of specific colors to identify phases is MAR-M 247, a nickel-base alloy, etched with etched with waterless Kalling’s and with a Ber- less reliable than whether the phase is colored glyceregia, the Lucas electrolytic reagent, and aha-type color etch (66 mL HCl, 33 mL water, when used either at room temperature or boiling. the ammonium molybdate reagent. All three re- 1gK2S2O5). Figure 12 shows the grain structure Murakami’s reagent has been used cold, warm, vealed the dendrites well, but the latter gave the of alloy X-750, a nickel-base superalloy in the or boiling to obtain various effects, but it must strongest contrast. Figure 5 shows the dendritic solution annealed and aged condition etched be used with care. It is best to check its results structure of CNK7, a cast Russian nickel-base with glyceregia, waterless Kalling’s, Marble’s ם with control specimens of the same or similar alloy, etched with four different, etchants. The reagent, aqua regia, HCl 1% Na2O2, and a composition where the thermal history is known Lucas reagent (Fig. 5c) and the color etch (Fig. Beraha tint etch (50 mL HCl, 50 mL water, 1 g and other more definitive characterization meth- 5d) gave the strongest contrast. Figure 6 shows K2S2O5). Figures 13 to 15 show the grain struc- ods have been used (e.g., x-ray diffraction, con- the dendritic structure of MAR-M 509 revealed ture of Waspaloy, a nickel-base superalloy, in the vergent-beam electron diffraction, etc.). by using three different etchants. solution annealed (1010, 1035, and 1065 C, or Electrolytic etching, when the time is con- Different etchants can reveal the structure of 1850, 1900, and 1950 F) and aged conditions, trolled, offers precision and reproducibility. The wrought alloys with different results, as dem- etched with glyceregia and with a Beraha-type specimen to be etched is usually made the anode; onstrated by Fig. 7 to 10, where alloy A-286 is color etchant (50 mL HCl, 50 mL water, 1 g stainless steel is often used as the cathode (ide- shown in different heat treatment conditions and K2S2O5). In each of these examples, the color ally, the cathode should be more noble than the anode). The current can be supplied by a variable voltage direct current (dc) power supply, although ordinary dry-cell batteries wired in series to provide outputs of 1.5, 3.0, 4.5, and 6.0 V can be used. Current density will range from less than 0.16 to 2 A/cm2 (1 to 13 A/in.2) or more. A wide variety of methods have been developed to electrolytically etch either mounted or unmounted specimens (Ref 1). Unmounted specimens are held with stainless steel tongs. If the specimen is mounted in a nonconducting material, the electrical connection can be conven- iently made using a brass machine screw that contacts the underside of the specimen through a tapped hole. The electric current at the anode surface promotes oxidation and therefore serves in place of the oxidizing agents that are added to hydroxide solutions. Some electropolishing procedures are useful for microstructural examination using scanning electron microscopy (SEM) or transmission Fig. 10 Alloy A-286 (AISI 660, 318 HV), solution annealed 1 h at 980 C (1800 F), oil quenched, aged 16 h at 720 electron microscopy (TEM) using replicas. The C (1325 F), and air cooled. (a) Glyceregia. (b) Tint etched. Only the matrix phase has been colored. 20 mL ן • electropolishing solution removes the cЈ phase HCl, 100 mL H2O, 1 g NH4F HF, and 0.5 g K2S2O5. Original magnification, both 100 but not the matrix (or more slowly), and subsequent etching is unnecessary. Backscattered electron imaging with the SEM can be very useful for viewing the second-phase particles without etching. For replica examination, it is often helpful to use a reagent or an electropolish that attacks cЈ so that this phase is readily distin- guished from other phases (Ref 16–18). In such cases, cЈ is recessed, but other second phases are in relief. Selective etchants and heat tinting have been commonly used to differentiate various carbide types and to identify phases. Borides, which are similar in appearance to metal carbides, can be discriminated by selective etching. Metal carbides are selectively colored; borides are unaf- fected (Ref 19). There are differences in how etchants bring up the microstructure of alloys and this can only be learned by experimentation. A few examples of the effect of different etchants in revealing the dendritic structure of cast alloys follow to illus- Fig. 11 Pyromet 31 (40 HRC), solution annealed and aged. (a) Etched using Kalling’s reagent 2 (waterless Kalling’s). ןtrate this. Figure 3 shows the microstructure of (b) Tint etched with 66 mL HCl, 33 mL H2O, and 1 g K2S2O5. Original magnification, both 100 Metallography and Microstructures of Heat-Resistant Alloys / 829 etch revealed the grain structure with strong con- weight plus the spring pressure applied will press men, as shown in Fig. 18(a) and (b), or an Alnico trast. on the suspension under the glass, keeping it thin horseshoe magnet can be placed alongside, as Identification of Ferrite by Magnetic Etch- and relatively uniform in thickness. shown in Fig. 18(c). The horseshoe magnet pro- ing. Because of the low contrast, or lack of con- An electromagnet or a permanent magnet can vides a horizontal and a vertical field. The field trast, after etching, it is sometimes difficult to be used to apply a magnetic field to the speci- is most intense through that part of the specimen differentiate ferrite contained in an austenite ma- men. If an electromagnet is used for applying the closest to the pole pieces of the magnet. trix. A technique using a magnetic field and magnetic field, it is a solenoid placed around the Transmission Electron Microscopy. Be- magnetic particles (smaller than 30 nm) in an specimen, as shown in Fig. 16. It has a winding cause some of the important constituents in organic or an aqueous colloidal suspension can of 2300 A turn and operates on batteries or on wrought heat-resistant alloys, such as cЈ phase, be used in conjunction with the light optical mi- direct current rectified from alternating current. are generally too small to observe using the light croscope to identify ferrite positively (Ref 20– The field can be cut off or reversed readily using optical microscope, considerable use has been 24). Details of the magnetic technique are de- switches. A variable autotransformer between made of TEM. Besides affording greater reso- scribed as follows. Additional information can the line and rectifier input or a rheostat in series lution and higher magnification, TEM provides be found in the article “Contrast Enhancement with a battery source controls the intensity of the means for phase identification by electron dif- and Etching” in this Volume. magnetizing field. The magnetic field will cause fraction and, when equipped with x-ray detec- A drop (5 lL measured in a micropipette) of a visible concentration of the colloid particles tors, can provide chemical analysis data. Such the suspension is deposited on the surface of a over the magnetic areas. Figure 17(b) shows the analytical procedures are necessary to under- specimen that has been polished and lightly magnetic pattern produced. Comparison of Fig. stand the strengthening mechanisms for heat-re- etched with HCl. A thin cover is placed over the 17(b) with Fig. 17(a) shows that the magnetic sistant alloys. Today, field emission SEMs pro- suspension. If the specimen is contained in a 25 pattern identifies the ferrite in the specimen. vide enough resolution to examine the cЈ mm (1 in.) diameter plastic mount, a cover glass For cursory ferrite identification, it is simpler particles in nearly all wrought superalloys. The of the same diameter is ideal. The specimen is to use a permanent magnet. Various shapes are cЈ in cast heat-resistant alloys is coarser than in placed on an inverted metallographic micro- available. A cylindrical Alnico permanent mag- wrought alloys and easier to study. Electron- scope. When the specimen is face down, its net can be placed on top of the inverted speci- backscattered diffraction can provide a useful al-

