Microstructure and mechanical characterization of in situ synthesized AA6061/(TiB₂+Al₂O₃) hybrid aluminum matrix composites

TiB₂ and Al₂O₃ particulates reinforced AA6061 aluminum matrix composites (AMCs) were synthesized by in-situ reaction of titanium (Ti) and boric acid (H₃BO₃) powders with molten aluminum. AMCs were fabricated using an electric stir casting furnace under a controlled environment. Heat flow curves of differential thermal analysis (DTA) showed that the synthesis temperature for the formation of TiB₂ and Al₂O₃ using Al-Ti-H₃BO₃ reaction system was 950 °C. The in-situ synthesized composites were characterized using XRD, FESEM, TEM and EBSD. XRD results revealed the formation of TiB₂ and Al₂O₃ particulates in the composite. FESEM micrographs revealed a homogenous distribution of both the particulates with good interfacial bonding. EBSD maps showed that the in-situ formed TiB₂ and Al₂O₃ particulates refined the grains of the aluminum matrix from 103 μm at 0 wt% to 14 μm at 15 wt%. Al₂O₃ particles exhibited spherical shape while TiB₂ particles displayed hexagonal and cubic shapes. The formation of ultrafine and nano scale thermodynamically stable TiB₂ and Al₂O₃ particles enhanced the microhardness and the tensile strength of the AMCs. The microhardness and the tensile strength were respectively 122 HV and 287 MPa at 15 wt%.