Thermal Expansion and Phase Transformations of Low-Expanding Cobalt-Iron-Chromium Alloys Peter Hidnert and Richard K

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Journal of Research of the National Bureau of Standards Vol. 55, No. 1, July 1955 Research Paper 2602

Thermal Expansion and Phase Transformations of Low-Expanding Cobalt-Iron-Chromium Alloys Peter Hidnert and Richard K. Kirby

Coeffi cients of lineal' thermal ex pansion of some cobalt-iron-chromium alloys are reported for various tempemture ranges between - 65 0 and + 800 0 C, and the effects due to tem perature, chemical composition, heat treatment, etc., were determined. Some of the alloys investigated have coefficien ts of ex pansion less than t hose for fu sed quartz and ordinary inval' between - 65° and + 60 0 C. Some of the low-expanding cobalt-iron-chromium alloys have ')'--> a transformations on cooling to low temperatures, and the resulting a-phase reverted to 'Y on heating to high temperatures. AI'" temperatures were observed as high as _ 10 0 C ancl Ac, tempemtures at abou t 600 0 C. The effects of various hea t treatments from - 196° to + ] ,0000 C on t he t ransformation s were investigated, and the resultillg changes of thermal ex pansion were correlated wi t h the structlll'e of these alloys.

1. Introduction During World "Val' II, Eas tman Kodak: Co., Rochester , N . Y. , obtained a Government contract In 1934 , Masumoto [1] ' reported the res ults of his from the Office of Scientific R esearch a nd D evelop investigation on the linear th ermal expansion of ment to produce a low-expanding alloy having a cobalt-iron-chromium alloys co ntaining more than coeffieien t of expansion less than th at of a good 49 percent of cobalt and 5 to 20 percent of chromium. grade of invar at atmospheric temperatu res. After These alloys were prepared by mixing suitable pro a search of the li terature and some preliminary tests portions of electrolytic cobalt, electrolytic iron, and with the cooperation of other companies, Eastm an chromium (with 2.7 % of impurities) and melting Kodak Co. authorized th e Unexcelled Manufacturing these metals in alumina crucibles in a hydrogen Co., Cambrid ge, }.tIass. , to prepare some low-expand atmosphere. The melts were cast in iron molds 26 ing cobalt-iron-chromium alloys similar to those cm long and 0.6 em in diameter. Samples 10 cm in investigated by Masumoto. Peter Hidnert and length were cut from the ingots and heated for 1 hI' \iVilmer Souder of th e N'ational Bureau of Stand ard s at 1,000° C in a vacuum fmnace and then cooled made determinations of linear thermal expansion on slowly in the furnace. The chemical composi tions a number of th ese alloys over the range from - 65 ° indicated by 'Mas umoto for these alloys were not to + 60 0 C. A restrictedrepolt of the results of this obtained by chemical analyses [2] but were computed work was preps,red by Edgar D . Seymour of Eastman by him from th e initial charges. Kodak Co. for th e Office of Scientific R esea rch and Some of these alloys investigated by Masumoto D evelopment. The following summary was taken have very small coefficients of expansion. For from an unclassifi ed extract of the report : example, the coefficient of expansion of an alloy From a sea rch of t he li terature for a low-expandin g wi th a nominal composition of iron 36.5 percent and metal, Masumoto's co balt, iron, a nd chromium alloy gave chromium 9.5 per ce nt is 0.1 X 1O-6/deg C between the most promi se, if the meltin g, heat treatment, a nd a na lysis problems co ul d be solved. We beli evc these 20° and 60 ° C. This expansion coe ffwi ent is less problems have bee n solved and the techniq ue has been than those of such well-known low-expanding perfected so t hat it is possible to specify t he composition materials as fu sed quartz [3] and the usual grade of to 0.1 % in chromium and iron. An all oy has been made invar [4] . The investigation of these low-expanding with a coeffi cient of ex pa nsion of less than ± 0.5 X 10- 6/° C over t he range from - 65° to + 60 0 C wi t h the coefficient co balt-iron-chromimum alloys was made over the nearly zero from 20° to 45° C and also in t he region of range from liquid-ail' temperature to about + 250° C. - 35° C. This is from t wo to three t imes better t han Masumoto stated that the observations above room invar obtainable on t he ma rket. temperature were taken during heating, as the The data reported by Seymour 011 thermal expan expansion was almost reveri sble for heating and sion of cobalt-iron-chromium alloys is included in the cooli ng, and the observations below room temper present paper. Add itional data by th e present ature were taken during cooling. a uthors on these alloys and other cobalt-iron In b is paper l\l[asumoto also reported results of chromium alloys are given for various temperatures investigations on some magnetic, electrical, and between - 65 ° and + 950° C. corrosion-resisting 2 properties of one of these low expanding alloys. Data on Young's modulus and 2 . Alloys Investiga ted the temperature coefficien t of Young's modulus for various cobalt-iron-chromium alloys were reported The cobalt-iron-chromium alloys listed in table 1 by Masumoto and Saito [5]. were prepared by John Wulff of the Unexcelled Manufacturing Co., Cambridge, :Mass. The alloys I Figures in brackets indicate the literature references at the end of this paper. were made from cobalt, iron, and chromium powders. % 'rhis all oy res isted co rrosion in a d ilute sodium chloride solution to a much greater extent than invar, a nd t herefore IVf asumoto named it "Sta inlcss- Tn var." A master alloy of cobalt and chromium was first

