Phenomenology of precipitation in copper-20 pct nickel-20 pct manganese
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T H E f i r s t published work on alloys of the c o p p e r m a n g a n e s e - n i c k e l s y s t e m s a p p e a r e d in the e a r l y 1940's. Dean e t a l ~-4 in the United States, and Cook and Alexander s in the United Kingdom, r e p o r t e d on the p r e c i p i t a t i o n phenomena of this alloy s y s t e m . Relying p r i m a r i l y on h a r d n e s s , mechanical p r o p e r t y m e a s u r e ments and optical m e t a l l o g r a p h y these i n v e s t i g a t o r s studied the effects of v a r i a t i o n s in the nickel and m a n ganese contents on the aging kinetics and the r e s u l t a n t p r e c i p i t a t e morphologies at s e v e r a l t e m p e r a t u r e s . They concluded that maximum hardening was achieved at equal nickel and manganese contents and that an a l loy containing a p p r o x i m a t e l y 60 pct copper was optimum with r e g a r d to p r o c e s s i n g c a p a b i l i t y and heat t r e a t e d p r o p e r t i e s . Two p r e c i p i t a t e morphologies, dependent on aging t e m p e r a t u r e , w e r e r e p o r t e d . At 400~ and below a finely divided p r e c i p i t a t e nucleated at the g r a i n boundaries. After long t i m e s this p r e c i p i tate consumed the e n t i r e s t r u c t u r e . Aging at 450~ produced a minimum amount of boundary p r e c i p i t a t e which had l i t t l e tendency to grow. The p r e c i p i t a t e r e sponsible for hardening was not identified but it was suggested 4 that an o r d e r e d phase c o r r e s p o n d i n g to MnN1 might be r e s p o n s i b l e . Samans and his c o - w o r k e r s 6 studied an alloy containing 20 pct each of manganese and nickel. Utilizing diff e r e n t i a l d i l a t o m e t r y supplemented by h a r d n e s s , r e s i s t i v i t y m e a s u r e m e n t s and optical m e t a l l o g r a p h y ; they concluded that an o r d e r i n g reaction, producing a f a c e c e n t e r e d t e t r a g o n a l s t r u c t u r e , was r e s p o n s i b l e for the hardening in this alloy. STANLEY SHAPIRO,DEREK E. TYLER and RICHARD LANAM are Metal Structures Group Supervisor,Process MetallurgySection Chief, and EngineeringSpecialist,respectively,at Olin Corporation, Metals Research Laboratories, New Haven, Conn. 06504. Manuscript submitted April 26, 1974. METALLURGICALTRANSACTIONS
B a r t s c h and Schmidt 7 confirmed the p r e s e n c e of a f a c e - c e n t e r e d t e t r a g o n a l s t r u c t u r e in a 30 pct c o p p e r 70 pct NiMn alloy aged at 400~ The s i m i l a r i t y b e tween the m a t r i x (fcc) and the p r e c i p i t a t i o n (fct, c / a = 0.955) s t r u c t u r e s led them to suggest that the l a t t e r was an o r d e r e d v a r i a n t of the f o r m e r . In a s e r i e s of p a p e r s Rolland and o t h e r s 8-1~ r e p o r t e d on the mechanism and c r i t i c a l t e m p e r a t u r e s for h a r d ening in copper-20 pct manganese-20 pct nickel alloys. They c a t e g o r i z e d the aging r e a c t i o n s a c c o r d i n g to t h r e t e m p e r a t u r e r e g i m e s . The
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