Cyclic deformation behavior of single crystal NiTi

Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M_s (martensite start) temperature of - 30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress-strain response is governed by transformation from the austenite to the martensite phases and the dislocation structure evolution in the austenite domains. Two single crystals orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress-strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.