29 30 .}

ex pansion of cobalt-iran-chromium allays

Average coetlicients of expansion per d eg C Residual change in length after 20° to 60° to JOOo to 150° to 200° to 250° to 20° to 20 ° to 20° to 20° La -60° to -60° to each test f -20° C d + 20° C • 60° C 100° C 150° C 200° C 250° C 300° C 100° C 200° C 250° C 300° C 1------1------1------1-----1 X 10-6 X 10- 6 X JO-6 X JO- 6 X 10- 6 X IO-6 X 10-6 X 10- 6 X 10- 6 X 10- 6 X 10-6 X 10- 6 X 10-6 X IQ-6 %

1.1 0. 000 1.2 } (1',=-67°) (Ara=- JOO) \ +.228 6.7 4.8 5. 7 J 5.2 -. 002 5.6 }

.8 .000 .8 } ('1' ,=-64°) h. 1 (Ara= -35°) +- 170 6.6 3.8 5.2 } 4.3 -. 001 4.5 } 4.6 5. 4 7.7 10.6 5.0 6. 0 7. 3 9.7 +. 003 4.4 5.4 7.8 JO. 7 4. 9 6.0 7.3 9. 7 + .001 4.3 5.8 7.8 11 . 1 5. I 6. 1 7.5 9.7 .000 2. 6 4.0 3.3 6.7 -. 001 .2 1. 5.2 10.9 .8 2.5 4.8 8. 5 -. OOl -.2 1. . 6 . 000 - .2 1. .7 . 000

'. 4 . 000 '. 4 (T ,=-65°) .7 (Ara= - 40°) +.019 2.5 1.0 1. 7 .8 2.0 4.9 9.8 1.4 2.8 4. 7 8.3 -.001 1.0 1.9 1. 5 .000 . 4 1.6 4.6 9.9 14. 4 15.4 1. 0 2. 4 4. 4 6.6 8. 2 -.002 .2 1.8 1. 0 .000 .2 -.OOl . 4 (1',=-64°) . 2 (Ar, =-100) +- 146 5.5 2.8 4.2 3.4 -.001 3.7 3.7 4. i 7.3 II. 2 4.2 5. 4 7. 0 9.7 +.001 3.7 4.8 7. I J I. 6 L3.7 14.9 4.2 5. a 7. I 8.5 9.6 . 000 5.5 6.4 8.5 J I. 3 13. 4 14 .6 5.9 6.9 8. 1 9.3 10_2 +.001 2.4 4. 0 3.2 6.7 .000 .3 2.0 5. 0 10.7 14.6 15.7 1. 1 2.6 4. 9 7.0 8.5 -. OO [ .3 1. 8 1.1 .000 .6 2.0 5.6 10. I 13.9 15.6 1.3 3.0 4.9 6.9 8.4 011 4.6 8.2 5.9 7. 4 6.6 8.7 10.6 276 4.5 7.3 5.9 9.3 II. 3 4. 4 7.2 5.8 II. 4 .6 2. I 1.3 .000 .6 .6 . 000 .4 .3 .4 +-004 1.2 .5 .8 .8 +.001 .7 .8 2.5 5. 6 JO.9 1.7 3.2 5.3 8.8 -. 001 1. 1 .000 1.2 } . 000 1. 3 -.2 -. 1 -.2 } +.002 . 0 . 0 . 0 1.3 } .000 1.2 1.4 4.2 2.8 .000 8.2 13.0 15.2 16.0 2.8 4.8 7. [ 8. 9 10. [ -.001

