Evolution of microstructure, texture, and mechanical properties in a multi-directionally forged ZK60 Mg alloy

An extruded ZK60 magnesium alloy with the nominal composition of Mg-6Zn-0.5Zr was processed by multi directional forging (MDF) for 2, 4, 6 and 8 passes at 473 K. Microstructural studies showed that the uniform microstructure of the as-extruded material transforms to a duplex structure consisting of fine recrystallized grains surrounding some patches of unrecrystallized grains after 2 and 4 passes of MDF. Further MDF passes, however, resulted in a homogeneous equiaxed structure in such a way that the as-extruded grain size of 12.7 mu m was refined to 1.9 mu m after 8 passes. Evaluation of crystallographic texture revealed that deformation by MDF changed the conventional fiber texture of the extruded condition to a new texture, in which the basal planes tend to align almost 45 degrees to the transverse direction. Mechanical properties of the extruded and MDFed samples were studied by shear punch testing (SPT) at room temperature. The results indicated that the 182.6 MPa shear yield stress (SYS) and 184.9 MPa ultimate shear strength (USS) of the extruded condition were, respectively, reduced to 170.0 and 172.3 MPa after 2 passes of MDF. This drop in strength is attributed to the presence of coarse patches of unrecrystallized grains. Further pressing of the alloy was accompanied by a decrease in the volume fraction of coarse grains and an increase in the number density of fine grains. Textural softening that occurred at higher strain levels of the final MDF passes, however, offset the strengthening effect of these fine grains.