.b) Etched using Kalling’s reagent 2) .ןFig. 12 The effects of different etchants on solution-annealed and aged alloy X-750. (a) Etched using glyceregia. Original magnification 100 Na2O2. Original magnification 1% ם e) Etched using HCl) ןd) Etched using aqua regia. 100) .ןc) Etched using Marble’s reagent. 100) .ןOriginal magnification 100 f) Tint etched in 50 mL HCl, 50 mL H2O, and 1 g K2S2O5) .ן100 830 / Metallography and Microstructures of Nonferrous Alloys ternative to the more tedious convergent-beam (1550 F), the cЈ is finer and spherical in shape grain structure of Astroloy and cЈ in a specimen electron diffraction with the TEM for identifi- with two different sizes, as shown in Fig. 24. If solution annealed 1 h at 1150 C (2100 F). The cation of the crystal structure of phases. Energy- an IN-738 casting is aged at 815 C (1500 F) cЈ is fine and spherical. Figure 29 shows the dispersive spectroscopy provides chemical infor 1000 h, the cЈ is spherical with a single size same nickel-base alloy, Astroloy, solution an- formation and can be used with either the SEM distribution (Fig. 25). Other shapes and phases nealed 4 h at 1150 C (2100 F), air cooled, aged or the TEM. can be observed in cast superalloys, as illustrated 4 h at 1080 C (1975 F), oil quenched, aged 4 Several types of specimens can be prepared in Fig. 26. This shows light optical and TEM hat845C (1550 F), air cooled, aged 16 h at for TEM examination; each type has advantages views of a MAR-M 246 casting held at 980 C 760 C (1400 F), and air cooled. Light micros- and disadvantages. The replica method, which (1800 F) for 5000 h. There is needlelike M6C copy reveals grain-boundary MC carbides and had been prevalent, is being replaced by use of carbide and cЈ in the matrix. In this case, the cЈ coarse cЈ, while the TEM replica reveals irreg- the SEM (Ref 17, 18). A well-polished and prop- is not uniform in shape. Figure 27 shows an ex- ularly shaped cЈ in the grain boundaries and two erly etched specimen can be examined with a ample of the structural changes in cast MAR-M sizes of cЈ from the aging treatments at 845 and or 246 heated above 980 C (1800 F). Compare 760 C (1550 and 1400 F). Figure 30 shows ןstandard SEM at magnifications of 50,000 more. Therefore, much of the structural exami- these micrographs to those in Fig. 26. differences in cЈ precipitation in alloy X-750. In nation role of TEM replicas can be accomplished Transmission electron microscopy examina- Fig. 30(a), the specimen was solution annealed without replica preparation and the complication tion is also crucial in the examination of the fine at 1150 C (2100 F) for 2 h, air cooled, then of replica interpretation or artifact control. strengthening phases in wrought alloys. Figure aged 24 h at 815 C (1500 F). The cЈ is small In addition, the contrast mechanisms operable 28 shows light optical and TEM views of the and uniformly dispersed. M23C6 carbide can be in the SEM are valuable for structural examination. Because chemical analysis using SEM is limited to features larger than a few microns, TEM examination and analysis of extracted constituents remains an important procedure. Pro- cedures for preparing structural and extraction replicas are discussed in Ref 25 to 33. Direct examination of the fine structure of heat-resistant alloys is also performed by TEM examination of thin foils. As with extraction replicas, electron diffraction and chemical analysis can be performed. Because the beam size in a transmission electron microscope or a scanning transmission microscope is much smaller than in a scanning electron microscope, much finer particles can be analyzed using thin foils without interference from the surrounding matrix. Extremely small particles are difficult to analyze even with a transmission electron microscope. Extraction replicas are useful, because matrix effects can be eliminated. In addition, using a transmission electron microscope, microdiffraction patterns Fig. 13 Waspaloy (42 HRC), solution annealed4hat1010 C (1850 F), water quenched, aged 4 h at 845 C (1550 ן are obtainable from individual particles rather F), air cooled, aged 16 h at 760 C (1400 F), and air cooled. (a) Glyceregia. Original magnification 200 . ןb) Tint etched to color matrix phase. 50 mL HCl, 50 mL H O, and 1 g K S O . Original magnification 100) than many particles. The microdiffraction pat- 2 2 2 5 tern is of great value in basic structural studies of the constituents. Thin foils are prepared by the window method or the disk method described in Ref 25 to 27 and 33 to 36. These methods involve careful sectioning to obtain a relatively thin slice of the material free of artifacts, followed by mechanical, chemical, or electrolytic thinning until a small area is thin enough for electron transmission. Table 9 lists several popular electropolishing procedures for preparing thin foils of heat-resistant alloys. Figure 19 shows the microstructure of cast B- 1900 nickel-base alloy containing carbides and coarse cЈ precipitates in a c matrix revealed by light microscopy and with a replica viewed with the TEM. The large angular particle in the center of Fig. 19(c) is an MC carbide, while the cЈ is finer in size and in high concentration. Similar examples are given in Fig. 20 and 21 for cast IN- 100 and in Fig. 22 for alloy 713C, both nickel- base alloys. Figure 23 shows coarse cЈ in cast IN-738 alloy where the shape is basically cubi- Fig. 14 Waspaloy (37 HRC), solution annealed4hat1035 C (1900 F), water quenched, aged 4 h at 845 C (1550 cal. However, after solution annealing (1120 C, F), air cooled, aged 16 h at 760 C (1400 F), and air cooled. (a) Etched in glyceregia. (b) Tint etched in 50 ןor 2050 F, for 2 h) and aging 24 h at 845 C mL HCl, 50 mL H2O, and 1 g K2S2O5. Original magnification, both 100 Metallography and Microstructures of Heat-Resistant Alloys / 831 seen in the grain boundary. In Fig. 30(b), the phase, Laves, and the hexagonal close-packed g Iron-Nickel-Chromium-Base Alloys. Many specimen was given the same solution anneal, phase (Ni3Ti). Nitrides are also commonly ob- cast and wrought Fe-Ni-Cr-base heat-resistant but it was double aged, first at 845 C (1550 F) served, and borides may be present in some al- alloys (the wrought alloys are usually classified for 24 h and then at 705 C (1300 F) for 24 h. loys. as iron-nickel-base, even though they all contain The grain-boundary M23C6 carbide is stabilized, The physical metallurgy of these systems is substantial chromium) have been developed. and there is a denser precipitation of cЈ in the quite complex, perhaps more challenging than These alloys contain at least 10% Fe but gener- matrix. that of any other commercial alloy system. In ally 18 to approximately 55%. The cast Fe-Ni- Bulk Extractions. X-ray diffraction studies of addition, as demonstrated in Tables 1 to 3, the Cr alloys are not strengthened by cЈ. The most phases extracted electrolytically are widely prac- compositions of these alloys are complex as important of the wrought Fe-Ni-Cr-base alloys ticed. X-ray diffraction is an important tool for well. References 46 to 57 provide basic review are those with an austenitic matrix that are phase identification in heat-resistant alloys (Ref articles on the metallography and physical met- strengthened by cЈ, such as A-286. Some of 37–45). Because of the complex nature of these allurgy of these alloys. Table 11 summarizes the these alloys are quite similar to wrought austen- alloys, such techniques must be carefully con- functions of elements in heat-resistant alloys. In itic stainless steels with the addition of the cЈ trolled to ensure good results. Qualitative iden- general, the Fe-Ni-Cr-base alloys tend toward strengthening agent; therefore, metallographic tification of the phases by this method is consid- formation of tcp phases, such as r, l, Laves, and procedures for these alloys are identical to those erably easier than quantitative evaluations. The v phase. The nickel-base alloys are prone to pre- for wrought austenitic stainless steels. Other Fe- extraction method must be designed to permit cipitation of ordered geometrically close-packed Ni-Cr-base alloys, such as Inconel 718, contain separation of the carbides, nitrides, cЈ, and top- phases, such as cЈ and g. Such phases are not much less iron and additions of niobium and tan- ologically close-packed (tcp) phases. Once sepa- common in cobalt-base alloys, because cЈ is not talum to obtain strengthening from cЉ. Another rated, the phases can be analyzed using x-ray a suitable strengthening agent. The cobalt-base group of Fe-Ni-Cr-base alloys contains rather diffraction, chemical analysis (elemental), and alloys contain various carbides, nitrides, r, and high carbon contents and is strengthened by car- light and electron microscopy procedures. l, depending on composition, processing, and bides, nitrides, carbonitrides, and solid-solution Considerable research has been conducted to exposure conditions. strengthening. Other Fe-Ni-Cr-base alloys, such establish reliable procedures for bulk extraction in heat-resistant alloys (Ref 37–45). Anodic dis- solution using 10% HCl in methanol, which dissolves cЈ and the austenitic matrix, is imple- mented to extract carbides, borides, nitrides, and tcp phases. If the alloy to be digested contains substantial amounts of tungsten, tantalum, or niobium, 1% tartaric acid is added to prevent contamination of the residue. To extract cЈ from nickel-base alloys, two electrolytes have been used: 20% aqueous phos- phoric acid (H3PO4), or an aqueous solution containing 1% ammonium sulfate [(NH4)2SO4] and 1% citric acid or tartaric acid. The latter electrolyte produces better recovery of cЈ. When the ammonium sulfate/citric or tartaric acid electrolyte is used, the residue will also contain carbides, nitrides, and borides (if present in the alloy). All the cЈ morphologies are extracted using this electrolyte. Details concerning the use of these electrolytes and others are given in Table 10 and provided in Ref 37 to 45. Fig. 15 Waspaloy (35 to 36 HRC), solution annealed 4 h at 1065 C 1950 F), water quenched, aged 4 h at 845 C (1550 F), air cooled, aged 16 h at 760 C (1400 F), and air cooled. (a) Etched in glyceregia. (b) Tint etched ןusing 50 mL HCl, 50 mL H2O, 3 g NH4F•HF, and 1.5 g K2S2O5. Original magnification, both 100 Microstructures of Heat-Resistant Alloys

Heat-resistant alloys are designed for use above approximately 540 C (1000 F). In general, they have an austenitic (c-phase) matrix and contain a wide variety of secondary phases. The most common second phases are metal carbides Ј (MC, M23C6,M6C, and M7C3) and c , the ordered face-centered cubic strengthening phase [Ni3(Al, Ti)] found in age-hardenable Fe-Ni-Cr and nickel-base superalloys. In age-hardenable alloys containing niobium or niobium and tantalum, the primary strengthening phase is cЉ,a body-centered tetragonal phase. Other phases, generally undesirable, may be observed due to variations in composition or processing or due to high-temperature exposure. Included in this group are orthorhombic d phase (Ni3Nb), r Fig. 16 Electromagnet setup for identification of ferrite in Fe-Cr-Ni alloys 832 / Metallography and Microstructures of Nonferrous Alloys as Hastelloy X, derive most of their strength solution-annealing temperatures plus the stan- creasing the aluminum/titanium ratio improves from solid-solution alloying, with a minor influ- dard double-aging treatments. Note that the high-temperature properties. The volume frac- ence from carbide precipitation. lower solution-annealing temperature does not tion, size, and spacing of cЈ (or cЉ) are important Microstructures of as-cast Fe-Ni-Cr grades dissolve all the delta, and the grain size remains parameters to control. Alloys with low amounts (HF-33, HH, HK-35, HW, HN, HT-44, and HT- fine for good cryogenic properties, while the of cЈ require greater attention to spacing than 57) are shown in Fig. 31 to 37. Figures 7 to 10 higher solution-annealing temperature dissolves alloys with high amounts. Other factors, such as show the structure of A-286, and Fig. 11 shows all of the delta, resulting in a coarse grain size coherency strain due to the lattice mismatch be- the structure of Pyromet 31, both wrought Fe- and good high-temperature properties. Figure 45 tween c and cЈ, appear to be important in certain Ni-Cr heat-resisting alloys. Figures 38 and 39 shows alloy 718 after solution annealing at 954 alloys such as Waspaloy. show the microstructures of wrought martensitic C (1750 F), followed by aging 100 h at 871 C Grain size is an important microstructural pa- Moly Ascoloy and Greek Ascoloy. Figure 40 (1600 F) to grow large delta needles while rameter. Fine grain sizes normally provide su- shows wrought alloy 330 in the solution an- coarsening the cЉ. perior room-temperature properties, such as nealed (from 996 to 1080 C, or 1825 to 1975 Nickel-Base Alloys. Nickel-base high-tem- toughness, strength, and fatigue resistance. F) and aged condition. Figure 41 shows the mi- perature alloys, cast or wrought, are basically of Coarse grain sizes generally yield better creep crostructure of nonrotating-quality alloy 718 in two types: those strengthened only by solid-so- resistance at elevated temperatures, although the solution-annealed and aged condition etched lution alloying and those that are also precipi- properties under other types of loading may suf- with glyceregia; the 15 HCl, 10 acetic acid, and tation hardenable. The solid-solution alloys con- fer. Duplex grain structures generally are unde- 10 HNO3 reagent; and the Lucas electrolytic re- tain little or no aluminum, titanium, or niobium; agent (2 V dc, 20 s). Note the primary carbide the precipitation-hardenable alloys contain sev- and the fine dispersion of delta phase. Figure 42 eral percent aluminum and titanium, and a few shows the microstructure of premium, rotating- contain substantial niobium. grade alloy 718 in the forged and aged condition The age-hardenable alloys are strengthened by after etching with glyceregia; the 15 HCl, 10 ace- cЈ precipitation by the addition of aluminum and tic acid, and 10 HNO3 reagent; and the Lucas titanium, by carbide, and by solid-solution alloy- electrolytic reagent (2 V dc, 20 s). In this case, ing. The nature of the cЈ is of primary impor- glyceregia only outlined the large primary car- tance in obtaining optimal high-temperature bides. Figure 43 shows alloy 718 after solution properties. Compositionally, the aluminum and annealing at 954 and 1066 C (1750 and 1950 titanium content and the aluminum/titanium ra- F), while Fig. 44 shows alloy 718 after the same tio are very important, as is heat treatment. In-