.6 . 000 .6 +- 1 -. 4 -.2 +.004 .6 -. 1 +-3 . 7 -.001 .8 1.0 2.7 6. 3 11. 9 15.0 16.2 1. 8 3.6 5.9 7.9 9.3 -. 001

1. 0 .000 1.0 .8 +-5 .7 .000 1.0 . 4 .7 1.0 .000 . 9 1.0 2.7 6. 2 11. 4 14.6 15.8 1.9 3.6 5. 7 7.7 9. 1 -.OOl

31 TABLE 1. Coefficients of linear thermal

Ohemical composition a

Sample 'rreatment Test c Phase 00 b Fe Or o Mn Si

% % % % % %

lH 'Y 10 'Y 20 'Y 1i67 ______{HYdrOgen-annealed at 1,000° 0 for 1 hr and fur- 2H 53. 40 36.9, 9.30 .06 .08 .24 nacc-cooled in 20 hr. 'Y 3H 'Y 30 'Y I 4H 'Y 1770 ______53.50 36.70 9. :), .08 .07 .29 ____ _do ______IH 'Y

IH 'Y 10 'Y 20 1i72 ______do ______'Y 53.5, 36.6, 9.4, .08 .07 .28 ------. - - -.------2H 'Y 3H 'Y 30 'Y I 4H 'Y Ifi 'Y 10 'Y 20 li69______do ______-_- ___ 'Y 53. 56 36. 5, 9. ':6 .06 .07 .22 --- 2H 'Y 3H 'Y 30 'Y I 4H 'Y IH I i64 , ______53.30 30.6, 9.57 . 09 .08 .33 ____ do ______'Y - { 2H 'Y

IH 'Y 10 'Y 20 'Y 17GjA e ______• ___ ._ do ______53. l; 36. i s 9.57 .-----.------.--- -- 2H 'Y 3H 'Y 30 'Y 4H 'Y

IH 'Y 10 'Y 20 'Y 1773 ______53.9, 36.2, . 07 .07 .05 ____ do ______2H 'Y 3H 'Y 30 'Y 4H 'Y

t IH 'Y 10 'Y 20 .27 _____ do ______-______'Y li65______53. 1, 36.5, 9.81 . 11 .10 2H 'Y 3H 'Y 30 'Y 4H 'Y

• Fe and Or by J. I. Shultz and R . K. Bell of the National Bureau of Standards: ; Before this test. cooled to - 1960 C. 0, Mn, and Si by Walter M. Saunders, Jr. Data on two samples (1752 and 1753) k Before this test, hydrogen-annealed at 850 0 0 for 10 hr and at 1,000 0 0 for 1 were discarded on account of segregation. 11r, and furnace-cooled in 22 hr. b By difference. I From 200 to 50 0 O. " I-I indicates that the coefficients of expansion were obtained on heating and m During this test, beated to 700 0 C (sec table 2). o indicates that they were obtained on cooling. " Before this test, cooled to - 190 0 C . d TL indicates the lowest observed temperature during the test. o During this test, heated to SOOO C (see table 2). e An indicates the initial temperature of the 'Y----.a transformation as evidenced o During tbis test, heated to 9500 C (see table 2 for coefficients of expansion to by sndden expansion on cooling. 800 0 0 ). f Determined from the initial and final observations at room temperature. q On cool bog a duplicate sample rapidly, an A J'3 transformation was observed The plus sign indicates an increase in length and the minus sign a decrease in at abont -RO° C. length 'No An transformation was observed on cooling a dnplicate sample to -1960 O. • Approximate. • Tbis sample is not a duplicate of sample li65 . h From _~O O to +200 O. t Before this test, heated and cooled between -650 and +600 O. i Before this test, cooled to -800 O. made in a high-frequency furnace. Using this as analyses were generally made on pieces cut from the stock material, a series of runs was then made with ends of the samples. The iron contents varied from various iron additions to determine by chemical 36.22 to 37.21 percent, and the chromium contents analyses the losses met with in melting the same from 8.5 6 to 9.8 7 percent by weight. weight of material each time. In order to compen sate for these losses, desired amounts of the metal powders were added to the remelts before casting. 3. Apparatus Before pouring, the melts were deoxidized with definite amounts of calcium-silicon alloy. The cast The micrometric thermal-expansion apparatus rods C%-in. diam.) were then heat-treated. described by Hidnert and Souder [6] was used for Table 1 gives information about the heat treat the determinations of linear thermal expansion of ments and the chemical compositions of the cobalt the cobalt-iron-chromium samples. Each sample iron-chromium samples investigated. The chemical was about 300 mm long. 32 I L expansion of cobalt-iron-chromi t!m alloys- Continued

