Negative yield stress temperature anomaly and structural stability of Pt_3Al

It is well established that the yield stress anomaly (the increase of the yield stress with temperature) is an intrinsic property of L1₂ intermetallics resulting from features of the structure of their superdislocations. Pt_3X (X=Al, Ga, Ge) are the most studied L1₂ alloys with a strong negative temperature dependence of the yield stress. Despite numerous research efforts, the nature of this unusual behavior remains unclear. Based on ab-initio DFT total energy calculations within the GGA, we establish that for stoichiometric Pt_3Al, the D0^'_c structure is energetically preferable, and demonstrate the electronic origin of the instability of the L1₂ phase, and show that small deviations from stoichiometry (>3% excess Al) stabilize the L1₂ phase. We analyse the structure and mobility of dislocations in the D0^'_c phase within the modified Peierls-Nabarro model with first-principles generalized stacking fault energetics. Our results allow an understanding of the main peculiarities of the mechanical behavior of Pt_3Al such as its negative yield stress low temperature anomaly, and the sensitivity of the yield stress to crystal orientation and to deviation from stoichiometry.