Precipitation of chromium in copper and copper-nickel base alloys

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zation was c o m p l e t e d by cold r o l l i n g and then a n n e a l ing for 72 h at 1000~ u n d e r a p r o t e c t i v e a t m o s p h e r e . The r e s i s t i v i t y s p e c i m e n s were p r e p a r e d by d r a w i n g wire down to 0.46 m m d i a m and after heat t r e a t m e n t m e a s u r e m e n t s w e r e c a r r i e d out by c o n v e n t i o n a l pot e n t i o m e t r l c m e a n s . D u r i n g m e a s u r e m e n t the s p e c i m e n s w e r e i m m e r s e d in liquid n i t r o g e n to r e m o v e a n y effects f r o m l a r g e and v a r y i n g t e m p e r a t u r e c o m p o n e n t s of r e s i s t i v i t y . 1~ Strip m a t e r i a l for e l e c t r o n m i c r o s c o p y s p e c i m e n s was obtained by cold r o i l i n g hom o g e n i z e d m a t e r i a l down to 0.5 m m while h a r d n e s s s p e c i m e n s were cut f r o m the homogenized 3.8 m m thick sheet. Specimens w e r e solution t r e a t e d u n d e r a p r o t e c t i v e a t m o s p h e r e for 1 h at 1000~ and quenched into an aqueous s o l u t i o n of 10 pct NaOH. 2. RESULTS a) C u - C r Alloy The a s - q u e n c h e d s t r u c t u r e showed no s i g n s of p r e cipitation when e x a m i n e d in the e l e c t r o n m i c r o s c o p e . P r e c i p i t a t i o n o c c u r r e d d u r i n g aging in the t e m p e r a t u r e r a n g e 300 to 550~ The f i r s t p r e c i p i t a t i o n was shown b y the a p p e a r a n c e of a homogeneous d i s t r i b u t i o n of s m a l l c o h e r e n t p r e c i p i t a t e s (Fig. 1) r a n g i n g in size f r o m 5 to 10 m m . The p r e c i p i t a t e s c o a r s e n e d slowly d u r i n g aging and as they did so the i m a g e s lost s p h e r ical s y m m e t r y , the p r e c i p i t a t e s taking the f o r m of s m a l l rods lying along (110) m a t r i x d i r e c t i o n s (Fig. 2). No r e f l e c t i o n s other than those f r o m the m a t r i x w e r e v i s i b l e on s e l e c t e d a r e a diffraction p a t t e r n s at any stage. The r e s i s t i v i t y m e a s u r e m e n t s indicated that the a s -

Table I. Analysis of Alloys

Element Cr Ni Impurities Ni $1 Ti W Fe Others

Cu-CrCu-Cr 0.t5 pct

q ppm 4 ppm

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