Improvement of strength and ductility of an EZ magnesium alloy by applying two different ECAP concepts to processable initial states

The commercial magnesium alloy EZ33A was processed by Equal Channel Angular Pressing (ECAP). Two different tools were used: a conventional one with two channels and one intersection (90 degrees), and a so-called double ECAP (D-ECAP) tool with two intersections (90 degrees and 120 degrees). Four passes were conducted at temperatures subsequently lowered from 350 degrees C down to 240 degrees C using route Bc. The as-cast, pre-deformed material and both ECAP conditions were investigated. The microstructure was revealed by means of optical microscopy, scanning and transmission electron microscopy (SEM, TEM), and X-ray diffraction. Mechanical properties were studied by Vickers hardness measurements as well as by compressive and tensile tests. The microstructure investigations show a significant grain refinement as well as fragmentation and redistribution of the hard intermetallic (Mg,Zn)(12) RE phase by ECAP. This effect is even more pronounced in the material processed by D-ECAP where this phase is two times finer compared to the size after conventional ECAP. Additionally, small Zn-Zr particles of a few nanometers size within grains or subgrains were observed. Microstructure changes evoked by the ECAP processing significantly affect the mechanical properties. This is especially the case after 4 passes in the D-ECAP tool where hardness (90 HV2) is almost 70% higher than in the as-cast condition (54 HV2) and the tensile yield strength is enhanced by nearly 300% (82 MPa up to 327 MPa). Hence, the D-ECAP tool is more efficient than the conventional one in improving mechanical properties.