Low-dose damage evolution in pure magnesium under electron irradiation: Effect of foil orientation and pre-existing dislocations

Abstract

Low-dose damage evolution in commercial purity (99.95 wt%) magnesium has been investigated, under 200 keV electron irradiation at room temperature, up to ∼0.03 displacements per atom. Quantitative defect production and evolution statistics were obtained for two types of prism foils (z = [$\text{1}\overline{\text{2}}\text{10}$], [$\text{10}\overline{\text{1}}\text{0}$]), in samples of as-received and heat-treated (400 °C/1 h) condition. An incubation period was found to produce visible damage in all samples, in correspondence with about ⅓ of the maximum dose. The damage microstructure consisted of basal-plane $\frac{1}{6}$<$20\overline{2}3$> loops, exclusively; while no voids were observed throughout the course of irradiations. Steady-state accumulation of dislocation loops was found at doses beyond the incubation limit in heat-treated samples. Higher loop number density and large loop average size were confirmed in [$\text{1}\overline{\text{2}}\text{10}$] than in [$\text{10}\overline{\text{1}}\text{0}$]. One-dimensional loop rafts were developed via elastic interaction. In as-received samples, the presence of pre-existing dislocations (on the order of 10¹⁴ m⁻²) gave rise to suppressed build-up of loop population. Saturation of loop growth was confirmed, when loop average size reached ∼20 nm. Underlying mechanisms of foil orientation effect and pre-existing dislocation effect upon microstructure development are discussed. The paper concludes with a brief comparison between electron irradiation and fission neutron irradiation in magnesium, aiming to bring new insights upon displacement damage studies in materials with hexagonal close-packed structure.