Low-temperature high-speed extrusion of a multicomponent Mg–Bi base alloy and its heat resistance during hot tension

In this work, the microstructure and room/high-temperature (RT/HT) mechanical properties of Mg-1.0Bi-1.0Mn-1.0Al-0.5Ca-0.3Zn (BMAXZ11110, wt%) alloy, subjected to extrusion at low temperature of 300 °C and high ram extrusion speed of 9 mm/s, have been systematically investigated. This alloy exhibits excellent microstructural thermo-stability during high-speed extrusion without obvious grain coarsening, especially for the edge of alloy bar. Profuse micro/nano-scale second phases (Mg2Bi2Ca, Al8Mn5 and α-Mn) and the co-segregation of Ca and Zn atoms at grain boundaries (GBs) /residual dislocations can be observed. The co-effects of nano-scale phases and solute segregation facilitate the formation of bimodal grain structure in the BMAXZ11110 alloy. At RT, it possesses high tensile yield strength (TYS, ∼ 327.1 MPa), and GB strengthening is the most primary mechanism. At 250 °C, this alloy still can maintain relatively high TYS (∼ 187.8 MPa), indicating that it also exhibits high heat resistance. Even though the contribution of GB strengthening on the TYS decreases at 250 °C, solute segregation strengthening increment makes up for its reduction to a large extent. Hence, this study will be helpful for developing a new generation of high-strength high-speed extruded Mg-Bi base alloys accompanied with good heat resistance.