Surface segregation in an engineering alloy: Hastelloy C-276
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T H E composition of a g r a i n boundary or free surface of an alloy can be quite different from its bulk composition.1 One or more elements may segregate t o the interface. This segregation is of considerable importance t o a number of metallurgical problems such as t e m p e r embrittlement and hydrogen embrittlement,e Since H a r r i s3 f i r s t showed that A u g e r Electron Spectroscopy (AES) call be used t o study this s e g r e g a tion, it has been the subject of widespread interest. Both engineering alloys2 and m o d e l binary systems have been examined. 4 The kinetics 5 and thermodyn a m i c s6 of segregation have been studied in the model binary systems and a r e reasonably well understood. The studies of engineering alloys have dealt primarily with the g r a i n boundary or surface segregation of specific elements a f t e r some particular heat treatment.2 Often surface segregation, r a t h e r than g r a i n boundary segregation, has been studied. Surface s e g r e gation has been found to be closely related t o g r a i n boundary segregation in some systems, though t h e r e are also important differences. 7 Surface segregation experiments are more suited to identifying fundamental physical phenomena than are g r a i n boundary experiments because of the feasibility of continuous in situ monitoring of the interface composition during prolonged high temperature-annealing. In this paper, we r e p o r t on a systematic study of the surface composition of a cold-worked nickel-base superalloy, HASTELLOY C-276,* u s i n g a wide r a n g e *Registered trademark of Cabot Corporation. of annealing temperatures. In general, w e find that s u r f a c e composition of this alloy depends, in a quite complicated way, on the temperature at which it is annealed, the time spent at that temperature, and the previous cold-work and annealing history of the sample. Depending on the interplay of these variables, a very wide r a n g e of surface compositions can be produced. Therefore, as w e will show, the surface composition, and presumably also the g r a i n boundary composition, of a multicomponent engineering alloy in s e r v i c e can vary quite extensively depending on the p r i o r treatment of the alloy. This suggests that a thorough underJ. J. BURTON and B. J. BERKOWITZ are both with Exxon Research ~mdEngineering Company Linden,NJ 07036. R. D. KANE, formerly with Exxon Production Research Company, is now with Battelle, Houston Operations, Houston, TX. Manuscript submitted May 30, 1978. METALLURGICAL TRANSACTIONS A
standing of the relation of the p r i o r treatment of the sample to its interface composition may ultimately lead to control of interface composition and thus improved alloy properties. In this paper, we make no c l a i m s to fully understanding this relation of treatment t o interface composition. Rather, this paper is, a f i r s t and necessary step in which we attempt t o identify some of the fundamental effects which are important. EXPERIMENTAL The alloy examined in this work is HASTELLOY C276.
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