Unexpected Thermal Transport, Expansion and Storage Behavior via Reversible Martensitic Transformation

We recently demonstrated unique functionalities in shape memory alloys (SMAs) by engineering chemistry, microstructure, crystallographic texture, structural disorder, and their coupling to the martensitic transformation. Certain magnetic SMAs show unexpected transport responses: thermal and electrical conductivities change abruptly, by as much as threefold, across the transformation, exceeding typical metallic alloys. We can also program the switching temperature and direction (on heating or on cooling) through alloy choice, atomic ordering, and applied fields.

These alloys permit isothermal, field- or stress-induced phase switching: modest magnetic fields or stresses trigger an instantaneous jump from a low-conductivity martensite to a high-conductivity austenite, or vice versa, enabling fast, solid-state thermal switches and regulators with no moving parts. By combining compositions with opposite switching polarity, we create directional thermal diodes and thermal transistors that bias heat flow and support basic thermal logic.

Beyond transport, we tailor thermal expansion. Through thermo-mechanical processing that imposes strong texture, we achieve both the classical Invar response and wide-range negative thermal expansion. Several SMAs also deliver higher heat-storage figures of merit than conventional phase-change materials because of their greater density and thermal conductivity.

Together, these results show that reversible martensitic transformation can serve as a general design tool to concurrently tune thermal, electrical, mechanical, and magnetic properties. The emerging platform enables “designer” solids with dynamic, multi-field control for thermal management, energy conversion, sensing, and actuation.