Effect of titanium additions on the aging characteristics of an al-zn-mg alloy
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THE r o l e
of fourth e l e m e n t additions on the s t r u c t u r e and m e c h a n i c a l p r o p e r t i e s of A I - Z n - M g alloys has b e e n r e v i e w e d by Mondolfo ~ in his r e c e n t e x t e n sive r e v i e w a r t i c l e on the A 1 - Z n - M g alloy s y s t e m . Mondolfo 1 indicated that additions of t i t a n i u m have b e e n r e p o r t e d to have the following effects: to slow down the diffusion of zinc and m a g n e s i u m ; to p r o d u c e g r a i n r e f i n e m e n t ; and to have a v e r y l i m i t e d effect on the mode or k i n e t i c s of p r e c i p i t a t i o n in the a g e - h a r d ening p r o c e s s . The r e p o r t e d effects of s l o w e r solute diffusion and no change i n p r e c i p i t a t i o n k i n e t i c s a r e in a p p a r e n t conflict. The p r e s e n t i n v e s t i g a t i o n was und e r t a k e n to study the effect of a t i t a n i u m addition i n g r e a t e r d e t a i l in o r d e r to d e t e r m i n e the exact r o l e that t i t a n i u m plays i n l i m i t i n g solute diffusion. MATERIAL
The alloy used in this i n v e s t i g a t i o n was an A1-5.32 pct Z n - l . 6 6 Mg-0.041 pct Ti alloy* p r e p a r e d by *Thisalloywillbe referred to as AI-Zn-Mg-Tifor the remainderof this paper. m e t h o d s s i m i l a r to H a r d y 2 f r o m high p u r i t y (99.99 pct) e l e m e n t s . B i l l e t s w e r e h o m o g e n i z e d at 580 ~ + 3~ and then p r o c e s s e d by cold r e d u c t i o n and i n t e r m e d i a t e heat t r e a t m e n t s to 0.080 by 0.80 by 0.60 in. for use i n h a r d n e s s t e s t s and e l e c t r o n p r o b e m i c r o a n a l y s i s (EPM). F u r t h e r cold r e d u c t i o n with an i n t e r m e d i a t e heat t r e a t m e n t was p e r f o r m e d to p r o d u c e thin s t r i p s 0.004 to 0.006 in. for use in m a k i n g thin foils for t r a n s m i s sion e l e c t r o n m i c r o s c o p y (TEM). E X P E R I M E N T A L PROCEDURE All the s a m p l e s i n v e s t i g a t e d i n this study w e r e s o lution heat t r e a t e d in an a i r c i r c u l a t i o n f u r n a c e at 578 ~ + 3~ for 2 h and then e i t h e r a i r cooled or b r i n e quenched (10 pct NaC1 at - 1 0 ~ Aging t r e a t m e n t s at v a r i o u s t i m e s w e r e p e r f o r m e d on the b r i n e - q u e n c h e d s a m p l e s at 22.0 ~ + 2~ 122 ~ + 2~ (oil bath), and 150 ~ + 2~ (oil bath). The f i r s t
C. A. GROVE, formerly Postdoctoral Associate, Rensselaer Polytechnic Institute, Troy, N. Y. 12181, is now Engineer, Materials Performance Analysis, Knolls Atomic Power Laboratory, Schnectady, N. Y. 12301. G. JUDD is Associate Professor, Materials Division, Rensselaer Polytechnic Institute, Troy, N. Y. 12181. Manuscript submitted July 14, 1972. METALLURGICALTRANSACTIONS
two aging t e m p e r a t u r e s w e r e below the G . P . solvus . e m p e r a t u r e ( e s t i m a t e d to be ~ 145~ 3) while the t h i r d was above it. At the c o m p l e t i o n of aging, the s p e c i m e n s w e r e quenched into liquid n i t r o g e
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