Chance 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 degrees 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 vim) was attained in Mg-3Bi alloy. Then, a microstructure of gradient grain size (similar to 18 mu m near the center and similar to 8 mu m 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 (similar to 7 mu m). We attribute this change in grain size microstructure homogeneity mainly to the effect of different levels of undissolved constituent Mg3Bi2 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.