Fig. 17 (a) HE-14 alloy, creep tested at 4.5 MPa (650 psi) and 980 C (1800 F) for 336 h. Structure: islands of ferrite (darker gray) in an austenite matrix (lighter gray). White constituent is carbide particles. Compare appearance of ferrite in (b). (b) Same alloy and condition as in (a), showing the magnetic pattern (dark) on ferrite as influenced by a vertical magnetic field from a concentric solenoid. Dark areas with diffuse edges and no mosaic pattern indicatesubsurface ןferrite. Striped pattern shows magnetic domains. 50% HCl. Original magnification, both 100

Table 9 Electropolishing solutions for transmission electron microscopy thin foils of wrought heat-resistant alloys

Composition Comments

1. 950 mL acetic acid and 50 mL HClO4 Popular electropolish for wrought superalloys for perforation; use at 70–80 V dc, 100–120 mA, 15 C (60 F) 2. 133 mL acetic acid, 25 g CrO3, 7 mL H2O Best for window method; opacity makes jet thinning difficult; use at 10–12 V dc, 20 C (70 F) 3. 77 mL acetic acid and 23 mL HClO4 For cobalt-base superalloys; keep temperature below 30 C (85 F), stainless steel cathode; used with the window method; use at 22 V dc, 0.08 A/cm2 (0.5 A/in.2) 4. 600 mL methanol, 250 mL butanol, 60 mL HClO4 Two step procedure: (a) 0.13 mm (0.005 in.) disk, polished 15–30 Permanent-magnet setups for identification of Fig. 18 70מ to 60מ min at 30 V. (b) Final thinning at 16–24 V; use at ferrite in an austenite matrix. (a) and (b) Use of מ מ C( 75 to 95 F) cylindrical magnets. (c) Use of horseshoe magnet Metallography and Microstructures of Heat-Resistant Alloys / 833 sirable. Grain size also affects carbide precipi- in Fig. 46 to 53. The use of different etchants to 39,000 h of service (forged turbine blade). Fig- tation at the grain boundaries. Coarse grain sizes reveal the microstructure of wrought X-750 and ure 58 shows the microstructure at the center of have less grain-boundary surface area; therefore, Waspaloy were given in Fig. 12 to 15. Figure 54 an as-forged 30.5 cm (12 in.) diameter bar of carbide precipitation will be more continuous shows Waspaloy in the as-forged condition; Fig. alloy 600; note the grain-boundary carbides. and thicker, thus impairing properties. Due to 55 shows the grain structure of rotating-grade Figure 59 shows the grain structure of alloy these problems, a uniform, intermediate grain Waspaloy in the solution-annealed and aged con- 617 strip. Note the fine carbides strung out in the size is generally preferred. dition (grains are still elongated from forging), rolling direction. Figure 60 shows the carbides Microstructures of cast nickel-base alloys IN- while Fig. 56 shows the grain structure of rotat- in alloy 625 after solution annealing at 982 C 738, MAR-M 247, and CNK7 were shown in ing-grade Waspaloy in the solution-annealed and (1800 F) (as-polished condition, revealed by in- Fig. 3 to 5. Microstructures of other cast grades aged condition (note the duplex, necklace-type troducing a slight amount of relief in final pol- (Hastelloy B and C, IN-100, 713C, MAR-M grain structure). Figure 57 shows the grain struc- ishing). Figure 61 shows the microstructure of 246, TRW-NASA VI A, and U-700) are shown ture of Russian wrought alloy ZMI-3U after as-forged Custom Age 625 PLUS. Figure 62

Fig. 19 B-1900 nickel-base alloy, as-cast. (a) Structure consists of nickel-rich c solid-solution matrix containing a few light-etching carbide particles and dispersed cЈ. Kalling’s b) Higher magnification. The light-etching carbide particles are dispersed and at grain boundaries. The fine constituent within grains) ןreagent. Original magnification 100 c) Higher magnification and a replica electron micrograph showing details of a large MC carbide particle and particles of cЈ) .ןis cЈ. Kalling’s reagent 2. Original magnification 500 ןin the c matrix. HCl, ethanol, CuCl2, and H2O2. Original magnification 7500

Fig. 20 IN-100, as-cast. (a) Small, white islands are primary (eutectic) cЈ; peppery gray constituent is precipitated cЈ; black constituent is probably perovskite, a complex carbide. b) Higher magnification. Light constituent (A) is primary (eutectic) cЈ; dark (B), probably) .ןNi3(Al,Ti)C; matrix is nickel-rich c. Marble’s reagent. Original magnification 100 c) Higher magnification) .ןperovskite, Ni3(Al,Ti)C. Dispersed carbide particles are shown at C. Gamma matrix contains precipitated cЈ (D). Marble’s reagent. Original magnification 500 and a replica electron micrograph showing islands of primary cЈ (A), a large particle of primary carbide (B), and dispersed particles of precipitated cЈ in c matrix. Marble’s reagent. ןOriginal magnification 5000 834 / Metallography and Microstructures of Nonferrous Alloys shows the microstructure of Custom Age 625 Niobium and tantalum (8 to 10%) as well as crostructures of selected wrought cobalt-base su- PLUS with finish hot rolling temperatures of 916 titanium and zirconium (less than 0.5%) form peralloys are given in Fig. 79 to 82. and 1007 C (1680 and 1845 F). Figure 63 carbides of the MC type. Molybdenum and tung- shows these specimens with finishing tempera- sten form M6C in the Co-Cr-C alloys when the tures of 916 and 1007 C (1680 and 1845 F) content of either element is great enough to pre- Phases in Wrought after the standard solution-anneal and double- clude substitution for chromium in M23C6. Heat-Resistant Alloys age treatment. Figures 64 to 67 show the micro- Cobalt-base superalloys are strengthened by structure of wrought U-520, U-700, U-710, and solid-solution alloying and carbide precipitation. The microconstituents observed in iron-nickel U-720 nickel-base superalloys, respectively. The The grain-boundary carbides inhibit grain- and nickel-base wrought heat-resistant superal- latter is in the as-forged condition and contains boundary sliding. Unlike the Fe-Ni-Cr- and loys are identical, with a few exceptions. The substantial very coarse cЈ. Figures 68 and 69 nickel-base alloys, no intermetallic phase has cobalt-base alloys are not strengthened by pre- show the grain structure of Hastelloy B and G30 been found that will strengthen cobalt-base al- cipitated intermetallics but share many common alloys, respectively. The microstructure of Rene´ loys to the same degree that cЈ or cЉ strengthens features. All the alloys have an austenitic (c- 41 is shown in Fig. 70. Figure 71 shows the mi- the other superalloys. Gamma is not stable at phase) matrix that is strengthened by solid-so- crostructure of a forged Russian turbine blade high temperatures in cobalt-base alloys. lution alloying and by carbide precipitation. made from E1893L alloy. Powder metallurgy The microstructures of selected as-cast cobalt- Most of the phases discussed subsequently have processes can also be used to produce high-tem- base superalloys are shown in Fig. 74 to 78. Mi- some degree of solubility for other elements; perature alloys. Figure 72 shows coarse cЈ in as- hot isostatically forged Rene´ 95. Figure 73 shows the microstructure of EP741NP, a Russian powder metallurgy turbine disc grade, after hot isostatic forging and heat treatment. Cobalt-Base Alloys. Pure cobalt is allotropic and has a phase change from the low-temperature e (hexagonal close-packed) to the high-temperature ␣ (face-centered cubic) at 427 C (801 F). However, cobalt-base superalloys are designed to have a stabilized face-centered cubic matrix at all temperatures, and any reversion of the matrix to hexagonal close-packed crystal structure is undesirable. The alloys are charac- terized by chromium contents of 18 to 35%, principally to provide resistance to oxidation and sulfidation and secondarily as a carbide (M7C3 and M23C6) former and solid-solution strength- ener. Other elements common to the alloys are carbon, tungsten, tantalum, titanium, and zirconium for solid-solution strengthening and carbide (MC and M6C) formation, small amounts of silicon and manganese for improved oxidation resis- Fig. 21 IN-100 casting, held at 815 C (1500 F) for 5000 h. (a) Structure consists of massive MC particles, platelets Ј tance, and boron for solid-solution strengthening of r phase, and primary and precipitated c in the c matrix. HCl, ethanol, and H2O2. Original magnification .b) Replica electron shows a massive particle of MC, Widmansta¨tten platelets of r phase, and cЈ in the c matrix) ן500 ןand boride formation. The carbides are seldom HCl, ethanol, and H O . Original magnification 4500 binary compositions; chromium, tungsten, tan- 2 2 talum, silicon, zirconium, nickel, and cobalt may be present in a single particle of carbide. Molyb- denum, although used extensively in nickel alloys, is used only sparingly in cobalt-base alloys (Haynes 21); in cobalt alloys, tungsten is more effective and less detrimental. Compared to the wrought alloys, cobalt-base casting alloys have higher contents of high-melting-point metals, such as chromium, tungsten, tantalum, titanium, and zirconium, and higher carbon contents. Wrought and cast alloys, however, derive their strength from the dispersion of complex carbides in a highly alloyed matrix. Depending on chemical composition and heat treatment, the microstructure of cobalt-base alloys consists of a cobalt-rich solid-solution matrix containing carbide within grains and at grain boundaries. In the Co-Cr-C system, M7C3 and M23C6 are common. The ratio of chromium to carbon in this ternary system is important in de- termining which carbide will predominate. In Fig. 22 Alloy 713C, as-cast. (a) The massive white particles are primary cЈ; the grain-boundary film is MC particles. -b) Replica electron micrograph. Struc) .ןmore complex alloy systems, cobalt, tungsten, Gamma matrix contains cЈ. Glyceregia. Original magnification 500 and molybdenum replace some of the chromium ture shown consists of large particles of carbide (A) and cЈ in the matrix of c solid solution. Marble’s reagent. Original ןin the carbide phases. magnification 5000 Metallography and Microstructures of Heat-Resistant Alloys / 835

