Effect of precipitation hardening treatment on hardness and tensile behaviour of stir cast LM4 hybrid composites through TEM and fractography analysis

LM4 hybrid composites reinforced with TiB2 and Si3N4 are fabricated using a two-stage stir casting method and are subjected to precipitation hardening treatment. The efficacy of TiB2, Si3N4, and precipitation hardening treatment on LM4 was investigated. When compared to as-cast LM4, all hybrid composites had displayed better hardness and UTS values. This increase in mechanical properties of hybrid composites may be attributed to the presence of hard reinforcement particles, appropriate stirring parameters, which resulted in uniform reinforcement distribution free of porosity and agglomeration. It was observed that in as-cast condition hybrid composites with a higher wt.% of TiB2 had displayed improved hardness (12.5% higher) and UTS (4.4% higher) values than composites with a higher wt.% of Si3N4, because Si3N4 particles are 3.8 times larger than TiB2 particles. Precipitation hardening treatment enhanced the mechanical properties of hybrid composites, with multistage solutionizing (MSHT) and artificial aging at 100 °C producing the best results. Peak aged (MSHT + aging at 100 °C) hybrid composites exhibited 160, 178, and 202% improvement in hardness and 46, 34, and 30% improvement in UTS when compared to as-cast LM4. This increase in age hardened specimens properties is due to the formation of metastable phases, which were validated using XRD and TEM. Fracture analysis of these as-cast and peak aged hybrid composites revealed a mixed mode of fracture-mostly brittle type, because of the presence of reinforcements, intermetallic phases, and strong interface bonding between matrix and reinforcements.