Enhancing tensile strength and ductility of high-speed-extruded Mg–5Bi–2Al through trace Mn addition

The Mg–5Bi–2Al (BA52, wt%) has shown excellent extrudability despite its high alloying content. This work is conducted to enhance the strength and ductility of BA52 extrudate, which is fabricated at a very high speed, through the addition of a trace amount of Mn. The addition of 0.2 wt% Mn to BA52 causes substantial changes in the microstructure of a homogenized billet. These variations include grain refinement, formation of a semi-continuous coarse-particle network along grain boundaries, and formation of fine-particle-depletion regions near the grain boundaries. These microstructural variations provide abundant nucleation sites for recrystallization near the grain boundaries during high-speed extrusion (exit speed: 70 m/min). Consequently, grain refinement of the extrudate occurs from 29.7 to 20.0 μm with Mn addition. The BAM520 extrudate has similar basal fiber texture and internal strain energy with the BA52 extrudate. However, the former exhibits an improved tensile strength (~20 MPa greater) than the latter, which is primarily ascribed to the decreased grain size. This work demonstrates that minimal Mn addition considerably facilitates dynamic recrystallization during hot extrusion, thus improving the strength and elongation of high-speed-extruded BA52.