Name /bam_asmint_104738/6072_003j/Mp_1 08/26/2002 10:01AM Plate # 0 pg 1 # Metallographic Techniques in Failure Analysis George F. Vander Voort, Buehler Ltd. METALLOGRAPHIC EXAMINATION is purpose is to determine whether processing or niques and examinations using the light micro- one of the most important procedures used by service conditions have produced undesirable scope (LM) in failure analysis. Metallographic metallurgists in failure analysis. Development of microstructural conditions that have contributed examination typically should follow nondestruc- powerful electron metallographic instruments, to the failure, such as abnormalities due to ma- tive and macroscopic examination procedures such as the scanning electron microscope, has terial quality, fabrication, heat treatment, and and should precede use of techniques of electron not diminished the importance of light micros- service conditions. Examples are given in this microscopy. copy. Basically, the light microscope is used to article to demonstrate such analytical work. Examination of fractured components should assess the nature of the microstructure and its Conducting a materials failure analysis, a begin with the low-power stereomicroscope. influence on the failure mechanism. The purpose common activity for many metallurgists, re- Hand-held magnifying lenses are still widely in using the light microscope may be twofold. quires a carefully planned series of steps (Ref 1, used to study fractures but mainly in the field. One purpose may be to determine the relation- 2) designed to arrive at the cause of the problem. While the light microscope has limited value for ship between the microstructure and the crack Proper implementation of light microscopy is of direct observation of fracture surfaces (more lim- path (in failures involving fracture) and/or the critical importance in failure analysis, and this ited for metals than nonmetals), a great deal can nature of corrosion or wear damage. The second article focuses on the use of metallographic tech- be learned by indirect examination, that is, by Fig. 1 Illustration of a cleavage fracture in a quenched and tempered low-carbon steel examined using three direct methods and three replication methods. (a) LM cross section (nickel plated). Etched with Vilella’s reagent. (b) LM fractrograph (direct). (c) SEM fractograph (direct). (d) LM replica. (e) SEM replica. (f) TEM replica Name /bam_asmint_104738/6072_003j/Mp_2 08/26/2002 10:01AM Plate # 0 pg 2 # 2 / Tools and Techniques in Failure Analysis Fig. 2 Light microscope fractographs taken with (a) bright-field and (b) dark-field illumination compared to (c) a SEM secondary-electron image fractograph of the same area. Sample is an Fe-Al-Cr alloy. examination of the fracture profile and secondary graphs published by Zapffe and coworkers (Ref ture and a replica of the fracture, and a TEM cracking. 4) beginning in the early 1940s, although a few replica of the fracture. Although Zapffe used Detailed observation of the fracture surface is studies of historical value predated their efforts. bright-field illumination for this work, dark-field best accomplished by use of the scanning elec- Zapffe’s work, however, was almost exclusively illumination often produces superior results. Fig- tron microscope (SEM) or by examination of confined to observation of cleavage facets on ure 2 illustrates the use of bright-field and dark- replicas with the transmission electron micro- rather brittle, coarse-grained specimens. The field illumination for viewing a brittle fracture in scope (TEM). However, lack of access to a SEM technique, basically an interesting academic ex- an Fe-Cr-Al alloy, plus a SEM fractograph of the or TEM should not be viewed as a crippling ob- ercise, did stimulate interest in fracture exami- same area. Dark-field illumination is better at stacle to performing failure analysis, because nation as part of failure analysis. However, the collecting the light scattered from the fracture such work was done successfully prior to the de- depth-of-field limitation of the light microscope features; glare is reduced, and image contrast is velopment of these instruments. In many studies, has restricted its use for such work. Aside from improved. such equipment is not needed, while in other the published light optical fractographs made by Examination of fracture replicas with the light cases, they are very important tools. In most Zapffe (see Ref 5 for a review of many of these), microscope (Ref 5–7) can extend the use of the cases, a more thorough job can be accomplished very few optical fractographs of metallic mate- method only to a limited extent, because the rep- using such tools. rials have been published by others. Microfrac- lica collapses slightly, producing less depth of The LM, on the other hand, is a virtually in- tography gained momentum with the develop- field. Also, with a replica, the risk of damaging dispensable tool for the failure analyst. The SEM ment of TEM replication methods and became the objective is eliminated. and