࿣ therefore, their true compositions will vary from is usually spherical. Optimal strength results and {100}cЉ {100}c. Strengthening is due to the alloy to alloy and may be altered by heat treat- when cЈ is in the size range of 0.01 to 0.05 lm, coherency strains produced by the low degree of ment and thermal exposure. Not all phases per- much too small to be seen using the light optical c/cЉ lattice mismatch. Although cЉ and cЈ are mit substitution, however. Eta phase (Ni3Ti) has microscope. If the aluminum/titanium ratio is present in Inconel 718 after aging, the amount of no significant solubility for other elements. Table equal to or greater than 1, extended high-tem- cЈ is much less, and cЉ is the primary strength- 12 summarizes data on the commonly encoun- perature exposure results in replacement of cЈ by ening agent. Other alloys strengthened by cЉ in- tered second-phase constituents in heat-resistant Ni2AlTi, NiAl, or Ni(Al,Ti). These phases over- clude Inconel 706 and Udimet 630. alloys. age rapidly at moderately high temperatures, Because cЉ is not a stable phase, application Gamma prime, a geometrically close-packed forming massive platelike precipitates. of alloys such as Inconel 718 is restricted to be- Ј phase, has an ordered face-centered cubic L12 Alloys with c contents below 20%, such as low 700 C (1290 F). Above this temperature, crystal structure and is Ni3Al or Ni3(Al,Ti), al- Nimonic 80A, are heat treated using a simple extended exposure produces a loss of strength though considerable elemental substitution oc- two-step process of solution annealing and ag- due to rapid coarsening of cЉ, solutioning of cЉ curs. For example, cobalt and chromium will re- ing. The solution anneal recrystallizes the aus- and cЈ, and formation of the stable orthorhombic Ј place some of the nickel, and titanium will tenitic matrix and dissolves any c and M23C6 form of Ni3Nb, which has an acicular, platelike replace part of the aluminum. Iron can replace carbides present. Aging precipitates cЈ uniformly shape. Љ nickel or aluminum. The lattice parameters of c throughout the matrix and precipitates M23C6 Positive identification of bct c is more diffi- and cЈ are similar, resulting in coherency, which carbides at grain and twin boundaries. Alloys cult than cЈ, because x-ray diffraction of bulk accounts for the value of cЈ as the principal with cЈ contents of approximately 30%, such as extraction residues will not detect cЉ. The failure strengthening agent in Fe-Ni-Cr- and nickel-base Waspaloy or Udimet 500, are solution treated superalloys. and then given two aging treatments. Alloys with Gamma prime is spherical in Fe-Ni-Cr-base 40 to 45% cЈ, such as Udimet 700, are solution alloys and in some of the older nickel-base al- treated and then given three aging treatments. loys, such as Nimonic 80A and Waspaloy. In the Positive identification of cЈ is usually per- more recently developed nickel-base alloys, cЈ is formed by x-ray diffraction of the residue of bulk generally cuboidal. Experiments have shown extractions or by electron diffraction using ex- that variations in molybdenum content and in the traction replicas. Some of the electrolytes that aluminum/titanium ratio can change the mor- selectively attack cЈ can be quite useful, because phology of cЈ. With increasing c/cЈ mismatch, the cЈ will be recessed below the matrix, and the the shape changes in the following order: spher- other second phases will be in relief or plane ical, globular, blocky, cuboidal (Ref 58). When with the surface, depending on the preparation the c/cЈ lattice mismatch is high, extended ex- procedure. posure above 700 C (1290 F) causes undesir- Gamma double prime has an ordered body- able g (Ni3Ti) or d (Ni3Nb) phases to form. centered tetragonal (bct) DO22 crystal structure The volume fraction, size, and distribution of with an Ni3Nb composition and is found in iron- cЈ are important parameters for control of prop- nickel-base alloys containing niobium. It gained erties. The volume fraction of cЈ increases with prominence as the strengthening phase with the the addition of aluminum and titanium, but the introduction of Inconel 718 (Ref 59). Early stud- amounts of each must be carefully controlled. ies of the strengthening mechanism produced Љ Gamma prime contents above approximately conflicting results until the precise details of c - Fig. 24 IN-738 solution annealed 2 h at 1120 C (2050 45% render the alloy difficult to deform by hot phase formation, composition, crystallography, F), held 24 h at 845 C (1550 F), replica elec- or cold working. This is not a restriction, of and stability were determined (Ref 60–65). tron micrograph. Gamma-prime particles in c; the smaller Ј particles formed in cooling. Electrolytic etch: H2SO4 and ןcourse, in cast alloys, where the c content can Gamma double prime has a disk-shaped mor- methanol. Original magnification 25,000 reach 70%. In the Fe-Ni-Cr-base alloys, the vol- phology and precipitates with a well-defined re- Ј ࿣ ume fraction of c phase is less than 20%, and it lationship to the austenite matrix: [001]cЉ 001 c

Fig. 23 IN-738, as-cast. (a) The structure consists of primary, or eutectic, cЈ islands (shown at A), dispersed carbide Fig. 25 IN-738 casting, after holding at 815 C (1500 particles (shown at B), and precipitated cЈ in the matrix of c solid solution. Marble’s reagent. Original mag- F) for 1000 h. A replica electron micrograph. -b) Higher magnification and a replica electron micrograph showing randomly distributed precipitated Structure consists of rounded cЈ particles in c matrix. Com) .ןnification 500 cЈ Ni3(Al,Ti) and a carbide particle (at right edge) in matrix of c solid solution. Marble’s reagent. Original magnification pare with Fig. 24. Electrolytic etch: H2SO4 and methanol. ןOriginal magnification 20,000 ן5000 836 / Metallography and Microstructures of Nonferrous Alloys to detect bct cЉ is attributed to line broadening perature exposure. Eta phase has no solubility sistant alloys. Four basic types are encountered: due to the very fine particle size that obscures for other elements and will grow more rapidly MC, M23C6,M6C, and M7C3 (where “M” rep- the peaks of interest (Ref 62). Electron diffrac- and form larger particles than cЈ, although it pre- resents one or more metallic elements). Carbides tion will, however, detect the superlattice lines cipitates slowly. Coarse g can be observed using in these alloys serve three principal functions. of bct cЉ. Bright-field TEM examination is un- the light optical microscope. First, grain-boundary carbides, when properly satisfactory for resolving cЉ, due to the high den- Two forms of g may be encountered. The first formed, strengthen the grain boundary, prevent sity of the precipitates and the strong contrast develops at grain boundaries as a cellular con- or retard grain-boundary sliding, and permit from the coherency strain field around the pre- stituent similar to pearlite, with alternate lamel- stress relaxation. Second, if fine carbides are precipitates. However, dark-field TEM examination lae of c and g; the second develops intragranu- cipitated in the matrix, strengthening results. provides excellent imaging of the cЉ by selective larly as platelets with a Widmansta¨tten pattern This is important in cobalt-base alloys that can- imaging of precipitates that produce specific su- (Ref 66–68). The cellular form is detrimental to not be strengthened by cЈ. Third, carbides can perlattice reflections (Ref 62). In addition, cЉ can notched stress-rupture strength and creep ductil- tie up certain elements that would otherwise pro- be separated from cЈ using the dark-field mode, ity, and the Widmansta¨tten pattern impairs mote phase instability during service. Carbide because the cЉ dark-field image is substantially stress-rupture strength but not ductility. Eta precipitation in nickel-base alloys has a stronger brighter than that of cЈ (Ref 62). phase is relatively easy to identify, due to its tendency to form at grain boundaries than in Fe- Eta phase has a hexagonal DO24 crystal struc- characteristic appearance. Most of the general- Ni-Cr- or cobalt-base alloys. Although grain- ture with a Ni3Ti composition. Eta can form in purpose reagents will reveal g, as will x-ray dif- boundary carbides, depending on their morphol- Fe-Ni-Cr-, nickel-, and cobalt-base superalloys, fraction of bulk-extracted residues. ogy, can degrade properties, reducing carbon especially in grades with high titanium/alumi- Carbides, which are important constituents, content to low levels substantially reduces creep num ratios that have had extended high-tem- are present in all the cast and wrought heat-re- life and ductility in nickel-base alloys. Aging of Fe-Ni-Cr- and nickel-base superalloys causes M23C6 to form at the grain boundaries. The optimal situation is a chain of discrete globular M23C6 particles at the grain boundaries. This form benefits creep-rupture life. However, if the carbides precipitate as a continuous grain- boundary film, properties will be seriously de- graded. It is not uncommon to observe zones around the grain boundaries that are devoid of cЈ. Such precipitate-free zones can significantly influence stress-rupture life, depending on the width of the zones (Ref 69). In these alloys, the MC-type carbide is most frequently titanium carbide; other types, such as niobium carbide, tantalum carbide, or hafnium carbide, are less common. Titanium carbide has some solubility for other elements, such as nitrogen, zirconium, and molybdenum. They are large, globular particles observable on the as- polished surface, particularly if some relief is introduced during final polishing. Metal carbides usually are irregular in shape or cubic. They can Fig. 26 MAR-M 246 casting, held at 980 C (1800 F) for 5000 h. (a) The structure consists of needlelike particles of be preferentially colored by certain etchants. .b) Higher magnification) .ןM6C and cЈ in the c matrix. HCl, ethanol, and H2O2. Original magnification 500 A replica transmission electron micrograph. Structure consists of needlelike particles of M6C, and cЈ in the c matrix. HCl, The most important carbide in superalloys is ן ethanol, and H2O2. Original magnification 4500 M23C6, because it forms at the grain boundaries

