Molecular dynamics simulation of uniaxial fatigue deformation mechanism in single-crystal β-Ti under thermo-mechanical coupling

This study systematically investigates the microscopic deformation mechanism of single-crystal β-Ti under cyclic tension–compression loading using molecular dynamics (MD) simulations. The effects of crystallographic orientation ([100], [010], [001]), cycle number (1, 5, 10, 15, 20 cycles), and temperature (300, 473, 673, 873 K) on the mechanical response and dislocation evolution are examined in detail. The results demonstrate that the fatigue behaviour of β-Ti exhibits pronounced anisotropy and temperature dependence. At 300 K for the [100] orientation, the total dislocation density decreases anomalously with increasing cycle number, accompanied by a transition in the dominant plastic carrier from partial dislocations to perfect dislocations. The underlying mechanism is identified as the annihilation of opposite-signed dislocations driven by cyclic reverse loading and the reaction 1/6 < 112 > +1/3 < 100 > →1/2 < 110 > , which converts partial dislocations into low-mobility perfect dislocations. Increasing temperature significantly enhances dynamic recovery; at 873 K, the saturated dislocation density is reduced by approximately 40% compared with 300 K, the stress–strain hysteresis loop becomes narrow and symmetric, and the deformation mechanism shifts from cyclic hardening to a dynamic equilibrium between dislocation multiplication and annihilation. Orientation comparison reveals that the [100] orientation exhibits the optimal cyclic stability due to multi-slip coordination forming a stable tangled network, whereas the [010] orientation displays the poorest stability with pronounced stress serrations and microstructural instabilities caused by slip localisation. This study provides atomic-scale insights into the fatigue damage accumulation mechanism of β-Ti under thermo-mechanical coupling and offers theoretical guidance for the design of high fatigue-resistant titanium alloys.