Heterogeneous nucleation of plastic defects and tension-compression asymmetry in the presence of vacancies in W single crystals

The tension–compression asymmetry with pre-existing vacancies is investigated for tungsten using molecular dynamics (MD). The tension–compression asymmetry is revealed by means of uniaxial tension and compression along [100], [110], [111], and [112] crystallographic orientations with different strain rates (10⁸ ∼ 10¹¹ s^-1). Results show that except for [110] loading orientation, the yield stresses in compressive are generally greater than those in tensile loading. Vacancy narrows the tension–compression asymmetry as it reduces the gap between tensile and compressive yield strengths when the vacancy concentration goes up. This is through the coalescence of individual vacancy into vacancy clusters before yielding. Aggregation and coalescence of vacancies before yielding lead to the formation of different types of defects, facilitating plastic deformations at yielding. Thus, various plastic deformation mechanisms, like vacancy dislocation loops, twinning, anti-twinning, etc., are observed in tension and compression along different crystallographic orientations. Owing to the non-planar cores, $1/2〈111〉$ screw dislocation is identified as the manipulator behind those plastic deformations. The critical resolved shear stress (CRSSs) on the maximum resolved shear stress plane (MRSSP) for the $1/2\left[111\right]$ screw dislocation loaded in tension and compression are determined and responsible for the origin of tension–compression asymmetry in tungsten.