A verage coeffi cients of expansion per deg C Residual change in _ 60° to - 20 0 to -600 to 20 0 to 60 0 to 1000 to 150 0 to 200° to 250 0 to 20 0 to 20 0 to 20 0 to 20 0 to 20 0 to length after 0 0 0 0 0 0 0 0 0 0 2500 C 3000 C each tes t f - 20 C d +200 C +20 C 0 60° C 100 C 150 C 200 C 250 C 300 C 100 C 150 C 200 C 1------1-----1------1---- X I0-6 X IO-6 X 10-6 X l0-6 X IO-6 X JO-6 X 10-6 X 10-6 X I0-6 X 10-6 X lO-6 X 10-6 X 10-6 X 10-6 % 2. 0 1.8 } +. 001 -. 2 .5 . 2 . 0 . 8 . 4 } +. 002 1.8 1.7 } . 000 1. 7 4. 4 8.8 13.4 15. 4 16.6 3.0 5.3 7. 5 9. 2 10. 6 - .001 3. 2 5.8 10. 5 14. 0 15. 8 16.4 4. 5 6. 8 8. 8 10. 3 11. 4 +. 002 2. 6 2.8 } - . 001 . 0 . 8 .4 001 +. 2 . 9 . 6 +. 2. 8 2. 8 . 000 2. 7 6.0 10. 1 14.2 15. 9 16. 0 4. 3 6. 6 8.7 10. 3 11. 3 ~ +. 001 2.1 2.3 } -. 001 -. 1 . 3 . 1 -. 1 . 3 . 1 } . 000 2. 0 1.8 } . 000 2.2 4. 5 9. 4 14. 1 15.5 16. 1 3. 3 5. 7 . 0 9. 6 10. 8 . 000 4. 6 8.1 12. 1 15. 2 6. 4 8.6 10. 4 12. 6 - . 001 4.6 8. 2 12. 3 14.9 6. 4 8. 7 10. 4 12. 6 . 000 4. 5 4. 6 } . 000 .9 . 0 1.8 001 +. 2 1. 9 1.0 } +. 4. 5 4. 5 } . 000 4. 5 8. 0 12. 3 14. 9 15. 9 16. 3 6. 3 . 6 10. 3 11. 6 12. 4 . 000 3.8 3.8 } . 000 . 3 1.2 . 7 . 000 . 3 1.3 . 8 } 3. 6 3. 8 } - . 001 3. 6 6. 8 11 . 2 14. 4 5. 2 7.5 9. 4 11. 8 . 000 7.2 7. 3 } . 000 1.5 4.0 2. 8 . 000 1.5 4. 0 2. 8 7. 3 7. 4 } . 000 7. 6 10. 6 13. 7 15. 4 16. 4 17.4 9. 1 10. 9 12. 1 13. 0 13. 8 -. 002