Fig. 27 MAR-M 246 casting, exposed to temperatures above 980 C (1800 F). (a) Chainlike M23C6 particles at grain boundaries and needlelike M6C within grains. Matrix is c. b) Replica electron micrograph. Needles of M6C (A), M23C6 at grain boundaries, and cЈ Ni3(Al,Ti). Original magnification) .ןMarble’s reagent. Original magnification 500 ןc) Replica electron micrograph. A large particle of M23C6 and a connecting carbide (B), and a cЈ envelope (C). The matrix is c. Marble’s reagent. Original magnification 5000) .ן5000 Metallography and Microstructures of Heat-Resistant Alloys / 837

during aging and, when properly formed, in- grain boundaries in a blocky form or intragran- tation of M23C6 due to the previously formed creases the strength of the grain boundaries to ularly in a Widmansta¨tten pattern and can be Cr7C3, which generally exhibits a blocky shape balance the matrix strength. Although chromium preferentially stained by certain etchants. when present at grain boundaries. is the primary “M” element, other metallic ele- The Cr7C3 carbide is likely to appear in the Borides. Boron is added in small amounts to ments, such as nickel, cobalt, iron, molybdenum, higher-carbon Fe-Cr-Ni cast alloys as spinelike many heat-resistant alloys to improve stress-rup- and tungsten, can substitute for it. The discrete crystals of roughly hexagonal cross section, fre- ture and creep properties or to retard formation globular form is the most desirable morphology; quently with a hole in the center. It can be easily of g phase, which would impair creep strength. films, platelets, lamellae, and cells have also stained with Murakami’s reagent. In the alloys Boron retards formation of the cellular grain- been observed. with 0.20 to 0.75% C, the Cr7C3 carbide is likely boundary form of g but has no influence on the The M6C carbide is generally rich in molyb- to be the eutectic carbide. Although M7C3 is not intragranular Widmansta¨tten g. Consequently, denum or tungsten, but other elements, such as widely observed in wrought superalloys, it is boron influences grain-boundary structures. Bo- chromium, nickel, or cobalt, may substitute for present in some cobalt-base alloys and in Ni- ron also reduces the solubility of carbon in aus- it to some degree. It is the most commonly ob- monic 80A, a Ni-Cr-Ti-Al superalloy, when tenite, which increases precipitation of finer- served carbide in the cobalt-base superalloys and heated above 1000 C (1830 F). Additions of sized MC and M23C6 carbides. If the boron in nickel-base alloys with high molybdenum such elements as cobalt, molybdenum, tungsten, addition is sufficiently high, detrimental borides and/or tungsten contents. In these alloys, M6Cis or niobium to nickel-base alloys prevent for- will form. Borides are hard and brittle and pre- often observed in the as-cast condition randomly mation of M7C3. Massive Cr7C3 is formed in Ni- cipitate at the grain boundaries. Borides are gen- distributed throughout the matrix. In wrought al- monic 80A in the grain boundaries after heating erally of M3B2 composition with a tetragonal loys, it will usually be dissolved during heating to 1080 C (1975 F). Subsequent aging at 700 structure (Ref 70). Molybdenum, tantalum, nio- before hot working. It may precipitate at the C (1290 F) to precipitate cЈ impedes precipi- bium, nickel, iron, or vanadium can be “M” elements. The identification of borides in Udimet 700 has been documented (Ref 70). Laves phase, a tcp phase, has a MgZn2 hexagonal crystal structure with a composition of the AB2 type. Typical examples include Fe2Ti, Fe2Nb, and Fe2Mo, but a more general formula is (Fe,Cr,Mn,Si)2(Mo,Ti,Nb). They are most commonly observed in the iron-nickel-base alloys as coarse intergranular particles; intragranular precipitation may also occur. Silicon and niobium promote formation of Laves phase in alloy 718. Excessive amounts will impair room- temperature tensile ductility; creep properties are not significantly affected. Laves phases have been observed in certain cobalt-base alloys and have been identified as Co2W, Co2Ti,orCo2Ta. Sigma phase is a tetragonal intermetallic tcp phase that forms with a wide range of compositions. Sigma is a hard, brittle compound usu- Fig. 28 Astroloy forging, solution annealed 1 h at 1150 C (2100 F) and air cooled. (a) View showing grain boundaries ally formed from ferrite (but sometimes directly ן and fine MC carbides in a c-phase matrix. Kalling’s reagent 2. Original magnification 100 . (b) Replica electron micrograph showing a clean grain boundary (diagonal). cЈ precipitate is visible in the c matrix. Electrolytic: from austenite) between approximately 650 C .(F) and slightly above 870 C (1600 F 1200) ןH2SO4,H3PO4, and HNO3. Original magnification 10,000

Fig. 29 Astroloy forging, solution annealed 4 h at 1150 C (2100 F), air cooled, aged4hat1080 C (1975 F), oil quenched, aged 4 h at 845 C (1550 F), air cooled, aged 16 h b) Higher magnification. MC carbides are precipitated at grain boundaries; the) .ןat 760 C (1400 F), and air cooled. (a) Kalling’s reagent 2. Original magnification 100 c) Replica electron micrograph showing intergranular cЈ precipitated at 1080 C (1975) .ןsolid-solution matrix contains cЈ particles. Kalling’s reagent 2. Original magnification 1000 F) as well as fine cЈ precipitated at 845 C (1550 F) and 760 C (1400 F). Carbide particles are visible at grain boundaries. Electrolytic: H2SO4,H3PO4, and HNO3. Originalmagnification ן10,000 838 / Metallography and Microstructures of Nonferrous Alloys

It develops most rapidly near 870 C (1600 F). exhibit an acicular pattern. A frequently encoun- with carbonitrides, chromium nitride, or chro- The classic sigma-phase compound has an ap- tered lamellar constituent that resembles pearlite mium carbide. Etching in Murakami’s reagent at proximate FeCr composition, but in the alloys has been identified as an aggregate of austenite room temperature for approximately 10 s will discussed in this article, it has a more complex, variable composition. The addition of silicon promotes the formation of r, and a ternary composition of 43Fe43Cr-14Si (at.%) has been sug- gested. Because r imparts ambient-temperature brittleness and a loss of creep-rupture strength, its presence is generally undesirable. Various morphologies may be encountered, some of which are quite detrimental to properties. How- ever, the presence of r in superalloys is not necessarily damaging to properties. Sigma, in the form of plate precipitation, can improve creep properties. Considerable effort has been devoted to de- termining how composition influences r-phase formation, particularly in nickel-base superalloys. References 71 to 75 present examples of the many studies that have been conducted. This work has substantially influenced alloy devel- opment. Sigma can be preferentially attacked or stained by a number of reagents. However, be- Fig. 30 Replica electron micrograph of Inconel X-750, solution annealed2hat1150 C (2100 F) and air cooled. (a) cause of the wide range of alloy compositions Aged 24 h at 815 C (1500 F). Structure is small, uniformly dispersed cЈ precipitate and large, discontinuous that may contain r and the variable nature of its M23C6 carbide at the grain boundary. (b) Aged 24 h at 845 C (1550 F), then 24 h at 705 C (1300 F). Grain-boundary Ј composition, positive identification by etching is M23C6 carbide is stabilized, and precipitation of fine c particles has increased. Both glyceregia. Original magnification, ןnot always possible. X-ray diffraction of bulk both 15,000 extraction residues is a more reliable technique. Etching procedures are best applied when they Table 10 Techniques for bulk electrolytic extractions can be tested for response on specimens of the Solution Comments alloy known to contain r phase. 1. 10% HCl in methanol For extraction of carbides, borides, topologically close-packed phases, and Mu phase is a rhombohedral (triagonal) in- geometrically close-packed phases from nickel- and iron-nickel-base termetallic tcp phase with a W6Fe7 structure (Ref alloys; solution may dissolve Ni3Ti; maintain bath at 0–30 C (32–85 F); 76). In general, it has little influence on proper- for alloys containing tungsten, niobium, tantalum, or hafnium, add 1% 2 2 ties. Mu precipitates as coarse, irregularly tartaric acid; use 0.05–0.1 A/cm (0.33–0.65 A/in. ) for 4 h or longer (additional details in ASTM E 963) 2 shaped platelets in a Widmansta¨tten pattern. A 2. 0.5–2% citric acid and (NH4)2SO4 in H2O Used to extract cЈ phase in nickel-base Udimet 700; use at 0.03 A/cm (0.2 2 general formula for l is (Fe,Co)7(Mo,W)6. (1% of each is most common) A/in. ) for 3 h at room temperature; minor amounts of carbides and Nickel can substitute for part of the iron or the borides will also be extracted (additional details in Ref 35–38) Ј Ј cobalt. 3. 10 or 20% H3PO4 in H2O Used to extract c in nickel-base alloys; may etch the c phase, although results have been contradictory are commonly observed in heat-re- 2 Nitrides 4. 50 mL HNO3, 20 mL HClO4, 1000 mL For extraction of cЈ and g in nickel-base superalloys; use at 0.1 A/cm 2 sistant alloys containing titanium or niobium as H2O (0.65 A/in. ), 25 C (75 F) titanium nitride (most common) or niobium ni- 5. 300 g KCl (potassium chloride), 30 g citric For extraction of carbides from nickel-base superalloys; use at 0.1 A/cm2 2 tride. Nitrides are not influenced by heat treat- acid, 50 mL HCl, 1000 mL H2O (0.65 A/in. ), 25 C (75 F) ment and are insoluble to the melting point. Ni- trides are easily identified in the as-polished condition or after etching due to their regular, Table 11 Role of elements in superalloys angular shapes and distinct yellow-to-orange Effect(a) Iron-base Cobalt-base Nickel-base color. Nitrides are quite hard and will appear in Solid-solution strengtheners Cr, Mo Nb, Cr, Mo, Ni, W, Ta Co, Cr, Fe, Mo, W, Ta relief after polishing. They have some solubility Face-centered cubic matrix stabilizers C, W, Ni Ni ... for carbon and may be referred to as Ti(C,N), Carbide form: Nb(C,N), and so on. They should not be con- MC Ti Ti W, Ta, Ti, Mo, Nb fused with carbonitrides, which are much richer M7C3 ... Cr Cr M23C6 Cr Cr Cr, Mo, W in carbon and lower in nitrogen. Nitrides, often M6C Mo Mo, W Mo, W duplex, include an embedded phase or a sur- Carbonitrides: M(CN) C, N C, N C, N rounding film; this second phase is generally a Promotes general precipitation of carbides P ...... Ј darker colored nitride containing considerable Forms c Ni3(Al,Ti) Al, Ni, Ti ... Al, Ti Retards formation of hexagonal g (Ni3Ti) Al,Zr ...... carbon. The usual amounts present in superal- Raises solvus temperature of cЈ ...... Co loys generally have little influence on properties. Hardening precipitates and/or intermetallics Al, Ti, Nb Al, Mo, Ti(b), W, Ta Al, Ti, Nb The acicular constituent that occurs near creep Oxidation resistance Cr Al, Cr Al, Cr fractures in cast Fe-Ni-Cr alloys is likely to be Improves hot corrosion resistance La, Y La, Y, Th La, Th Sulfidation resistance Cr Cr Cr chromium nitride, which originates by diffusion Improves creep properties B ... B of nitrogen from the atmosphere. The acicular Increases rupture strength B B, Zr B(c) pattern is the result of precipitation on crystal- Causes grain-boundary segregation ...... B, C, Zr lographic planes. However, nitrides are not al- Facilitates working ... Ni3Ti ... ways acicular, and acicular platelets are not nec- (a) Not all these effects necessarily occur in a given alloy. (b) Hardening by precipitation of Ni3Ti also occurs if sufficient nickel is present. (c) If essarily nitrides; r phase, and even carbides, can present in large amounts, borides are formed. Metallography and Microstructures of Heat-Resistant Alloys / 839

stain the carbide but not the nitrides or carboni- that of r phase, coexists with r (Ref 8), and is 54% Fe by weight, approximating Fe3CrMo. Chi trides. hard and brittle. Ternary diagrams have been de- phase can be revealed with a brief etch in Vi- Chi phase may be encountered in Fe-Cr-Ni veloped at 815 and 900 C (1500 and 1650 F) lella’s reagent, followed by electrolytic etching heat-resistant casting alloys containing molyb- for the Fe-Cr-Mo system that identify v phase as in concentrated NaOH at 1.5 V. Chi phase is ﬁrst denum. Chi phase has a composition similar to containing approximately 18% Cr, 28% Mo, and stained light brown, but after approximately 10

