Molecular dynamics insights into the temperature-dependent cyclic stability of gradient nanocrystalline NiTiCu shape memory alloys

NiTi-based shape memory alloys (SMAs) are widely used for their exceptional superelasticity and shape memory effects, yet their application is limited by cyclic stability degradation due to irreversible deformation. The introduction of Cu element and gradient grain structures demonstrates potential for enhancing their performance. In this work, molecular dynamics simulations were conducted to explore the phase transformation behavior of polycrystalline Ni 48 Ti 50 Cu 2 SMAs under cyclic loading. The synergistic effects of gradient grain size distribution and temperature conditions, and Cu concentration on cyclic stability were elucidated. Results indicate that elevated temperatures impede reverse phase transformation, promoting residual strain accumulation and impairing cyclic stability. However, increasing the gradient rate effectively suppresses cyclic instability. Compared to homogeneous nanocrystalline structures, the gradient grain structure exhibits a lower grain boundary density, thereby alleviating stress concentration during transformation. Simultaneously, the coarse-grained regions provide sufficient space for phase transformation, resulting in a more uniform and coordinated martensitic transformation and consequently enhancing the recoverable deformation capacity at elevated temperatures. Simultaneously, a moderate increase in Cu content can also improve cyclic stability, though this effect is constrained by the dual influence of gradient structure and temperature. This study clarifies the micro-mechanism by which gradient structures mitigate cyclic instability at high temperatures, offering insights for optimizing the cyclic stability of NiTiCu alloys.