Influence of bimodal microstructure on strength and ductility of as-extruded Mg-Gd-Y-Zr alloy

This paper studied the role of layered bimodal structure in the strength-ductility balance of Mg-8Gd-4Y-0.4Zr (wt %) alloys under as-extruded conditions. The various bimodal microstructure, consisting of fine dynamically recrystallized grains and elongated un-dynamically recrystallized grains along extrusion direction, were controlled by different extrusion ratios. The mechanical properties were then discussed in detail. The local strain evolution was investigated in the alloys based on in situ testing and analyzed with electron backscattered diffraction and digital image correlation. The fracture behavior of the alloys was characterized by 3-Dimensional X-Ray Microscopy. The alloys with an extrusion ratio of 7.5 exhibited high tensile yield strength of 338 MPa and ductility of 13.2%, resulting from grain boundary strengthening of fine DRXed grains, and dislocation strengthening of elongated un-DRXed grains and Mg5RE particles. The strain evolution analysis showed that the slips and rotations of DRXed grains contributed to relieving strain localization at the interface. Additionally, the layered structure assisted local strain dispersion and uniformed strain distribution, improving the strength and ductility of alloys. The microcracks primarily nucleated and propagated at the un-DRXed regions. The DRXed regions can refine microcracks and enhance deformation compatibility.