Diffusion Couple Study on the Mechanism and Kinetics of TiAl3 Formation in the Ti-Al System

Abstract

Introduction: In the Ti-Al diffusion system, intermetallic formation is reasonable to be controlled by diffusions of the titanium and aluminum through the compound layer. The current understanding of diffusion process in this system is not clear. However, most of the researchers have suggested that aluminum is the only or dominant diffusing component through the TiAl3 layer. On the other hand, reported data for kinetics of TiAl3 layer formation such as kinetic exponent and activation energy vary widely among different works, which has made the phenomena more complicated. <br><br>Experimental Procedure: A simple diffusion couple was prepared by using one pure aluminum foil and two pure titanium sheets in a form of tri-layer sandwich held by a steel fixture. Diffusion heat treatments were performed at 550, 575, 600, 625, 640 and 650 °C. Microstructural observation and EPMA were employed to find the impurity diffusion coefficient of aluminum through TiAl3 and the kinetics of TiAl3 formation and growth.<br><br>Results and discussion: Variation of TiAl3 layer thickness by time showed a parabolic behaviour. EPMA results indicated that intermetallic layer roughly had a composition of Ti-75at.% Al which corresponds to TiAl3. Considering aluminum as the dominant diffusing component, the impurity diffusion coefficient of aluminum in the TiAl3 layer was calculated by measuring the layer growth rate and the concentration gradient in the compound layer based on the Stefan condition. As results, diffusion coefficient and activation energy for aluminum diffusion through the TiAl3 layer were calculated.