Extending the Operational Window of NiTi Shape-Memory Alloys into the Deep-Cryogenic Regime

Abstract

This work experimentally demonstrates the thermally activated shape-memory behavior of a near-equiatomic NiTi alloy under extreme cryogenic conditions down to 20 K. Cylindrical samples were subjected to controlled thermal cycling between 20-150 K under constant tensile loadings, revealing a sharp, non-linear increase in Young's modulus below 50 K and a significant reduction in the shape-restoring coefficient from 98% at 300 K to 62% at 20 K. The novel alloy exhibited an impressive recovery strain of 8.2% at 10 K, marginally surpassing the conventional limit of 8.1%, while maintaining a functional response across an extended temperature range up to 350 K with a residual recovery strain of approximately 0.9%. The actuation strain was found to evolve monotonically from 0.8% at 100 K to a maximum of 7.0% at 400 K, with a discernible thermal hysteresis loop indicating stable and reversible phase transformations. These findings confirm the feasibility of SMA operation in deep-space and superconductor environments, challenging traditional design paradigms by demonstrating that the alloy can retain substantial shape-memory functionality even when atomic mobility is severely restricted. This extended operational window, enabled by microstructural optimization, positions the new alloy as a promising candidate for robust, wide-range actuators and structural components requiring reliability under extreme thermal conditions.