• PDF / 1,989,713 Bytes
• 10 Pages / 612 x 792 pts (letter) Page_size
• 87 Downloads / 220 Views

DOWNLOAD

REPORT

---

R. DeIASI, AND

G. GESCHWIND

The significance of m a t r i x and grain boundary m i c r o s t r u c t u r a l c h a r a c t e r i s t i c s on the m e c h a n i c a l 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 s u s c e p t i b i l i t y of 7075 aluminum alloy has been evaluated. Maximum strength was found to be a s s o c i a t e d with a G u i n i e r - P r e s t o n zone m a t r i x . The p r e c i p i t a t e - f r e e - z o n e adjacent to high angle grain boundaries had only a slight effect on yield and tensile strength but a g r e a t e r influence on h a r d n e s s . S t r e s s c o r r o s i o n s u s c e p t i b i l i t y was studied in an aqueous chloride environment over a 0.7-3.5 pH r a n g e . F o r m a t e r i a l of highest strength, g r a i n boundary p r e c i p i t a t e spacing was found to be of p r i m a r y i m p o r t a n c e to s u s c e p t i b i l i t y . The effect of g r a i n boundary p r e c i p i t a t e spacing is most significant to the c r a c k propagation stage of s t r e s s c o r r o s i o n . These r e s u l t s indicate that improved p r o p e r t i e s for A1-Mg-Zn type alloys could be attained by a d e s i r a b l e combination of m a t r i x and grain boundary s t r u c t u r e .

S T R E N G T H and s t r e s s c o r r o s i o u s u s c e p t i b i l i t y of c o m m e r c i a l 7000 s e r i e s alloys a r e r e l a t e d to the p r e cipitation r e a c t i o n s accompanying heat t r e a t m e n t . In its highest strength t e m p e r (T6), the widely used 7075 alloy (basically A1-Mg-Zn-Cu) is susceptible to i n t e r g r a n u l a r s t r e s s c o r r o s i o n attack; overaged m a t e r i a l (T73 t e m p e r ) is not susceptible but has lower strength.l,2 It is d e s i r a b l e to obtain m a t e r i a l that is not susceptible to s t r e s s c o r r o s i o n but which has optimum strength. To d e t e r m i n e the f e a s i b i l i t y of a t t a i n ing this dually advantageous condition, knowledge of the m a t e r i a l f a c t o r s c r i t i c a l to mechanical p r o p e r t i e s , as well as s t r e s s c o r r o s i o n attack, is n e c e s s a r y . Precipitation in AI-Mg-Zn type alloys generally occurs in the following sequence: 3-6 Supersaturated solid solution - - Guinier-Preston (G.P.) Zones - - 77'(MgZn2) -- ~ (MgZne). The semicoherent intermediate MgZn2 phase, ~7', has recently been described in terms of a monoclinic unit cellv while the incoherent equilibrium MgZn2 phase, 77, is hexagonal.5,6 Highest strength in 7000 s e r i e s a l l o y s has been found to coincide with a m a t r i x containing G.P. zones with slight amounts of 7/' phase, a Although G.P. zones have been suggested as the source of optimum s t r e n g t h ening,9'l~ maximum strength in A1-Mg-Zn type alloys has a l s o been attributed to other m a t r i x s t r u c t u r e s . 3'~x'lz Study of s t r e s s c o r r o s i o n attack in A1-Mg-Zn type a l l o y s has focused on three p r i n c i p a l m i e r o s t r u c t u r a l f e a t u r e s : 1) the p r e c i p i t a t e