The mechanism of pre-twinning on enhancing strength of AZ31 magnesium alloy

A strong basal texture is typically developed in magnesium alloy sheets that have been subjected to the rolling process. Consequently, their mechanical properties and formability are significantly impaired, which in turn restricts potential applications. In this study, an innovative texture-altering technique, which involves in-plane free compression and width-constrained rolling (FCWR), is used for AZ31 alloy sheets in order to manufacture magnesium alloy sheets with both high strength and high ductility. During FCWR deformation process, a substantial number of tensile twins and a small quantity of (1012) - (0112) twin-twin boundaries are induced to coordinate plastic deformation. It is precisely the presence of such crossed twins that impedes the detwinning of partial twins under low-stress conditions. The retained twins hinder the motion of dislocation, thereby enhancing the strength. Specifically, the yield strength of preset crossed twins sample along rolling direction, 45° and transverse direction has increased by 105%, 16.8% and 23%, respectively. Additionally, the ultimate tensile strength along these three directions has increased by 42.7%, 25.5% and 34.8%, respectively. The twin boundaries in FCWR sample consist of steps, which correspond to basal - prismatic (BP/PB) boundaries that connect straight terraces which are parallel to theoretical {1012} twin boundaries. Furthermore, as the number of processing passes increases, the step features become more pronounced. Compared with the as-received sample, the YS enhancement in the sample prepared via the second pass of the FCWR process is attributed to two primary mechanisms: grain refinement strengthening contributes 61 MPa, while dislocation strengthening accounts for 90 MPa.