Title

Diffusivity and intrinsic diffusivity measurements on multicomponent alloys

Date of Completion

January 1992

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

The diffusivity and intrinsic diffusivity of multicomponent alloys can be determined from diffusion couple experiments, when concentration differences in each diffusion couple are small, by using the Square Root Diffusivity analysis. However, this analysis assumes that the diffusivity is independent of composition. Therefore an important question is what diffusivity is produced by the Square Root Diffusivity analysis when applied to real systems that normally have variable diffusivity. The present study investigates this question with perturbation theory and a numerical analysis. The results show clearly that the Square Root Diffusivity analysis gives the diffusivity at the average composition with negligible error as long as the diffusivity is linearly dependent on concentration and the data is analyzed with respect to the Matano plane. Also, analyses are presented that shows how the error is affected by diffusion couple design and by symmetric pairing of data.^ In another study, it is shown how the Square Root Diffusivity equations can be used to model diffusion phenomena in constant intrinsic diffusivity systems, e.g. thermodynamically ideal solutions. This work extends a previous model by van Loo to higher order systems.^ The diffusivities of Ni-4at%Cr-6at%Al-4at%Mo quaternary alloy at 1100$\sp\circ$C and 1200$\sp\circ$C were determined from three diffusion couples at each temperature with the Square Root Diffusivity analysis. By combining equations from the Square Root Diffusivity analysis and the analysis proposed by Krishtal et al., the diffusivity was also obtained from two diffusion couples and the results were compared with three couple values.^ Finally, the intrinsic diffusivities at 1200$\sp\circ$C were measured with the diffusivities and marker movement data. However the intrinsic diffusivities were subject to a relatively large error under the present experimental conditions. ^