Title

Interdiffusion microstructures in gamma/gamma + beta Ni-Cr-Al diffusion couples

Date of Completion

January 2000

Keywords

Engineering, Industrial|Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

Diffusion induced microstructure changes may either enhance or reduce the material performance, it is important to predict diffusion behavior especially in the design of coatings and intermetallic composites. The study of diffusion is done through the use of diffusion couples, which consist of two alloys that are placed in intimate contact with one another. When diffusion couples are heated at high temperature, the difference in the chemical potentials of the two alloys will cause transference of elements from one alloy to the other, forming unique microstructures in the vicinity of their common interface. The microstructure is called “interdiffusion microstructure” because it forms through the interdiffusion of atoms near the interface. ^ This project is to study the interdiffusion between a single phase, γ, Ni-Cr-Al alloy with various γ + β Ni-Cr-Al alloys. The approaches include computer simulation, diffusion experiments and mathematical modeling. Three types of interdiffasion microstructures, γ > γ β + β, γ < γ + β:γ + β, and γ < γ + β < γ > γ + β, were found in DICTRA simulations and diffusion experiments. However, DICTRA simulated diffusion paths for γ < γ + β:γ + β type microstructure always came with some artifacts at type 0 boundary. Thus, a mathematical model was build up for this type of diffusion path. ^ An interdiffusion microstructure map (IMM) was constructed by denoting the composition of non-base alloys with different symbols for different microstructures on the phase diagram, then drawing lines to separate different microstructure fields. The lines between different IMM regions were modeled mathematically and compared with DICTRA and experimental results. It was found that mathematical modeled IMMs agreed with DICTRA and experimental IMM. ^