4 . Results and Discussion perature and - 65° C (test 2); (c) again heating and cooling between room termperature and + 60° C D eterminations of linear thermal expansion of the (test 3). Several of the samples in figure 1 have samples of cobalL-iron-chromium alloys were made coefficients of expansion that are very nearly zero at various temperatures between - 65° and + 950° C for the range from - 65° to + 20° C. and of t he eff ects of various heat treatments (from Figure 2 shows the expansion of a low-expansion - 196° to + 1,000° C) on their thermal expansion . alloy over the range from - 65 ° to + 300° C. While The resulLs are summarized in 3 tables and 8 figures. ~ h e average coefficient of expansion of this sample Table 1 gIves the coeffi cients of expanSIOn that IS less than 1 X 10- 6jdeg C over the range from - 65° were determined for various temperature ranges to + 60° C, it rapidly increases to 15.0 X lO- 6jdeg C from - 60° to + 300 ° C. The phase of the samples over the range from 200° to 300° C. It is seen that for each test and the residual change in length after the low coefficients of expansion for such alloys each tes t are al 0 given in table l. generally apply for temperatures between - 65° Figure 1 shows expansion curves of nine samples and + 60° C. of cobalt-iron-chromium alloys between - 65 ° and Figure 3 indicates graphically the relations be tween chemical composition and the average coef + 60 0 C. These samples are arranged, from the bottom to the top, in increasing values of their coef ficien ts of expansion of the samples in the first tests ficients of expansion for the range from + 20° to for the range from 20° to 60° C . All of the coef fi cien ts of expansion apply for the samples in the + 60 0 C. For all of the samples that were investi gated between - 65° and + 60° C the observations original annealed conditions ('Y phase). The curve were m ade in the following order: (a) H eating and in this figure represents an isodil 3 equal to 1.0 X cooling between room temperature and + 60 ° C 3 The word "isodU" was derived in 1931 by Ridnert, from "iso" (Greek i,o$, meaning equal) and from the first three letters of "dilatation", and denotes a (te t 1); (b) cooling and heating between room tem- curve representing compOSitions having equal expansion. 33 ... --

+- COBALT FIG U RE 3. Portion of ternary diagram indicating the effects of composition (percentage by weight) on the coefficients of linear expansion (in millionths per degree C) of annealed cobalt iron-chromium alloys for the range 20° to 60° C.

1O- 6/deg C, and appears to approximate part of an ellipse as shown by Masumoto (1]. The results indicate that the low-expanding alloys lie within t his elliptical-shaped isodil and t hat the alloys out side of it have coefficients that generally increase with increasing distance from it. These results indicate that close chemical control is necessary in the preparation of these low-expanding alloys. The samples with chromium contents between 8.5~ to 9.15 pereenthave very low coefficients of expansion at room temperature but "( -7a transformations started between - 10° and - 80° C. Ara tempera tures for,five samples may be found in table 1 (see -60 -40 -20 o +20 +40 +60 also footnote q). These values indicate that for the TEMPERATURE. ·C samples investigated the Ar;J temperature tends to FIGURE 1. Linear thermal ex pansion of nine annealed cobalt decrease as the chromium content increases. No iron-chromium alloys (Fe 36.2, to 36.9" Cr 9.0. to 9.87 %), "(-7a transformation was found on cooling a sample The initial observation for each alloy was taken at about 20° C and is plotted (see table I, foo tnote r) containing 9.5 7 percent on the zero ordinate. 0, Heating; . , cooling. chromium, to as low as - 196° C. Masumoto (1) reported that his low-expanding alloys showed no "(-7a transformations even when cooled to liquid T hydrogen temperature (-253° C), but four of his ~ alloys expanded slightly (less than 0.02 percent) on d 17 81 1 / cooling to low temperatures. >z Figure 4 shows the "(-7a transformation that w V o a: occurred on cooling sample 1762 to - 67° C. This w a. / transformation was not completed after reaching z o this low temperature. The expansion on cooling if> z ~L caused by the "(-7a transformation proceeded with ~ ~.---- ~ 0 discrete rapid in creases in length. These discrete increases in length were accompanied by audible TESTS 1 TO 3 clicks and slight increases in temperature of the o sample. Upon heating the sample from - 67 ° C. it - 100 -50 o 100 200 300 was found that the rate of expansion was larger than TEMPERATURE. · C it had been for the sample in its original annealed FIG URE 2. Linear thermal expansion of annealed cobalt-iron co ndition. It was found that for temperatures chromium alloy (Fe 36.55, Cr 9.06 %). below 200° C the rate of expansion increases with O. Heating; • • cooling;_.initial observation. increasing amounts of a phase. 34 ~ pes ...

