Supercritical Martensitic Phase Transformations for Ferromagnetic Shape Memory Alloys

Many multiferroics including ferromagnetic shape memory alloys (FSMAs) owe their unique functionality to reversible martensitic phase transformations. Recent research indicates that, in a supercritical thermodynamic state, the phase transformation occurs through a gradual distortion of the crystal lattice, i.e., without any phase interfaces. Consequently, these supercritical FSMAs exhibit exceptional reversibility and low hysteretic or “anhysteretic” behaviors, despite up to 15% recoverable deformation. In this work, we seek: (1) A fundamental understanding of differences in the microstructure evolution in subcritical vs. supercritical transformations; (2) A way to predict when these supercritical martensitic phase transformations are possible in term of crystal orientation, temperature, and composition. We use in-situ X-ray microscopy (near-field and far-field high-energy diffraction microscopy), in-situ optical microscopy (differential interference contrast), and ex-situ transmission electron microscopy on Ni-Fe-Ga-Co FSMAs to reveal systematic differences between subcritical vs. supercritical transformations, shifting from distinct phases with sharp interfaces to smoothly blended phases without interfaces. We enrich these observations using explanations involving the Landau theory of phase transitions to develop a unified understanding of supercritical behavior in multiferroics, informing future strategies for designing multiferroic materials with exceptional functional properties.