Temperature-modulated differential scanning calorimetry for the study of reversing and nonreversing heat flow of shape memory alloys

The measurement of the thermophysical parameters associated with the thermoelastic martensitic transformation (TMT) of shape memory alloys (SMAs) is fundamental for the correct simulation of the process describing the outcome of SMA actuation. In particular, there are two important energy components during a TMT: the first one is a reversing form related to the nucleation and accommodation of martensite twin boundaries upon cooling, and it is restored when the martensite is heated up to austenite finishing transformation temperature. The second one is represented by a nonreversing form associated with the movement of the grain interfaces, with the defect forming and with the work raised when a plastic transformation occurs. This nonreversing component is dissipated in the form of heat and internal frictional work. Standard differential scanning calorimetry (DSC) measures the sum of these two contributions; therefore, DSC is not able to identify the reverse component that is the one useful in simulation and mathematical modeling of the SMA actuation. Temperature-modulated DSC (MDSC) is really able to separate the two energy forms by the superimposition of a sinusoidal temperature signal to the standard DSC linear temperature ramp. In this paper, MDSC is used to investigate the evolution of reversing and nonreversing heat flows of several NiTi and NiTi-based wires and sheets depending on the microcrystalline state. Observations on the reversing specific heat capacity trend are also drawn.