Dislocation velocity enhancement under electron irradiation in magnesium

The softening of materials subjected to irradiation in reactors is known for a long time but still difficult to interpret. One reason for such a situation is that post-irradiation microstructural observations mainly show different types of irradiation defects which are potential sources of hardening, not softening. Irradiation softening must accordingly be studied in situ, as a function of temperature, irradiation energy and flux. Simple sources of irradiating particles can be used, such as ions or electrons, but in situ experiments remain difficult to perform, and to combine with clear microstructural observations. For those reasons, and in order to measure irradiation effects at the scale of individual dislocations, a simple procedure has been used: straining magnesium micro-samples in a transmission electron microscope working between 80kV (the irradiation threshold of magnesium) and 180kV. The in situ observations reveal a huge effect of the electron beam corresponding to a 10000 times enhancement of the velocity of screw dislocations in the prismatic planes under a 160kV electron beam, and the dependence of this enhancement factor with the accelerating voltage. These results are interpreted in terms of something equivalent to a temperature increase of 100K or more, but clearly different from a true heating process.