A structural investigation of multiple aging of Al-7 wt pct Zn-2.3 wt pct Mg
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f i r s t studied n e a r l y two d e c a d e s ago. 1-3 As a r e s u l t of this e a r l y X - r a y work on alloys n e a r A1-6 Zn-2 Mg, an aging s e q u e n c e was p r o p o s e d : Sequence I OtSupersat ~
ot + G.P.Z.~
o~ + 7' ~
~ + 7
where Guinier-Preston zones (G.P.Z.) and 7' are transition phases and the 77 is the equilibrium phase. The G.P.Z. are spherical, fcc, solute rich clusters while the 7' and 7 are hexagonal precipitates.* The lattice *It has beensuggestedthat the r~'cellmay be monochnicwith7 = 120 deg rather than hexagonal6
p a r a m e t e r s of t h e s e two p h a s e s have b e e n r e p o r t e d a s : 7': a = 4.97A, C = 5.54/~ (Ref. 6) 7 ' : a = 4.89.~, C = 13.74A (Ref. 9) 7 : a = 5.21A, C = 8.60A (Refs. 6, 10). The n o m e n c l a t u r e of Gj~nnes and S i m e n s e n c o n c e r n i n g the 7' + 7 will be used throught the p a p e r . Recent 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 s t u d i e s have yielded m o r e i n f o r m a t i o n on the d e t a i l s of the aging p r o c e s s . T h o m a s and Nutting 4 showed that the 7' p r e c i p i t a t e s w e r e thin p l a t e s on { l l l } a and were r e s p o n s i b l e for the m a x i m u m h a r d n e s s at 130~ This was l a t e r c o n f i r m e d by Gj~nnes and S i m e n s e n . 8 Two r e c e n t t r a n s m i s s i o n m i c r o s c o p y s t u d i e s of A I - Z n - M g have led to the i d e n t i f i c a t i o n of at l e a s t nine different v a r i a n t s of the 7 Phase.~'6 It should be noted that these nine v a r i a n t s of 7 were all MgZn~ and had d i f f e r e n t o r i e n t a t i o n r e l a t i o n s h i p s with the m a t r i x . Another r e sult of t h e s e two i n v e s t i g a t i o n s has been the p r o p o s a l of other aging s e q u e n c e s to d e s c r i b e the p r e c i p i t a t i o n p r o c e s s . T h a c k e r y , 5 s i n c e he could not identify the 7' phase, suggested that Sequence I should be modified: Sequence II
ass ~
~ + G.P.Z. ~
~ +7
Gj~nnes and S i m e n s e n 6 have c o n f i r m e d the X - r a y r e s u l t s of Mondolfo e t a l . , 1 except that they chose to i n t e r p r e t the c a u s e of the s t r e a k s in the ( l l l ) a as b e i n g an e a r l y f o r m of 7' r a t h e r than G . P . Z . Gj~nnes and S i m e n s e n also s u g g e s t e d that one of the nine types of 7, t h e i r 772 p l a t e s on { I l l } a , could have f o r m e d d i r e c t l y f r o m the 7'. They also suggested that m a n y of the o t h e r types of 7 were d i r e c t l y n u c l e a t e d f r o m the a . These o b s e r v a t i o n s led to the following aging s e quences : VOLUME 4, OCTOBER 1973-2413
Sequence III
aSS - - c~ + ~?' { 1 1 1 } ~
Sequence IV
a s s ~ a + ~7.
90
~ + ~72 {111}~
Gjg~nnes and S i m e n s e n also noted that the ~ ' ~ ~ 2 t r a n s f o r m a t i o n a p p a r e n t l y o c c u r r e d just past m a x i mum hardness. U n f o r t u n a t e l y t h e r e has not been as much s t r u c t u r a l work done on the A1-Zn-Mg s y s t e m after multipl
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