Production of ultrafine-grain microstructure in Mg alloy by alternate biaxial reverse corrugation

Magnesium alloys usually have poor formability at ambient temperatures due to a lack of sufficient number of slip systems associated with hexagonal close-packed crystal structure. Twinning is often found to accommodate plastic deformation in addition to slip in Mg alloys. Grain refinement of Mg alloys can significantly alter these processes. In the present work, severe plastic deformation is imparted to Mg alloy plate AZ31B using a process of alternate biaxial reverse corrugation followed by flattening. By progressively decreasing the deformation temperature from 250 to 170 °C during repeated deformation, an initial bimodal grain structure is progressively subdivided by slip and twinning. Concurrent dynamic recovery and some amount of recrystallization lead to the formation of nearly uniform ultrafine microstructure of average size 1.4 μm at ε = 5.0. Twinning deformation causes grain refinement in the early stages, but as the grain size becomes finer twinning is increasingly inhibited even though the deformation temperature is lowered. Deformation twins disappear when the average grain size reaches about 2–3 μm. Texture evolution during processing is seen to be affected by the amount of strain and instantaneous grain size. A strong basal texture is formed after several flattening processes, but its intensity drops again as the grain size becomes finer.