Redetermination of the invariant compositions in the Co-Coal eutectic

A eutect ic reac t ion , in which the liquid decomposes on cooling into a fcc s tab i l ized Co solid so lut ion and an off s to i ch iome t r i c CoAl i n t e r m e t a l l i c phase, occurs in the cobalt r i ch end of the A1-Co sys t em. As r epor t ed in s t andard r e f e r e n c e s , ~,~ the compos i t ions , at the eutect ic invar ian t , of the liquid, sol id solut ion and i n t e r m e t a l l i c phases a r e 90, 92 and 89 wt pct Co respec t ive ly . These repor ted composi t ions a re based p r i m a r i l y on the work of S c h r a m m . a The al loys u t i l ized in S c h r a m m ' s study, however , were not p a r t i c u l a r l y pure; spec i f ica l ly , some of the base cobalt u t i l i zed had a total combined Fe , Ni and Cu content of 2.4 wt pct. To our knowledge, the most r e c e n t publ ished work on this eutect ic was that of Cline ~ who inves t iga ted the mechan ica l p r o p e r t i e s and other c h a r a c t e r i s t i c s of a d i rec t iona l ly sol idif ied eutect ic . In his study Cline noted that the volume f rac t ion of the Co sol id so lut ion was 65 pct by vo lume whereas , accord ing to the accepted phase d i ag ram, this volume f rac t ion should only have been about one-ha l f of this amount at the eutect ic inva r i an t . To account for this contradic t ion, Ctine pos tu la ted that the s t r u c t u r e obse rved at r oom t e m p e r a t u r e was a r e su l t of epi taxia l p rec ip i t a t ion of the two phases , one upon the other, subsequent to sol id i f ica t ion. The n o n s y m m e t r i c a l solvus curves of the two phases , as r epor t ed in the l i t e r a tu re , would then yield a s t r u c t u r e composed of approxi mate ly twoth i rds by volume of the Co sol id solut ion below about 1300 K. Whether this postulated reac t ion occurs on cooling f rom the eutec t ic t e m p e r a t u r e depends on the r e l a t ive r a t e of epi taxia l growth compared to that of p r ec ip i ta t ion within the s u p e r s a t u r a t e d phases . At f i r s t glance it would appear that the coarse na tu re of the d i r ec t iona l ly sol idif ied eutect ic (typical i n t e r l a m e l l a r spac ings of 30 gm) would favor the l a t t e r p r o c e s s e s . However, typical growth condit ions dur ing d i r ec t iona l so l id i f ica t ion (rate of growth ~2 cm/h , t h e r m a l grad i ent ~150 K /cm) a re such that following eutect ic so l id i f ica t ion the m a t e r i a l r e m a i n s at e levated t emp e r a t u r e s for r easonab le pe r iods of t ime . The char a c t e r i s t i c post so l id i f ica t ion diffusion d is tance X (Dr) ~/=, where D is the average diffusivi ty subse quent to sol idif icat ion, can be ca lcula ted s if the t e m p e r a t u r e va r i a t ion of the d i f fus iv i t ies in the cons t i tuent phases a re known. The diffusivi ty of CoAl has been repor ted ~ and the diffusivi ty of A1 in Co can be approximated by that of A1 in Ni.~ On so doing, one f inds that X is comparab le to half of the individual l amel lae th icknesses . Thus addi t ional in fo rmat ion is r equ i r ed to ru le out the p roces s of ep i tax ia l growth. Recent s tudies in our l abora to ry on this s ame a l -