Preferential site occupancy of alloying elements in TiAl-based phases

TL;DR

This study uses first principles calculations to analyze how different alloying elements prefer specific sites in TiAl phases, revealing trends in site occupancy and phase stabilization effects.

Contribution

It provides detailed insights into the site preferences of various alloying elements in TiAl phases, including anomalies and the influence on phase stability, based on first principles calculations.

Findings

Early transition metals prefer Ti sites in most phases.

Silicon prefers Al sites across all phases.

B and C prefer interstitial positions rather than substitution.

Abstract

First principles calculations are used to study the preferential occupation of ternary alloying additions into the binary Ti-Al phases, namely $\gamma$-TiAl, $\alpha_2$-Ti$_3$Al, $\beta_{\mathrm{o}}$-TiAl, and B19-TiAl. While the early transition metals (TMs, group IVB, VB , and VIB elements) prefer to substitute for Ti atoms in the $\gamma$-, $\alpha_2$-, and B19-phases, they preferentially occupy Al sites in the $\beta_{\mathrm{o}}$-TiAl. Si is in this context an anomaly, as it prefers to sit on the Al sublattice for all four phases. B and C are shown to prefer octahedral Ti-rich interstitial positions instead of substitutional incorporation. The site preference energy is linked with the alloying-induced changes of energy of formation, hence alloying-related (de)stabilisation of the phases. We further show that the phase-stabilisation effect of early TMs on $\beta_{\mathrm{o}}$-phase has a different origin depending on their valency. Finally, an extensive comparison of our predictions with available theoretical and experimental data (which is, however, limited mostly to the $\gamma$-phase) shows a consistent picture.