A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy
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copy w a s u s e d to study the aging s e q u e n c e an unusual c o n t r a s t r e v e r s a l w a s found 6'7 f o r G P zones i m a g e d by s t r u c t u r e f a c t o r c o n t r a s t . 8 E m b u r y and N i c h o l s o n 6 a t t r i b u t e d the c o n t r a s t r e v e r s a l to e i t h e r o r d e r e d GP z o n e s o r to a h e x a g o n a l p h a s e , w h e r e a s Gjonnes and S i m e n s e n 7 s u g g e s t e d that the c o n t r a s t effect was due to the a p p e a r a n c e of a s p h e r i c a l l y s h a p e d 7/' p h a s e . G j c n n e s and S i m e n s e n found s t r e a k s t h r o u g h 7 ' diff r a c t i o n s p o t l o c a t i o n s and d a r k f i e l d i m a g i n g of t h e s e r e g i o n s of d i f f r a c t i o n s p a c e r e v e a l e d b r i g h t c o n t r a s t f o r the p r e c i p i t a t e s . Thus, t h e r e a p p e a r s to b e s o m e c o n t r o v e r s y o v e r the e x a c t n a t u r e of the s p h e r i c a l p a r t i c l e s o b s e r v e d in t r a n s m i s s i o n e l e c t r o n m i c r o s copy a f t e r s h o r t aging t i m e s at low t e m p e r a t u r e s . M o s t of the p r e s e n t knowledge of G P zone f o r m a t i o n , zone s i z e , and zone k i n e t i c s in A I - Z n - M g h a s been deduced from X-ray scattering or resistivity measu r e m e n t s . A s y s t e m a t i c o b s e r v a t i o n of e a r l y s t a g e precipitates by transmission electron microscopy is p r e r e q u i s i t e both f o r d e t e r m i n i n g t h e n a t u r e and for e l u c i d a t i n g t h e b e h a v i o r of t h e s e e a r l y s t a g e p r e c i p i tates. EXPERIMENTAL
PROCEDURE
The high purity AI-5 wt pct Zn-2 wt pct M g alloy used in this investigation was obtained from Kaiser A l u m i n u m and Chemical Co. and had an actual composition of 4.84 wt pct Zn and 2.06 wt pct Mg. To facilitate sample preparation for electron microscopy, 3 m m diam disc specimens were punched from 0.I m m thick sheet and encapsulated prior to solution treatment. The solution treatment temperature of 450~ w a s m a i n t a i n e d in the hot zone to • 5~ f o r the l a s t hour of t h e f o u r - h o u r h e a t t r e a t m e n t . The s a m p l e s w e r e t h e n quenched into w a t e r at 20~ The s o l u t i o n - t r e a t e d s p e c i m e n s w e r e then q u i c k l y r e m o v e d to a liquid n i t r o g e n c o n t a i n e r . Aging w a s c a r r i e d out at 30, 70, 100, and 150~ in o i l b a t h s c o n t r o l l e d to • 2~ A f t e r aging, s p e c i m e n s w e r e a g a i n s t o r e d in liquid n i t r o g e n . H e a f - t r e a t e d s p e c i m e n s w e r e l i g h t l y e t c h e d f o r 30 s in K e l l e r ' s R e a g e n t (1 m l HF, 1.5 m l HC1, 2.5 m l HNO3, 95 m l H20) p r i o r to Knoop h a r d n e s s m e a s u r e m e n t s (100 g load). R e p l i c a t e s a m p l e s w e r e u s e d f o r h a r d n e s s m e a s u r e m e n t s and e l e c t r o n m i c r o s c o p y . VOLUME 7A, AUGUST 1976-1211
RESULTS M i c r o h a r d n e s s m e a s u r e
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