Ideal Shear Strengths of fcc Aluminum and Copper

The ideal shear strength is the minimum stress needed to plastically deform an infinite dislocation-free crystal and is an upper bound to the strength of a real crystal. We calculate the ideal shear strengths of Al and Cu at zero temperature using pseudopotential density functional theory within the local density approximation. These calculations allow for structural relaxation of all five strain components other than the imposed shear strain and result in strengths on ${111}$ planes of 1.85 and 2.65 GPa for Al and Cu, respectively ( $8%$-- $9%$ of the shear moduli). In both Al and Cu, the structural relaxations reduce the ideal shear strengths by $35%$ to $45%$, but the directions of relaxation strain in each are qualitatively different.