Microstructure and hardness inhomogeneity of fine-grained AM60 magnesium alloy subjected to cyclic expansion extrusion (CEE)

In the present paper, the effect of cyclic expansion extrusion (CEE) process on the microstructure inhomogeneity and microhardness distribution of AM60 magnesium alloy is investigated. The results demonstrated that after the first pass of CEE at 300 °C, ultrafine grains nucleated along initial grain boundaries which caused a bimodal structure. The area fraction of the newly formed grains and Mg17Al12 dissolution was higher in the outer region rather than the central areas which is related to the different contribution of shear and normal strains. By increasing the number of passes to three, the level of inhomogeneity decreased and the average grain size of samples reduced from ∼185 μm to ∼6 μm. After the second pass of CEE, an excellent combination of ultimate strength (∼367 MPa) and elongation to fracture (∼20%) was improved with the initial values of 261 MPa and 13.8%, respectively. To show the variations of hardness, Vickers microhardness measurements were performed over the cross-section. After the first pass, heterogeneous microhardness distribution was seen. However, the microhardness inhomogeneity decreased by increasing the number of passes. Harness results were in good agreement with the microstructure results.