Optimizing Mg-4.5Tm-0.7Zr alloy performance through coarse-fine grain synergy via extrusion temperature control

The study investigates the effects of different extrusion temperatures on the microstructure and mechanical properties of Mg-4.5Tm-0.7Zr alloy. Among the tested extrusion temperatures, the microstructure of the alloy extruded at 380 °C was the most favorable, characterized by continuous fine-grain bands. At this temperature, the alloy achieved optimal mechanical properties, with an elongation of 49.7% while maintaining high strength. To explore the deformation mechanisms under different strain stages, quasi-in situ dynamic tensile tests were conducted. During the early stages of deformation, stress was primarily concentrated in the fine-grain bands, leading to the activation of limited tensile twinning, elongation of the fine grains, and a rapid increase in dislocation density, accompanied by the activation of non-basal slip systems. In the intermediate stages of deformation, a balance between dynamic recrystallization and grain growth was achieved, resulting in a reduction in dislocation density and a shift of stress to the coarse-grain regions. At this point, non-basal slip in the fine grains began to extend to basal slip in the coarse grains. In the later stages of deformation, coarse grains became the primary contributors to strain, with increased dislocation density and basal slip becoming the dominant deformation mechanisms. Additionally, grain boundary sliding within the fine-grain bands served as a complementary deformation mechanism, further enhancing plasticity. The coordinated deformation between the coarse and fine grains significantly improved the overall performance of the alloy.