Comparison of the influence of chromium and zirconium on the quench sensitivity of Al-Zn-Mg-Cu alloys
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c o m m e r c i a l a l l o y s of the A l - Z n - M g - C u type a r e " q u e n c h - s e n s i t i v e " , that is they m a y lose some h a r d e n i n g c a p a b i l i t y when slowly quenched f r o m the s o lution t e m p e r a t u r e . S e v e r a l w o r k e r s have studied the " q u e n c h - s e n s i t i v i t y " p r o b l e m , but m a i n l y f r o m the point of view of the e v a l u a t i o n of its effects on the p r o p e r t i e s of the a l l o y s ; in r e l a t e d s t u d i e s it has been shown that low quench r a t e s m a y s e r i o u s l y r e d u c e the s t r e n g t h of s t r u c t u r a l c o m p o n e n t s and that the degree of " q u e n c h r a t e s e n s i t i v i t y " of A 1 - Z n - M g - C u alloys is dependent on the a l loying c o n t e n t and on the n a t u r e of the a n c i l l a r y e l e m e n t s they c o n t a i n . 1 In this s e n s e , c h r o m i u m e x e r t s the s t r o n g e s t i n f l u e n c e , followed by m a n g a n e s e , while z i r c o n i u m - b e a r ing alloys show little or no " q u e n c h - s e n s i t i v i t y " . 2 M o r e o v e r , it has been shown that z i r c o n i u m p r o d u c e s s i m i l a r , o r m o r e d e s i r a b l e effects than c h r o m i u m or m a n g a n e s e in c o n t r o l l i n g the r e c r y s t a l l i z a t i o n b e h a v i o r of A 1 - Z n - M g - C u a l l o y s ; 3 z i r c o n i u m also i m p r o v e s the toughness and, in some c a s e s , the s t r e s s - c o r r o s i o n r e s i s t a n c e of these a l l o y s . 2-4 The b e n e f i c i a l effect of z i r c o n i u m as an a n c i l l a r y e l e m e n t h a s s u g g e s t e d the d e v e l o p m e n t of high s t r e n g t h A 1 - Z n - M g - C u , z i r c o n i u m - b e a r i n g a l l o y s , Z e r g a l - 3 and Zergal-4. 5 Little work has as y e t been devoted to the u n d e r s t a n d i n g of the m e t a l l u r g i c a l and s t r u c t u r a l f a c t o r s affecting the " q u e n c h - s e n s i t i v i t y " p h e n o m e n o n ; F i n k and Willey 6 suggested that low q u e n c h i n g r a t e s would r e sult in the f o r m a t i o n of c o a r s e n o n h a r d e n i n g p a r t i c l e s that, by depleting the m a t r i x of the s o l u t e , r e d u c e the r e s p o n s e to s u b s e q u e n t aging. More r e c e n t work h a s proposed that the s t r o n g e s t " q u e n c h - s e n s i t i v i t y " , a s s o c i a t e d with the p r e s e n c e of c h r o m i u m , is to be a t t r i b u t e d to the n u c l e a t i o n of MgZnz--type i n c o h e r e n t p a r t i c l e s in c h r o m i u m - r i c h high t e m p e r a t u r e p r e c i p i tates.7 However, t h e r e is no quite c o n v i n c i n g e x p l a n a t i o n of the p h e n o m e n o n , and in p a r t i c u l a r , of the d i f f e r e n t r o l e s played by c h r o m i u m and z i r c o n i u m on the " q u e n c h s e n s i t i v i t y " , i n a s m u c h as v e r y little is k n o w n about the d i s t r i b u t i o n m o d a l i t
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