4.7 Article

Twinning in rolled AZ31B magnesium alloy under free-end torsion

Publisher

ELSEVIER SCIENCE SA
DOI: 10.1016/j.msea.2020.140405

Keywords

Magnesium alloy; Torsion; Swift effect; Strain hardening; Twinning

Funding

  1. U.S. National Science Foundation [CMMI-1762312]

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The study experimentally characterized the mechanical response and microstructure evolution of a rolled AZ31B magnesium alloy using companion thin-walled tubular specimens under free-end monotonic torsion. It was found that the shear stress-shear strain response exhibits a sigmodal shape with four distinctive stages of strain hardening, involving basal slips, tension twinning, and compressing twinning. The formation of twin-twin boundaries (TTBs) and the collective hardening effects by twin boundary (TB) and TTB contribute to a unique rise in strain hardening rate in certain stages. Further observations of different types of twins and their interactions provide insights into the deformation mechanisms during different stages of the process.
The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stresss-hear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains; all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain.

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