The Alpha z --> Gamma Transformation in Iron Powders

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1 The Alpha z --> Gamma Transfrmatin in Irn Pwders ALAIN COUTURE and ROCH ANGERS The alpha ~- gamma transfrmatin in varius size fractins f three different types f irn pwder was studied by differential thermal analysis. The alpha-t-gamma transfrmatin temperature increases cnsiderably with decreasing particle size and with the number f thermal cycles thrugh the transfrmatin fr a given particle size. The gamma-t-alpha transfrmatin temperature decreases with decreasing particle size and remains abut cnstant during thermal cycling thrugh the transfrmatin. I. INTRODUCTION NUMEROUS studies n the alltrpic transfrmatins f irn have been published. Mst f them were made with the purpse f determining the transfrmatin temperatures f pure irn using dilatmetric and/r calrimetric techniques. Others were carried ut t identify the transfrmatin mechanisms; they ften led t cntradictry results. The alpha-gamma transfrmatin temperature is nw fairly well knwn, but dubts still remain abut the transfrmatin mechanisms. The present study f the alphagamma transfrmatin f irn based n the differential thermal analysis (DTA) f irn pwders is believed t cast sme new light n this subject. II. A. Irn Pwders EXPERIMENTAL PROCEDURE Three irn pwders were used. Tw f them are cmmercially available: the B-214 type frm Glidden Metals and the Atmet 28 frm Quebec Metal Pwders Ltd. The third ne was made in ur labratries by the rtating electrde prcess (REP). The B-214 pwder is a high purity reduced and hydrgen annealed pwder. The Atmet 28 is a spnge-type pwder annealed under dissciated ammnia and grund t its final size. The REP pwder is f spherical shape. The particle shape f these pwders is illustrated in Figure 1. Chemical analyses f the pwders and f the wrught Armc irn used t make the REP pwder are given in Table I. The pwders were selected n the basis f tw main criteria: purity and wide particle size distributin. Gd purity was necessary t minimize the effect f impurities n the alltrpic transfrmatins. Wide particle size distributins were preferred t allw the preparatin f the many size fractins necessary fr the present study (Table II). Size fractins dwn t 325 mesh were btained by standard sieving. Smaller fractins were prepared with micr-sieves. B. Apparatus The transfrmatin temperatures were determined by differential thermal analysis. A Standata 6-25 apparatus* was *Stantn Instruments Ltd. used t btain DTA curves. The sample hlder was made f sintered alumina, and the tw cylindrical sample wells were 7 mm in diameter and 11 mm deep. ~ Temperatures in the tw wells were measured by means f Pt/Pt-13 pct Rh thermcuples using the temperature f the center f the reference sample well as the reference temperature. C. Sample Preparatin The reference material was alpha alumina f 0.3 /zm nminal size. This alumina pwder (0.10 g) was lsely pured int the reference well befre each experiment. The samples t be analyzed cnsisted f a mixture f 0.30 g f irn pwder with 0.08 g f alumina pwder. The use f a mixture instead f pure irn pwder minimized the differential temperature (AT) signal drift caused by the difference in heat capacity between the sample and the reference material 2'3 and prevented sintering f irn particles during the DTA runs, allwing measurement f transfrmatin temperatures f a lse pwder rather than f a sintered aggregate. This mixture was lsely pured int the sample well. D. DTA Parameters The DTA furnace cntaining the reference and sample wells was purged by circulating argn befre every run. The sample was first heated at the rapid rate f abut 0.67 ~ per secnd up t 850 ~ At 850 ~ a temperature cntrller maintained the slwer heating rate f 0.12 ~ per secnd up t 965 ~ In all cases,, the endthermic DTA peak crrespnding t the alpha-t-gamma transfrmatin culd be