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

Abstract

Dislocation motion governs the strength and ductility of metals, and the Peierls stress (${\ensuremath{\sigma}}_{p}$) quantifies dislocation mobility. ${\ensuremath{\sigma}}_{p}$ measurements carry substantial uncertainty in face-centered cubic (fcc) metals, and ${\ensuremath{\sigma}}_{p}$ 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 ${\ensuremath{\sigma}}_{p}$ for the motion of $\frac{1}{2}\ensuremath{\langle}110\ensuremath{\rangle}\left\{111\right\}$ dislocations in fcc Al. We predict the ${\ensuremath{\sigma}}_{p}$s of screw and edge dislocations (dissociated in their equilibrium state) to be $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}G$ and $4.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}G$, respectively ($G$ is the shear modulus). These values fall within the range of measurements from mechanical deformation tests (10${}^{\ensuremath{-}4}$--10${}^{\ensuremath{-}5}G$). 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 ${\ensuremath{\sigma}}_{p}$ for this undissociated dislocation is predicted to be $1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}G$, which agrees with typical Bordoni peak measurements (10${}^{\ensuremath{-}2}$--10${}^{\ensuremath{-}3}G$). The calculated ${\ensuremath{\sigma}}_{p}$s 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 ${\ensuremath{\sigma}}_{p}$ measurements.