An electron microprobe analysis of solute segregation near grain boundaries in an Al-Zn-Mg alloy
- PDF / 426,151 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 78 Downloads / 185 Views
PROPOSEDm e c h a n i s m s
of s t r e s s c o r r o s i o n c r a c k ing in aged A I - Z n - M g a l l o y s have u s u a l l y a s s u m e d a u n i f o r m solute d i s t r i b u t i o n f r o m g r a i n to g r a i n a c r o s s the g r a i n b o u n d a r i e s with only the solute d i s t r i b u t i o n at the p r e c i p i t a t e - f r e e z o n e s left as a m a t t e r of debate. It has been suggested that, except for the c a s e s of slow quenching no solute d e p l e t i o n o c c u r s at the p r e c i p i t a t e f r e e zones.1 T h e s e z o n e s have b e e n a s s u m e d to be f o r m e d p r e d o m i n a n t l y by a vacancy depletion m e c h a n i s m .2-4 Clark5 employed an electron microprobe to determine the solute concentration in the precipitate-free zones in an AI-Ag alloy aged for several days at two different temperatures. From his results, he concluded that the precipitate-free zones formed by a vacancy depletion process at the lower aging temperature whereas a solute depletion mechanism was operative at the higher aging temperature. By m i c r o h a r d n e s s m e a s u r e m e n t s , R u t t e r and Aust 6 and Aust and c o w o r k e r s 7 i n d i r e c t l y showed that solute r e d i s t r i b u t i o n over a c o n s i d e r a b l e d i s t a n c e n e a r g r a i n b o u n d a r i e s o c c u r r e d in a n u m b e r of quenched dilute b i n a r y alloys. Anthony8 developed a t h e o r e t i c a l m o d e l to explain these r e s u l t s in which e i t h e r solute e n r i c h m e n t or solute depletion at vacancy sinks o c c u r s depending upon the m a g n i t u d e of s o l u t e - v a c a n c y b i n d i n g energy. In view of the p o s s i b i l i t y that c o n s i d e r a b l e solute r e d i s t r i b u t i o n could o c c u r d u r i n g quenching a n d / o r aging, the p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n to study the solute d i s t r i b u t i o n in the g r a i n b o u n d a r y r e g i o n of an A I - Z n - M g alloy following d i f f e r e n t quench r a t e s and heat t r e a t m e n t s . Heat t r e a t m e n t v a r i a b l e s such as aging t i m e and t e m p e r a t u r e w e r e c h o s e n so a s to obtain the a p p r o x i m a t e peak h a r d n e s s condition for this alloy. The i n t e r e s t in the A I - Z n - M g s y s t e m was due to both its c o m m e r c i a l i m p o r t a n c e and the w e l l - d o c u m e n t e d p r e c i p i t a t i o n and p r e c i p i t a t e - f r e e zone f o r m a t i o n c h a r a c t e r i s t i c s . C. R. SI-IASTRY,formerly Graduate Research Assistant at Rensselaer Polyte6hnic Institute, Troy, N. Y., is now Resident Research Associate, Army Materials and MechanicsResearch Center, Watertown, Mass. G. JUDD is Assistant Professor of Materials Engineering, Rensselaer Polytechnic Institute. Manuscript submitted April 28, 1971. METALLURGICALTRANSACTIONS
SPECIMEN PREPARATION An AI-Zn-Mg alloy of average composition 6.86 wt pct Zn and 2.35 wt pct Mg was obtained in the form of a 50 mil. thi
Data Loading...
