The transgranular SCC of a Mg-Al alloy: Crystallographic, fractographic and acoustic-emission studies
- PDF / 3,359,213 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 15 Downloads / 150 Views
IT
is well established that Mg-base alloys containing more than about 2.5 pct A1 undergo transgranular stress-corrosion cracking (SCC) at room temperature when tested in the solution treated and quenched condition.1 Mechanisms proposed to account for the failure fall into two classes. The first considers that the crack propagates continuouslyby preferential anodic dissolution at the crack tip. Logan2 attributed the preferential dissolution to film rupture, while Priest e t al s suggested it resulted from preferential attack of FeA1 (Fe being present as an impurity) precipitated selectively on basal planes. More recently, Oryall and Tromans4 proposed that preferential dissolution occurs at MgITAI~zprecipitated at the interfaces between the matrix and twins produced ahead of the advancing crack. The second class considers that the crack propagates discontinuouslyby a series of mechanical f a i l u r e s . F a i r m a n and West 5 p r o p o s e d that the f r a c t u r e s t a g e s involve c l e a v a g e in the m a t r i x . A l t e r n a t i v e l y , Pugh e t a l 6 s u g g e s t e d that b r i t t l e f r a c t u r e o c c u r r e d through an e m b r i t t l e d s u r f a c e l a y e r , e i t h e r an oxide or a p o r o u s l a y e r f o r m e d by s e l e c t i v e d i s s o l u t i o n of Mg o r A1. F r a c t o g r a p h y would a p p e a r to p r o v i d e a m e a n s of d i s t i n g u i s h i n g b e t w e e n the two b a s i c a l l y d i f f e r e n t t y p e s of m o d e l s . However, p r e v i o u s f r a c t o g r a p h i c s t u d i e s 4-6 have b e e n c a r r i e d out on p o l y c r y s t a l s and the r e s u l t i n g f r a c t u r e s u r f a c e s w e r e c o m p l e x and d i f f i c u l t to i n t e r p r e t . To avoid this difficulty, the p r e s e n t work was r e s t r i c t e d to a s i n g l e c r a c k i n i t i a t e d and p r o p a g a t e d in a s i n g l e g r a i n in c o a r s e - g r a i n e d s p e c i m e n s . T h e s e s t u d i e s also p e r m i t t e d the o r i e n t a t i o n of the f r a c t u r e s u r f a c e to be d e t e r m i n e d , r e s o l v i n g p r e v i o u s u n c e r t a i n t y . 3'G'7 In addition, a c o u s t i c - e m i s s i o n e x p e r i m e n t s w e r e u n d e r t a k e n to i n v e s t i g a t e the q u e s D. G. CHAKRAPANI,formerly a Research Assistant in the Department of Metallurgy and Mining Engineering, University of Illinois, Urbana, Ill. 61801, is now Corrosion-Control Analyst with the Physical Plant at that University. E. N. PUGH is Professor of Metallurgical Engineering and member of the Materials Research Laboratory at the University of Illinois. Manuscript submitted September 12, 1974. METALLURGICALTRANSACTIONSA
tion of whether propagation is continuous or discontinuous. EXPERIMENTAL The s t u d i e s w e r e c a r r i e d out on a h i g h - p u r i t y b i n a r y a l l o y c o n t a i n i n g 7.61 wt pct Al, k i n d l y p r o v i d e d by The Dow C h e m i c a l Company. The a l l o y was r e c e i v e d in the form of ingots,
Data Loading...
