Unveiling ultra-stable cyclic martensitic transformation behavior of an additively manufactured NiTiCu shape memory alloy

Abstract

Additively manufactured (AM) nickel-titanium (NiTi) shape memory alloys (SMAs) often suffer from functional fatigue due to intrinsic large hysteresis and process complexities during cyclic operation. This study, by optimizing lattice compatibility between the parent and martensitic phases, presents a laser-directed energy deposited (LDED) Ni40-Ti50-Cu10 (at%) SMA that simultaneously achieves low hysteresis and enhanced cyclic stability in martensitic transformation behavior. Only 0.31°C of martensitic transformation temperature shift was observed after 50 thermal cycles, significantly outperforming conventional SMAs. The non-equilibrium solidification of the LDED process generates cross-scale microstructures and a distinct (100) growth texture, while excellent lattice compatibility restricts martensitic variants to favorable orientations of (010)_B2 || (001)_B19 and (101)_B2 || (010)_B19. Dominant (011) type I twin relationship between martensitic variants is observed with coherent twin interfaces. These characteristics reduce the energy barrier at the phase transition interface, effectively mitigating interface stress and preventing irreversible defects, minimizing hysteresis, and ensuring ultra-stable cyclic martensitic transformation. Additionally, Ti₂Cu precipitates, formed at twin ridges due to cyclic heating during fabrication, pin dislocations and prevent their movement across twin interfaces, ensuring repeatability and reversibility in each cycle. The utilization of pre-alloyed powder guarantees macroscopic uniformity in phase composition and transition behavior, resulting in consistent cycle stability along the build direction. This study provides new insights into the high cyclic stability of LDED-fabricated NiTiCu10 SMAs and advances the development of high-performance SMAs with degradation-free cyclic phase transformation using AM technology.