recrded cmpletely befre reaching 965 ~ Frm 965 ~ the sample was cled at the same rate f 0.12 ~ per secnd dwn t 870 ~ allwing the exthermic DTA peak fr the gamma-t-alpha transfrmatin t be recrded cmpletely. After this first cycle, a secnd heating cycle was started frm 870 ~ t 965 ~ and, then, back again t 870 ~ During the experiments, argn was circulated at 0.33 cm 3 per secnd arund the sample hlder. The thermal cycle described abve is shwn schematically in Figure 2; it was used fr all the DTA runs. Precisin n the temperature was ---1 ~ ALAIN COUTURE, Graduate Student, and ROCH ANGERS, Prfessr, are with the Department f Mining and Metallurgy, Laval University, Ste-Fy, Quebec, PQ G1K 7P4, Canada. Manuscript submitted March 29, III. EXPERIMENTAL RESULTS Every selected size fractin (Table II) f the three types f irn pwder was subjected t 3 t 5 cmplete DTA runs depending n the precisin f the results. A new sample was METALLURGICAL TRANSACTIONS A VOLUME 17A, JANUARY

2 Fig. 1--Scanning electrn micrgraphs f: (a)b-214 pwder, (b) Atmet 28 pwder, and (c) REP pwder. used fr each run. The results presented fr each size fractin are then average values. In all cases, the transfrmatin temperature was taken n the DTA curves at the pint where the base line (AT signal) lses its linearity, i.e., at the beginning f the endthermic r exthermic peak. 1,3.4 As explained earlier, each DTA run included fur transfrmatins: tw n heating and tw n cling. The relatin between the temperatures f these fur transfrmatins and the size fractin (r particle diameter) is illustrated in Figures 3, 4, and 5 fr the three irn pwders used. Linear regressin was used t fit thse curves thrugh the experimental pints. DTA runs including mre than tw thermal cycles and fur transfrmatins were als carried ut during this wrk and shwed that the transfrmatin temperatures remain stable after tw thermal cycles. Fr this reasn mst DTA experiments were limited t tw thermal cycles, and data n experiments cvering mre than tw cycles are nt reprted here. As it can be seen in Figures 3, 4, and 5, the transfrmatin temperature n heating (a ~ 30 rises as the particle diameter decreases. This effect is mre imprtant fr the B-214 and the Atmet 28 pwders especially during the secnd cycle. On the ther hand, the transfrmatin temperature n cling (7--* a) decreases with the particle diameter fr bth cycles. In accrdance with results reprted by ther authrs fr massive irn, 6'7 hysteresis ccurred between the transfrmatin temperature f irn pwders n heating and cling fr each f the tw cycles. The influence f the particle diameter n the magnitude f this hysteresis is illustrated in Figure 6 fr the tw thermal cycles. The curves f Figure 6 were btained by the same linear regressin technique used fr thse f Figures 3, 4, and 5. Fr the first cycle f each pwder, particles larger than abut 70 micrns exhibit a hysteresis f the rder f magnitude reprted fr massive irn 6'7 fr the same heating rate. In the 50 t 70/xm range, hysteresis increases when the particle size decreases. During 38--VOLUME 17A, JANUARY 1986 METALLURGICAL TRANSACTIONS A

3 Table I. Chemical Cmpsitin f Varius Particle Size Fractins f the Starting Irn Pwders B-214 Atmet 28 REP Pwder Pwder Pwder Element 106 t 53 t 20 t 125 t 75 t 37 t 180 t 90 t 53 t Wrught r Oxide 125/xm 63 /xm 30/xm 150 p,m 90/zm 45 /xm 212/zm 106/xm 63 p,m Armc Irn Detected (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) C S Ca SiO2 A1203 MgO TiO2 MnO V205 Cu Ni C Cr M Zn P Ag Zr : nt available <2000 < < 95 < 95 < < 90 < 90 < < 10 < 10 < < 20 < 20 < 20 < 10 < 10 < < 20 < 20 < 20 < 10 < 10 < ,5 2,5 5,0 < 5 < 5 < < 95 < < < Table II. Pwder Size Fractins Studied by DTA (Indicated by an X) Size Fractin Pwder (/zm) B-214 Atmet 28 REP Pwder ~ 94c 250 t 212 X X 212 t 180 X X ~ t 150 X X 150 t 125 X X X n= 125 t 106 X X X.