Role of amorphous phase in phase transformation of nanograined NiTi alloy: Insights from molecular dynamics simulations

This study employs molecular dynamics simulations to investigate how amorphous layer thickness affects martensitic transformation in homogeneous and gradient nanograined (NG and GNG) NiTi shape memory alloys (SMAs). Amorphous-homogeneous nanograined and amorphous-gradient nanograined models are constructed to examine temperature- and stress-induced transformations, as well as loading-direction dependence. With increasing amorphous layer thickness, the onset temperature of temperature-induced transformation decreases and the transformation rate weakens. In GNG NiTi, transformation primarily occurs in large-grain regions, where low-temperature shear strain localizes. For stress-induced transformation, thicker amorphous layers lead to higher critical stress, peak stress, Young's modulus, energy dissipation, and residual strain, but lower martensite content. After unloading, residual shear strain concentrates at grain boundaries and amorphous regions. Under different loading directions, martensite variant types vary within the same grain, and the tensile transformation plateau is longer than the compressive one. Increased amorphous layer thickness reduces the critical and peak stress ratios between compression and tension, narrows the transformation region, and alleviates tension–compression asymmetry. The difference in martensite content between tension and compression is small in GNG alloys but large in NG alloys. Dislocation density under compression is significantly higher than under tension and decreases with amorphous layer thickness. In GNG NiTi, dislocations mainly distribute in large-grain regions. This atomic-scale study reveals the microscopic mechanism by which amorphous layer thickness regulates martensitic transformation and mechanical response, offering a theoretical basis for designing high-performance SMAs with low asymmetry.