Fig. 31 Alloy HF-33. (a) As cast. Austenite matrix contains eutectic carbide chains (at grain boundaries) and scattered carbide particles. Note the patch of lamellar constituent at a grain boundary. The globular inclusions are chiefly sulfide and silicate. (b) Creep-test specimen, a view of the structure at the interior. Traces of fine carbide precipitation are evident at slip regions in the austenite matrix. (c) Interior of creep-test specimen fractured after 13,690 h at 650 C (1200 F) and 110 MPa (16 ksi). Cored structure is more fully developed; eutectic carbide is unchanged. (d) Interior of creep-test specimen fractured after 13,680 h at 760 C (1400 F), 41 MPa (6 ksi). Carbide precipitation is more general than in Fig. 31(c) (coalescence of precipitates has begun); eutectic carbide is essentially unchanged. (e) Fracture produced after 1210 h at 870 C (1600 F) and 28 MPa (4 ksi). Secondary- carbide precipitation throughout, especially near particles of eutectic carbide and dendrite boundaries; some coalescence. Subgrain boundaries are prominent. (f) Fracture produced after 13,300 h of 870 C (1600 F) and 17 MPa (2.5 ksi). Eutectic and secondary carbides have coalesced to form a nearly continuous network and paths for subsurface oxidation. Some nitride platelets. (g) Same as (f), but at the interior of the specimen. The cored structure is much less marked than in (d), and the carbide network is less continuous than in (f). Some ןcoalescence of carbide has occurred but, again, less than in (f). All glyceregia. Original magnification, all 250 840 / Metallography and Microstructures of Nonferrous Alloys

s, it develops a blue-gray tint, distinguishing it loys similar to A-286, G phase (Ni18 Ti8Si6) has steels, can also be found in these alloys. How- from sigma phase, which etched brown. been observed (Ref 77, 78). This phase has a ever, in the nickel-base alloys, titanium sulﬁdes Other Phases. A few other phases are less globular shape and precipitates in grain bound- may be observed. Oxides, such as Al2O3 or mag- frequently observed in wrought heat-resistant al- aries. It is detrimental to stress-rupture life. A nesia, may also be present. Oxides and sulﬁdes loys. For example, a few cobalt-base alloys have chromium-iron niobide, Z phase, has been ob- may be observed at the surface of components been developed that attain some degree of served in an Fe-18Cr12Ni-1Nb alloy after creep due to environmental effects. Coatings are also strengthening by precipitation of intermetallic testing at 850 C (1560 F) (Ref 79). Inclusions, used on some alloys, and their microstructures phases, such as CoAl, Co3Mo, or Co3Ti. In al- some of which are similar to those found in may be of interest (Ref 80, 81).

Fig. 32 Alloy HH, as-cast. Austenite matrix grains are Fig. 33 Alloy HK-35, as-cast. Scattered eutectic car- surrounded by nearly continuous envelopes of bide in austenite matrix and at grain bound- primary carbide. Primary carbide also occurs as interden- aries; patches of the lamellar constituent also are associated dritic islands. Patches of lamellae, such as the one at upper with grain boundaries. No fine particles of carbide have right, are not clearly resolved at this magnification. Gly- precipitated during freezing and mold cooling. Glyceregia. ןOriginal magnification 250 ןceregia. Original magnification 250

Fig. 35 Alloy HN, as-cast. The microstructure consists Fig. 34 Alloy HW, as-cast, showing pattern of interdentritic eutectic carbide segregation. (a) 5 mL conc HCl and 1 of an austenite matrix containing chains of eu- b) Higher magnification. Austenite matrix containing massive tectic carbide between the dendrites. Note that in some) .ןmL conc HNO3. Original magnification 50 interdendritic eutectic carbide and some small precipitated carbide particles. 5 mL conc HCl and 1 mL conc HNO3. portions of the eutectic carbide a duplex or lamellar struc- ןture is present. Glyceregia. Original magnification 250 ןOriginal magnification 500 Metallography and Microstructures of Heat-Resistant Alloys / 841

Fig. 36 Alloy HT-44, as cast. The austenite matrix con- Fig. 37 Alloy HT-57, as-cast. This structure is similar to tains a complex network of eutectic carbide that shown in Fig. 36 (as-cast alloy HT-44), but that outlines the boundaries of the original dendrites. Note the higher carbon content has caused additional primary that the larger patches of primary carbide have a lamellar eutectic carbide to form. Also, the larger carbide shapes structure. Compare with Fig. 37. K3Fe(CN)6. Original mag- are coarser than those in Fig. 36. K3Fe(CN)6. Original mag- ןniﬁcation 250 ןniﬁcation 250

Fig. 38 Lath martensitic grain structure of Moly Ascoloy (329 HV) revealed using Vilella’s reagent, (a) Original mag- ןb) Original magniﬁcation 500) .ןniﬁcation 100

Fig. 39 Lath martensitic grain structure of Greek As- coloy (335 HV) revealed using Vilella’s re- ןagent. Original magniﬁcation 400 842 / Metallography and Microstructures of Nonferrous Alloys

Fig. 40 Austenitic grain structure in alloy 330 revealed using 10% oxalic acid (6 V dc, 10 s) for specimens solution annealed at: (a) 996 C (1825 F), (b) 1024 C (1875 F), (c) 1038 C (1900 F), (d) 1052 C (1925 F), (e) 1066 C (1950 F), and (f) 1080 C (1975 F). Note that only with the 1080 C (1975 F) solution anneal is the grain structure fully recrystallized and of approximately normal size distribution. The specimens were water quenched after solution annealing and aged at 677 C (1250 F) for 2 h. Original ןmagniﬁcation, all 100

Fig. 41 Grain structure of nonrotating-quality alloy 718, forged and given a standard solution-anneal and aging treatment, revealed using: (a) glyceregia, (b) 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3, and (c) the Lucas electrolytic reagent (2 V dc, 20 s). Note the substantial ﬁne delta content, as well as some large primary carbides. Original ןmagniﬁcation, all 1000 Metallography and Microstructures of Heat-Resistant Alloys / 843

Fig. 42 Grain structure of premium-quality alloy 718 in the as-forged and aged condition revealed using different etchants. (a) Glyceregia (it did not reveal the grain or twin c) The Lucas electrolytic reagent (2 V dc, 20) .ןb) 15 mL HCl, 10 mL acetic acid, 10 mL HNO3. Original magnification 1000) .ןboundaries). Original magnification 500 ןs). Original magnification 1000

Fig. 43 Grain structure of hot-rolled alloy 718 in the solution-annealed condition from: (a) 954 C (1750 F), and (b) 1066 C (1950 F). Revealed using glyceregia. This reagent is good for revealing the second-phase precipitates but is not suitable for fully revealing the grain structure. Note that the lower solution-annealing temperature is used for cryogenic applications (fine grain size), while the higher solution-annealing temperature is used for high-temperature ןb) Original magnification 100) .ןapplications (coarse grain size). (a) Original magnification 1000

Fig. 45 Copious delta phase precipitated along spe- Fig. 44 Grain structure of hot-rolled alloy 718 in the solution-annealed and double-aged condition (719 C, or 1326 cific crystal planes in specimen of alloy 718 F, for 8 h, furnace cool to 621 C (1150 F), hold 8 h, air cool). Revealed using glyceregia. (a) Solution that was solution annealed at 954 C (1750 F) and then -Note aged 100 h at 871 C (1600 F). Glyceregia. Original mag .ןb) Solution annealed at 1066 C (1950 F). 100) .ןannealed at 954 C (1750 F). Original magnification 1000 ןthat delta is present in (a) but not in (b). nification 500 844 / Metallography and Microstructures of Nonferrous Alloys

Fig. 46 Hastelloy B. (a) As-cast. Structure consists of M6C at grain boundaries and as Fig. 47 Hastelloy C. (a) As-cast. Structure consists of M6C at grain boundaries and as islands in the c matrix. (b) Casting, annealed at 1175 C (2150 F) for 2 h and islands in the c matrix. (b) Casting, annealed at 1230 C (2250 F) for 2 h and water quenched. M6C islands in the matrix. Both electrolytic etch: HCl and CrO3. Original water quenched. M6Cinc matrix. Both electrolytic etch: CrO3. Original magniﬁcation, ןboth 300 ןmagniﬁcation, both 300

Fig. 48 IN-100, as-cast. (a) The white islands are primary, or eutectic, cЈ. Precipitated cЈ is barely visible in the c (b) Higher magnification. Island of primary (eutectic) .ןmatrix. Marble’s reagent. Original magnification 100 ןcЈ (A), dispersed carbide (B), and precipitated cЈ in c matrix. Marble’s reagent. Original magnification 500

Fig. 49 Alloy 713C. (a) Ingot, 250 mm (10 in.) diam, as-cast. Structure is a matrix of c solid solution containing“script” MC carbide in a characteristic dendritic arrangement. HCl, methanol, and CuCl2. (b) As-cast. The MC ar- ןrangement is in a “script” pattern. The matrix is c solid solution. As-polished. Original magniﬁcation, both 100 Metallography and Microstructures of Heat-Resistant Alloys / 845

