Yield Stress Reversibility and the Operation of Frank-Read Sources in Li₂ Alloys in the Anomalous Regime for (111) Slip

Abstract

The yield stress τ_y at small strains (≈ 0.01%) is strain rate independent, and has the same anomalous temperature dependence as that of the 0.2% strain. τ_y is considered to be the stress at which Frank-Read sources operate in a virgin crystal. For successful operation, τ_y must exceed the stress τ_s at which screws propagate dynamically through the crystal, and the source dislocation must pass rapidly through the unstable Frank - Read configuration. This can be achieved by the bowing edge dislocation overcoming local obstacles before reaching that configuration. Loops elongated along the screw direction are expected to be formed in the microstrain region. Under certain conditions such loops are unstable on unloading, thereby generating long edge dislocations which can operate successfully as sources at low but not at high temperatures, explaining the reversibility phenomenon.