Evidence for crack-arrest markings on intergranular stress corrosion fracture surfaces in Al-Zn-Mg alloys
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Evidence for Crack-Arrest Markings on Intergranular Stress Corrosion Fracture Surfaces in AI-Zn-Mg Alloys
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Fig. 4-Ultimate compressive strength, measured at room temperature, as a function of isothermal exposure at 1121~ in air or argon. Growth rate7• 10 -3 m m . s -1. 2.16 X 10 s S and 3.24 • 10 ~ s, r e s p e c t i v e l y . T h e s e l e v e l s a r e h i g h e r than t h o s e in the p r e s e n t study (Fig. 1) by a f a c t o r of about two. B o o m g a r d and V an Run s g i v e an a v e r a g e v a l u e ~ 1.37 m g / c m 2 as the weight gain a f t e r 3.7 x 10 ~ s at 900~ in a i r ; this is c l o s e to the r a t e we o b s e r v e at 1100~ T o date, a d e t a i l e d m e t a l l o g r a p h i c e x a m i n a t i o n of ingot c r o s s s e c t i o n s has not b e e n m a d e following e l e v a t e d t e m p e r a t u r e e x p o s u r e . It is t h e r e f o r e not p o s s i b l e to c o m m e n t on the depth of p e n e t r a t i o n of o x i d e . W o r k of f r a c t u r e Gf and peak load Pl m e a s u r e d in the t o u g h n e s s t e s t a r e p l o t t e d as a function of t h e r m a l e x p o s u r e in F i g . 2. D a t a r e f e r to c o m p o s i t e s gr ow n at 7 • 10-~ m//s. While the Gf v a l u e s w e r e c o m p a r a b l e f o r a i r and a r g o n e x p o s u r e , the Pl v a l ues w e r e s i g n i f i c a n t I y i n f e r i o r as a r e s u l t of e x p o s u r e at 1121~ in a i r . P e a k load (Pt) is a m e a s u r e of the s t r e n g t h of the c o m p o s i t e and the d e c r e a s e i s c o n s i s t e n t with the o b s e r v e d c h a n g e s in h a r d n e s s and c o m p r e s s i v e s t r e n g t h . F i g u r e s 3 and 4 show that both h a r d n e s s and c o m p r e s s i v e s t r e n g t h d e c r e a s e as a r e s u l t of i s o t h e r m a l e x p o s u r e in a i r at 1131~ The i n c r e a s e in the w o r k of f r a c t u r e at r o o m t e m p e r a t u r e ( c o m p a r e d to the a s - g r o w n condition) has b e e n shown 9 to be the r e s u l t of f i b e r c o a r s e n i n g with an attendant i n c r e a s e in i n t e r f i b e r s p a c i n g and f i b e r d i a m e t e r . Since the e x t e n t of f i b e r c o a r s e n i n g should be s i m i l a r in a i r and a r g o n , the s i m i l a r i t y in the c u r v e s f o r Gf a f t e r e x p o s u r e in a r g o n or a i r m e a n s that any m i c r o s t r u c t u r a l c h a n g e s o c c u r r i n g d u r i n g oxidation a r e not d e t r i m e n t a l to t o u g h n e s s . T h i s w o r k was s u p p o r t e d by the Office of N a v a l R e s e a r c h u n d er C o n t r a c t No. N 0 0 0 1 4 - 7 6 - C - 0 2 0 5 . 1. J. Stringer: A GARD Conference Proceedings No. 156, p. 131, Hartford House, London, 1974. 2. J. G. Smeggil: Conference on In-Situ Composites-lI, M. R. Jackson, J. L. Walter, F. D. Lemkey and R. W. Herzberg, eds. p. 397, Xerox Individualized Publishing, Lexington, MA, 1976. 3. F. R. M
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