Development of NiTi Alloys for Energy-Efficient Elastocaloric Applications Through Nanostructural Analysis

Elastocalorics is a sustainable, non-greenhouse gas cooling and heating technology based on solid-state endothermic and exothermic reactions driven by stress-induced phase transformations in superelastic NiTi. The efficiency and fatigue behavior of NiTi elastocaloric systems are governed by intrinsic microstructural parameters—grain size, orientation, precipitation reactions, and internal defect structures—that evolve with thermomechanical processing and cyclic loading. This study employs advanced (scanning) transmission electron microscopy, (S)TEM, to directly correlate nanoscale structure with functional performance. Imaging, diffraction, and spectroscopy, including 4D-STEM, are applied to characterize dislocations, phase interfaces, and precipitates. TEM specimens are prepared using electropolishing to obtain thin foils from both as-processed and fatigued NiTi sheets, to ensure minimal damage and to preserve nanoscale defect structures for analysis. The resulting nanostructural data are quantitatively correlated with calorimetric (ΔH), mechanical (W), and fatigue measurements to determine the Coefficient of Performance (COP = ΔH/W) and to identify the processing routes that enhance both efficiency and durability. Results from preliminary tests indicate that modifications in material processing can lead to COPMAT values that range from ~2 to >10, consistent with differences in transformation enthalpy and mechanical work that arise from microstructural improvements. By revealing the structural and functional fatigue mechanisms that limit current NiTi performance, this work establishes microstructural design principles for long-lifetime elastocaloric materials.