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<p>A gradient nanostructure is introduced on the material surface through surface strengthening, providing a new pathway to tune the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs). Based on a grain size (GS)-dependent phase-field model, the influences of surface-layer thickness and initial core-region GS (GS<sub>C</sub>) on the martensitic transformation and eCE in polycrystalline NiTi SMAs are systematically investigated, along with the underlying microstructural mechanisms. The results show that surface strengthening produces a gradient microstructure characterized by continuous martensitic transformation in the surface layer coexisting with nucleation–growth transformation in the core, which suppresses the overall martensitic transformation. This suppression becomes more pronounced as the surface-layer thickness increases. Macroscopically, these changes manifest as higher stress levels, reduced adiabatic temperature change (|Δ<i>T</i><sub>ad</sub>|), and decreased stress–strain hysteresis loop area (Δ<i>W</i>) under the same applied strain, resulting in a significant enhancement of the material’s coefficient of performance (<i>COP</i><sub>mat</sub>), approximately 98.5%. Further analysis reveals that GS<sub>C</sub> critically governs the regulatory effect of surface strengthening: when GS<sub>C</sub> is relatively large, the intrinsic transformation capacity of the system is strong, and the suppressive effect of the surface layer is limited. As GS<sub>C</sub> decreases, the grain boundary density increases, and the martensitic transformation gradually shifts from a nucleation–growth mechanism to a spatially continuous mode, while the additional grain boundary constraints introduced by the surface layer further enhance transformation suppression. Under the cooperative regulation of surface-layer thickness and GS<sub>C</sub>, <i>COP</i><sub>mat</sub> increases as GS<sub>C</sub> decreases, reaching a maximum of 34.6. This study elucidates the microstructural mechanisms by which surface strengthening modulates the eCE in NiTi SMAs, providing a theoretical basis for optimizing SMA-based solid-state cooling materials via surface engineering.</p>

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Phase-Field Simulation of the Elastocaloric Effect of Surface-Strengthened Nanocrystalline NiTi Shape Memory Alloys

  • 期刊论文

  • 发表日期：

    2026年5月6日

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  • Yiyuan Tang,
  • Yuhao Wang,
  • Xi Xie,
  • Bo Xu,
  • Chong Wang,
  • Qingyuan Wang

摘要

A gradient nanostructure is introduced on the material surface through surface strengthening, providing a new pathway to tune the elastocaloric effect (eCE) of NiTi shape memory alloys (SMAs). Based on a grain size (GS)-dependent phase-field model, the influences of surface-layer thickness and initial core-region GS (GSC) on the martensitic transformation and eCE in polycrystalline NiTi SMAs are systematically investigated, along with the underlying microstructural mechanisms. The results show that surface strengthening produces a gradient microstructure characterized by continuous martensitic transformation in the surface layer coexisting with nucleation–growth transformation in the core, which suppresses the overall martensitic transformation. This suppression becomes more pronounced as the surface-layer thickness increases. Macroscopically, these changes manifest as higher stress levels, reduced adiabatic temperature change (|ΔTad|), and decreased stress–strain hysteresis loop area (ΔW) under the same applied strain, resulting in a significant enhancement of the material’s coefficient of performance (COPmat), approximately 98.5%. Further analysis reveals that GSC critically governs the regulatory effect of surface strengthening: when GSC is relatively large, the intrinsic transformation capacity of the system is strong, and the suppressive effect of the surface layer is limited. As GSC decreases, the grain boundary density increases, and the martensitic transformation gradually shifts from a nucleation–growth mechanism to a spatially continuous mode, while the additional grain boundary constraints introduced by the surface layer further enhance transformation suppression. Under the cooperative regulation of surface-layer thickness and GSC, COPmat increases as GSC decreases, reaching a maximum of 34.6. This study elucidates the microstructural mechanisms by which surface strengthening modulates the eCE in NiTi SMAs, providing a theoretical basis for optimizing SMA-based solid-state cooling materials via surface engineering.