- 92C 106t 90 X X X 90 t 75 X X X 75 t 63 X X X 9 91 Iz 63 t 53 X X X h 53 t 45 X X X ca z t 37 X X,a n,- 37 t 30 X X =-- 30 t 20 X X 89 <20 X X A 95C t g j I J ~ ~ ~ I T =, = 't '~ B-214 POWDER cx-~ (ISTCYCLE) \ _- - ~-,.0~(... ) ",, -. ~,.-.-u (2NOcYCLE) b., ~.~ 1,.-~, (.,, ) "~,..,.,.~ 0 50 I00 MEAN DIAMETER OF PARTICLES (MICRONS) I +,-- Fig. 3--Influence B-214 pwder. f particle size n the transfrmatin temperatures f 25 TIM E---~.-- Fig. 2--Schematic DTA curve fr a first cmplete thermal cycle.... 0~- the secnd cycle, hysteresis values are quite imprtant even fr large particles. As in the first cycle, these values increase as particle diameter decreases. Anther interesting result which can be drawn frm the curves f Figures 3, 4, and 5 is that the a ~ y transfrmatin temperatures during the secnd heating cycle were always higher than during the first ne. During cling (y ~ a), transfrmatin temperatures f the secnd cycle were at abut the same as thse f the first ne. The transfrmatin temperature differences between the first and secnd cycle are illustrated in Figure 7 as a functin f particle METALLURGICAL TRANSACTIONS A VOLUME 17A, IANUARY

4 .9 95(; 94O 950 I,,, ' l l,,, l, l,, I \ ATOMET 28 POWDER ~ =e-~'~" (tltcycle) H Ir'~'=(( " " 1 "~, ~-~ ~...~l (2k~t'CLE) ".. ~-- tr.~-=~(.,. ) ",% '.. 94C 930 I ' ' ' ' I ' I,,, ' I 92(3 9 REP POWDER ~ t~(istcycle ) H lr~*~(,,,, ) ~-~ 0(~11" (2NOCYQ.E) >-~ I~(... } 1-92C " - =-... ~'.,~9 ~ ~... ~... ~_... ;... _m I,, = ~ i n,, = I,, a I I MEAN DIAMETER OF PARTICLES (MICRONS) Fig. 4--Influence f particle size n the transfrmatin temperatures f Atmet 28 pwder. diameter. In the case f the 7 --~ t transfrmatin, n significant temperature difference culd be bserved. Hwever, fr the transfrmatin n heating (a ---* y), imprtant temperature differences were bserved with the B-214 and the Atmet 28 pwders. These differences increase with decreasing particle diameter. In the case f the REP irn pwder, the temperature difference des nt exhibit the same trend. These transfrmatin temperature differences between the first and secnd cycle suggest that the particle diameter is nt the nly parameter affecting the transfrmatin behavir f the irn pwders particularly during heating. A pssible effect f cntaminatin in the curse f the experiments was cnsidered at first but was eliminated by the results f DTA experiments including up t ten thermal cycles instead f tw as usual.~ As mentined earlier, the transfrmatin temperatures during these tests stabilized after the secnd cycle at different values depending n the particle size fractins. This behavir excludes prgressive cntaminatin as an explanatin fr the change in transfrmatin temperatures with the number f thermal cycles. Indeed, if cntaminatin were respnsible fr the shift in transfrmatin temperatures, large particles wuld require mre thermal cycles than small nes t reach the same cnstant transfrmatin temperature and particles f all sizes wuld eventually reach the same transfrmatin temperature, which was nt the case. An examinatin f particle surfaces befre and after thermal cycling was als made by scanning electrn micrscpy and shwed n significant surface differences due t cntaminatin. Systematic cnventinal metallgraphic examinatin f the pwders' internal micrstructure was then carried ut and led t sme interesting bservatins. Figure 8 shws the micrstructures f the same size fractin f each f the three irn pwders investigated, befre and after tw thermal cycles. It is bvius that grain grwth tk place during thermal cycling thrugh the alpha-gamma phase transfrmatin. In the case f smaller size fractins (nt illustrated here), mst particles were single crystals after thermal cycling which reveals a tendency t reach the single crystal state thrugh repeated transfrmatins. I I ~,,, l,.,, I, n i i I i i n 50 I MEAN DhIIdMETER OF PARTICLES 1MICRONS) Fig. 5--Influence f particle size n the transfrmatin temperatures f REP pwder. IV. DISCUSSION