Microstructure evolution and mechanical properties of Mg–Mn–RE alloy processed by equal channel angular pressing

Abstract

Equal Channel Angular Pressing (ECAP) has been used to successfully produce ultrafine-grained ME21 alloy with a good strength-ductility synergy. The microstructure of the as-cast ME21 alloy and its evolution during the ECAP processing were characterized using a combination of optical microscopy (OM) and scanning electron microscopy (SEM). Fracture surfaces after tensile test were also characterized using SEM. Chemical analysis and phase identification were done by energy dispersive spectroscopy (EDS) and X-ray diffraction analysis, respectively. The combination of these techniques enabled the interpretation of the mechanical behavior during the various processing stages of the alloys. The ECAP process was performed up to 16 passes, producing a uniform ultra-fine grain (1 µm) ME21 alloy resulting from full dynamic recrystallization. At room temperature, the yield strength after 16 ECAP passes (191.4 MPa) was 251% higher than that of the as-cast ME21 alloy, accompanied by a good elongation of 8.1%. The hardness increased to 60.7HV which is about 52% higher than that of the as-cast alloy. These improvement in mechanical properties is due to the ultra-fine grains with high angle grain boundaries (HAGBs) and the precipitate strengthening effects of the uniformly dispersed fine precipitates of Mg12Ce and Mn formed as a result of the intense shear during the ECAP process.