Microscopic mechanisms of low-temperature yield stress anomaly in solids

Abstract

The effect of temperature ( T=0.5–300 K ), stress rate ( 10 −4–10 4 MPa s −1 ) and shear stress ( τ) on the mean path of mobile dislocations ( l), slip bands ( L), macroscopic strain, and relaxation stress have been studied in pure (P) and high purity (HP) single crystals: LiF, NaCl, KBr, KJ, RbJ. The temperature-dependencies of the starting stresses for dislocation motion, multiplication and the yield stress are common for various crystals. They increase with decreasing temperature and drastically drop at T< T 0. It is the deformation stress that governs T 0; T 0 usually increases with crystal softening and decreases with the rise of crystal hardening. It is governed by the competing mechanisms of dislocation bowing around obstacles, climb, and dislocation double cross-slip (DDCS). DDCS is initiated by local stresses at the interfaces of matrix-impurity micro-precipitates due to their thermal expansion mismatch. The stress (strain) rate controls the contribution of thermal DDCS, climb, or athermic bowing to the temperature dependence of crystal hardening or softening and irrefutably contradicts the quantum and inertial mechanisms in the low-temperature anomaly of plastic flow in solids. The data confirm the extremely low estimates of the Peierls–Nabarro stress in various crystals: τ P <(10 −8–10 −6) G , where G is the shear modulus.