Development of AA2024/TiB₂ + Y₂O₃ Hybrid Composites with Improved Mechanical Properties Through Stir Rheocasting

Semisolid cast AA2024 matrix composites reinforced with 2.0 wt% titanium boride (TiB₂) and varying amounts of yttria (Y₂O₃) (0.5 wt%, 1.0 wt%, 1.5 wt%) were developed using stir rheocasting. The metallurgical characterisation was done by X-ray diffraction, optical microscopy, and scanning electron microscope, and mechanical characterisation by Vickers's hardness testing machine and universal testing machine. The microstructure evolution and mechanical properties such as hardness, tensile strength, and fracture toughness (K_Q) of the developed composites were correlated. Pure AA2024 and AA2024 reinforced with TiB₂ samples were also fabricated as a benchmark to compare the effect of yttria. The highest hardness, ultimate tensile strength, and yield strength achieved were 122 HV, 310 MPa, and 230 MPa, respectively, for the AA2024/2 wt% TiB₂ + 1.5 wt% Y₂O₃ composite. They are 25%, 45%, and 57% higher compared to the unreinforced sample. The maximum conditional fracture toughness (K_Q) achieved was 25.56 MPa m^1/2 for the unreinforced specimen and 19.77 MPa m^1/2 for AA2024/2 wt% TiB₂ + 1.5 wt% Y₂O₃ composite. With the addition of Y₂O₃ particles in the matrix, the porosity in the samples increased, resulting in lower elongation. The processing route enhanced the mechanical properties of the composite in comparison with the liquid cast matrix due to a uniform distribution of particles and grain refinement.