High dose electron irradiation damage in MgAl2O4 spinel

Samples of MgAl2O4 spinel single crystal have been irradiated with high energy electrons in a high-voltage electron microscope at temperatures in the range 900 - 1130K. The structural damage accumulates in three stages, beginning with dislocation loop nucleation and growth, followed by precipitation of metallic magnesium on the external surfaces, and gross shrinkage of the material, manifest as orthogonal microcracks, and finally metal precipitation within the bulk, coupled with nucleation and growth of gas bubbles, material dilation, and healing of the microcracks. A mechanism is proposed whereby irradiation generated lattice defects can segregate, even in the absence of biased point defect sinks. The rate of segregation is proportional to the second derivative of the damage rate profile and the square of the ballistic range of the recoiling particles. The temperature dependence of metal or gas bubble precipitation can be substantially below that for thermally activated migration of the respective point defects. (author)