Title

Non-linear diffusion paths in two-phase ternary diffusion couples

Date of Completion

January 2005

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Prediction of diffusion paths facilitates the understanding of interdiffusion microstructure development at the vicinity of a common interface between two alloys. Understanding the influence of interdiffusion on microstructure is critically important to the design of many advanced materials systems such as high temperature coatings. ^ The current study using DICTRA finite difference software predicts non-linear features formed on the diffusion path as the initial interface is approached. The non-linear diffusion path deviates from the linear zigzag shape predicted by an error function model for multiphase diffusion couples. The deviations appear as "horns" that protrude from the linear paths. The horns were found to be of two types. When the two outer legs of the diffusion path bend in the same direction, a "single-horn" is formed. When they bend in opposite directions a "double-horn" is formed. The formation of horns is attributed to the concentration dependence of the diffusivity. It results in a shift on the maximum of the flux profile from the initial interface, which accordingly leads to a rapid rise or decrease of the precipitate fraction as the interface is approached. ^ It was found that the horn length is proportional to the composition vector component along the major eigenvector of the effective diffusivity matrix. Applying these results to a study on Ni-Cr-Al diffusion couples prepared from γ + β alloys, it also was found that the formation of single-phase β layers could be attributed to the horns pointing away from each other, in which case the diffusion path could intersect the single phase β region of the phase diagram. ^ Comparison between EPMA data and DICTRA simulation shows that existence of second phase could introduce microstructure effect on diffusion. This microstructure effect may be taken into account for promoting or blocking the diffusion. ^