Grain refinement and Hall-Petch strengthening of magnesium alloy via alloying and hot extrusion

The effects of the addition of Al, Zn and Mn along with the application of the hot extrusion process on the microstructural refinement and enhancement of mechanical properties of magnesium alloy were studied. Based on the Mg-2Al alloy, it was found that the addition of 0.5 wt% Zn to form Mg-2Al-0.5Zn alloy or 0.5 wt% Mn to form Mg-2Al-0.5Mn alloy is the effective way for grain refinement of α-Mg in the as-cast state. Moreover, further remarkable refinement of grain size can be achieved by the extrusion process in such a way that the average grain size of the extruded Mg-2Al-0.5Mn alloy was determined to be 1/165 that of as-cast Mg. The obtained refined alloys showed significant enhancement of yield stress and tensile strength, where the former was successfully related to the average grain size by the Hall-Petch relationship with the slope of ~ 309 MPa/µm0.5. By grain refinement, firstly the yield ratio did not change considerably while tensile strength, the work-hardening exponent, and the uniform elongation increased. However, after a transition grain size (~ 32 µm), the yield ratio increased sharply due to the large increase in the yield stress, and hence, it was not possible to further enhance the uniform elongation by grain refinement despite obtaining higher yield and tensile strengths.