Evidence for cavitation in the superplastic Zn-22 Pct Al eutectoid
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n-
o 9 o t,
Cost from Cost from Cost from Cast from
:316*C 488"C 150 ~ 3200C superheat
ing a cadmium a r s e n i d e phase which a p p e a r e d to nucleate both lead- and t i n - r i c h p h a s e s . The two-phase s t r u c t u r e s in P b - S n - C d a l l o y s were dominated by l i n e a r a r r a y s , which g e n e r a l l y contained r e c o g n i z a b l e l a m e l l a r s t r u c t u r e s in t h e i r core. L a m e l l a r branching is m o r e difficult in these a l l o y s than is branching in s i n g l e - p h a s e d e n d r i t e s , but i r r e g u l a r branching may occur, e s p e c i a l l y when the duplex volume fraction is high. The p e r c e n t a g e of columnar growth was high in eut e c t i c alloys, but d e c r e a s e d in monovariant a l l o y s with i n c r e a s e d f r e e z i n g range. This may be due to the m o r e i r r e g u l a r branching in these alloys, r a t h e r than the formation of true equiaxed g r a i n s . This work was supported by the National R e s e a r c h Council of Canada.
I
4-I
80,
Z D -J O o
60
4O
20
0
I0 Wt. pct. Cd
15
18.5
20
Fig. 4 - - E f f e c t of c o m p o s i t i o n and c a s t i n g t e m p e r a t u r e on the c o l u m n a r p e r c e n t a g e of P b - S n - C d r n o n o v a r i a n t e u t e c t i c a l loys.
f r e e z i n g range, branching is l e s s likely. Thus, t h e r e is apparent c o r r e s p o n d e n c e between lack of branching and columnar g r a i n s . Using l i n e a r s t r u c t u r e s v s i r r e g u l a r s t r u c t u r e s to m e a s u r e the columnar to equiaxed t r a n s i t i o n may be misleading, however, because of the p o s s i b i l i t y of i r r e g u l a r b r a n c h e s . The intertwining of i r r e g u l a r b r a n c h e s can give r i s e to an equiaxed a p p e a r a n c e , (which because of p o s s i b l e r o t a t i o n s 13 would be supported by p o l a r i z e d light studies on plane sections), without r e q u i r i n g the nucleation of new o r i e n t a t i o n s . In o r d e r to p r o p e r l y d e t e r m i n e what is truly equiaxed, s u c c e s s i v e polishing must be done to d e t e r m i n e what s t r u c t u r e s a r e connected. We have done this on some s p e c i m e n s and have shown that t h e r e is cons i d e r a b l e connectivity of i r r e g u l a r s t r u c t u r e s . This p r o b l e m does not a r i s e with s i n g l e - p h a s e d e n d r i t e s , because of the specific branch d i r e c t i o n s , and is l e s s s e v e r e in b i n a r y or t e r n a r y eutectic compositions. The effect of casting t e m p e r a t u r e on the equiaxed percentage in the b i n a r y eutectic composition in Fig. 4 is consistent with o b s e r v a t i o n s in s i n g l e - p h a s e a l loys. It might be explainable by both constitutional supercooling or nucleation on pouring theories.7 The s a m e effect may also have o c c u r r e d in the 5 and 10 pct Cd alloys, if the noncolumnar s t r u c t u r e s were indeed equiaxed. The columnar to equiaxed t r a n s i
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