Synergistic effects of initial tensile twins with various volume fraction and strain on plasticity enhancement in ZM21 Mg alloy during screw twist extrusion

Abstract

To address the poor room-temperature plasticity of Mg alloys, this study proposes a method called twist strain-induced twin orientation regulation (TITOR). Firstly, compressive strain is applied along the extrusion direction (ED) to generate preset {10–12} tensile twins, and then torsional deformation is introduced by screw twist extrusion (STE) to regulate the initial twin orientation. The results show that the TITOR process is dominated by continuous dynamic recrystallization (CDRX). Among them, STE will weaken the basal texture effectively. Under the continuous torsional stress, the c-axis of grains will deviate 30° in the transverse direction (TD), thereby refining the grains and promoting the movement of the basal slip obviously. By introducing controllable torsional deformation to conduct secondary regulation on the twin orientation at a large angle, the problem of poor low-temperature plasticity of Mg alloys is improved. As the increase of volume of pre-twinned, the mechanical properties and orientation regulation of Mg alloys have been improved significantly. Especially, the ultimate tensile strength (UTS) of 3% preset twins after 8 passes sample increased from 295.3 MPa to 375.5 MPa, while the fracture elongation (FE) improved from 17.23 % to 31.84 %, which increased by 27.2 % and 84.9% respectively.

Keywords

ZM21 Mg alloy / pre-twinned / twist strain induce twin orientation regulation / orientation regulation

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