Mechanical properties of TiNbTa single crystals at cryogenic temperatures

Mechanical properties of b.c.c. TiNbTa alloy single crystals, which have excellent superconducting properties, were studied by compression tests at cryogenic temperatures. A {332} twin occurred at 4.2 K in the as-annealed specimens of alloy 1 (Ti-40.3Nb-13.3Ta in wt%) at χ = -15 degree and alloy 2 (Ti-26.4Nb-34.2Ta in wt%) at χ = 0 degree in which slip deformation also occurred. The specimens with other orientations in alloys 1 and 2 at 4.2 K and whole orientations at 77 and 290 K deformed by {112} slip. (bar 101) slip was not observed. Curves of the yield or twinning stress vs χ at 4.2 and 77 K were different from those of typical b.c.c. metals and alloys. A new dislocation model for the {332} twinning mechanism is proposed whereby that after a pair of stacked twinning dislocations move, shuffling of an atomic group consisting of three atoms occurs, forming one elementary twinned layer of four {332} planes. This model could well explain the shape of the twin. The analysis of the twin interface coincides with observations of dense dislocation structures within the twin and on the twin interfaces