Possible origin of the discrepancy in Peierls stresses of fcc metals: First-principles simulations of dislocation mobility in aluminum

Dislocation motion governs the strength and ductility of metals, and the Peierls stress (σ_p) quantifies dislocation mobility. σ_p measurements carry substantial uncertainty in face-centered cubic (fcc) metals, and σ_p values can differ by up to two orders of magnitude. We perform first-principles simulations based on orbital-free density functional theory (OFDFT) to calculate the most accurate currently possible σ_p for the motion of 12 111 dislocations in fcc Al. We predict the σ_ps of screw and edge dislocations (dissociated in their equilibrium state) to be 1.9×10^-4G and 4.9×10^-5G, respectively (G is the shear modulus). These values fall within the range of measurements from mechanical deformation tests (10 ^-4-10^-5G). OFDFT also finds a new metastable structure for a screw dislocation not seen in earlier simulations, in which a dislocation core on the glide plane does not dissociate into partials. The corresponding σ_p for this undissociated dislocation is predicted to be 1.1×10^-2G, which agrees with typical Bordoni peak measurements (10^-2-10^-3G). The calculated σ_ps for dissociated and undissociated screw dislocations differ by two orders of magnitude. The presence of undissociated, as well as dissociated, screw dislocations may resolve the decades-long mystery in fcc metals regarding the two orders of magnitude discrepancy in σ_p measurements.