Degraded Reversibility in Modulated Martensite Driven by Kinetic Arrest

Abstract Achieving complete reversibility in martensite–austenite phase transitions is challenging due to structural degradation during thermal cycling, yet the atomistic pathways driving this degradation remain unresolved. Using in situ atomic-resolution scanning transmission electron microscopy on a Ni2MnGa shape-memory alloy, we reveal that although structural modulation forms through phonon softening to relieve mismatch strain between martensite and austenite, it significantly compromises reversibility. While the global lattice shear in nonmodulated martensite is reversible, the localized atomic shuffles in the modulated structure do not reverse collectively. Instead, they follow locally divergent pathways that create a kinetically arrested austenite state. Upon cooling, this arrested state triggers irregular atomic shuffling and residual defects that produce thermal hysteresis and functional fatigue. Linking this degradation to the multidirectional energy landscape of atomic shuffling provides a framework for understanding kinetic trapping and reversibility in complex materials.