Microstructure and thermal stability of ultra fine grained Mg-based alloys prepared by high-pressure torsion

The microstructure of ultra fine grained (UFG) Mg and the Mg–9.33 wt.% Gd (Mg10Gd) alloy and its development with temperature were studied in this work. The UFG specimens were prepared by high-pressure torsion (HPT) and investigated using positron lifetime spectroscopy combined with transmission electron microscopy, X-ray diffraction and microhardness measurements. It was found that pure Mg has undergone a substantial recrystallization already in the course of HPT treatment which resulted in a binomial type microstructure. On the other hand, a homogenous UFG structure with a high density of dislocations was formed in HPT-treated Mg10Gd. Investigations of the development of the microstructure with increasing temperature revealed that recovery of dislocations takes place at similar temperatures in both specimens. In the pure Mg it is accompanied by grain growth, while the Mg10Gd exhibits grain growth only at significantly higher temperatures. Moreover, the precipitation sequence in HPT-treated Mg10Gd differs significantly from that in the coarse-grained alloy.