The Influence of [110] Compressive Stress on Kinetically Arrested B2-R Transformation in Single-Crystalline Ti-44Ni-6Fe and Ti-42Ni-8Fe Shape-Memory Alloys

Ti-(50-x)Ni-xFe alloys exhibit a thermally induced B2-R martensitic transformation (MT) when x is between 1.5% and 5.7%, whereas this transformation is suppressed when x is 6 at% and higher. We studied the reason for this suppression by applying compressive stress in the [110]B2 direction to single-crystalline Ti-44Ni-6Fe and Ti-42Ni-8Fe (at%) alloys. Under stress, these alloys exhibit a B2-R MT with a large temperature hysteresis of >= 50 K. The B2-R MT in these alloys is probably thermally arrested, and a small entropy change is a possible reason for this arrest. The Young's modulus E[110] of these alloys significantly decreases with decreasing temperature, and the B2-R MT under stress occurs at a temperature where E[110] is approximately 50 GPa. Presumably, lattice softening assists the B2-R MT.