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

• Damage evolution in Mg has been studied with 200 keV electron irradiation at RT. • The occurrence of visible damage required incubation due to low electron flux. • Irradiation defects consisted of basal-plane ⅙< 20 2 ¯ 3 > loops, exclusively. • The [ 1 2 ¯ 10 ] foil orientation favoured loop growth and population increase. • Pre-existing dislocations (10 14 m −2 ) suppressed the build-up of loop population. 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 = [ 1 2 ¯ 10 ], [ 10 1 ¯ 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 1 6 < 20 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 [ 1 2 ¯ 10 ] than in [ 10 1 ¯ 0 ]. One-dimensional loop rafts were developed via elastic interaction. In as-received samples, the presence of pre-existing dislocations (on the order of 10 14 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.