Enhanced superplasticity achieved by disclination-dislocation reactions in a fine-grained low-alloyed magnesium system

Superplastic forming, as an advanced manufacturing method, is especially important for producing complex-shaped components for Mg-alloys. Superplasticity can be achieved through grain boundary sliding at elevated temperatures in high-alloyed Mg systems, with a typical grain size < 10 mu m stabilized by high densities of intermetallic precipitates. However, it is difficult to stabilize small grains and facilitate grain boundary sliding in low-alloyed systems due to insufficient precipitates. Here, by utilizing a distinctive design strategy, i.e. introducing solute segregation to enhance stability of the fine-grained structure, we obtained an Mg-1Zn-0.2Ca-0.2Zr-0.1Ag (wt.%) alloy achieving similar to 450% superplastic strain. Through quasi-in-situ Electron Backscatter Diffraction analyses, we investigated systematically microstructure evolutions during superplastic deformation of the alloy at different strains. It has been found that superplasticity is achieved through the absorption of intragranular dislocations by grain boundaries, which are associated with stressdriven grain boundary migration mediated by disclination-dislocation reactions. By using a theoretical description based on stress-driven grain boundary migration and disclinationdislocation reactions, our model can capture multiple superplasticity mechanisms (e.g., grain boundary sliding and grain rotation), which establishes a fundamental relationship between the evolution of microstructural defects and the strain accommodation during superplastic deformation. Our results not only suggest an effective way to achieve superplasticity in low-alloyed Mg systems, but also provide a new insight to elucidate the nature of superplasticity mechanism by revealing the intrinsic correlation between stress-induced grain boundary migration and disclination/dislocation motion.