On the microstructural origin of yield point phenomenon and high work-hardening response in fine-grained Mg-3Gd alloy

Mechanical properties of Mg-3Gd (wt.%) samples with average grain sizes ranging from 3 to 45 μm were characterized by room temperature tensile test. A reversal of the trade-off, simultaneously high yield strength and large tensile elongation, was observed in the fine-grained samples. The microstructures and hardening response were analyzed in terms of the viewpoint of strain evolution, including local strain evolution by tensile digital image correlation strain measurement, and lattice strain by using synchrotron-based in-situ high energy X-ray diffraction technique. The dislocation-based deformation mechanisms were investigated to underpin the microstructural origin of the yield point phenomenon and enhancement in work-hardening. The occurrence of the yield point phenomenon represented by a yield drop and propagation of the Lüders band is related to the absence of mobile dislocations at an early stage and to the slip transmission between the adjacent grain. The extraordinary work-hardening enhancement over an extended range can be ascribed mainly to the increases in dislocation multiplication and accumulation capabilities by the activation and interaction of multiple slip systems including <a> and <c+a> types. These results contribute to the design of strong and ductile Mg alloys.