Non-linear temperature dependences of pseudoelastic stress and stress hysteresis of a nanocrystalline Ni47Ti50Fe3 alloy

This paper reports a study on non-linear temperature dependences of the critical pseudoelastic stress and stress hysteresis for stress-induced martensitic transformation deformation of a nanocrystallined Ni47Ti50Fe3 alloy. Such non-linear temperature dependences were found over a wide low temperature range from -196 degrees C to 20 degrees C and they defy the predictions of the Clausius-Clapeyron law for the transformation. The pseudoelastic behaviour exhibited two unusual characteristics, including a progressively decreasing d6/dT slope and a rapidly widening stress hysteresis with decreasing the temperature. In-situ X-ray diffraction analysis indicated that the starting microstructure of the alloy for pseudoelastic deformation was a mixture of the B2 and a crystallographically suppressed R phase (denoted R*) within -8 degrees C to -196 degrees C. The volume fraction, the rhombohedral lattice distortion and the internal atomic shuffle of the suppressed R phase increased upon cooling. These continued evolutions of the crystal structure of the R* phase and the microstructure of the alloy upon cooling are ascribed to be responsible for the abnormal non-linear temperature dependences of the pseudoelastic stresses and the widening of the stress hysteresis.