Effect of mechanical cycling on stabilizing the transformation behaviour of NiTi shape memory alloys

The influence of mechanical cycling on the behaviour of NiTi shape memory alloy (SMA) wire in the martensitic and R-phase transformation range is analyzed as a function of the cycling temperature in order to obtain the best cycling conditions: that is to say, the material stabilizes after few cycles and the post-cycling mechanical properties are good. Mechanical cycling, irrespectively of temperature, tends to lower the transformation stresses and move the transformation lines on the stress-temperature plane in the direction of low stress and high temperature as cyclic deformation progresses. The elastic modulus also changes significantly with cycling but the trend is different at lower temperatures than at higher ones. If the mechanical cycling is performed at temperatures well above or below the R-phase transformation temperatures, this phase tends to vanish or diminish with the mechanical cycling. The main constitutive parameters that describe the SMA mechanical behaviour depend significantly on the temperature at which the wire is cycled and it is best to obtain the parameters at the lowest temperature at which the material is to be used. The sample cycled at the lowest temperature presents the best mechanical properties, which tend to be stable for at least 100 mechanical cycles. (C) 2008 Elsevier B.V. All rights reserved.