Zener pinning-induced suppression of dynamic recrystallization in CaO-added AM30 magnesium alloy under hot torsion

초록

This study examines the effect of CaO addition on the high-temperature torsional deformation behavior of the wrought AM30 magnesium alloy. CaO introduced during melting partially reduces in the Mg-rich melt, releasing Ca that preferentially reacts with Al to form the thermodynamically stable Laves phase Al2Ca. Electron probe micro-analyzer (EPMA) mapping confirmed that Al2Ca forms predominantly along grain boundaries, with an areal fraction of similar to 4-5% and a boundary coverage of similar to 20%, providing a quantitative basis for its role in deformation. Torsion tests conducted at 300-500 degrees C and strain rates of 0.01-1 s(-1) showed that AM30-0.5CaO exhibits consistently higher flow stresses and lower fracture strains compared with base AM30. Constitutive analysis revealed hyperbolic-sine behavior for both alloys, with the apparent activation energy increasing from similar to 181 kJ mol(-1) (AM30) to similar to 190 kJ mol(-1) (AM30-0.5CaO), indicating reduced grain-boundary mobility. The higher flow stresses in AM30-0.5CaO are attributed to Zener pinning by Al2Ca and to the immobilization of grain boundaries, which decreases the effective area available for migration. These effects collectively suppress dynamic recrystallization (DRX), resulting in delayed flow softening and higher steady-state stresses. The experimental trends align with theoretical predictions derived from the measured Al2Ca fraction and morphology. Overall, the results establish that CaO-induced Al2Ca formation governs both strengthening and DRX kinetics in AM30, highlighting the trade-off between improved melt processability and reduced high-temperature ductility in CaO-modified Mg alloys.

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