The mechanism of anneal hardening in dilute copper alloys
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AND H. WARLIMONT
Copper alloys containing 4 at. pet of a solute (A1, Au, Ga, Ni, Pd, Rh, and Zn) were cold worked and annealed below the r e c r y s t a l l i z a t i o n t e m p e r a t u r e to study the effect of different solute s p e c i e s on the anneal hardening. By m e a s u r i n g the flow s t r e s s at s m a l l bending s t r a i n s , it was found that anneal hardening occurs with all solutes and that the magnitude i n c r e a s e s with the misfit of the solute a t o m s . This indicates that solute s e g r e g a t i o n to d i s l o c a t i o n s and the r e s u l t i n g binding force is the p r i m a r y cause of anneal hardening.
C O P P E R a l l o y s a r e widely used as spring contact m a t e r i a l s because of t h e i r conductivity, c o r r o s i o n r e s i s t a n c e , f o r m a b i l i t y , nonmagnetic behavior, and l a r g e yield strength to e l a s t i c modulus r a t i o . One of the m e c h a n i s m s employed to improve the mechanical p r o p e r t i e s of s i n g l e - p h a s e copper alloys is anneal h a r d e n hag whereby c o n s i d e r a b l e strengthening is attained when a l l o y s in the cold r o l l e d state a r e annealed at 425 to 575 K. However, the mechanism r e s p o n s i b l e for this hardening effect is incompletely understood. ~ The effect has been investigated mainly in copper base a l loys z-9 and some o b s e r v a t i o n s have been i n t e r p r e t e d as indicating that atomic o r d e r i n g is p r i m a r i l y r e s p o n s i ble for the hardening effect. 7'8 On the other hand, in a r e c e n t detailed investigation of anneal hardening in Cu-A1 alloys, 9 it was concluded that solute s e g r e g a t i o n to d i s l o c a t i o n s gives r i s e to the predominant hardening m e c h a n i s m . If this is the case, dilute alloys should a l s o show anneal hardening, and the magnitude should be r e l a t e d to those p r o p e r t i e s of the solute atoms which d e t e r m i n e the extent and effectiveness of their s e g r e gation to d i s l o c a t i o n s . In o r d e r to test the v a l i d i t y of this conclusion, the p r e s e n t study was undertaken in which the anneal hardening behavior of a s e r i e s of copp e r alloy solid solutions with constant solute content was investigated.
EXPERIMENTAL PROCEDURE Seven b i n a r y copper a l l o y s , each weighing a p p r o x i mately 0.2 kg and containing nominally 4 at. pct of a solute (A1, Au, Ga, Ni, Pd, Rh, Zn) were melted in an induction furnace and c a s t in chilled copper molds. Chemical a n a l y s e s of the s a m p l e s a r e given in Table I. The cast alloys were homogenized at 1123 K for six days in an evacuated quartz tube and subsequently cold r o l l e d with i n t e r m e d i a t e anneals at 773 K, from 10 mm to a p p r o x i m a t e l y 0.5 ram; in o r d e r to obtain a compar a b l e initial s t a t e in all s p e c i m e n s , a heat t r e a t m e n t of 723 K for 2 h followed by an i c e - w a t e r quench was J. M. VITEK, formerly with the Max Planck lnstitut f
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