A. Effect f Impurities n Transfrmatin Temperatures The chemical analyses f ur pwders (Table I) shw that their impurity cntent shuld lead t transfrmatin temperatures n heating and cling belw the generally accepted value f 911 ~ fr pure irn. 6 They als shw that smaller size fractins f all pwders are richer in y-stabilizing elements (C, Ni, C, etc.) than the larger size nes, which shuld lead t slightly lwer transfrmatin temperatures fr smaller size fractins than fr larger size nes. Figures 3, 4, and 5 cnfirm these assumptins fr the transfrmatin n cling (y ~ a) since transfrmatin temperatures f bth thermal cycles stay belw 911 ~ and becme lwer as particle diameter decreases r, as said earlier, as y-stabilizing impurity cntent increases. Hwever, the impurity cntent cannt explain the a --~ y transfrmatin behavir f the pwders since, even if the transfrmatin temperature f the large particles stays belw 911 ~ during the first heating, it increases as the panicle diameter decreases cntrary t what shuld be expected frm the impurity effect. This cnclusin is supprted by the behavir f the pwders during the secnd cycle when the a ~ y transfrmatin temperatures f all the size fractins f the three pwders stay well abve 911 ~ This suggests that anther parameter was acting strngly against the influence f the impurity cntent. B. Effect f Particle Size and Number f Thermal Cycles n Transfrmatin Temperature Because f the elevated temperatures at which the alphat-gamma and the gamma-t-alpha transfrmatins ccurred and the relatively small verheating (r undercling) bserved during the DTA runs, these transfrmatins must invlve a nucleatin and grwth mechanism. 8,9'~~ The alpha-t-gamma alltrpic transfrmatin f irn usually takes place ver a range f temperature when impurities are present. Gamma nucleatin begins at preferred sites such as grain bundaries, inclusins, and ther crystalline defects. Generally, grain bundaries are cnsidered as the mst prbable nucleatin sites since they can accmmdate defrmatins and stresses prduced by the 1 pct vlume 40--VOLUME 17A, JANUARY 1986 METALLURGICAL TRANSACTIONS A

5 ,, i, 1 ' ' I ~ I ~ ' ' ' I.... I, ], _ "'~'x~ _- c B-214 POWDER IISTcYCLE) 60 \ =--'--= ATOMET 28 POWDER (. " I- \... REP POWDER ( " " ) % B-214 POWDER (2NOCYCLE) 50 \ ATOMET 28 POWDER ( " " )-... REP POWDER (. 9 I 40 I-- ~ "... --~ - "",~... >'z ZO '~... ~ \ ~ =-... "~ "~"~, ,~. "--~ ~ ~ - ~.. I0 ~ ~,.~. --= ~--r ".:... ~... O "~" ~,, a" x "l i i A J,,, n i I 1 l,,, 50 I MEAN DIAMETER OF PARTICLES IMICRONS) Fig. 6--Influence f particle size n hysteresis between transfrmatins n heating and cling. ' ' ' ' I.... I ' ' ' ' I ' ' ' ' I ' ' ' -- 4D : : POWDER "--'4 ATOMET 28 POWDER 9... REP POWDER w t2 n 1,1 n, 0 w.."" A"""..,.,..' "--Z_.,,... " t "z'--~,-- T - ~. il. IE 50 I MEAN DIAMETER OF PARTICLES (MICRONS) Fig. 7--Influence f particle size n temperature differences between the transfrmatins f the first and secnd cycle n heating and n cling. (Fr heating, pltted values crrespnd t the transfrmatin temperature f the first cycle minus thse f the secnd ne. Fr cling, the negative values f that difference are pltted.) cntractin which takes place during nucleatin f the gamma phase inside the alpha matrix. It Als, the free energy f frmatin f a critical nucleus at a grain bundary is smaller than within the matrix because sme intergranular energy is recvered when the nucleus appears at the bundary, t~ The intergranular energy gain is larger when nucleatin takes place at a triple pint and is even mre imprtant when the nucleus is situated at a quadruple pint. The frmatin f a nucleus is then mre prbable at grain intersectins. The abve cnsideratins abut nuclei frmatin culd explain t a certain extent the influence f pwder particle size n the a ---> y transfrmatin temperature reprted in the present paper. As bserved earlier in Figures 3, 4, and 5, the ct -* 