Effects of strain rate and temperature on the superelastic-plastic behaviors of NiTi polycrystalline-amorphous composite structure based on molecular dynamics

Abstract

This study employs molecular dynamics simulations to construct a NiTi polycrystal-amorphous composite structure, and systematically investigates the effects of amorphous layer thickness (1–2.5 nm), strain rate (5×10⁸–2×10⁹ s⁻¹), and temperature (400–500 K) on its superelastic-plastic behaviors. The findings indicate that as the thickness of the amorphous layer thickens, there is an increase in the overall stress level, which encompasses both the critical transformation stress and the yield stress. Furthermore, the incorporation of the amorphous phase leads to an elevation in residual strain. This heightened residual strain is primarily due to the plastic deformation occurring within the amorphous phase, which concurrently obstructs the martensitic reverse transformation. As strain rate increases, the yield strength rises monotonically regardless of amorphous phase presence, while the residual deformation gradually decreases. Meanwhile, grain boundary sliding becomes less prominent at higher strain rates. On unloading to zero stress, the composite structure displays a higher concentration of residual martensite. For the NiTi polycrystalline, the rate of transformation from austenite to martensite slows down as the temperature increases. However, the composite structure with an amorphous layer exhibits minimal fluctuation in martensite transformation suppression between 400–500 K, demonstrating higher thermal stability than the polycrystalline structure. Simultaneously, as the temperature rises, the yield strength decreases while the residual deformation increases.

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