Investigation on plastic deformation mechanism of gradient nano-polycrystalline pure titanium by atomic simulation

A gradient-structure nano-polycrystalline (GS) model and two pure nano-polycrystalline (PS1 and PS2) models with different grain sizes of pure titanium are studied deeply via molecular dynamics (MD) simulations. The plastic deformation mechanism of these three models is investigated in depth through analyzing atomistic details during loading process. In this research, the inverse Hall-Petch relationship is found in pure titanium since the tensile strength is positively correlated with average grain size. The transition from hexagonal close-packed (hcp) to face-centered cubic (fcc) structure is delayed by the gradient structure during loading, leading to the attenuated lattice distortion and thus preventing crack generation. The GS model allows plastic deformation through grain reorientation and grain boundary migration, which effectively improves the plasticity of pure titanium. The 1/3<1-100> type dislocation is proved to be dominant during tensile deformation and the gradient structure can provide dislocation networks interacted with grain boundaries, thus enhancing the tensile strength of the pure nano-polycrystalline titanium. In addition, a weaker temperature dependence on tensile stress is found in GS model compared with PS1 and PS2 models. This work opens a novel avenue for fabricating bulk GS materials with expected mechanical properties through microstructural design.