Planar fault energies and sessile dislocation configurations in substitutionally disordered Ti-Al with Nb and Cr ternary additions

Variations in planar fault energies, generated by changes in alloy composition, can influence thermally activated processes which govern plasticity in intermetallic alloys. Predicting variations in defect energy as a function of alloy composition would aid both alloy design and the interrogation of models of yield stress. In this paper, layered Korringa-Kohn-Rostoker coherent potential approximation calculations are reported for the superlattice intrinsic stacking fault (SISF) and antiphase-boundary (APB) energies in binary and ternary Ti-Al alloys. The planar fault energies were calculated over a range of alloy composition: (Ti1-xAlx)(1-y) M(y) with 0.48 less than or equal to x less than or equal to 0.51, 0.00 less than or equal to y less than or equal to 0.02 and M = Cr, NE. For the Ti-rich alloys, ternary additions up to 4at.% were also considered. APB (010) energies were calculated for the binary alloy while the SISF and APB (111) energies were calculated for all the binary and ternary alloys. The compositions Ti50Al50 and (Ti50Al50)(1-y)Cr-y have the maximum defect energies for this range of alloy compositions. Cr additions appear to have little influence on the defect energies of the Ti-rich alloys, while slightly reducing the APB (111) in the Al-rich alloys. The defect energies for the Nb alloys are reduced relative to the binary alloys, with the fault energies increasing monotonically with increasing Al concentration. The variation in defect energies, both trends and magnitude, are used with anisotropic elasticity theory to estimate the forces needed to produce two possible [101]{111} superdislocation glide barriers. Also, the anisotropic elastic energy of the nonplanar equilibrium superdislocations are compared. The formation of [101]{111} superdislocations with portions on cross-slip octahedral planes is favoured over cross-slip onto the cube plane for conservative estimates of the planar fault energies and applied stress. These results suggest that alloy designers can expect only moderate improvements in the high-temperature yield stress, because of the formation of [101]{111} superdislocation barriers, with changes in alloy composition.