TEM find application in microscopy when commonplace after the commercial introduction Considerable information concerning the frac- the required magnification/resolution exceed the of the SEM in approximately 1965. ture mode and the relationship of the microstruc- but the A flat, brittle fracture can be examined with ture to the fracture path can be obtained by LM ,(ןcapabilities of the LM (about 1000 first tool of choice is the LM. Hence, it is used the light microscope by orienting the fracture examination of the profile of partially fractured for fine-structure examination and identification. perpendicularly to the optical axis. It is best to (Ref 8–10) or completely fractured (Ref 11–15) Thus, TEM and LM are complementary tools. start with a low-power objective; long-working polished metallographic specimens. Such ex- Microstructural examination can be performed distance types are preferred. Focusing reveals aminations have been conducted for many years, with the SEM over the same magnification range the limitations of the method, because only part long before the development of electron metal- as the LM, but examination with the latter is of the fracture is in focus at any setting. Thus, lographic techniques, and continue to be used more efficient. Contrast mechanisms for viewing photographs reveal only a portion of the fracture because of the value of the method. If the frac- microstructures are different for LM and SEM. in focus, depending on the coarseness and ori- ture has progressed to complete rupture, so that Many microstructures, for example, tempered entation of the fracture facets. Figure 1 shows an only one side of the fracture is to be examined, martensite, exhibit poor contrast in the SEM and example of a brittle fracture in a low-carbon steel it may be best to nickel plate the fracture to en- are best viewed by light microscopy. When examined in this manner. Figure 1 also shows a hance edge retention. This is not required if the atomic number contrast or topographic contrast LM image of the fracture profile, a LM image of crack has not separated the component into two is strong, the SEM provides good structural im- a replica of the fracture, SEM images of the frac- pieces, or if a secondary crack is to be examined. ,Ref 3). Again) ןages, particularly above 500 because of the limitations and advantages of each instrument, they are complementary rather than competitive tools. All studies of microstruc- tures and fractures should begin at the lowest magnification level, the unaided human eye, and progress upward, first using the stereomicro- scope for fractures and the LM for fracture path and microstructural studies, before using elec- tron metallographic equipment. Examination of Fractures Microfractography is a relatively new field; its Fig. 3 Light micrographs of section profiles of (a) a nickel-plated ductile fracture and (b) a nickel-plated brittle fracture. roots can be traced to the light optical fracto- Both are carbon steels etched with 2% nital. Name /bam_asmint_104738/6072_003j/Mp_3 08/26/2002 10:01AM Plate # 0 pg 3 # Metallographic Techniques in Failure Analysis / 3 Figure 3(b) shows a nickel-plated brittle im- exhibits some minor branching and does not tra- case, the impact strength at room temperature pact fracture of a low-carbon steel with a ferrite- verse the ferrite grains in a straight-line fashion, was only 7% of that of an as-welded sample con- pearlite microstructure. The crack consists of nu- as with cleavage (Fig. 3b). Also, there are no taining austenite and delta ferrite. Note that the merous connected straight-line segments in the spherical cavities near the crack, as observed fracture path is microscopically flatter and con- ferrite phase. Several subsurface cleavage cracks with ductile fractures (Fig. 3a). The crack shows sists of numerous connected short straight-line are also present. In comparison, a ductile impact no preference for either the ferrite or pearlite dur- segments. The SEM fractograph shows numer- fracture in a quenched and tempered carbon plate ing its growth, and the macroscopic appearance ous small, flat fracture regions indicative of the steel is shown in Fig. 3(a). Note that the fracture of the crack is rather flat, which is typical of embrittlement due to sigma. The high-magnifi- surface exhibits a much rougher appearance due fatigue cracks. cation LM profile view of the completely frac- to the linking up of the microvoids. Below the Figure 5 shows partially broken and com- tured surface reveals the presence of extensive fracture surface, spherical ruptures are frequently pletely broken sensitized (649 ЊC, or 1200 ЊF, for sigma along the crack path. seen, which are also indicative of a ductile frac- 4h) impact specimens of American Iron and Examination of the crack path using cross sec- ture mechanism. Steel Institute (AISI) 304 stainless steel. Scan- tions is also very useful for study of fractures Fatigue fractures can also be examined using ning electron microscope examination of the due to environmental problems.