Comparative Study on the Microstructural Features and Mechanical Properties of an Mg-Zn Alloy Processed by Squeeze Casting and ECAP

Abstract

In the present work, Mg-4Zn alloys were produced by the squeeze casting method and subsequently processed by Equal Channel Angular Pressing (ECAP) to enhance their mechanical properties. The ECAP setup incorporated a constant channel diameter of 10 mm, a channel angle (ϕ) of 90°, and a corner angle (Ψ) of 20°. Microstructural evolution was studied using optical microscopy (OM), scanning electron microscopy (SEM), and x-ray diffraction (XRD). The ɑ-Mg and precipitated secondary Mg_x-Zn_y phases were observed in the as-cast condition; however, only the ɑ-Mg phase was observed in the ECAPed condition. ECAPed processing led to a refinement of the crystallite size from 38.28 nm in the as cast condition to 36.22 nm. Compared to the as cast state, the ECAPed specimens exhibited higher average hardness in both the transverse and longitudinal directions. An increase in hardness levels was attributed to grain refinement generated by deformation. The maximum load deformation for specimen lengths of 50 and 72 mm was estimated at 33 and 34 kN, respectively, using upper bound theory; however, during ECAP, these values reached 43.60 and 59.60 kN. The higher coefficient of friction, increased dislocation density, and material trapped between the punch and die clearance were the primary reasons for the higher ECAP load values. To assess the contribution of grain boundary strengthening, a semi-quantitative analysis based on the Hall-Petch relationship was carried out. The average grain size decreased from 136 μm in the as cast condition to 19.65 μm after ECAP, resulting an increase in from 0.0857 to 0.225 μm^−1/2. The evaluated increase in yield strength was 38 MPa. Grain refinement strengthening might have been slightly influenced by texture evolution and the recovery effect during ECAP at room temperature.