ZERO-FLUX PLANES, FLUX REVERSALS AND DIFFUSION PATHS IN TERNARY DIFFUSION
Abstract
Ternary diffusion was investigated in the Cu-Ni-Zn system at 775(DEGREES)C with both single phase and multiphase couples for the development of zero-flux planes (ZFP) and flux reversals of the individual components and the determination of diffusion paths and diffusion structures. Several series of diffusion couples were assembled with (alpha)(fcc), (beta)(bcc) and/or (gamma)(cubic) Cu-Ni-Zn alloys and were characterized by terminal alloys of similar thermodynamic activity of either Cu, Ni or Zn. The couples were annealed at 775(DEGREES)C and analyzed for the concentration profiles and diffusion paths by energy dispersive X-ray analysis on the SEM. Interdiffusion fluxes were determined directly from concentration profiles and the diffusion paths were represented analytically in terms of parameters characteristic of paths. Interdiffusion coefficients were also determined at several compositions in the (alpha), (beta) and (gamma) phase regions of the Cu-Ni-Zn isotherm at 775(DEGREES)C. ZFP's where the interdiffusion flux of either Ni, Cu or Zn goes to zero were identified in both single phase and multiphase couples. The ZFP compositions were found to correspond to intersection points of diffusion paths and isoactivity lines drawn through the terminal alloy compositions on the Cu-Ni-Zn isotherm. Discontinuous flux reversals were also identified at interfaces with or without ZFP's in adjacent phases in multiphase assemblies. The flux reversal of a component at a planar interface was observed when the activity of a component at the interface coincided with its activity in one of the terminal alloys of the couple. The transition from planar to nonplanar morphologies reflected a mismatch among the interface velocities determined from concentrations and fluxes of each component on either side of the interface and indicated a disagreement between the experimentally observed interface velocity and the calculated values. The generality of the ZFP phenomenon was also checked in several past studies of ternary diffusion and ZFP's and flux reversals have been identified in several systems including Cu-Ag-Au, Cu-Zn-Sn, Fe-Ni-Co, Fe-Ni-Al and Co-Ni-Cr.
Degree
Ph.D.
Subject Area
Metallurgy
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