Fig. 50 Alloy 718. (a) Vacuum cast, solution annealed 2 h at 1095 C (2000 F), air cooled, reannealed 1 h at 980 C (1800 F), air cooled, aged 16 h at 720 C (1325 F), air cooled. Structure: chainlike precipitate of M2(Cb,Ti) Laves phase in the c matrix. (b) Vacuum cast and heat treated as for (a), except solution annealing at 1095 C (2000 F) was for only 1 h, and all furnace heating was done under a protective atmosphere of argon. Laves phase (white islands) ןhas precipitated at dendrites in the c matrix. Both HCl, methanol, and FeCl3. Original magniﬁcation, both 250

Fig. 52 TRW-NASA VI A, as-cast. Blocky, light constit- Fig. 51 MAR-M 246, as-cast. Structure consists of pre- uent is primary cЈ; the black spots are carbide cipitated cЈ in the c matrix (A) and fine particles particles; the mottled gray areas are precipitated cЈ in a of carbide or primary cЈ at grain boundaries (B). Marble’s matrix of c solid solution. Electrolytic etch: H2SO4 and ןmethanol. Original magnification 250 ןreagent. Original magnification 500

Fig. 53 U-700. (a) As-cast. Small, white crystals are carbide particles; darkened areas include primary (eutectic) cЈ; matrix is c solid solution. Kalling’s reagent. Originalmagnification ;b) Casting, solution annealed and furnace cooled from 1080 C (1975 F), then aged at 760 C (1400 F) for 16 h and air cooled. Small, white crystals are carbide) .ן100 c) Same heat treatment as (b), but at a higher magnification. M23C6 has precipitated along grain) .ןmottled gray areas include cЈ. Kalling’s reagent. Original magnification 100 ןboundaries. Large carbide particles dispersed within grains. Remainder is cЈ in c matrix. Kalling’s reagent. Original magnification 1000 846 / Metallography and Microstructures of Nonferrous Alloys

Fig. 54 Grain structure of as-forged Waspaloy revealed using the 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3 etch. Note that the ﬁnish forging temperature was high enough to permit full recrystallization. Original ןmagniﬁcation 200

Fig. 55 Grain structure of rotating-quality, forged Waspaloy that was solution annealed and aged. Revealed using: (a) glyceregia, (b) 15 mL HCl, 10 mL acetic acid, and 10 mL ןHNO3, and (c) the Lucas electrolytic reagent (1.2 V dc, 40 s). Original magniﬁcation, all 200

Fig. 56 Grain structure of rotating-quality, forged Waspaloy that was solution annealed and aged but exhibited a necklace-type duplex grain size distribution. Revealed using: (a) ןglyceregia, (b) 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3, and (c) the Lucas electrolytic reagent (2 V dc, 20 s). Original magniﬁcation, all 200 Metallography and Microstructures of Heat-Resistant Alloys / 847

Fig. 57 Grain structure of a Russian forged turbine blade made from ZMI-3U grade after 39,000 h of service. Revealed using: (a) Kalling’s No. 2, (b) the Lucas electrolytic reagent (2 V dc, 10 s), and (c) Beraha’s tint etch (50 mL HCl, 50 mL water, 0.8 g K2S2O5,4gNH4F•HF, 1 g FeCl3). The latter image was taken close to a test fracture made in a post ןservice study and reveals the deformation in the gage section. Original magniﬁcation, all 100

Fig. 58 Grain structure of alloy 600 taken at the center Fig. 59 Grain structure of alloy 617 strip revealed us- of an as-forged 30.5 cm (12 in.) diameter bar ing the 15 mL HCl, 10 mL acetic acid, and 10 and revealed by glyceregia. There is a considerable amount mL HNO3 etch. Note the fine carbide strung out in the of fine carbide decorating the grain boundaries. Original rolling direction (horizontal in the image). Original mag- ןnification 100 ןmagnification 200

Fig. 60 Carbides and grain structure in a 14 cm (5.5 in.) diameter bar (midradius location shown) of alloy 625 that was solution annealed at 982 C (1800 F). The carbides in (a) were revealed without etching by introducing a slight amount of relief during final polishing, while the grain structure in (b) was revealed by etching with acetic ןb) Original magnification 200) .ןglyceregia. (a) Original magnification 100 848 / Metallography and Microstructures of Nonferrous Alloys

Fig. 61 Duplex necklace-type grain structure in as-forged Custom Age 625 PLUS grade revealed using: (a) the 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3 etch, and (b) the Lucas electrolytic reagent (2 V dc, 20 s). Original ןmagniﬁcation, both 100

Fig. 62 As-hot-rolled grain structures of Custom Age 625 PLUS with finish rolling temperatures of: (a) 916 C (1680 F) (not fully recrystallized), and (b) 1007 C (1845 F). Revealed using the 15 mL HCl, 10 mL acetic acid, and ןmL HNO3 etch. Original magnification, both 100 10 Fig. 64 Grain structure of U-520 in the solution annealed and aged condition revealed using the 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3 etch. ןOriginal magnification 100

Fig. 63 Grain structure of Custom Age 625 PLUS with finish rolling temperatures of: (a) 916 C (1680 F) and (b) 1007 C (1845 F) after solution annealing at 1038 C (1900 F) and aging at 732 C (1350 F) for 8 h, furnace cooling to 621 C (1150 F), holding 8 h, and air cooling. The grain structure is more uniform with the 1007 C (1845 F) Fig. 65 Grain structure of U-700 strip revealed using finish rolling temperature. The 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3 etch was used. Original magnification, the 15 mL HCl, 10 mL acetic acid, and 10 mL ןHNO3 etch. Original magnification 100 ןboth 100 Metallography and Microstructures of Heat-Resistant Alloys / 849

Fig. 66 U-710 bar. (a) Solution annealed 2 h at 1120 C (2050 F), aged 1.5 h at 1040 C (1900 F), and air cooled. Longitudinal section shows recrystallized bands (light) and bands containing residual primary cЈ. (b) Solution annealed 4 h at 1175 C (2150 F) and air cooled. Structure is dispersed primary MC carbide and M3B2 boride in a c matrix. cЈ is in solution. (c) Solution annealed same as (b), aged 24 h at 845 C (1550 F), air cooled, aged 16 h at 760 C (1400 F), and air cooled. M23C6 precipitate along grain and ןtwin boundaries. All Kalling’s reagent. Original magniﬁcation, all 100

Fig. 67 Coarse cЈ in as-forged U-720 revealed using Fig. 68 Grain structure and carbides in Hastelloy B re- the 15 mL HCl, 10 mL acetic acid, and 10 mL vealed using the 15 mL HCl, 10 mL acetic acid, ןand 10 mL HNO3 etch. Original magniﬁcation 100 ןHNO3 etch. Original magniﬁcation 500

Fig. 69 Grain structure of Hastelloy G30 alloy revealed using: (a) the 15 mL HCl, 10 mL acetic acid, and 10 mL ןHNO3 etch, and (b) the Lucas electrolytic reagent (2 V dc, 20 s). Original magniﬁcation, both 200 850 / Metallography and Microstructures of Nonferrous Alloys

Fig. 70 Rene´41. (a) Solution annealed 4 h at 1065 C (1950 F) and air cooled. Structure consists of stringers of b) Same processing) .ןcarbide in a c solid-solution matrix. Kalling’s reagent 2. Original magnification 100 as in (a), but at higher magnification. Light, globular particles are M6C; gray particles are MC carbide; grain-boundary c) Solution annealed 4 h at 1065 C) .ןenvelopes are M6CorM23C6. Kalling’s reagent 2. Original magnification 500 (1950 F), air cooled, aged 16 h at 760 C (1400 F), and air cooled. Particles of mixed carbides are present in the c solid- solution matrix, which was darkened by the formation of cЈ at 760 C (1400 F). Kalling’s reagent 2. Original magnification d) Same as (c) but at a higher magnification, showing particles of M6C (white), MC (gray), and M23C6 (at grain) ן110 ןboundaries). Grain-boundary borders are darkened by cЈ. Kalling’s reagent 2. Original magnification 540

Fig. 71 Grain structure of Haynes HR-160 revealed using the 15 mL HCl, 10 mL acetic acid, and 10 Fig. 72 Coarse cЈ in as-hot-isostatically pressed, powder metallurgy processed Rene´95 revealed using glyceregia. (a) ןb) Original magnification 1000) .ןOriginal magnification 200 ןmL HNO3 etch. Original magnification 100 Metallography and Microstructures of Heat-Resistant Alloys / 851

Fig. 73 Grain structure and coarse, cuboidal cЈ and decorated prior-particle boundaries in powder processed, hot isostatically pressed, and heat treated EP741NP (1210 C, or 2210 F, for 8 h, furnace cool to 1160 C, or b) The Lucas) .ןF, air cool, reheat to 871 C, or 1600 F, hold 32 h, air cool). (a) Molybdic acid reagent. 100 2120 c) The 15 mL HCl, 10 mL acetic acid, and 10 mL HNO3) .ןelectrolytic reagent (2 V dc, 20 s). Original magnification 100 ןd) Molybdic acid reagent. Original magnification 1000) .ןetch. Original magnification 1000

Fig. 74 Haynes 21 cobalt-base alloy, as-cast. Structure Fig. 75 Haynes 31, as-cast. Structure consists of large, consists of primary M7C3 particles in an ␣ primary M7C3 particles and grain-boundary (face-centered cubic) matrix. Electrolytic etch: HCl. Origi- M23C6 in an ␣ (face-centered cubic) matrix. Electrolytic ןetch: 2% CrO3. Original magniﬁcation 400 ןnal magniﬁcation 200 852 / Metallography and Microstructures of Nonferrous Alloys

␣ Fig. 76 Haynes 151, as-cast. (a) Structure consists of dispersed islands of large primary carbide (M6C) in the (face- -b) Higher magnifi) .ןcentered cubic) matrix. Electrolytic etch: HCl and CrO3. Original magnification 200 cation, which reveals details of the M6C (note the lamellar form) in the ␣ (face-centered cubic) matrix. Electrolytic etch: ןHCl and CrO3. Original magnification 500

