The effect of strain rate on the microstructural evolution of pure Mg during ECAE

Abstract

Samples of 99.9% pure, as-cast magnesium (Mg) were processed by Equal Channel Angular Extrusion (ECAE) using up to four passes of the BC route, a temperature of 250 °C, and three different extrusion rates of 0.127, 0.381, and 0.762 mm/s. The microstructures and textures of the resulting samples were characterized by Electron Backscatter Diffraction (EBSD) as a function of extrusion pass and rate. The average grain size was reduced significantly to approximately 30 μm after the second pass when extruded at all three rates, but then it varied depending on extrusion rate after the third and fourth passes. The two slower rates yielded a 30–40% increase in grain size after the third and fourth passes, while the fastest extrusion rate of 0.762 mm/s maintained a relatively consistent grain size after the third and fourth passes. The presence of twins in the resulting microstructures also varied with extrusion rate, with the slowest and fastest rates producing a greater volume fraction of twins than the intermediate rate. The larger fraction of twins is attributed to the microstructure maintaining favorable orientations for twinning at the lower rate and to the need for greater strain accommodation at the higher rate.