Thermomechanical treatments on high strength Al-Zn-Mg(-Cu) alloys
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ments we have p r o p o s e d the use of the t e r m ITMT (Intermediate Thermomechanical Treatments), whereas when low t e m p e r a t u r e a g e - h a r d e n i n g phenomena a r e involved, the t e r m FTMT ( F i n a l T h e r m o m e c h a n i c a l T r e a t m e n t s ) is suggested. In the p r e s e n t work, the application of FTMT to high strength A1-Zn-Mg(-Cu) alloys has been investigated; our goal was to e s t a b l i s h the conditions in which the d i s l o c a t i o n s introduced by plastic deformation i n t e r a c t most favorably with the age-hardening p r o c e s s e s to produce m a t e r i a l s that have improved p r o p e r t i e s c o m p a r e d to conventionally aged m a t e r i a l s . I) A P P R O A C H A n overall view of the tensile properties of the most widely used commercial Al-Zn-Mg(-Cu) alloys is shown in Fig. I. It can be seen from this figure that with the standard processed alloys, yield strength (YS) levels of 90,000 psi can be approached with elongation values of about I0 pct. Recently, considerable effort has been directed towards improving the strength of wrought Al-Zn-Mg(-Cu) alloys and some investigators produced these alloys with YS values that were higher than 100,000 psi, Fig. I. Various combinations of strength and elongation have been achieved by Di Russo 3 by controlling the solidification conditions, chemical composition and the homogenization, solution treatment and aging cycles. Flemings and coworkers 4 obtained similar results on alloys that were solidified under controlled conditions, completely homogenized and then cold worked (H) after normal solution (T) and aging (A) treatments. The use of different combinations of aging and work hardening was tested by Mercier et al. 5 using a process based on plastic deformation after complete T6 heat treatment, followed by a partial recovery treatment. Studies were also made by Pavlov et al.6 in which the solution treatment was followed by warm deformation (Hc) and then by artificial aging (A). From Fig. I it can be seen that the increase in strength is always accompanied by a decrease in elongation which, at the highest YS level, drops to a very low value. This is more apparent for the treatments involving combinations of deformation and aging. Neglecting the T-AH treatments, in which no signifi-
VOLUME 4, APRIL 1973-1133
a b l e a l u m i n u m a l l o y s . 32'33 T h i s p a p e r p r e s e n t s a s u m m a r y of the d a t a obtained on the effect of TMT on the s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p s in A 1 - Z n - M g ( - C u ) a l l o y s . In m o r e d e t a i l , s e l e c t e d r e s u l t s showing the t e n s i l e p r o p e r t i e s , notch and f r a c t u r e t o u g h n e s s , fatigue and s t r e s s - c o r r o s i o n b e h a v i o r of d i f f e r e n t a l l o y / t r e a t m e n t s a r e p r e s e n t e d and d i s c u s s e d . O t h e r p a p e r s w i l l be p u b l i s h e d in the n e a r f u t u r e d e a l i n g with the m o r e f u n d a m e n t a l a s p e c t s of
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