Site occupation behavior of sulfur and phosphorus in NiAl, TiAl and FeAl

The site occupation behavior of sulfur and phosphorus in the binary NiAl, TiAl and FeAl alloys is investigated by first-principles calculations. Using a supercell approach, the enthalpies of formation of vacancies, antisites, and substitutional defects are calculated for the three binary alloys. The distributions of S and P in the three alloys and their dependence on Al composition are investigated based on a thermodynamical model. The predicted existence of the dominant defects agrees well with available experimental results and theoretical results. At 1273 K, P atoms occupy exclusively the Al sites in the three alloys, and S atoms occupy exclusively the Al sites in FeAl alloys. However, the concentration of S atoms occupying Ni (Ti) sites increases and that located at Al sites decreases as the Al content increases in NiAl (TiAl). These results cannot be determined from considering only the effects of atomic size and electronegativity on the site occupations. Instead, these results can be understood in terms of differences between the enthalpies of formation for impurity I occupying different sublattices coupled with the configurational entropy differences.