Abstract: High performance is of great importance to expand the application of magnesium alloys, and the inherent strength-plasticity synergic mechanism during a specific process should be unveiled. In this paper, a multi-degrees of freedom (multi-DOF) forming process is conducted on initially extruded AZ91 magnesium alloy at different deformation degrees, including small deformation with deformation amounts of 10% and 20%, medium deformation with deformation amounts of 30% and 40%, and large deformation with deformation amounts of 60% and 70%. Simultaneous enhancement of ultimate tensile strength (UTS) and plasticity is achieved in all these multi-DOF processed alloys in comparison to the initially extruded one. As deformation degrees increase, both UTS and elongation of the multi-DOF processed alloy gradually increase in small and medium deformation and then slightly decrease in large deformation, exhibiting a superior strength (401 MPa) and plasticity (16.3%) combination at deformation amount of 40%. The evolution of mechanical properties varying with deformation degrees is closely dependent on microstructure and texture characterization. The microstructures of multi-DOF processed AZ91 alloy are increasingly refined and heterogeneous as deformation degrees gradually increase, which consist of the predominant equiaxed coarse grains (CGs) and a few fine grains (FGs) in small deformation, some CGs (equiaxed or slightly elongated) and some FGs in medium deformation, and some remarkably elongated CGs and the predominant FGs in large deformation. The area fraction of basal texture gradually decreases while that of prismatic texture gradually increases with increasing deformation degrees, finally resulting in a complete disappearance of basal texture at a deformation amount of 70%. Thus, the strength-plasticity synergic mechanism related to increasingly obvious heterogeneous structure, gradually refined microstructure, and gradually decreased basal texture contribute to the constantly simultaneous improvement of UTS and plasticity until in medium deformation, and the remarkably elongated CGs play a significant role in the slight decrease of UTS and plasticity in large deformation even with further increasing grain refinement and decreasing basal texture. This research provides an efficient and novel way to achieve strength-plasticity synergic magnesium alloy via optimizing microstructure and texture.

Key words: AZ91 magnesium alloy, Multi-degrees of freedom forming, Strength-plasticity synergic mechanism, Microstructure, Texture