Grain boundary segregation of an Al-Zn-Mg ternary alloy
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tron beam in a TEM, Doig and Edington were able to d e t e r m i n e the spatial distribution of Mg as the electron beam was t r a v e r s e d a c r o s s a grain boundary. The situation is depicted by the t r a c e a-a' in Fig. l(a). F o r as-quenched s p e c i m e n s , Doig and Edington found an accumulation of Mg at the grain boundary. On the other hand, for o v e r - a g e d s p e c i m e n s , their m e a s u r e m e n t s showed a depletion of Mg in the P F Z , the Mg content d e c r e a s i n g f r o m the bulk value of 3.2 pet to about 0.2 pet at the boundary. Some of Doig and Edington's data 14 a r e r e p r o d u c e d in F i g . l(b) and (c). The above r e s u l t s were i n t e r p r e t e d with the model that, in the o v e r aged case, a m a j o r portion of Mg at the grain boundary combine to f o r m the MgZn2 p r e c i p i t a t e s , leaving the region between the p r e c i p i t a t e s depleted in Mg. Recently, Green et al Is also have m e a s u r e d the grain boundary composition of Ag-Zn alloys of s i m i l a r composition in the as-quenched, peak and o v e r - a g e d conditions. AES and argon ion sputtering were used to obtain the chemical depth p r o f i l e s of grain boundary s u r f a c e s . The AES m e a s u r e m e n t s showed, however, an accumulation of Mg at the grain boundary under all conditions. In this p a p e r , we d e m o n s t r a t e that the apparent contradiction between the T..lvI/plasmon l o s s r e s u l t s of Doig and Edington and the AES r e s u l t s of Green et al, can be r e s o l v e d by exploiting the plasmon l o s s f e a t u r e s of the AES s p e c t r a to help elucidate the s e g r e g a t i o n / p r e c i p i t a t i o n p r o b l e m . Specifically, it is found that the p l a s m o n l o s s energy m e a s u r e m e n t s in the TEM and the AES techniques provide different, but c o m p l e m e n t a r y , information about the grain boundary. A combination of these two techniques has led to a b e t t e r understanding of s e g r e g a t i o n effects at grain boundaries.
EXPERIMENTAL J. M. CHEN is Head, T. S. SUN is Research Scxentist,Physics Department, R. K. VISWANADHAMis Research Scientist, Material Science Department, and J. A. S. GREEN is AssociateDirector, Martin Marxetta Laboratories,Baltimore,MD 21227. Manuscript submitted April 15, 1977. METALLURGICALTRANSACTIONSA
METHODS
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MATERIALS
The Auger electron spectrometer used in this study is a Physical Electronics, double-pass cylindrical mirror analyzer (system model 548). The spectrometer includes an ultra high vacuum system (UHV), where the experiments were conducted under a basal VOLUME 8A, DECEMBER 1977-1935
G.B.
P,ITATE
gZn2 PRECI
solution t r e a t e d s p e c i m e n s was about 0.08 m m . The quenched s a m p l e s were then heat t r e a t e d at 130 and 160~ for 4.5, and 22 h to obtain the u n d e r , peak, and o v e r - a g e d conditions. Small s t r i p s were then cut f r o m the s a m p l e s , notched, and t r a n s f e r r e d to the s p e c t r o m e t e r . RESULTS
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