Effect of Grain Size on the Thermal Properties of Nickel-Titanium Shape Memory Alloys Across the Martensite-Austenite Phase Transition

In this work, the thermal conductivity and volumetric heat capacity of 50-50 NiTi shape memory alloys (SMA) with varying grain size are investigated as a function of temperature. SMAs, and NiTi in particular, have a critical role to play in next-generation solid-state refrigeration systems and long-term thermal storage applications. However, their performance in these applications is predicated on their latent heat, thermal conductivity and heat capacity. Complicating the prediction of their performance is the wide variability in reported results for each of these properties within the scientific literature and a lack of information regarding their temperature-dependence, particularly as SMAs experience a martensitic-austenitic phase transition. We use Frequency-Domain Thermoreflectance (FDTR) to probe the thermal transport properties of NiTi as a function of average grain size and temperature and report our results here. Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM) are used to characterize the phase transitions and microstructure of the NiTi, respectively. Collectively, these measurements provide a better understanding of the impact of grain size and phase transition on thermal transport and storage within NiTi as it changes phase, permitting improved predictions of their behavior in a host of important applications.