Influence of the precipitate-free zone width on the tensile properties of an Al-6 Wt pct Zn-1.2 Wt pct Mg alloy
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attention has been paid to p r e c i p i t a t e - f r e e zones (PFZ) along grain boundaries in high strength A I - Z n - M g a l l o y s , with p a r t i c u l a r r e f e r e n c e to mechanical p r o p e r t i e s and s t r e s s - c o r r o s i o n cracking. It has been d e m o n s t r a t e d ~-8 that the width of the P F Z is affected by s e v e r a l f a c t o r s such as the homogenization t e m p e r a t u r e , aging t e m p e r a t u r e , quenching p r o c e s s , and the addition of t r a c e e l e m e n t s . G e n e r a l l y the influence of these f a c t o r s on the width of the P F Z has been explained by a s s u m i n g a p a r t i c u l a r distribution of quenched-in v a c a n c i e s and solute atoms near the g r a i n boundary and also a s s u m i n g a c e r t a i n c r i t i c a l t e m p e r a t u r e for the homogeneous nucleation of p r e c i p i t a t e s . Asano, Abe, and F u j i w a r a s have r e c e n t l y suggested a different model that the lack of vacancy c l u s t e r s , which act as the heterogeneous nucleation site for p r e c i p i t a t e s , near g r a i n boundaries is one of the o r i g i n s of the P F Z . The effect of the P F Z on mechanical p r o p e r t i e s of aluminum a l l o y s was f i r s t examined by Thomas and Nutting, 9 who suggested that p l a s t i c flow o c c u r s p r e f e r e n t i a l l y within the P F Z since this r e g i o n is softer than the p r e c i p i t a t e d m a t r i x . This is supported by Sedriks e t al. ~~ F o r s y t h e t al. n have pointed out that, in the initial stage of deformation, d i s l o c a t i o n s g e n e r ated at g r a i n boundaries move along the P F Z ' s and work hardening takes place within them so that f r a c ture eventually o c c u r s t h e r e . On the c o n t r a r y , Ryum and his c o w o r k e r s *a''~ have p r o p o s e d an a l t e r n a t i v e model for the deformation p r o c e s s , a c c o r d i n g to which deformation does not occur p r e f e r e n t i a l l y within the P F Z , but takes place in the m a t r i x by the formation of slip bands. M. ABE, K. ASANO, and A. FUJIWARA are Researchers and Assistant Chief Researcher,respectively,Central Research Laboratory,
Kobe Steel, Ltd., Kobe, Japan. Manuscript submitted September 18, 1972. METALLURGICAL TRANSACTIONS
According to Unwin and Smith, 14 elongation does not n e c e s s a r i l y depend on the width of the P F Z in an A1-6 wt pct Zn-3 wt pct Mg alloy having the P F Z of 0.1 to 0.6 ~ in width; however, the elongation shows a m i n i mum at some i n t e r m e d i a t e width, say, about 0.25 g. The strength and the proof s t r e s s have been shown to be p r a c t i c a l l y independent of the P F Z width. 1%14 In these investigations, however, little attention was paid to the influence of the p r e c i p i t a t e d i s t r i b u t i o n in the m a t r i x and at g r a i n boundaries on the m e c h a n i c a l p r o p e r t i e s . Ryum is intended to get a fixed distribution of p r e c i p i t a t e s in the m a t r i x by u
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