Ab Initio Study of Ideal Shear Strength

Ideal strength, which can be defined as the stress necessary to induce permanent deformation in a material without prior imperfections, is one of the important materials characterizations. In this study we calculate the ideal pure shear and simple shear strengths of fcc (Al, Cu, Ni, Ag) and bcc (Fe, Mo, W) metals in their common slip systems using density functional theory. We find the critical shear strains (CSS) of bcc metals (∼0.18) do not depend sensitively on the material, and they are higher than fcc metals except Al. In contrast, the CSS of fcc metals spread over a wide range (0.13∼0.2), with Al having extremely high CSS (0.2). As a result, although Al has smaller moduli than Cu in {111}〈112̄〉 shear, its ideal pure shear and simple shear strengths are higher than Cu. By comparative analyses of the generalized stacking fault energy and valence charge (re)distributions in Al, Ag and Cu, we conclude that the abnormally large CSS, ideal shear strength and intrinsic stacking fault energy in Al are all related to directional bonding. Cu and Ag do not have strong directional bonding. Generally, bcc metals have stronger directional bonding than the fcc metals except Al. By turning off spin polarization in our calculations, we find magnetization is a main source of bond directionality in Ni and Fe.