Cyclic deformation behavior of single crystal NiTi

Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loadin g conditions at room temperature which is above the M,(martensite start) te mperature of - 30 degreesC. 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 compre ssion. The increase in stress range is primarily due to the increasing stra in hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was under taken with a strain range of 3% which is far below the theoretical transfor mation strains levels exceeding 6%. The maximum stress levels reached in th e experiments are below those that cause martensite slip. Therefore, the st ress-strain response is governed by transformation from the austenite to th e martensitic phases and the dislocation structure evolution in the austeni te domains. Two single crystal orientations [148] and [112] were examined d uring the experiments with single and double CVP (correspondent variant pai r) formations respectively. The strain hardening in compression cases is ra ther substantial with the stress range in the double CVP case surpassing th e single CVP case. Two heat treatments were selected to produce coherent an d 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 a t the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue. (C) 2001 Elsevie r Science B.V. All rights reserved.