Concerning the role of mechanical twinning on the stress-strain behavior of Cu-4.9 At. pct Sn alloy
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MANY
m e t a l s whicfi twin p r o f u s e l y d u r i n g d e f o r m a tion exhibit a d e s i r a b l e c o m b i n a t i o n of high s t r e n g t h and high d u c t i l i t y . T h i s is p a r t i c u l a r l y t r u e of fee m e t a l s of low stacking fault e n e r g y . T h a t d e f o r m a t i o n twinning i n f l u e n c e s the m e c h a n i c a l r e s p o n s e of fee m e t a l s has b e e n d e m o n s t r a t e d by a n u m b e r of i n v e s t i g a t o r s . ~-~ However, the r o l e that t w i n n i n g p l a y s in the d e v e l o p m e n t of the d e s i r a b l e p r o p e r t i e s in p o l y c r y s t a l line fee m e t a l s is f a r f r o m r e s o l v e d . L a t e l y , two d i v e r g e n t points of view have evolved. In one due to V g h r i n g e r , 1-4 it is a s s u m e d that the f o r m a t i o n of a twin adds an i n c r e m e n t to the s t r a i n but does not i n fluence the flow s t r e s s thus lowering the work h a r d e n tug. In the other due to R~my 7 and o t h e r s , 9 it is a s s u m e d that the twinning i n c r e a s e s the work h a r d e n i n g by d e c r e a s i n g the effective g r a i n s i z e . In this p a p e r , a d d i t i o n a l i n f o r m a t i o n is p r e s e n t e d c o n c e r n i n g this p r o b l e m . A c o p p e r - t i n alloy c o n t a i n i n g 4.9 at. pet Sn was c h o s e n for this study b e c a u s e in this a l l o y m e c h a n i cal twins a r e r e s o l v a b l e in an optical m i c r o s c o p e and a r e thus s u i t a b l e for q u a n t i t a t i v e m e t a l l o g r a p h i c a n a l y sis. E X P E R I M E N T A L PROCEDURE 1. S p e c i m e n p r e p a r a t i o n The C u - 4 . 9 at. pct Sn alloy used in t h i s work was p r e p a r e d by the M a t e r i a l s R e s e a r c h C o r p o r a t i o n u s i n g V . P . g r a d e (99.998 pct p u r i t y ) e l e m e n t a l m a t e r i a l s . The a s - r e c e i v e d c a s t r o d s were f i r s t g i v e n a 24 h hom o g e n i z a t i o n a n n e a l at 993 K. T h e y w e r e next m a c h i n e d f r o m 25.4 to 24.4 m m in diam and then swaged to 6.35 m m with one i n t e r m e d i a t e a n n e a l at 993 K for 4 h which was given when the d i a m e t e r had b e e n r e d u c e d b y about 25 pct. The r e s u l t i n g r o d s were m a c h i n e d into c y l i n d r i S. KRISHNAMURTHYand R. E. REED-HILL are Graduate Student and Graduate Research Professor, respectively, Department of Materials Science and Engineering, University of Florida, Gainesvllle, FL 32611. Manuscript submitted May 24, 1979. METALLURGICALTRANSACTIONSA
cal t h r e a d e d - e n d t e n s i l e s p e c i m e n s with 3.8 by 20.3 m m gage s e c t i o n s and c y l i n d r i c a l c o m p r e s s i o n s p e c i m e n s of 6.35 m m d i a m by 15.9 m m long. The gage l e n g t h s of four of the c o m p r e s s i o n s p e c i m e n s were v a r i e d to o b tain d i f f e r e n t do//ho r a t i o s b e t w e e n 0.4 and 3.2 (d o = d i a m e t e r and h o = height of the c y l i n d e r ) . A l l t e s t s
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