Relaxation of a Rotational Defect in a Titanium Nanocylinder

The investigation of defects in nanostructured materials is of great importance in addressing the issues of material and device reliability. This study focuses on a rotational defect called disclination, which has been directly observed in nanostructured materials [1]. The disclination is inserted into a bicrystalline nanocylinder of titanium and its stability is investigated at zero temperature. This is done by merging two bicrystalline wedges containing [1 100] symmetrical tilt grain boundaries with the misorientations θ 1 and θ 2, respectively, resulting in a misorientation misfit which characterizes the disclination strength ω θ 1–θ 2 [2]. The disclinated cylinder is then relaxed via the Materials Explorer software [3]. The results for nanocylinders with diameters 10–20 nm show that (1) a critical disclination strength ω c exists, (2) above ω c the defect is unstable and relaxes, and (3) ω c demonstrates a strong size effect. Moreover, the relaxation mechanisms are complex, including both cleavage and new grain nucleation near the disclination. The cleavage can occur on the grain boundary, along the basal plane, as well as along the prismatic plane. The particular relaxation mechanism that occurs depends on ω. The figure below shows grain boundary cleavage, basal plane cleavage, as well as new grain nucleation in the region bounded by the crack bifurcations in a cylinder of 16 nm diameter. Such results may have implications for the development of nanocrystalline metals processed by the method of severe plastic deformation.