7 transfrmatin temperature during the first thermal cycle remains nearly cnstant fr all particles larger than abut 70 p.m. There is a prgressive rise f that temperature frm 70 micrns t the smaller size fractins. A systematic metallgraphic examinatin shwed that the number f grains and the amunt f grain bundary surface within particles f the starting pwders decrease with particle diameter s that the prbability f finding a triple r quadruple pint in the 50 t 70/~m diameter range is very small and becmes nil in finer particles (20 t 30 p~m) which METALLURGICAL TRANSACTIONS A VOLUME 17A, JANUARY

6 Fig. 8--Optical micrgraphs f pwder particles shwing grain grwth after tw thermal cycles: (a) as-received B-214 pwder, (b) cycled B-214 pwder, (c) as-received Atmet 28 pwder, (d) cycled Atmet 28 pwder, (e) as-atmized REP pwder, and (f) cycled REP pwder. (Each micrgraph illustrates a different particle.) 42--VOLUME 17A, JANUARY1986 METALLURGICALTRANSACTIONSA

7 d nt cntain mre than tw r three grains. Large particles cntain many nucleatin sites where the free energy f frmatin f gamma nuclei is minimal and can then transfrm at temperatures near the equilibrium temperature f a wrught sample f similar chemical cmpsitin and micrstructure. In the case f particles smaller than 50 t 70 tzm which cntain few triple and quadruple pints and even nne fr the finer particles, the gamma nuclei must eventually frm at tw grain interfaces r inside the alpha grains where the free energy f frmatin is higher. The rise in the a ~ 3' transfrmatin temperature during the first cycle wuld then be due t an verheating phenmenn caused by a delay in nucleatin. Figures 3, 4, and 5 shw a similar phenmenn during the secnd heating cycle, but since thermal cycling f pwders prduces grain grwth inside particles as mentined earlier, the particles within which triple r quadruple pints can still be fund are larger. Figure 8 als shws that the prbability f finding such pints, even in the larger particles, is very lw, sme f thse particles being already single crystals after tw thermal cycles. This culd explain the rise in the a ~ 3' transfrmatin temperature frm the first t the secnd cycle. As in the first cycle, this rise in temperature can be ascribed t verheating caused by the disappearance f the easy nucleatin sites during thermal cycling. The influence f particle size n the a ~ 3' transfrmatin temperature can, hwever, be nly partially explained by cnsideratins abut the number f grain bundaries. Indeed, in the case f B-214 and Atmet 28 pwders, metallgraphic examinatin shwed that mst particles smaller than 100/xm are single crystals after the secnd thermal cycle, but Figures 3 and 4 shw that the alpha-t-gamma transfrmatin temperature fr the secnd cycle is nt cnstant fr thse particles but increases with decreasing particle diameter. Althugh it is easily cnceivable that nucleatin can take place inside single crystals, prbably hetergeneusly n an impurity r a dislcatin, an explanatin is still needed t clarify the difference in transfrmatin temperature fr single crystal particles in the 25 t 100 txm range. Slidificatin experiments made by Turnbull ~2'~3"14 can thrw sme light n thse questins. Turnbull's experiments cnsisted in measuring the undercling f small liquid metal drplets (10 t 100/xm), expecting that sme f the drplets wuld be free frm impurity particles because f their small size and that hetergeneus nucleatin wuld be minimized. Turnbull experimented n many metals and btained in mst cases maximum underclings f 200 t 300 ~ as expected. He als nted, frm a statistical pint f view, that the slidificatin temperatures f smaller particles were lwer than thse f 12 the large nes. Turnbull's results culd als explain the imprtant verheating bserved by Zerwekh and Wayman t5 during the alpha-t-gamma transfrmatin f irn whiskers smaller than 50 p~m and the 200 ~ verheating f highly perfect irn whiskers reprted by Christian. 