α-Al₂O₃ nanoslab fracture and fatigue behavior

Abstract Strain rate effects and cyclic loading behavior, relevant for fatigue initiation processes, of single crystalline α-Al₂O₃ nanoslab structures in vacuum have been characterized and compared under finite temperature dynamic loading and incremental static loading conditions by reactive molecular dynamics and ionic relaxation simulations, respectively. Different failure mechanisms for each loading case are observed and compared in view of known applicable material failure mechanisms. In particular, structure size effects along with unit cell and bulk defect distribution based mechanisms are considered. The effects of lateral pre-strain are assessed. Conclusions are drawn regarding conditions which could lead to low cycle failure of the material. The results indicate that finite temperature and strain rate result in lower failure strains, as compared to static relaxation calculations, however low strength enhancement of ductility with kinematic strain hardening upon repeated loading may occur. We suggest that the latter facilitates a shakedown mechanism. Positive pre-straining results in increased stress triaxiality (hydrostatic vs. equivalent stress), which significantly reduces crack healing probability, due to single sharp crack propagation. Instead, volume pre-relaxation results in multiple crack branching and/or amorphous band formation, which facilitates crack healing and deformation induced transition to purely elastic response (shakedown) possibility. Amorphization, which manifests as small strain plasticity enhancement, is found to occur ahead of propagating cracks due to multiple dislocation mechanism as a low energy barrier partially reversible low density phase transition, both at static and finite temperature/strain rate conditions. The observed property changes and phase change related defect healing mechanisms have been investigated further by bulk unit cell simulations and validated against DFT calculations.