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now c l e a r they show all the other m a j o r a t t r i b u t e s of s u p e r p l a s t i c b e h a v i o r s viz, s t a b i l i t y of g r a i n shape, a b s e n c e of d i s l o c a t i o n a c t i v i t y (slip), GBS, g e n e r a t i o n of curved g r a i n b o u n d a r i e s (see Fig. 1) also, low n v a l ues in the equation d = A(rn (A is a constant, ~ the s t r a i n r a t e , ~ the s t r e s s ) , and a s i g m o i d a l plot for log vs log ( r - s h o w n in Fig. 2, which c o n t a i n s p r e v i o u s l y published r e s u l t s together with new ones b a s e d on the s a m e e x p e r i m e n t a l p r o c e d u r e s . 1 T h e r e was no r e c r y s t a l l i z a t i o n , cavitation or d e v e l o p m e n t of t e x t u r e .2 The g r a i n - s i z e dependence, except at v e r y c o a r s e g r a i n s i z e s , is well r e p r e s e n t e d by ~ cc 1//d 2 (d being m e a s u r e d by the a v e r a g e l i n e a l i n t e r c e p t ) , s e e Fig. 3. The s t r u c t u r a l changes in these l e a d - t h a l l i u m a l loys have b e e n r e g a r d e d as t y p i c a l of c r e e p t'3 r a t h e r than s u p e r p l a s t i c i t y , thus linking the two f i e l d s . M e a s u r e m e n t s show GBS to occur throughout the r a n g e of s t r e s s in F i g . 2, the c o n t r i b u t i o n s m a k e s to o v e r a l l s t r a i n E b e i n g given as values of ~ = Egb/E in the boxed n u m b e r s in F i g . 2, where y is quoted as a p e r c e n t a g e . Fig. 4 s u m m a r i z e s the s t r u c t u r a l changes s c h e m a t i c a l l y , by dividing a typical g r a i n into a c e n t r a l " c o r e " in which s i n g l e - c r y s t a l b e h a v i o r o c c u r s and a

2. S U P E R P L A S T I C I T Y AND L E A D - T H A L L I U M ALLOYS The a l l o y s contained up to 13 pct T1, most of the work b e i n g on an alloy c o n t a i n i n g ~2.5 pct T1; they w e r e made with v e r y high p u r i t y lead and t h a l l i u m and t r e a t e d with hydrogen to r e m o v e oxygen (less than 5 • 10 -5 wt pct r e m a i n e d ) . 4 B e s i d e s high ductility, it is R. C. GIFKINS is Senior Principal Research Scientist, CSIRO Division of Tribophysics, at the Metallurgy Department, University of Melbourne, Parkville 3052, Australia. Manuscript submitted December 3, 1974. METALLURGICALTRANSACTIONSA

Fig. 1-Illustrating microstructural features developed during the creep of a lead-thallium alloy which are typical of superplastie deformation, magnification 173 times. VOLUME 7A, AUGUST 1976-1225

p e r i p h e r a l " m a n t l e " w h e r e the p r e s e n c e of the g r a i n b o u n d a r i e s m o d i f i e s the p a t t e r n of d e f o r m a t i o n . At high enough s t r e s s , the core d e f o r m s by slip as p r e dicted by the c r i t e r i o n of c r i t i c a l r e s o l v e d s h e a r s t r e s s but, Fig. 4(a), the m a n t l e has additional slip s y s t e m s ~

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3. GBS AND ITS ACCOMMODATION

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which can be viewed in t e r m s of " g e o m e t