9 A parallel between Turnbull's results and thse f the present wrk can als be established. Indeed, the verheating f irn particles bserved during a secnd thermal cycle can be cmpared t Turnbull's undercling since it increased rapidly when particle diameter decreased belw 100/~m. In the case f the REP irn pwder, the interpretatin f the size effect is nt s easy (Figures 5, 6, 7). As mentined earlier, the starting REP particles and specially the small particles are already in mst cases in the single crystal state due t their manufacturing prcess. The existence f this micrstructure in the starting particles partly eliminates the gradual changes in micrstructure and ther phenmena bserved in the B-214 and Atmet 28 pwders. A precise explanatin f the REP pwder behavir wuld prbably require an exact knwledge f their internal structure which is ut f the scpe f the present wrk. One can simply nte that the REP particles generally behave quite like the B-214 and Atmet 28 particles. Fr the transfrmatin n cling (3' ~ ~) the transfrmatin temperature decreases as the particle diameter becmes smaller (Figures 3, 4, and 5). Als, the number f thermal cycles des nt affect the gamma-t-alpha transfrmatin temperature (Figure 7). Althugh it was assumed that the gamma-t-alpha transfrmatin prceeds by a nucleatin and grwth mechanism, the alpha nucleatin sites in the gamma matrix are different frm thse f the alphat-gamma transfrmatin. Accrding t ur bservatins and the infrmatin available in the literature, ~6 alpha nucleatin wuld ccur randmly at the free surface f particles. The decreasing trend f transfrmatin temperature with particle diameter culd then be explained nly by the difference in chemical cmpsitin between the different size fractins as mentined earlier. C. Effect f Particle Size and Number f Thermal Cycles n the Transfrmatin Temperature's Hysteresis Different values f hysteresis were measured fr each type f pwder (Figure 6), but fr a given type f pwder, these values increase when particle diameter decreases and when the number f thermal cycles increases. It was als bserved that, fr the first thermal cycle, hysteresis increases slwly in the 50 t 70/.Lm range and reaches maximum values fr still finer particles. Since the heating and cling rates were kept cnstant during all ur experiments and the imprtant hysteresis measured in this wrk cannt be explained nly by chemical cmpsitin variatins, the nly plausible explanatin fr this behavir is that the hysteresis is affected by the micrstructure which varies with the type f pwder, the particle size, and with the number f thermal cycles. It shuld als be nted that since the nucleatin f the gamma-t-alpha transfrmatin is relatively easy, it ccurs near the equilibrium temperature, and cnsequently hysteresis is a gd measure f the ver~ heating during the alpha-t-gamma transfrmatin. The increase in hysteresis with the decrease in particle size can then be attributed t a decrease in the number f grain bundaries and in easy nucleatin sites fr the alphat-gamma transfrmatin. Fr the secnd cycle, there is a larger increase in hysteresis values with decreasing particle size. This increase can be attributed t grain grwth caused by thermal cycling. Indeed, as mentined earlier, when particle diameter becmes smaller, the prprtin f single crystal particles within a given size fractin increases. The REP pwder behaves smewhat differently because f its particular micrstructure. METALLURGICAL TRANSACTIONS A VOLUME 17A, JANUARY

