On the formation and evolution dynamics of electron irradiation defects in magnesium using high-voltage electron microscopy

Utilizing a high-voltage electron microscope (HVEM), we conducted 1.25 MeV electron irradiations (RT, up to 0.54 dpa) and performed in-situ observations of defect formation and evolution dynamics in commercial-purity magnesium. The irradiation induced both interstitial and vacancy-type loops. For the prism-foil (z = [1$\bar{2}$$\bar{2}$10]), preferential nucleation and growth of $⅙$$⅙$<$20\bar{2}3$$20\bar{2}3$> loops were observed. Conversely, for the basal-foil (z = [0001]), $⅓$$⅓$<$11\bar{2}0$$11\bar{2}0$> loops were favoured. Prism-foil exhibited a higher yield of loop number density (1.3×10²¹ m^-3) and larger mean loop size (69.4 nm), exceeding those in basal-foil by factors of 5.9 and 2.3, respectively. Meanwhile, the loop population reached steady state after 90 minutes of irradiation, regardless of foil orientation. The irradiation induced defects in turn promoted diverse dynamic events, viz. spontaneous line-to-helix transitions of pre-existing dislocations; growth, shrinkage, and habit plane rotation of dislocation loops; and loop coalescence and splitting reactions. Defect evolution kinetics depended on the incident electron energy: at 1.25 MeV, irradiation accelerated loop population toward steady state and promoted a loop growth dominated microstructure; whereas at 300 keV, the microstructure exhibited loop nucleation dominated behaviour, as informed from literature. Natural ageing resulted in significant growth of interstitial-type $⅙$$⅙$<$20\bar{2}3$$20\bar{2}3$> loops at a rate of ~2.5×10^-4 nm/s. Given the potential application of Mg/Mg alloys in brachytherapy and space, this finding necessitates evaluating their long-term material behaviour under the synergistic effects of defect production and evolution (driven by electron irradiation) and thermal recovery.