Hydrogen effects on martensite formation
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orientation . ~'o v a l u e (at z e r o s t r a i n ) . The e x p e r i m e n t a l p r o c e d u r e of e x t r a p o l a t i o n , and t h e p h y s i c a l m e a n i n g of the thus d e t e r m i n e d ~'o c o e f f i c i e n t w e r e d e s c r i b e d in p r e v i o u s p a p e r s . 5-~~ It i s t r u e that c a l c u l a t i n g of the r o c o e f f i c i e n t i s l a b o r i o u s , n e e d s a c o n s i d e r a b l e n u m b e r of s t r a i n r a t i o d e t e r m i n a t i o n s , and that the i n t e r p o l a t i o n of e x p e r i m e n t a l p o i n t s i s difficult, e s p e c i a l l y within the r a n g e of s m a l l e v a l u e s . A l l t h e s e d i f f i c u l t i e s a r e due to the e x p e r i m e n t a l s c a t t e r of r v a l u e s which r i s e s with the d i m i n i s h i n g of s t r a i n and d e p e n d s on the g r a i n s i z e of the s p e c i m e n : the s m a l l e r the g r a i n , the b e t t e r the a c c u r a c y of the e d e t e r m i n a t i o n . I n D r H u ' s r e s u l t s on s t e e l the s c a t t e r w a s not g r e a t , due t o a r e l a t i v e l y s m a l l g r a i n s i z e . The s c a t t e r in a c o a r s e - g r a i n e d s p e c i m e n i s s c h e m a t i c a l l y p r e s e n t e d in F i g . 3. In 1969 we p r o p o s e d az-~s the Ap c o e f f i c i e n t a s a f u l l y q u a n t i t a t i v e m e a s u r e of p l a s t i c a n i s o t r o p y . T h i s v a l u e i s b e i n g obtained f r o m the s t a t i s t i c a l a n a l y s i s of a p o l a r plot of r0 (tim a n g u l a r d i s t r i b u t i o n of r o v a l u e s in a n y p l a n e of t h e e x a m i n e d m e t a l ) . T h e r i g h t n e s s of t h i s p r o p o s i t i o n f i n d s e x p r e s s i o n in the e q u a l i t y of Ap v a l u e s c a l c u l a t e d f r o m the a n a l y s i s of p o l a r p l o t s of s t r a i n r a t i o in d i f f e r e n t p l a n e s of t h e m a t e r i a l . The e x p e r i m e n t s c a r r i e d out on p o l y c r y s t a l l i n e c o p p e r , a l u m i n u m and n i c k e l have c l e a r l y shown the p h y s i c a l m e a n i n g of Ap c o e f f i c i e n t but only in the c a s e when the ~'o v a l u e s w e r e a p p l i e d f o r i t s c a l c u l a t i o n . The use of t h e s t r a i n r a t i o v a l u e s c a l c u l a t e d at the l i m i t of the u n i f o r m e l o n g a t i o n c a u s e s the 100 t i m e s r i s e of d i f f e r e n c e (exp r e s s e d a s the p e r c e n t a g e of the s m a l l e s t value) b e t w e e n Ap v a l u e s d e t e r m i n e d f o r s e v e r a l p l a n e s of an examined metal. The r e s u l t s of o u r r e c e n t i n v e s t i g a t i o n s ~8 on the v a r i a t i o n of s t r a i n r a t i o in a l u m i n u m s i n g l e c r y s t a l s c o n s t i t u t e a s t r o n g s u p p o r t f o r the p h y s i c a l
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