Fig. 77 WI-52, as-cast. The solid gray islands are com- Fig. 78 MAR-M 302, as-cast. (a) Structure consists of primary, or eutectic, M6C particles (dark gray) and MC particles .ןplex chromium-tungsten carbide; eutectic car- (small white crystals) in the matrix of Co-Cr-W solid solution. Kalling’s reagent. Original magnification 100 bides are niobium carbide. The dark dots are silicate inclu- (b) Higher magnification, with better resolution of constituents. The mottled gray islands are primary eutectic carbide; the sions in the matrix of cobalt-chromium solid solution. light crystals are MC particles; the peppery constituent within grains of the matrix is M23C6. Kalling’s reagent. Original ןmagnification 500 ןElectrolytic etch: 5% H3PO4. Original magnification 500 Metallography and Microstructures of Heat-Resistant Alloys / 853

Fig. 79 Haynes 25 (L-605). (a) Solution annealed at 1205 C (2200 F) and aged 3400 h at 650 C (1200 F). Structure is M6C and M23C6 carbides in a mixed face-centered cubic b) Solution annealed same as (a) and aged 3400 h at 815 C (1500) .ןfcc) and hexagonal close-packed matrix. Electrolytic: HCl and H2O2. Original magnification 500) c) Solution annealed same as (a) and aged) .ןF). Structure is precipitates of M6C and “Co2W” intermetallic in a fcc matrix. Electrolytic: HCl and H2O2. Original magnification 500 d) Solution annealed same as (a) and aged 3400 h at 925 C (1700) .ןh at 870 C (1600 F). Structure is the same as (b). Electrolytic: HCl and H2O2. Original magnification 500 3400 e) Solution annealed and cold) .ןF). Structure consists of M6C (primary) and “Co2W” intermetallic (secondary) in a fcc matrix. Electrolytic: HCl and H2O2. Original magnification 500 -Bright .ןworked to 35% reduction. Hardness, 486 HV. Longitudinal section. Etched with 15 mL HCl, 10 mL acetic acid, 5 mL HNO3, and 2 drops glycerol. Original magnification 100 field illumination. (f) Same as (e), except dark-field illumination. (g) Same as (e), except differential interference contrast. (h) Cold worked to 35% reduction and solution annealed 2.5 i) Same as (h) except solution annealed) .ןmin at 1150 C (2100 F); 261 HV. Etched with 15 mL HCl, 10 mL acetic acid, 5 mL HNO3, and 2 drops glycerol. Original magnification 100 4.25 min at 1150 C (2100 F); 254 HV. (j) Same has (i) except solution annealed 2.5 min at 1205 C (2200 F); 246 HV. Note in (h) through (j) that grain size increases with increasing annealing temperature and time. 854 / Metallography and Microstructures of Nonferrous Alloys

Fig. 80 Haynes 188. (a) Cold rolled 50%, heated to 815 C (1500 F) for 1 h, and water quenched. The partly recrystallized structure contains M6C and M23C6 carbides in a face- b) Cold rolled 20% and solution annealed at 1175 C (2150 F) for 10 min, then) .ןcentered cubic (fcc) matrix. Electrolytic: HCl and H2O2 Original magnification 1000 c) Solution annealed at 1175 C) .ןwater quenched. The fully annealed structure consists of M6C particles in a fcc matrix. Electrolytic: HCl and H2O2. Original magnification 500 d) Solution annealed) .ןF) and aged 3400 h at 650 C (1200 F). Structure is M6C and M23C6 particles in a fcc matrix. Electrolytic: HCl and H2O2. Original magnification 500 2150) at 1175 C (2150 F) and aged 6244 h at 870 C (1600 F). Structure is M23C6, Laves phase, and probably M6C in a fcc matrix. Electrolytic: HCl and H2O2. Original magnification e) Solution annealed at 1175 C (2150 F), aged 6244 h at 980 C (1800 F). Structure consists of M6C and M23C6, and probably Laves phase, in a fcc matrix. Electrolytic: HCl) .ן500 ןand H2O2. Original magnification 500 Metallography and Microstructures of Heat-Resistant Alloys / 855

Fig. 81 S-816 alloy. (a) As-forged bar. The randomly dispersed carbide particles in the solid-solution matrix are primarily NbC. The high hardness of the carbides has caused them to show in relief. As-polished. Original b) Solution annealed1hat1175 C (2150 F) and water quenched. Hardness 20 HRC. Particles) .ןmagnification 100 of NbC are dispersed in the face-centered cubic (fcc) matrix. There is no grain-boundary precipitate. 92 mL HCl, 5 mL c) Same material and annealing treatment as (b), but aged 16 h) .ןH2SO4, and 3 mL HNO3. Original magnification 500 at 760 C (1400 F) and air cooled. Grain-boundary precipitate (primarily Cr23C6) and dispersed NbC particles in the fcc d) Solution annealed 1 h at) .ןmatrix. 30 HRC. 92 mL HCl, 5 mL H2SO4, and 3 mL HNO3. Original magnification 500 1290 C (2350 F), water quenched, aged 16 h at 760 C (1400 F), and air cooled. Hardness 35 HRC. Structure is dispersed NbC particles and grain-boundary Cr23C6 in the fcc matrix. 92 mL HCl, 5 mL H2SO4, and 3 mL. HNO3. Original ןmagnification 1000 856 / Metallography and Microstructures of Nonferrous Alloys

Fig. 82 MP35N. (a) Solution annealed 1 h at 1065 C (1950 F), air cooled, and cold worked to 51% reduction. Hardness 492 HV. Longitudinal section. (b) Solution annealed same as (a), then aged4hat535C (1000 F) to increase hardness to 565 HV. Longitudinal section. Both 15 mL HCl, 10 mL acetic acid, 5 mL HNO3, and 2 drops glycerol. ןOriginal magniﬁcation, both 100 Metallography and Microstructures of Heat-Resistant Alloys / 857

Table 12 Constituents observed in wrought heat-resistant alloys

Phase Crystal structure(a) Lattice parameter(b), nm Formula Comments cЈ fcc (ordered, L12) 0.3561 for pure Ni3Al to NiAl Principal strengthening phase in many nickel- and nickel-iron-base 0.3568 for Ni3(Al0.5Ti0.5) Ni3(Al,Ti) superalloys; crystal lattice varies slightly in size (0 to 0.5%) from that of austenite matrix; shape varies from spherical to cubic; size varies with exposure time and temperature /Ni3Ti (no solubility for Found in iron-, cobalt-, and nickel-base superalloys with high titanium 0.5093 ס g hcp (DO24) a0 other elements) aluminum ratios after extended exposure; may form intergranularly 0.8276 ס c0 in a cellular form or intragranularly as acicular platelets in a Widmansta¨tten pattern Ni3Nb Principal strengthening phase in Inconel 718; cЉ precipitates are 0.3624 ס cЉ bct (ordered DO22) a0 coherent disk-shaped particles that form on the (100) planes (average 0.7406 ס c0 diam, approximately 600 A˚ ; thickness, approximately 50–90 A˚ ); metastable phase Ni3Nb Observed in overaged Inconel 718; has an acicular shape when formed 0.511–0.5106 ס Ni3Nb (d) Orthorhombic (ordered a0 between 815 and 980 C (1500 and 1800 F); forms by cellular 0.4251–0.421 ס Cu3Ti) b0 reaction at low aging temperatures and by intragranular precipitation 0.4556–0.452 ס c0 at high aging temperatures TiC Titanium carbide has some solubility for nitrogen, zirconium, and 0.470–0.430 ס MC Cubic a0 NbC molybdenum; composition is variable; appears as globular, HfC irregularly shaped particles that are gray to lavender; “M” elements can be titanium, tantalum, niobium, hafnium, thorium, or zirconium ,varies Cr23C6 Form of precipitation is important; it can precipitate as ﬁlms, globules) 1.070–1.050 ס M23C6 fcc a0 with composition) (Cr,Fe,W,Mo)23C6 platelets, lamellae, and cells; usually forms at grain boundaries; “M” element is usually chromium, but nickel-cobalt, iron, molybdenum, and tungsten can substitute. Fe3Mo3C Randomly distributed carbide; may appear pinkish; “M” elements are 1.175–1.085 ס M6C fcc a0 Fe3W3C-Fe4W2C generally molybdenum or tungsten; there is some solubility for Fe3Nb3C-Nb3CO3C chromium, nickel-niobium, tantalum, and cobalt. Ta3Co3C-Cr7C3 Cr7C3 Generally observed as a blocky intergranular shape; observed only in 1.398 ס M7C3 Hexagonal a0 (alloys such as Nimonic 80A after exposure above 1000 C (1830 F 0.4523 ס c0 and in some cobalt-base alloys Ta3B2 Observed in iron-nickel- and nickel-base alloys with approx. 0.03% B 0.620–0.560 ס M3B2 Tetragonal a0 V3B2-Nb3B2 or greater; borides appear similar to carbides but are not attacked by 0.330–0.300 ס c0 (Mo,Ti,Cr,Ni,Fe)3B2 preferential carbide etchants; “M” elements can be molybdenum, Mo2FeB2 tantalum, niobium, nickel, iron, or vanadium. TiN Nitrides are observed in alloys containing titanium, niobium, or 0.4240 ס MN Cubic a0 (Ti,Nb,Zr)N zirconium; they are insoluble at temperatures below the melting (Ti,Nb,Zr)(C,N) point; easily recognized as-polished, having square to rectangular ZrN shapes and ranging from yellow to orange NbN Co7W6 Generally observed in alloys with high levels of molybdenum or 0.0475 ס l Rhombohedral a0 Fe,Co)7(Mo,W)6 tungsten; appears as coarse, irregular Widmansta¨tten platelets; forms) 2.577 ס c0 at high temperatures Fe2Nb Most common in iron- and cobalt-base superalloys; usually appears as 0.495–0.475 ס Laves Hexagonal a0 Fe2Ti irregularly shaped globules, often elongated, or as platelets after 0.815–0.770 ס c0 Fe2Mo extended high-temperature exposure Co2Ta Co2Ti FeCr Most often observed in iron- and cobalt-base superalloys, less 0.910–0.880 ס r Tetragonal a0 FeCrMo commonly in nickel-base alloys; appears as irregularly shaped 0.480–0.450 ס c0 CrFeMoNi globules, often elongated; forms after extended exposure between CrCo 540 and 980 C (1005 to 1795 F) CrNiMo ˚10A ס Note: For more information on this subject, see the article “Crystal Structure” in this Volume. (a) fcc, face-centered cubic; hcp, hexagonal close-packed; bct, body-centered tetragonal. (b) 1 nm 858 / Metallography and Microstructures of Nonferrous Alloys

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