Effect of aluminium and calcium on the microstructure, texture, plastic deformation and related acoustic emission of extruded magnesium-manganese alloys

Alloying modifications with Ca and Al of a binary Mg-1 wt.% Mn have been used to investigate the microstructure development during extrusion and the relation between these microstructures and the mechanical properties. Acoustic emission (AE) measurements were used to analyse dislocation or twin dominated deformation in differently textured fractions of the microstructure during tensile and compression testing. Ca appears to have a texture modifying effect and leads to the formation of unusual texture components with a tilt of basal planes out of the extrusion direction. This texture development has previously been described for rare earth containing magnesium alloys after extrusion. Al as an alloying element does not lead to the formation of such a component. Both elements have in common, that the alignment of basal plane normals perpendicular to the extrusion direction is reduced, having a substantial effect on the mechanical properties. Partially recrystallised microstructures obtained in Mg-1 wt.% Mn-0.3 wt.% Ca and Mg-1 wt.% Mn-1 wt.% Al after slow extrusion exhibit bimodal grain size distributions. Only in the case of Mg-1 wt.% Mn-8 wt.% Al a fully recrystallised microstructure is found, indicating differences in the grain nucleation rate during recrystallisation. In this alloy, the weakest texture and, correspondingly, the lowest asymmetry in mechanical properties was also observed.