Development of high-performance Mg-Zn-Ca-Mn alloy sheets via the formation of optimized submicron precipitates

To develop room-temperature (RT) formable and strong Mg sheet alloys for automotive sheet applications, we investigated the effects of Zn content and annealing temperature on tensile properties, RT-formability, and microstructures of Mg-(0.4–2.3)Zn-0.5Ca-1Mn (mass%) alloy sheets. Annealing at a low temperature of 300 °C resulted in a dispersion of fine Ca₂Mg₆Zn₃ precipitates in the alloys with the Zn content larger than 0.8%, contributing to an enhancement of both RT-formability and tensile properties. In particular, the 1.5%Zn-containing alloy sheet annealed at 300 °C exhibited an excellent balance of 0.2% proof stress (0.2%PS) and Index Erichsen (I.E.) value. The I.E. value reached 8.2 mm, and the 0.2%PSs for rolling and transverse directions (0.2%PS_RD and 0.2%PS_TD) were 206 MPa and 161 MPa, respectively. The alloy sheet also showed an improved anisotropy of the 0.2%PSs (0.2%PS_TD/0.2%PS_RD = 0.78), which was better than that of recently designed Mg-Zn-Ca-/Mg-Al-Ca-Zn-based alloy sheets with good RT-formability and tensile properties. When the Zn content was 0.4% and annealing was conducted at 300 °C, the sheet also exhibited high 0.2%PSs, while the I.E. value was 6.1 mm by facilitating crack formation from Mg₂Ca precipitates. Although annealing at a high temperature of 400 °C resulted in a high activity of {101̅2} tension twinning during plastic deformation in the 1.5%Zn-containing alloy sheet, the I.E. value was not improved, and the 0.2%PSs decreased owing to the reduction of precipitation strengthening effect.