Effect of thermal and mechanical pretreatments on the guinier-preston zone state of a commercial 7075 aluminum alloy
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AND
R. W. GOULD
T h i s i n v e s t i g a t i o n was conducted to d e t e r m i n e the G u i n i e r - P r e s t o n zone state of a comm e r c i a l grade 7075 a l u m i n u m alloy, and the effects of both t h e r m a l and m e c h a n i c a l p r e t r e a t m e n t on the zonal state. T h e m a i n i n v e s t i g a t i v e t e c h n i q u e s used were X - r a y s m a l l angle s c a t t e r i n g and wide angle X - r a y d i f f r a c t i o n . The a g i n g s e q u e n c e was d e t e r m i n e d to be: Solid Solution ~ GP Z o n e s ~ 0 ' ~ ~7 and q ' ~ 7/ for the p a r t i c u l a r heat t r e a t m e n t s used d u r i n g this r e s e a r c h . T h i s a g i n g s e q u e n c e was not a l t e r e d by m e c h a n i c a l p r e t r e a t m e n t , but the i n i t i a l aging k i n e t i c s did i n c r e a s e . T h e r m a l p r e t r e a t m e n t s p r o d u c e d a GP zone state that r e m a i n e d stable for l o n g e r aging t i m e s and at higher t e m p e r a t u r e s than without the p r e t r e a t m e n t s .
I N r e c e n t y e a r s n u m e r o u s i n v e s t i g a t i o n s have been made on p r e p r e c i p i t a t i o n b e h a v i o r in A 1 - Z n - M g alloy s y s t e m s and much of this work has b e e n done on r e s e a r c h grade alloys u s i n g a wide v a r i e t y of i n v e s t i g a tive t e c h n i q u e s : 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 c o p y , ~-14 s m a l l angle X - r a y s c a t t e r i n g , 3,~ c a l o r i m e t r y , 15-~9 and e l e c t r i c a l r e s i s t a n c e . 2~ Studies on c o m m e r c i a l grade m a t e r i a l s have g e n e r a l l y b e e n r e s t r i c t e d to a c o r r e l a tion of heat t r e a t m e n t a n d / o r t h e r m o m e c h a n i c a l t r e a t ment32,33 to the r e s u l t i n g m e c h a n i c a l p r o p e r t i e s . The p u r p o s e of this study was to i n v e s t i g a t e the zone s t a t e of a c o m m e r c i a l A 1 - Z n - M g a l l o y u t i l i z i n g s m a l l a n g l e X - r a y s c a t t e r i n g and to i n v e s t i g a t e the effects of both t h e r m a l and m e c h a n i c a l p r e t r e a t m e n t s upon this zone state.
EXPERIMENTAL
PROCEDURE
The alloy chosen for this study was a commercial grade 7075 alloy. Spectrographic analysis showed the c o m p o s i t i o n to be: Zn Mg Cu Fe
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5.80 2.62 1.74 0.27
wt wt wt wt
pct pct pct pct
Cr Si Ti A1
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0.22 wt pct 0.19 wt pct 0.04 wt pct Balance.
The m a t e r i a l was r e c e i v e d as 19 m m thick A l c l a d 7075-T6 R e y n o l d s plate. The cladding and any effects f r o m it w e r e r e m o v e d by m i l l i n g 1.5 m m f r o m each face. The r e m a i n i n g m a t e r i a l was cold r o l l e d down to 0.82, 0.76 and 0.72 m m t h i c k n e s s e s with i n t e r m e d i a t e a n n e a l s of 1 h at 460~ S a m p l e s 10 • 1 cm w e r e cut f r o m the r o l l e d m a t e r i a l , s o l u t i o n heat t r e a t e d at 460~ for 2 h, then ice w a t e r quenched. All s a m p l e s w e r e s t o r e d at liquid n i
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