High-strength Cu-Ni-Sn alloys by thermomechanical processing
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The influence of p r i o r cold work on the aging c h a r a c t e r i s t i c s and mechanical p r o p e r t i e s r e s p o n s e for c o p p e r - r i c h alloys in the Cu-Ni-Sn s y s t e m has been investigated. It has been e s t a b l i s h e d is'x6 that there e x i s t s a spinodal mode of decomposition below a c r i t i c a l t e m p e r ature TR, 200 to 300~ below the equilibrium phase boundary in this s y s t e m . Significant age hardening r e s p o n s e is o b s e r v e d in this region; however, f r a c t u r e ductility is s e v e r e l y i m p a i r e d due to a grain boundary p r e c i p i t a t e network which develops after r e l a t i v e l y short aging t i m e s . Cold work p r i o r to low t e m p e r a t u r e aging is found to have r e l a t i v e l y little influence on the incubation t i m e for this e m b r i t t l i n g network. It does, however, profoundly enhance the kinetics of the continuous (spinodal) t r a n s f o r m a t i o n . It is o b s e r v e d that for b r o a d v a r i a t i o n s in composition, c r i t i c a l combinations of p r i o r cold work, aging time and t e m p e r a t u r e yield m a t e r i a l with unique combinations of. yield s t r e s s and f r a c t u r e ductility (for example, a Cu-9 wt pct Ni-6 wt pct Sn alloy may be p r o c e s s e d to exhibit an 0.01 pct offset yield of 174,000 psi in conjunction with a 55 pct R.A. on f r a c t u r e ; significantly higher 0.01 pct offset yield values may be achieved at some reduction in f r a c t u r e ductility for other Ni/Sn ratios). It is concluded that the r e s u l t a n t d u c t i l e / b r i t t l e p r o p e r t i e s r e s p o n s e is a consequence of a c r i t i c a l competitive balance between amplitude development in the modulated s t r u c t u r e and nucleation of the grain boundary network. The minimum level of p r i o r cold work r e q u i r e d to effect this balance in the Cu-9 wt pct Ni-6 wt pct Sn alloy is 75 pct R.A. The p r e s e n t l e v e l s of yield s t r e s s / f r a c t u r e ductility values r e p o r t e d , to the b e s t of our knowledge, a r e u n s u r p a s s e d by those of any other c o p p e r - b a s e alloy s y s t e m (at a significant cost reduction to the Cu-Be alloys) and suggest the potential yet to be r e a l i z e d in other s y s t e m s exhibiting this mode of decomposition.
PROMINENT
applications of high-strength, c o p p e r b a s e alloys include such items as connectors, d i a phragm m e m b e r s and s p r i n g components in e l e c t r o m e c h a n i c a l r e l a y packages. I n c r e a s e d m i n i a t u r i z a t i o n has imposed higher o p e r a t i n g s t r e s s e s on these c o m ponents and, hence, a need for i n c r e a s e d strength in these alloys. A v a r i e t y of copper alloys may be p r o c e s s e d 1-z to achieve yield l i m i t s (defined as the s t r e s s in tension r e q u i r e d to effect 0.01 pct p l a s t i c strain) in the region of 100 ksi (690 MN/me). Beyond this level of operating s t r e s s , however, the only choice
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