Dislocations in aluminium under stress observed by lang x-ray topography

Abstract

Dislocation densities and configurations as a function of stress were studied at elevated temperatures in two single crystals of zone refined aluminium using Lang X-ray topography. The initial dislocation densities ranged between (1 and 3.5) × 10 3 lines/cm 2. The topographs, exposed while the crystals were under stress, revealed no sources, but these are assumed to be at or near the crystal surfaces. A critical resolved shear stress for dislocation multiplication σ 0 = 0.8 × 10 −6 G, G being the shear modulus, was observed, corresponding to 2 g/mm 2 at room temperature. The dislocation density N increased with stress σ in units of G as Δ(σ/G)/ΔN 1 2 ~ b 1.3 , b being the Burgers vector. Neither the rate of work hardening nor σ 0 showed any systematic variation with temperature for temperatures between 160°C and 420°C. The dislocation movements were impeded mainly by pinning. However, the observed work hardening is attributed to properties of the surface sources. For stresses σ c ~ 1.8 × 10 −6 G the dislocations broke away from the pinning points located approximately 0.3 mm apart. The surface Frank-Read source described by Fisher can account for several features of the observations.