Quench rate effects in Al-Zn-Mg-Cu alloys
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B. S. SUBRAMANYA,
AND
S. A. LEVY
The effect of quench rate on a l l o y s with a base c o m p o s i t i o n of 6.75 pct Zn, 2.5 pct Mg, 1.2 pct Cu and v a r i o u s l e v e l s and c o m b i n a t i o n s of the m i n o r addition e l e m e n t s (MAE's) C r , Mn, and Z r i s r e p o r t e d . The l e v e l of c h r o m i u m and to a l e s s e r e x t e n t the l e v e l of z i r c o n i u m d e t e r m i n e quench s e n s i t i v i t y . Slow quenching of c h r o m i u m - o r z i r c o n i u m - b e a r i n g a l l o y s led to the n u c l e a tion of (MgZn) p l a t e s or laths at M A E - b e a r i n g p a r t i c l e s which had c e r t a i n o r i e n t a t i o n r e l a t i o n s h i p s with the m a t r i x . The d e n s i t y of the h a r d e n i n g p r e c i p i t a t i o n (100 to 200A in diana) was found to be high in slowly quenched, s e n s i t i v e a l l o y s . E v e n when the yield s t r e n g t h had f a l l e n to 50 pct of that of the f a s t - q u e n c h e d m a t e r i a l , a l a r g e f r a c t i o n of the fine p r e c i p i t a t e was s t i l l f o r m e d . A f r a c t u r e m e c h a n i s m is p r o p o s e d to a c c o u n t for the m a g n i t u d e of quench s e n s i t i v i t y in such m a t e r i a l s . An e m b r i t t l e m e n t , at low quench r a t e s , w a s found in a l l o y s c o n t a i n i n g m a n ganese o r v e r y low l e v e l s of MAE. This e m b r i t t l e m e n t was thought to be due to p r e c i p i t a t i o n in the g r a i n b o u n d a r i e s , and not to the width of the p r e c i p i t a t e - f r e e zone.
ALLOYS
of the A 1 - Z n - M g - C u s y s t e m a r e p r e c i p i t a t i o n - h a r d e n a b l e . At t e m p e r a t u r e s below the G.P. Zone solvus the p r e c i p i t a t i o n sequence is: solid solution G.P. Z o n e s (spherical) - - 77t (platelets) - - 7? (MgZn2) (plates o r laths). Only the G . P . Z o n e s o r rl' p r e c i p i t a t e s are effective h a r d e n e r s . A l i m i t i n g f a c t o r in producing heavy s e c t i o n s of high s t r e n g t h A 1 - Z n - M g - C u alloys is t h e i r quench s e n s i t i v i t y . By quench s e n s i t i v ity, we m e a n a drop in y i e l d s t r e n g t h or u l t i m a t e t e n sile s t r e n g t h at slow quench r a t e s . The drop in yield s t r e n g t h is g e n e r a l l y g r e a t e r than the a c c o m p a n y i n g drop in u l t i m a t e t e n s i l e s t r e n g t h and so is a b e t t e r ind i c a t o r of quench s e n s i t i v i t y . In heavy s e c t i o n s the quench r a t e i s l i m i t e d p r i n c i p a l l y by the t h e r m a l conductivity of the m e t a l ; this is e s s e n t i a l l y u n c o n t r o l l a b l e . The r e s p o n s e to a p a r t i c u l a r quench r a t e is c o n t r o l lable; in p a r t i c u l a r it is well known that the p r e s e n c e of m i n o r addition e l e m e n t s (MAE's) such as c h r o m i u m p r o m o t e high quench s e n s i t i v i t y . 1'2 All a g e - h a r d e n a b l e alloys suffer a loss in
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