The effect of quench rate and silver addition on the precipitate free zone width in an Al-4.3 pct Zn-1.3 pct Mg alloy
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The Effect of Quench Rate and Silver Addition on the Precipitate Free Zone Width in an AI-4.3 pct Zn-l.3 pct Mg Alloy
Two quench r a t e s which differed by three o r d e r s of m a g n i t u d e w e r e used. The r a p i d quench was a t t a i n e d u s i n g cold HzO (~10,000 ~ p e r m i n ) and the slow r a t e was a c c o m p l i s h e d by solution heat t r e a t i n g the s a m p l e between 0.5 in. thick a l u m i n u m p l a t e s and p e r m i t t i n g the e n t i r e a s s e m b l y to a i r cool. T h i s l a t t e r t r e a t m e n t r e s u l t e d in a quench r a t e of a p p r o x i m a t e l y 10~ p e r m i n . E l e c t r o n m i c r o s c o p y was p e r f o r m e d on a P h i l i p s E M - 3 0 0 at 100 kv u s i n g thin f i l m s p r e p a r e d by e l e c t r o polishing. A s u m m a r y of the e f f e c t s of quench r a t e and aging t e m p e r a t u r e on the P F Z width in both the s i l v e r - f r e e and s i l v e r - c o n t a i n i n g a l l o y s is shown in T a b l e I. The P F Z widths a r e the a v e r a g e of a m i n i m u m of five m e a s u r e m e n t s on high angle b o u n d a r i e s . PFZ
WIDTH
AFTER
A RAPID
QUENCH
A critical temperature exists in this system above whichhomogeneous nucleation of G P zones does not occur. H o m o g e n e o u s G P zone formation results in a homogeneous d i s t r i b u t i o n of the t r a n s i t i o n phase ~7' (MgZn2) upon f u r t h e r i s o t h e r m a l aging which is o b s e r v e d by e l e c t r o n m i c r o s c o p y . This t e m p e r a t u r e was found to be g r e a t e r than 120 ~ and l e s s than 135~ for the s i l v e r - f r e e alloy. As can be seen in Table I (rapid quench data) a B. C. P E T E R S l a r g e change in the P F Z width is also found between 120 ~ and 135~ It is not c l e a r w h e t h e r the i n i t i a l phase A c o n s i d e r a b l e a m o u n t of r e s e a r c h effort has been in the s i l v e r - c o n t a i n i n g a l l o y is a GP zone o r w h e t h e r expended by n u m e r o u s i n v e s t i g a t o r s on the study of 7/' i s n u c l e a t e d i n i t i a l l y . C o n s e q u e n t l y what is r e f e r r e d A1-Zn-Mg alloys. A particularly distressing problem to in this c o m m u n i c a t i o n as the c r i t i c a l t e m p e r a t u r e for with m a n y of these alloys i s t h e i r s u s c e p t i b i l i t y to the s i l v e r - c o n t a i n i n g alloy m a y in fact be the t e m p e r a s t r e s s c o r r o s i o n c r a c k i n g . The i n t e r a c t i o n between the t u r e below which the 7' is h o m o g e n e o u s n u c l e a t e d . The p r e c i p i t a t e f r e e zone along the g r a i n b o u n d a r i e s , and: r e s u l t a n t effect on the m i c r o s t r u c t u r e of aging above s t r e s s c o r r o s i o n c r a c k i n g h a s been s p e c u l a t e d upon in this t e m p e r a t u r e is that of c h a n g i n g the size and d i s t r i s e v e r a l p u b l i c a t i o n s . 1'2 Also, the effect of s i l v e r addibution of the p r e c i p i t a t e
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