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A cost-effective, high-performance Mg-2.0Zn-1.5Mn alloy was fabricated using closed forging extrusion (CFE). The results reveal that CFE can promote a dynamic recrystallization (DRX) ratio and enhance the residual dislocation density in deformed grains and subgrains. Grain refinement is achieved through multiple DRX mechanisms, particle-stimulated nucleation, and pinning effects. In addition, the basal texture and precipitation behavior are significantly influenced by pre-forging in CFE. Finally, grain refinement and substantial residual dislocations result in a good combination of strength and plasticity, with the ultimate tensile strength of 400 MPa and elongation of 22.1%, respectively. A novel strategy for regulating the bimodal grain structure is proposed to guide the optimization of the mechanical performance of rare-earth free Mg alloys.

Compared to conventional ceramic particle-reinforced Mg composites, incorporating Ti particles into the Mg molten melt poses greater difficulty due to their significantly higher density compared to Mg. To develop a feasible and effective method for incorporating Ti particles, the water flow visualization experiment was used to reveal the optimum parameters of the stirring velocity, the height of the stirrer, and the angle of the stirrer to the horizon. Based on the results, the Ti/AZ81 composite was successfully fabricated at a variable speed (100–950 rpm/min) and height (4–10 cm) with an angle of 75° via nearly semi-solid stirring casting and followed by hot extrusion. The distribution of Ti particles in the Mg matrix was more homogenous, exhibiting fewer flaws and a higher recovery rate. The tensile results indicated that the Ti/AZ81 composite exhibited excellent comprehensive mechanical properties with an ultimate strength (UTS), yield strength (YS), and elongation of 374.5 MPa, 263.5 MPa, and 11.5%, respectively, which were superior to other Ti particle-reinforced AZ alloys. The characterizations revealed that the interfacial product AlTi phase was additionally formed and the Ti/Mg interface showed a strong bonding strength. The dynamic recrystallization (DRX) was significantly promoted due to the particle-stimulated nucleation (PSN) and Ti particles weakened the precipitation of Mg17Al12. Based on the quantitative calculations, the increased strength was mainly attributed to grain refinement, the mismatch coefficient of thermal expansion (CTE), and the heterogeneous microstructures of Ti/AZ81 composite, in which heterogeneous deformation-induced (HDI) strengthening is dominant.

In the present study, pre-deformation at liquid nitrogen temperature (77 K) was applied to introduce twins into Ti–Mo alloys with different contents of molybdenum. The microstructural evolution and mechanical properties with different cryogenic pre-deformation amounts (from 0 to 9 %) were characterized and analyzed. The results show that the twinning activities and the evolution of twinning systems are mainly influenced by β phase stability and cryogenic pre-deformation amount. After cryogenic pre-deformation, the primary and secondary twinning of {332}twins are observed in Ti–12Mo and Ti–15Mo alloys but absent in Ti–20Mo alloy. With the cryogenic pre-deformation amounts increasing, the width and quantity of twins increase constantly. Moreover, {112}twinning is activated in Ti–15Mo alloy pre-deformed by 6 % and 9 %. Twins and high-density dislocations induced by cryogenic pre-deformation cause a significant enhancement in strength of all these three alloys. However, the effect of cryogenic pre-deformation on the elongation of alloys is complicated. For Ti–12Mo alloys, the yield strength increases monotonically (from 527 to 778 MPa) with the increase of cryogenic pre-deformation reduction, while the elongation is completely opposite (from 47 to 20 %). For Ti–15Mo alloys, the optimal matching of strength and ductility is achieved when the amount of cryogenic pre-deformation is 6 %, with a tensile strength of 815 MPa and an elongation of 24 %. Compared with the other two alloys, Ti–20Mo alloy gets a smaller increase in strength and a slight decrease in elongation with the amount of cryogenic pre-deformation increasing.

The recrystallization behavior of extruded Mg–Zn–Mn alloys was systematically investigated under different aging treatment prior to extrusion. Electron backscattered diffraction analysis was used to explore the recrystallization mechanism and corresponding effects on the basal texture of alloys during extrusion. The results show that the primary coarse particles can effectively promote recrystallization nucleation. Moreover, the precipitation of nanoscale particles during the aging process play a role in pinning the grain boundaries. Therefore, after peak-aged treatment (16 h), the extruded Mg alloy obtains uniform and fine microstructure, showing good mechanical properties: the tensile yield strength, tensile strength, elongation, compressive yield strength, and tension-compression yield asymmetry were 298 MPa, 363 MPa, 26%, 260 MPa, and 0.87, respectively.