• 26 l,---' o .24 'r-- L~---+----~----~---4-----~~L+--~~--~ ~ .22 ~ S---' \.---' .20 .... V p.-- 17 62 .18 V > z Ie<==< t--. w ~ . 16 ~ ~ o~---4----~~~~---+----~-=~~---+L---~ w a. u '\ z ffi . 14 o Q. ~ 0 1 ----4----~--<>oo=:i=----+----~""'7~~- -~---- 2 - . 12 1\ ~ 0 !:5 !Q . 10 \

.04 ~ + .02 o '., +. --....,!...... O.~...!..", _--'-____....L. ____ .L.... __ ---L ____ ....J \ 300 \ -100 o 100 200 0 \ TE MPERATURE. "C

- .02 FIGURE 5. Linear thermal expansion of anne'lled co balt-iron -60 - 40 -20 0 20 40 60 chromium alloy (F e 36.9" C r 8.8.%). TE MPERATURE :C Combined tests 1 to 3 show 'Y ....a tr,,;nsformation on cooling; before filth test. sample was cooled to -80° C; before SIxth test. sample was cooled to - 196° C; FIGUR E 4. Linear thermal ex pansion curve of annealed cobalt before seventh test. sample was hydrogen annealed at 850° C for 10 hr and at iron-chromium alloy (Fe 36.98, Cr 8.5a%) showing 'Y ...... '" 1.000° C for I hr. and furnace cooled in 22 hr. transformation on cooling. O. Hea ting; • • cooling; ..... initial observation. 0, Heating; e, cooling; ..... , initial observation . T - ,. The 1' ~ iX transformation III sample 1766A (c u t / ~ /' from same rod as sample 1766) proceeded iso ..,.. thermally when its temperature was maintained at '"o // - 70° 0 for 3 days. On rapid cooling from room ...L / /-:7 temperature to - 70° 0 , the sample expanded 0.12 VrEsi l i/-- percent. Its length at - 70° 0 in creased with time o ~ 1/ ~:/ V at a .dimin,ishing rate. At the end of 3 days, Lhe TESV additlOnal merease at - 70° 0 was 0.06 percent . TEST 2 V / V Figure 5 also shows an incomplete 1' ~ iX transfor ~STy: mation that occurred in sample 1758 on cooling to ..# ~ V - 64 ° O. Although the sample was cooled to - 80 ° V ~ST5 1760 C before the fifth test, the rate of expa nsion during o ~ the fifth test was practically Lhe same as the rate --- V during the third and fourth tesLs. This indicates / ~ that little or no additional 1' ~ iX transformation ,...-- occurred during cooling between - 64° and - 80° O. ."y' / The rate of expansion d Llring the sixth test after o cooling to - 196° 0, indicates that an additional TEST 6 o 1' ~ iX transformation occurred in the sample. After ao--

annealing at high temperatures (see table 1, footnote FIG URE 6. Lineal' theTmal expansion of annealed cobalt-iron k) and then cooled slowly to room temperature. chTo mium alloy (Fe 37.0" Cr 8.8, %), The smaller rate of expansion in the seventh test as Belore lo urth test. sample was cooled to - 196° C. compared to the rate in the sixth test of the sample O. H eating; • • coolin g, indicaLes Lhat the annealing treatment resulted in ~ partial transformation of the a to the l' condition. R ef - 190° 0 a nd held at this temperature several hours. erence should also be made to the coeffi cients of ex A comparison of the expansion curves on heating 111 pansion (between 20° and 60° 0 ) In table 1 for a the third and fomth tests indicates that the bettet comprehension of the comparisons between cooling to - 190° 0 transformed a part or all of the l' the test indicated in fi gure 5. to iX . An examination of the data in this paper ,,~ Figure 6 sho\vs th_e results obta,ined on six tests of indicates that the 'Y ~ a transformation in this sample sample 1760. The first three tests were made with was the most complete of any that were investigated th e sample in Lhe originaJ l' condition. The third In this work. On heating the sample in the fourth test did not show any irregularities to 700° O. After test, an a ~ 'Y transformation4 started at about 600° C. the third te t, the ample was cooled rapidly to • Confirmed with other samples. 35 ------~

