Influence of Torsion on Precipitation and Hardening Effects during Aging of an Extruded AZ91 Alloy

The effect of torsional deformation by various routes prior to artificial aging treatment on the microstructure of extruded AZ91 rods was investigated. The tension and compression properties along the extruded direction were tested to evaluate the hardening effect. The results showed that torsional deformation can largely enhance the subsequent age-hardening effect. A detailed microstructural characterization revealed that torsional deformation produced a high density of dislocations and {10-12} twins, which can promote continuous precipitation and increase the number density of the Mg₁₇Al₁₂ phase. In addition, part of dislocations and the {10-12} twins were retained after aging treatment. The combination of the increased precipitation hardening by Mg₁₇Al₁₂, dislocation strengthening and refinement strengthening contributed to the high hardening effect. Moreover, the hardening effect can be further increased by tailoring the torsion route. The torsion route exhibited little influence on the age-hardening effect, while it remarkably affected the texture of the twisted AZ91 rods. Reciprocating torsion had little influence on the texture. In contrast, unidirectional torsion caused the c-axis of the texture to rotate toward the extrusion direction, resulting in a texture softening effect. Therefore, combining reciprocating torsion and aging caused a higher hardening effect than combining unidirectional torsion and aging. The detailed strengthening mechanism was analyzed and is discussed.