Temperature effect on ideal shear strength of Al and Cu

According to Frenkel's estimation, at critical shear stress tau(c) = G/2p, where G is the shear modulus, plastic deformation or fracture is initiated even in defect-free materials. In the past few decades it was realized that, if material strength is probed at the nanometer scale, it can be close to the theoretical limit, tau(c). The weakening effect of the free surface and other factors has been discussed in the literature, but the effect of temperature on the ideal strength of metals has not been addressed thus far. In the present study, we perform molecular dynamics simulations to estimate the temperature effect on the ideal shear strength of two fcc metals, Al and Cu. Shear parallel to the close-packed (111) plane along the [11 (2) over bar] direction is studied at temperatures up to 800 K using embedded atom method potentials. At room temperature, the ideal shear strength of Al (Cu) is reduced by 25% (22%) compared to its value at 0 K. For both metals, the shear modulus, G, and the critical shear stress at which the stacking fault is formed, tau(c), decrease almost linearly with increasing temperature. The ratio G/tau(c) linearly increases with increasing temperature, meaning that tau(c), decreases with temperature faster than G. Critical shear strain, gamma(c), also decreases with temperature, but in a nonlinear fashion. The combination of parameters, G gamma(c)/tau(c), introduced by Ogata et al. as a generalization of Frenkel's formula, was found to be almost independent of temperature. We also discuss the simulation cell size effect and compare our results with the results of ab initio calculations and experimental data.