Micromechanisms of grain refinement during extrusion of Mg–0.3 at.% Al at low homologous temperature

Coarse grained Mg–0.3 at.% Al (0.33 in wt.%) alloy was processed by direct extrusion with a reduction ratio of 25:1 at a temperature of ~ 433 K. The extrusion remainder was removed from the die and analysed in three distinct zones: the cast billet, the conical zone of extrusion die, and the as-extruded rod. The zones were characterized by electron backscatter diffraction (EBSD) and light microscopy techniques to identify the processes responsible for grain refinement. Complex networks of {10–12} twins in practically all grains produced a noticeable microstructural fragmentation even before the material reached the conical zone of the die. Deformation twinning extended up to the entrance zone of the conical die where it was followed by a continuous dynamic recrystallization (CDRX) that gradually changed low angle boundaries to high angle boundaries. It is apparent that geometrically necessary dislocations play a crucial role in the formation of new grain boundaries. CDRX results in a bimodal structure with grain diameters ~ 3 and ~ 30 μm. As a material flows through the conical zone, the ratio of large to small grains is progressively decreased by CDRX in favour of fine grains. The as-extruded microstructure (a rod 8 mm in diameter), with an average grain diameter of ~ 2.1 μm, shows a strong texture where the vast majority of grains (99.99%) have the c-axis oriented at least 30° from the extrusion direction.