Effects of Solutionizing and Aging Alteration on Tensile Behavior of Stir Cast LM4-Si3N4 Composites

The main concern of this research is to identify the effect of multistage solutionizing and artificial aging behaviour on tensile behavior of LM4 + Si3N4 (1, 2, and 3 wt.%) composites. A two-stage stir casting method was employed to produce composites, which minimized the necessity for a lengthy and high-temperature preheating treatment of reinforcement and resulted in homogeneous reinforcement distribution. Cast composites were subjected to single-stage and multistage solutionizing heat treatment (SSHT and MSHT) followed by aging at 100 and 200°C. Peak hardness of the LM4 and cast composites was noted during artificial aging. With the increase in wt.% of reinforcement, the hardness of the composites increased. Cast composites subjected to MSHT and aging at 100°C displayed maximum hardness when matched to other combinations. Compared to as-cast LM4 hardness (70 VHN), L3SN (with MSHT + aged at 100°C) composite attained 124% higher hardness (157 VHN). UTS values followed a similar trend, compared to as-cast LM4 UTS (149 MPa), L3SN (with MSHT + aged at 100°C) composite attained 54% higher UTS (230 MPa). Major reasons for the improvement in mechanical properties of heat-treated composites are due to the existence of hard Si3N4 particles and the formation of θ'-Al2Cu and θ"-Al3Cu (metastable) phases. From the fracture surface analysis of LM4 and L3SN composite, it was concluded that the type of fracture experienced by LM4 is of ductile nature and that of the composite is of mixed nature.