TABLE 2. Coefficients of linear thermal expansion of cobalt-iron-chromium alloy (s ample 1760; Fe 37.04, Cr 8.87 %)

A verage coefficients of expansion per degree C Test · Phase 20° to 400° C 20° to 500° C 1200 to 600° C 1200 to 700° C 20° to 800° C

3H ...... l' 1O.4XlO-6 11. 9X1o-6 13.0XlO-6 13.9XlO-6 3C ...... l' 14. 0 4H b ••••••••••••• 11. 7 12.4 4C ...... 1'+"" 12. 6 13. 4 15. 3XlO-6 5H ...... 1'+" 13.5 14. 2 14. 9 15. 3

• H indicates that the coefficients of expansion were obtained on beating and C indicates that they were obtained on coo li ng. b Before this test, cooled to - 190° C.

The a -'7'Y transformation was completed during The coefficients of expansion in tables 1 and 2 heating to 950° C in the fifth test, as indicated by indicate the presence of a and 'Y phases in the samples the low expansion in the sixth test (compare with due to various heat treatments. Another indication results of tests 1 to 3 for the 'Y condition). of their presence is th e residual ch ange in length Table 2 gives the average coefficients of expansion at 20° C. Table 3 gives the changes in length at of sample 1760 for various temperature ranges from 20° C from the 'Y condition of five samples after 20° to 800° C . Figure 7 shows the coefficients of various heat treatments. The amount of a phase expansion versus temperature for the 'Y and IX phases present in a sample appears to be approximately of this sample. The marked change in slope of the proportional to its increase in length. curve for the sample in the 'Y condition at about 240° C is typical for these alloys and is apparently associated with the magnetic transformation at T ABLE 3. Changes in length caused by tmnsformations in 5 the Curie point.5 For various mixtures of a and 'Y samples of coba lt-iron-ch1'omium alloys in this alloy, the corresponding curves for the coeffi cients of expansion versus temperature generally lie Cumula tive changes' in length (at 20° C) after successive heat treatments between th e curves shown for a and 'Y (e. g., table 1, sample 1760). Heat treatment I Figure 8 reproduced from Koster [7] indicates 1______1 __ 1_ 7_57_ .-::... ~ 1766A b 1783A ~ that the boundary between IX and 'Y phases is a steeply ascending curve up to 600° C in the region of 9 As received (1' condi· % % % % % percent of chromium. The conclusion, deduced lion) ...... 0.000 0.000 0. 000 0.000 0. 000 Cooled to -65° C ... +.170 +. 146 from the data reported in table 1 of the present Cooled to _ 80° C . .. . 173 . 150 Cooled to - HJO o C .. +.265 investigation, that the Ar's temperatures of the alloys Cooled to - 196° C __ . 179 . 263 +.243 +- 182 decrease as the chromium content increases, confirms Heated to +700° C .. . 067 .070 Cooled to -196° C .. . 078 . 070 this equilibrium diagram. The AC1 temperatures Heated to 1.000° C .. . 069 . 068 . 006 -. 001 Held at -70° C for obtained on three samples at about 600° C are also 3 days...... 230 in agreement with this diagram. An examination Cooled to -196° C o. .232 +. 182 of this figure in conjunction with data in table 1 • Where values are not indicated, the heat treatments were not given. also indicates that IX phase should not be found in b Out from same rod as sample 1766. th ese alloys containing more t han 9.6 percent • Cut from same rod as sample 1783. chromium.

