Novel Mg-Bi-Mn wrought alloys: The effects of extrusion temperature and Mn addition on their microstructures and mechanical properties

Designing and developing the Mg alloys with low cost and high performance is of the great significance. Novel Mg-1Bi-xMn (x = 0, 1 and 2 wt.%) extruded alloys, in this work, were fabricated at different extrusion temperatures (220, 250 and 300 ℃). The effects of extrusion temperature and Mn addition on the microstructures and mechanical properties of extruded alloys at room temperature were investigated. The results showed that decreasing the extrusion temperature could refine the average grain size, weaken the basal fiber texture intensity and improve the microstructural homogeneity of extruded alloys. When the Mn element was added to the Mg-1Bi alloy, the average grain size further reduced. Simultaneously, the number fraction of low angle grain boundaries (LAGBs) increased, along with the occurrence of regions without dynamic recrystallization (unDRX). The combined effects of grain refinement and coarse unDRXed structure made the textures of the extruded Mg-1Bi-xMn alloys never obviously change. Besides few large size un-dissolved second phases, fine Mg₃Bi₂ and α-Mn phases were precipitated in the extruded Mg-1Bi-xMn alloys and partial nano-scale α-Mn particles pined at grain boundaries (GBs) to effectively impede the migration of GBs for grain refinement. Microstructural variations determined the extruded Mg-1Bi-2Mn alloy to exhibit the highest yield strength of ~ 319.2 MPa with the appropriate elongation-to-failure of ~ 13% at the extrusion temperature of 220 ℃, and they enabled the extruded Mg-1Bi-1Mn alloy to show the highest elongation-to-failure of ~ 26% without the obvious loss of yield strength of ~ 252.1 MPa.