Effect of bimodal size Ti particulates on microstructure and mechanical properties of AZ31B magnesium matrix composites

This study investigates the reinforcement of AZ31B magnesium alloy with bimodal (micron- and nano-sized) Ti particles. The influence of Ti content (0, 3, 6, and 9 vol%) on solidification behavior, ECAP-induced microstructural evolution, dynamic recrystallization (DRX), twinning, texture development, and resulting mechanical and electrochemical properties was examined. In the as-cast state, the addition of Ti particles transforms the β-phase from a continuous network into discrete particles and weakens the basal texture, a result of heterogeneous nucleation and solute redistribution at the solidification front. Following ECAP, all composites exhibit extensive DRX; however, the homogeneity of grain refinement is strongly composition-dependent. The 6 vol% Ti composite displays the finest, most uniform equiaxed grains, accompanied by the weakest basal texture and highest orientation randomization. Notably, the 6 vol% Ti composite features a distinct banded region of ultra-fine DRX grains. This microstructure arises from a synergistic mechanism: micron-sized particles induce localized high strain gradients and particle-stimulated nucleation (PSN), while nano-sized particles provide robust Zener pinning to stabilize the refined grains. Zener parameter analysis confirms that grain boundary pinning is predominantly governed by the nano-sized fraction, whereas micron-sized particles facilitate DRX activation and twinning. Crucially, the efficacy of these mechanisms hinges on particle dispersion, which is optimized in the 6 vol% Ti composite. Consequently, this composition achieves a superior strength–ductility balance and enhanced corrosion resistance, characterized by reduced current density and uniform degradation. Overall,the 6 vol% Ti addition offers the optimal trade-off among grain refinement, texture weakening, and multi-functional property enhancement in the AZ31B alloy.