Factors influencing phase transformations of NiTi wires for civil structure applications

NiTi shape memory alloys are smart materials which can be used as actuators for controls and strengthening of civil structures. The application of NiTi shape memory alloys as actuators is based mainly on the shape memory effect that arises from the phase transformation between martensite and austenite. In the design of structural actuator using NiTi wires, it is important that the characteristics of phase transformation in NiTi are understood and measured accurately. In this study, the effects of heat treatment conditions and heating/cooling rates on the phase transformation behaviours of NiTi alloys were investigated through the use of differential scanning calorimetry (DSC) methods. The results show that heat treatment must be applied to NiTi shape memory alloys in order to achieve satisfactory shape memory effect. Besides, it was found that the phase transformation temperatures of NiTi alloys are dependent on the heating/cooling rate, a variable which is important in determining the temperatures of structural elements at high temperatures. Upon heating at a lower rate, NiTi alloys tend to transform from martensite to austenite at lower temperatures. Likewise, NiTi shape memory alloys cooled at a lower rate transform to martensite phase at higher temperatures. Therefore, the transformation temperatures under different heating rates used for actuator design should be obtained under the working conditions for the intended usage