Review on ultrafined/nanostructured magnesium alloys produced through severe plastic deformation: microstructures

A review on the microstructural evolution in magnesium alloys during severe plastic deformation was presented. The challenges deserved to achieve ultrafine/ nanostructured magnesium were discussed. The characteristics of the processed materials are influenced by three main factors, including i) difficult processing at low temperatures, ii) high temperature processing and the occurrence of dynamic recrystallization and grain growth processes, and iii) a combined effect of grain refinement and crystallographic texture changes. Reviewing the published results indicate that there are two potential difficulties with severe deformation of magnesium alloys. First, it is very hard to achieve homogeneous ultrafined microstructure with initial coarse grains. The second is the dependency of microstructure development on the initial grain size and on the imposed strain level. It was clarified that different grain refining mechanisms may be contributed along the course of multi-pass severe deformation. It was clarified that discontinuous recrystallization takes places during the first stages of deformation, whereas continuous refinement of the recrystallized grain may be realized at consecutive passes. Shear band formation as well as twinning were demonstrated to play a significant role in grain refinement of magnesium alloy. Also, the higher the processing temperature employed the more homogeneous microstructure may be achieved with higher share of low angle grain boundaries.