Grain Refinement and Hall-Petch Strengthening of Magnesium Alloy Via Alloying and Hot Extrusion

In this paper, 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 the mechanical properties of magnesium alloy have been studied. Based on 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 the 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 the as-cast Mg. The obtained refined alloys showed significant enhancement of the 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. The use of the grain refinement process showed that at first the yield ratio did not change considerably, while the 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 using the grain refinement despite obtaining higher yield and tensile strengths.