INTERDIFFUSION IN THE IRON-NICKEL-CHROMIUM SYSTEM
Abstract
Isothermal ternary diffusion was investigated with diffusion couples assembled with (gamma)(fcc) and (alpha)(bcc) Fe-Ni-Cr alloys for the determination of diffusion paths and interdiffusion coefficients, and the development of diffusion structures and zero-flux planes for the individual components. The couples were annealed at 1100(DEGREES)C for 7 days, and analyzed for the concentration profiles and diffusion paths by SEM-EDAX analysis. Ternary interdiffusion coefficients were determined over a wide range of compositions in the (gamma)(fcc) region of the ternary isotherm at 1100(DEGREES)C from diffusion couples with intersecting diffusion paths. Computer programs were developed to curve-fit the experimental concentration profiles, to calculate the interdiffusion fluxes, and to determine the interdiffusion coefficients. Interdiffusion coefficients were also estimated from tracer diffusion data and thermodynamic data based on random alloy model and atomic mobility model. Zero-flux planes (ZFP) where the interdiffusion flux of a component goes to zero were identified in several couples with (gamma) terminal alloys characterized by similar thermodynamic activity of either Cr, Ni or Fe. The ZFP compositions corresponded to intersection points of diffusion paths and isoactivity lines drawn through the terminal alloy compositions on the Fe-Ni-Cr isotherm. An analytical representation of experimental diffusion paths was proposed with the aid of two path parameters, the cross-over composition Y(,c), and the path slope 1/n at that composition. These parameters were employed for the prediction of diffusion paths for several diffusion couples in the Fe-Ni-Cr system. Transitions from planar to nonplanar interfaces in the (alpha)/(gamma) two-phase diffusion couples were indexed by varying the compositions of the terminal alloys. Couples with planar (alpha)/(gamma) interfaces exhibited path crossings of the ((alpha)+(gamma)) two-phase region parallel to tie-lines; the two-phase path crossings for couples developing nonplanar interfaces were inclined to tie-lines. The interface velocities calculated from fluxes and concentrations of each of the components in the phases at the interface were observed to be in good agreement with the experimentally determined values for the couples developing planar interfaces; such an agreement between the calculated and observed interface velocities was not observed for nonplanar interfaces.
Degree
Ph.D.
Subject Area
Metallurgy
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