• T he aut hors found that a specimen cut from sample 1757 in the l' condition lost its magnetization at about 240° c. Masumoto [ll indicated that the mag· netic transformation point of low·expanding cobalt·iron·chromium alloys is in 1600 ,---,---,--"';'-..---r--= the vicinity of 250° C. 1400 ,. + '" 25 1200 C( 2 x !- o t5 20 ~ 1000 Vi f.--- f..------::;) z !-- ~ 800 ~ 15 w --- t-- !i"' 600 "- V o 10 A .."' en ...z 1760 w VI Q 5 Y " "w 8 0 ;/ o 10 20 30 40 50 60 0 10 20 3 0 40 50 60 70 o 100 200 300 400 500 600 700 800 900 1000 CH ROM IUM . PER.CENT TEMPERATURE, ·C FIGURE 7. Variation of coefficients of expansion versus tem FIGURE 8. Two sections of ternary equilibrium diagram of perature for 'Y and a phases of a cobalt-iron-chromium alloy cobalt-iron-chromium alloys for 40 and 30 percent iron (Fe 37.0" Cr 8.87 %), (Koster [7]). 36 FIGURE 9. Microstructure (X75) of two cobalt-il'on-chl'omium alloys. Etched with hydrochloric and picric acid s in alco hol [IOJ. A, Sample 1766, after annealing at 1,000° C; B , sample 1757, after cooli ng to - J96° C.

A meLallographic examination 6 was made of some proceeded with discrete rapid increases in length, ac of the samples in the "I and a + 'Y cond iLions. Typical companied by audible clicks and slight increases in tructures are shown in the photomicrographs of temperature. figure 9. This figure shows that the annealed 5. The coefficients of expansion below 200 0 C in sample (fig. 9A, "I) has large grains and dendrites crease with increa ing amounts of a phase present whereas t he ample that bad been cooled to - 196° C in the alloys. (fi.g. 9B, a+'Y) has a needle-like structure. The 6. Samples containing a started to transform to needle-like structure formed during the transforma· "I at about 600° C. tion on cooling and the kinetics of the transformation indicated by the dilatometric experiments, shows 6. References that tbis transformation is of tbe martensitic type [8, 9] . [1] H . Masumoto, On the thermal expansion of alloys of cobalt, iron, and chromium, and a new alloy, "Stain less-In val''', Science Reports T6hoku Univ. 23, 265 5. Summary (1934) . [2] Private communication from H. Masumoto. 1. The linear thermal expansion and pbase trans [3] W. Souder and P . Hidnert, Measurcments on the ther mal expansion of fused silica, Sci. Pap. BS 21, 1 (1926- formations of some cobalt-iron-chromium alloys 27) S524. were investigated at variou termperatures between [4] Invar and related nickel steels (2d edition), Circular BS - 65° and + 950° C. No. 58 (1923). [5] H. Masumoto and H . Saito, On Young's modulus and 2. Annealed alloys having chemical compositions its temperature coefficient of the alloys of cobalt, iron of about 54 percent cobalt, 36.6 percent iron, and and chromium, and a new alloy "Co-elinvar", Science 8.9 percent chromium have coefficients of expansion Reports Research Institutes, T6hoku Univ. A1, 17 les than 1 X 10- 6/deg C for the range from 20 ° to (1949). [6] P . Hidnert and W. Souder, Thermal expansion of solids, 60° C. Close control of chemical composition is a NBS Circular 486 (1950). prerequisite for success in the production of low [7] W. Koster, Das System Eisen-Kobalt-Chrom, Archiv expanding cobalt-iron-chromium alloys. Eisenhuttenwesen 6, 113 (1932) . [8] A. R. Troiano and A. B. Greninger, The martensite 3. For temperature above 60° C, the coefficients transformation, Metal Progress 50, 303 (1946) or of expansion of the alloys increase to about 16 X 10- 6/ Metals Handbook, Amer. Soc. Metals, p. 263 (1948). deg C for the range from 250° to 300° C. [9] M. Cohen, The martensite transformation, Symposium 4. Some low-expanding alloys were found to have on phase transformations in solids, p. 588 (John Wiley & Sons, Inc., New York, N. Y., 1951). a 'Y ---l> a transformation on cooling to low t.empera [10] Subcommittee on etching reagents for iron and steel, tures. This transformation is of the martensitic Etching reagents for the microscopic examination of type in that it produced a needle-like structure and iron and steel, Metals Handbook, Amer. Soc. Metals, p. 394 (1948) .

• By D . B. Ballard of the Metallurgy Division of the ational Bureau of Standards. WASHINGTON, December 27, 1954.