Grain size dependence of cracking performance in polycrystalline NiTi alloys

Abstract

Nickle-titanium (NiTi) alloys are widely considered as one of the most intelligent materials with great commercial values. This study focuses on investigating the grain size (GS) effect on crack propagation of polycrystalline NiTi alloys through molecular dynamics (MD) simulations combined with cohesive zone model (CZM). The GS distribution is statistically achieved by electron backscatter diffraction experiment, while the Voronoi tessellation is applied to characterize the microstructures. MD modeling is conducted to determine the traction-separation (T-S) law of CZM. The plastic behavior and phase transition are examined as well. Subsequently, the characteristic parameters extracted from the T-S law are embedded into the cohesive elements along grain boundaries, and then the intergranular fracture of NiTi alloys with various grain sizes are reproduced by conducting finite element simulations. Interestingly, an unexpected subcritical crack growth (SCG) is found during the simulations, and its mechanism is explored at the atomic scale. Moreover, the critical stress intensity factor (KIC) of compact tension specimens with average grain size from 17 to 45 µm is predicted. It is discoverable that the SCG duration and KIC increase as the grains become coarsened. Besides, the internal toughening mechanism of GS is probed from the aspects of stress-induced martensitic transformation and crack-path configuration. A comparison between numerical results and published experimental data demonstrates that this hybrid method could be used to successfully simulate the crack growth behavior.