Comparative Study on Mechanical Property and Fracture Behavior of Age-Hardened LM4 Monolithic Composites Reinforced with TiB₂ and Si₃N₄

The study aimed to compare and analyze the mechanical property and fracture behavior of LM4 composites reinforced with TiB₂ (1–3 wt.%) and Si₃N₄ (1–3 wt.%) ceramic powders. A two-stage stir casting process was employed for the effective preparation of monolithic composites. To further enhance the mechanical properties of composites, a precipitation hardening treatment (both single (SSHT) and multistage (MSHT), followed by artificial aging at 100 and 200 °C) was conducted. From mechanical property tests, it was understood that in both the monolithic composites, the properties improved with an increase in wt.% of reinforcements, and composite samples subjected to MSHT + 100 °C aging treatment bested other treatments in terms of hardness and UTS values. Compared to as-cast LM4, there was a 32 and 150% increase in hardness and a 42 and 68% increase in UTS for as-cast and peak-aged (MSHT + 100 °C aging) LM4 + 3 wt.% TiB₂ composites, respectively. Similarly, there was a 28 and 124% increase in hardness and a 34 and 54% increase in UTS for as-cast and peak-aged (MSHT + 100 °C aging) LM4 + 3 wt.% Si₃N₄ composites, respectively. Fracture analysis of the peak-aged composite samples confirmed the mixed mode of fracture in which brittle mode was dominating.