8 D. Transfrmatin in Wrught Armc Irn The abve results n the alpha ~ gamma transfrmatin in irn pwders are supprted by ther results n wrught Armc irn published elsewhere by the present authrs.~7 Thse results shwed that thermal cycling acrss the alpha ~ gamma transfrmatin caused imprtant grain grwth in wrught Armc irn and that the alpha-t-gamma transfrmatin temperature increased with the number f thermal cycles frm abut 892 t 911 ~ becming cnstant at 911 ~ after abut five t six thermal cycles instead f tw in the case f pwders. The micrstructure als stabilized after abut five t six cycles. Observatins n wrught irn cnfirm the explanatin given previusly fr the higher gamma nucleatin temperature, i.e., the decreasing in the number f intergranular nucleatin sites in irn pwders. The gamma-t-alpha transfrmatin temperature f wrught Armc irn remained cnstant at abut 902 ~ during thermal cycling nt being affected by the changes in the micrstructure. V. CONCLUSIONS The results f this wrk lead t sme interesting cnclusins: 1. The alpha-t-gamma transfrmatin temperature increases as the irn particle diameter becmes smaller. This phenmenn can be explained by a decrease in the number f grain bundaries within particles when their diameter decreases. Since grain bundaries are preferred nucleatin sites fr the alpha-t-gamma transfrmatin, it fllws that the start f the transfrmatin n heating is mre difficult in smaller particles. 2. Fr a given particle size, the alpha-t-gamma transfrmatin temperature is higher during a secnd thermal cycle than during a first ne. Since an imprtant grain grwth ccurs after tw cycles and fine particles even reach the single crystal state, it can be cncluded that the strng verheating bserved during the secnd cycle is due t an increasingly mre difficult nucleatin. 3. The gamma-t-alpha transfrmatin temperature decreases with particle diameter. This behavir can be explained by chemical cmpsitin differences between particle size fractins. It cannt be related t particle micrstructure. 4. There is n nticeable difference between gamma-talpha transfrmatin temperature during the first and the secnd thermal cycles. This cnfirms that that transfrmatin is nt affected by the micrstructure. 5. Frm the abve cnsideratins, it can be stated that the transfrmatin n cling als prceeds by a nucleatin and grwth mechanism, but that the preferred nucleatin sites fr this transfrmatin are different frm thse identified fr the transfrmatin n heating. Since the gamma-t-alpha transfrmatin temperature is nt affected by the grain micrstructure within particles and since material expansin ccurring during that transfrmatin requires accmmdatin f the nuclei, free surfaces f particles prbably act as preferred nucleatin sites in this case. 6. Fr the larger irn particles, hysteresis values bserved between transfrmatins n heating and n cling are cnsistent with thse reprted in the literature fr massive irn fr similar heating and cling rates. Finer particles display higher hysteresis values. REFERENCES 1. A. Cuture: Master's Thesis, Laval University, Ste-Fy, Quebec, Canada, R.W. Grimshaw and A. L. Rberts: Trans. Brit. Ceram. Sc., 1953, vl. 52, p G. Sabatier: Bull. Sc. Franc. Mineral., 1954, vl. 77. pp Y.G. Hill and R. Ry: J. Am. Ceram. Sc., 1958, vl. 41, p H. Shnaid: in Handbk f Differential Thermal Analysis, W.J. Smthers and Y. Chiang, eds., Chemical Publishing C., New Yrk, NY, 1966, p C. Bulanger: C. R. Acad. Sci. Paris, 1955, vl. 241, pp A.P. Wangsgard: Trans. Am. Sc. Metals, 1942, vl. 30, pp R.W. Cahn: Physical Metallurgy, Nrth-Hlland, Amsterdam, J.W. Christian: The Thery f Transfrmatin in Metals and Allys, Pergamn Press, Oxfrd, Y. Adda, J.M. Dupuy, J. Philibert, and Y. Quere: Eldments de mdtallurgie physique, C.E.N., Saclay, Paris, 1973, vl. 4. I1. F.W Clinard and O.D. Sherby: Acta Metall., vl. 12, pp D. Tumbull: J. Appl. Phys., 1950, vl. 21, pp D. Turnbull and R.E. Cech: J. Appl. Phys., 1950, vl. 21, pp D. Turnbull: J. Appl. Phys., 1949, vl. 20, p R.P. Zerwekh and C.M. Wayman: Acta Metall., 1965, vl. 13, pp J. Burke: The Kinetics f Phase Transfrmatin in Metals, Pergamn Press, New Yrk, NY, A. Cuture and R. Angers: Metall. Trans. A, 1983, vl. 14A, pp VOLUME 17A, JANUARY 1986 METALLURGICAL TRANSACTIONS A