Change of microstructure homogeneity with increasing Bi addition in extruded Mg-Bi alloys

Resultant microstructures of Mg-xBi (x = 3, 6 and 9 wt. %) extruded at 380 °C are studied to reveal the effect of increasing Bi content and possible dynamic recrystallization mechanisms. It was observed that a homogenous grain size microstructure (an average grain size of ~ 30 µm) was attained in Mg-3Bi alloy. Then, a microstructure of gradient grain size (~ 18 µm near the center and ~ 8 µm near the surface, of the extruded bar) was seen in the Mg-6Bi sample. With further increasing Bi, the microstructure becomes homogenized again but with a much finer grain size (~ 7 µm). We attribute this change in grain size microstructure homogeneity mainly to the effect of different levels of undissolved constituent Mg 3 Bi 2 phase in Mg-xBi before extrusion and possibly also the effect of plastic strain along the radial direction of the extruded bar based on finite element modelling results. These two factors, which we propose, impact the particle-stimulated-nucleation-driven dynamic recrystallization mechanisms and finally lead to the observed change of microstructure homogeneity.