Effect of hard-plate rolling and annealing treatment on the microstructure and mechanical properties of NbB₂ particle-reinforced AZ91 composite

In this study, a NbB₂/AZ91 composite exhibiting desirable mechanical properties was fabricated using a sample casting technique, followed by hard-plate rolling and short-term annealing. The effect of NbB₂ particles on the microstructural evolution of the AZ91 alloy was investigated. The presence of NbB₂ was shown to have a grain-refining effect on the AZ91 alloy and promoted dynamic recrystallization (DRX) and precipitation of fine Mg₁₇Al₁₂ phases via particle-stimulated nucleation (PSN). Tensile testing revealed substantial enhancements in the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) of the as-rolled AZ91 alloy, with values of 379 MPa, 292 MPa, and 14.7%, respectively, owing to the incorporation of NbB₂ particles. Annealing led to further enhancements in EL with slight reductions in UTS and YS (360 MPa, 252 MPa, and 16.8%, respectively). Owing to grain refinement and the PSN effect of the NbB₂ particles, a significant number of geometrically necessary dislocations (GNDs) were induced in the matrix during the rolling process, which reduces the nucleation barrier and increases the number of nucleation sites for the recrystallized grains and Mg₁₇Al₁₂ precipitates. Meanwhile, many residual dislocations and fine Mg₁₇Al₁₂ precipitates in the as-rolled alloys were annihilated during annealing, resulting in slight grain growth and coarsening. The strengthening mechanism of the NbB₂/AZ91 composite are mainly associated with grain-refinement strengthening, particle-induced dislocation strengthening, strengthening resulting from mismatching coefficients of thermal expansion (CTE), and hetero-deformation-induced (HDI) strengthening. Textural weakening, increased activation of non-basal slip systems, more-uniform strain patterns resulting from NbB₂ particles, and precipitation are mainly responsible for enhancing ductility.