Effect of extrusion speed on microstructure, mechanical properties and work-hardening of multi-microalloying MgAlCoCrFeNi alloy

In this study, a novel multi-microalloying MgAlCoCrFeNi alloy was extruded at 300 ˚C with different extrusion speeds (0.2, 1, and 2 mm/s). The microstructure evolution, tensile properties and work-hardening behavior of the alloy were studied. Results showed that increasing the extrusion speed could improve the homogeneity of the microstructure, transitioning from a bimodal microstructure (fine recrystallized grains and elongated un-recrystallized grains) at 0.2 and 1 mm/s to a fully recrystallized one at 2 mm/s. The lower extrusion speed could improve the YS of the alloy, which possesses a maximum YS of ~213 MPa at 0.2 mm/s, an increase of ~12% compared to the YS at 2 mm/s, which was mainly attributed to the fine grain strengthening and dislocations (residual dislocations) strengthening. Moreover, the work hardening capacity of the alloy increases with the extrusion speed. As the extrusion speed increases from 0.2 mm/s to 2 mm/s, the work hardening exponent n increased from 0.08 to 0.14, and the work hardening capability Hc increased from 0.31 to 0.52. The analysis results showed that the improvement of work hardening ability was mainly due to the increase of dislocation storage rate caused by grain coarsening.