Diffusion induced grain boundary migration and recrystallization during oxidation of a Ni- 48.5 Pet Cu alloy
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INTRODUCTION
DIFFUSION induced grain boundary migration (DIGM) is a process in which the solute atoms diffuse along the grain boundary to produce boundary migration, leaving behind an alloyed or dealloyed zone. I1-41DIGM has been observed in more than 20 binary systems. 15~It can be easily observed in the solvent when solute diffuses along the grain boundaries resulting in an alloyed z o n e . t2'6-9] DIGM has also been found in alloys when solute atoms are removed from grain boundaries; in this case migration is accompanied by the formation of de-alloyed zones, t''2'l~ Previous work on Cu-Ni Ilu revealed that Ni-rich DIGM occurs during high temperature sputtering. The formation of Ni-rich DIGM was explained in terms of preferential sputtering of Cu atoms at the migrating grain boundaries. Two different mechanisms have been proposed to explain DIGM. One of these, the coherency strain model, m states that stress due to compositional inhomogeneity in the vicinity of a boundary during diffusion provides a driving force for DIGM. The other t2'31 is based upon grain boundary dislocation climb caused by unequal boundary diffusivities of solute and solvent atoms (grain boundary Kirkendall effect). However, neither model can explain all the DIGM-related observations recorded in the literature. Recent studies have reported that the coherency strain driving force may be not enough to support the curvature of the DIGM boundaries in Cu-Zn ml and Ni-Cu I131 systems, suggesting that an additional driving force may be required. Preferential oxidation is a well-known phenomenon in many binary alloys. 1141 In a simple A-B binary alloy, if element A has a stronger oxygen affinity than element B, A will preferentially oxidize to form an A-rich oxide on the surface of the alloy. Also, an A-depleted zone will exist in the matrix beneath the metal-oxide (M/O) interface. During high temperature oxidation, the transfer of element A in the matrix to the M/O interface is controlled by lattice diffusion under the driving force of a free energy change for the chemical reaction: A + O = AO. However, when oxidation ocD. LIU, Graduate Student, W.A. MILLER, Professor and Chairman, and K. T AUST, Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada. M5S IA4. Manuscript submitted September 25, 1987.
METALLURGICALTRANSACTIONS A
curs at lower temperatures, grain boundary diffusion becomes more important for transferring A atoms to the M / O interface. Thus, in this case, A-depleted DIGM would be expected to occur in the matrix by removing A atoms from the grain boundaries to the M/O interface. Previous studies of oxidation in Cu-Ni alloys I'4'15''61 may be summarized as follows: (i) Ni has a stronger oxygen affinity than Cu, (ii) the oxides, NiO, Cu20, and CuO are virtually immiscible, and (iii) a displacement reaction of the form Cu20 + Ni = NiO + 2Cu occurs, controlling further oxidation. Recent work by the present authors tm indicated that Cu-rich DIGM occurs in oxidized, rapidly
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