Investigations of Ternary Interdiffusion in β (BCC) Phase Field of Ti-Al-Mo Alloy System

Abstract

Solid–solid diffusion couples were investigated at 1100 °C for determination of ternary interdiffusion coefficients at several compositions of the BCC phase field of Ti-Al-Mo system. Some of the diffusion couples developed zero flux planes (ZFP), which indicate the presence of strong diffusional interactions in this system. The composition of ZFP corresponds to the cross-over point of the diffusion path of the couple with the iso-activity line passing through one of the terminal alloys. Ternary interdiffusion coefficients were evaluated at several compositions at which the diffusion paths of the couples intersected each other. The strong diffusional interactions were also evident in large values of cross interdiffusion coefficients. Large negative values of $$ \tilde{D}_{\text{TiAl}}^{\text{Mo}} $$ indicate that the interdiffusion flux of titanium is enhanced up the gradient of aluminum and reduced down its gradient. An empirical relation is proposed between the ternary interdiffusivities and composition. The diffusion profiles were regenerated reasonably well using finite difference method based on empirically fitted composition dependent diffusion data. Availability of such composition–diffusivity relations in ternary systems would help in more accurate predictive design of the alloys and processes as it avoids the assumptions of negligible cross effects and need of complex extrapolations from lower order systems.