Shear instabilities in perfect bcc crystals during simulated tensile tests

This work demonstrates a simple but efficient way as to how to determine the existence of shear instabilities in ideal bcc crystals under uniaxial loading. The theoretical tensile strengths are derived from calculated values of the theoretical shear strength and their dependence on the superimposed normal stress. The presented procedure enables us to avoid complicated and time-consuming analyses of elastic stability of crystals. Results of first-principles simulations of coupled shear and tensile deformations for the two most frequent slip systems ({110} < 111 > and {112} < 111 >) in six ideal cubic crystals are used to evaluate the uniaxial tensile strengths in three low-index crystallographic directions (< 100 >, < 110 >, and < 111 >) by assuming a shear instability in the weakest shear system. While instabilities occurring under < 100 > tension are mostly related to the shear in the {112} plane, those occurring during loading in the other two directions are associated with {110} planes. The results are consistent with those predicted by available elastic analyses. The weakest tendency to fail by shear is predicted for uniaxial tension along < 100 >. This is consistent with the occurrence of {100} cleavage planes in bcc metals. DOI: 10.1103/PhysRevB.87.014117