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 com position, can influence thermally activated processes which govern pla sticity in intermetallic alloys. Predicting variations in defect energ y as a function of alloy composition would aid both alloy design and t he interrogation of models of yield stress. In this paper, layered Kor ringa-Kohn-Rostoker coherent potential approximation calculations are reported for the superlattice intrinsic stacking fault (SISF) and anti phase-boundary (APB) energies in binary and ternary Ti-Al alloys. The planar fault energies were calculated over a range of alloy compositio n: (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 0.0 2 and M = Cr, NE. For the Ti-rich alloys, ternary additions up to 4at. % were also considered. APB (010) energies were calculated for the bin ary alloy while the SISF and APB (111) energies were calculated for al l the binary and ternary alloys. The compositions Ti50Al50 and (Ti50Al 50)(1-y)Cr-y have the maximum defect energies for this range of alloy compositions. Cr additions appear to have little influence on the defe ct energies of the Ti-rich alloys, while slightly reducing the APB (11 1) in the Al-rich alloys. The defect energies for the Nb alloys are re duced relative to the binary alloys, with the fault energies increasin g monotonically with increasing Al concentration. The variation in def ect energies, both trends and magnitude, are used with anisotropic ela sticity theory to estimate the forces needed to produce two possible [ 101]{111} superdislocation glide barriers. Also, the anisotropic elast ic 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 s tress. These results suggest that alloy designers can expect only mode rate improvements in the high-temperature yield stress, because of the formation of [101]{111} superdislocation barriers, with changes in al loy composition.