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X P E R I M E N T A L RESULTS The e x p e r i m e n t a l p r o c e d u r e and t e s t i n g p r o g r a m was fully d e s c r i b e d in Ref. 2. The t e s t v a r i a b l e s w e r e s t r a i n r a t e , ~ (2.8 • 10 -5 to 2.3 • 10 -2 s-l), d e f o r m a t i o n t e m p e r a t u r e , T D (950 to 1350~ and p r i o r - a n n e a l i n g t e m p e r a t u r e , T A (1000 to 1350~ Compositional vari a t i o n was a c h i e v e d b y the u s e of a z o n e - r e f i n e d i r o n , a v a c u u m - m e l t e d e l e c t r o l y t i c i r o n , an F e - 0 . 0 5 C a l loy, and an F e - 5 . 2 Mn a l l o y . The p r i n c i p a l f e a t u r e s a r e a c o n s t a n t s t r a i n - r a t e t e n s i o n m a c h i n e , an e v a c u a b l e f u r n a c e c h a m b e r in which the g a s e o u s e n v i r o n m e n t can be c o n t r o l l e d , and the use of c y l i n d r i c a l s p e c i m e n s with a gage length to d i a m e t e r r a t i o of 11.5.

A) Plastic Instability The first stage of tensile failure is usually the onset of the rapid development of a necked region. At temperatures well below half the melting point, T m , the rapid growth of a single neck begins when Consid ~ r e ' s c r i t e r i o n is a p p r o x i m a t e l y s a t i s f i e d ~ 4 However, at t e m p e r a t u r e s a b o v e 0.5 Trn t h i s a p p r o x i m a t i o n is only v a l i d a t high s t r a i n r a t e s . In a s t u d y of the p l a s t i c i n s t a b i l i t y of two i n d i u m - b a s e d a l l o y s a t 0.69 Tms the e x t e n s i v e , c o n c u r r e n t growth of m u l t i p l e n e c k s was o b s e r v e d . This f i r s t m o d e of p l a s t i c i n s t a b i l i t y was t e r m i n a t e d by a s e c o n d i n s t a b i l i t y in which a s i n g l e , p r i m a r y neck b e c a m e m o r e a c t i v e than the o t h e r n e c k s and was e v e n t u a l l y d o m i n a n t . At the e l e v a t e d t e m p e r a t u r e of 0.69 T m, a p o s t p o n e m e n t of the s e c o n d i n s t a b i l ity was o b s e r v e d to i n c r e a s e with d e c r e a s i n g s t r a i n rate. In o r d e r to e x a m i n e the p l a s t i c i n s t a b i l i t y of a u s t e n i t i c i r o n , t h e p o s t - d e f o r m a t i o n p r o f i l e s of F e - 0 . 0 5 C s p e c i m e n s w e r e e x a m i n e d in the m a n n e r d e s c r i b e d p r e v i o u s l y . 4 ' 5 A r e p r e s e n t a t i v e s e l e c t i o n of p r o f i l e s i s p r e s e n t e d in F i g s . 1, 2, and 3. C o m p a r i s o n of Fig. 1 with F i g . 2 and c o m p a r i s o n of the two p r o f i l e s in Fig. 3 r e v e a l that the e x t e n s i v e growth of m u l t i p l e n e c k s was f a v o r e d b y d e f o r m a t i o n at h i g h e r t e m p e r a t u r e s . VOLUME 6A, JULY 1975-1379

F u r t h e r m o r e , the o c c u r r e n c e of m u l t i p l e n e c k i n g was t h e s a m e w h e t h e r the s p e c i m e n s w e r e p r i o r - a n n e a l e d a t 1200 o r 1350~ and t h e r e f o r e the p h e n o m e n o n i s not m e r e l y a g e o m e