• PDF / 2,207,657 Bytes
• 7 Pages / 612 x 792 pts (letter) Page_size
• 43 Downloads / 224 Views

DOWNLOAD

REPORT

---

IT

has b e e n w e l l e s t a b l i s h e d that, in A 1 - Z n - M g a l l o y s , high c o n t e n t s of z i n c and m a g n e s i u m a r e a s s o c i a t e d with e x t r e m e s t r e s s - c o r r o s i o n s u s c e p t i b i l i t y . 1' 2 T h i s has been u s e d a s an e x p e r i m e n t a l e x p e d i e n t by i n v e s t i g a t o r s who have studied s t r e s s - c o r r o s i o n p h e n o m e n a in this a l l o y s y s t e m . M o r e s p e c i f i c a l l y , s t r e s s - c o r r o sion s u s c e p t i b i l i t y is a s s o c i a t e d with r a p i d c o o l i n g f r o m s o l u t i o n heat t r e a t m e n t t e m p e r a t u r e s , 3'4 but w h e r e such t r e a t m e n t s a r e u n a v o i d a b l e , 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 can be a c h i e v e d with s o m e s a c r i f i c e in t e n s i l e p r o p e r t i e s by i n t e n t i o n a l l y o v e r - a g i n g . ~'6 Speidel 7 has d i s c u s s e d the r e l a t i o n s h i p b e t w e e n aging p r a c t i c e and s t r e s s - c o r r o s i o n life and pointed out that, in g e n e r a l , i s o t h e r m a l aging i n i t i a l l y r e s u l t s in a d e c r e a s e in s t r e s s - c o r r o s i o n l if e , but a t l o n g e r ag i n g t i m e s , the life i n c r e a s e s . I s o c h r o n a l aging has a s i m i l a r e f f e c t a s the t e m p e r a t u r e is i n c r e a s e d . A n o t h e r f a c t o r which p r o f o u n d l y i n f l u e n c e s s t r e s s c o r r o s i o n s u s c e p t i b i l i t y is g r a i n s t r u c t u r e . 8-~~ A g r e a t d e a l of e f f o r t has been d e v o t e d to finding the r i g h t c o m bination of such a d d i t i o n s as C r , Mn, and Z r to e n s u r e that a f i b r o u s u n r e c r y s t a l l i z e d s t r u c t u r e is r e t a i n e d w h e n e v e r p o s s i b l e . At the s a m e t i m e , the i n c r e a s e in precipitation rate during cooling (quench-sensitivity), c a u s e d by t h e s e a d d i t i o n s , m u s t be a m i n i m u m in o r d e r that full m e c h a n i c a l p r o p e r t i e s a r e o b t a i n e d . S e v e r a l r e c e n t a t t e m p t s 7' 11-13 have b e e n m a d e to c o r r e l a t e o b s e r v a t i o n s of the m i c r o s t r u c t u r e with s t r e s s - c o r r o s i o n b e h a v i o r , and t h e s e i n v e s t i g a t i o n s have a n u m b e r of c o m m o n f e a t u r e s . The a l l o y s u s e d w e r e a l l r a t h e r high in zinc and m a g n e s i u m (~5.5 pct Zn and 2.5 p ct Mg), and all w e r e a r t i f i c i a l l y aged. The i n v e s t i g a t o r s ' c o n c l u s i o n was that the d e f o r m a tion s t r u c t u r e was the m a j o r d e t e r m i n i n g f a c t o r in producing stress-corrosion susceptibility. Recently, i n v e s t i g a t o r s 7, n , 12 r e p o r t e d that a s m a l l a m o u n t of p l a s t i c s t r a i n p r o d u c e d c o p l a n a r a r r a y s of d i s l o c a t i o n s in s t r e s s - c o r r o s i o n s u s c e p t i b l e m a t e r i a l , an