High-temperature deformation behaviour of coarse-grained and fine-grained magnesium sheets: Insights from processing maps and constitutive modelling

Abstract

In recent years, magnesium (Mg) sheets have emerged as a prominent structural material in the transportation and aerospace industries, where they are exposed to significant mechanical and thermal stresses. To enhance their performance, it is critical to define optimal workability zones across different temperatures and strain rates, particularly for highly deformed Mg sheets. This study investigates high-temperature deformation behavior of coarse-grained (CG) and fine-grained (FG) AZ31 Mg alloy sheets produced through novel hot rolling (HR) process with 40 % thickness reduction per pass. The HR process reduced the grain size in FG materials to 8 μm, compared to an initial grain size of 160 μm in CG materials. The deformation characteristics were evaluated through a constitutive model that includes flow stress, deformation activation energy, and processing maps over a range of strain rates (0.001–10 s⁻¹) and temperatures (250 °C-450 °C). Processing maps identified both stable and instable zones during high-temperature tensile deformation, with dynamic recovery (DRV) governing stable regions of CG, while dislocation climb drives FG Mg sheet. The instability was marked by twinning, stress localization, and cracking for both materials. The FG Mg sheet exhibited a lower average activation energy (144 kJ/mol) than the CG material (156 kJ/mol). A comprehensive microstructural analysis using EBSD, SEM, and TEM provided visual validation of deformation mechanisms predicted by the constitutive model and processing maps.