Effect of reinforcement on the fracture surface of precipitation-hardened LM4 hybrid composites

The objective of this study was to compare the hardness, tensile strength, and fracture surface analysis of LM4, LM4+ (1, 2, and 3 wt.%) TiB2, LM4+ (1, 2, and 3 wt.%) Si3N4, and LM4+ (1, 1.5, and 2 wt.%) TiB2+ (1, 1.5, and 2 wt.%) Si3N4 hybrid composites. Composites were cast using a two-stage stir casting method, and as-cast samples were subjected to precipitation hardening treatment, including single-stage and multi-stage solutionizing, followed by artificial aging at 100 and 200 °C. When the as-cast and precipitation heat-treated samples were compared, the precipitation heat-treated samples of both the alloy and composites had greater hardness and tensile strength values than those of the as-cast samples. Scanning electron microscopy images were used to analyze and discuss the failure mechanisms related to the tensile strength of the samples. Crack propagation was also studied to understand failure mechanisms better. Because of the presence of coarser dimples, LM4 exhibited ductile failure, whereas monolithic and hybrid composites failed in a mixed manner, with both ductile and brittle failures prevailing equally. The precipitation heat-treated samples (both LM4 and composites) had a mixed failure mode, with dominant brittle failure.