A synergetic approach to enhancing mechanical properties of trace TiB2 particles reinforced AZ91D composites through α-Mg grain refinement and β-Mg17Al12 manipulation

Cast AZ91D magnesium alloys are plagued by coarse α-Mg grains and continuous β-Mg₁₇Al₁₂ phases that compromise mechanical performance. This work proposes a TiB₂ particle-reinforced AZ91D composite strategy that simultaneously addresses both challenges through interfacial engineering. Introducing trace 0.3 wt.% TiB₂ particles into the AZ91D alloy enables refinement via coupled heterogeneous nucleation and growth restriction effects. The identified crystallographic orientation relationship (${\text{[11}\overline{\text{2}}\text{0]}}_{\text{Mg}}\text{//}{\text{[11}\overline{\text{2}}\text{0]}}_{{\text{TiB}}_{\text{2}}}$, and ${\text{(01}\overline{\text{1}}\text{0)}}_{\text{Mg}}\text{//}{\text{(0001)}}_{{\text{TiB}}_{\text{2}}}$) confirms effective α-Mg nucleation on TiB₂ interfaces, reducing grain size by 64.4% to 68.2 μm. Concurrently, the β-Mg₁₇Al₁₂ phase morphology transitions from a typical continuous network to a dispersed small block or spherical structure through grain boundary pinning effects enabled by the refined microstructure. The yield strength, ultimate tensile strength, and elongation of AZ91D with 0.3 wt.% TiB₂ particles are 129.2 MPa, 247.0 MPa, and 8.5%, respectively, which are increased by 9.1%, 26.3%, and 63.4% compared to the particle-free AZ91D alloy. This interfacial design strategy not only deciphers the particle–matrix interaction mechanisms in Mg-based composites but also provides insight into the development of refiners for cast Mg alloys.