Effect of YB4 Reinforcement on the Microstructural Evolution and Mechanical Behaviour of AISI 420 Composites Produced by Vacuum Induction Melting

The influence of YB4 particle addition on the microstructure and the associated thermal and mechanical properties of AISI 420 stainless steel composites fabricated using the vacuum induction melting technique was investigated. Microstructural analysis using scanning electron microscopy (SEM) confirmed the presence of YB4 particles within the BCC-structured martensitic matrix and also along the grain boundaries across all weight fractions. In addition, YB4 addition resulted in a pronounced refinement of the martensitic matrix, as evidenced by a progressive reduction in the size of the packets, i.e., a group of martensitic laths/plates sharing the same habit plane variants with the parent austenite grain. The presence of YB4 particles induced internal stresses and microstrains, leading to peak shifting and broadening of the X-ray diffraction (XRD) peaks corresponding to that of the martensitic matrix phase. The coefficient of thermal expansion (CTE) decreased significantly from 13.4 × 10−6 K−1 for monolithic AISI 420 to 8.06 × 10−6 K−1 for the AISI 420/4 wt.% YB4 composite and was attributed to the excellent dimensional stability of YB4 particles. The maximum hardness (913.12 HV) and tensile strength (930 MPa) were achieved for the AISI 420/4 wt.% YB4 composite. Fractographic analysis using SEM indicated a transition from ductile to brittle fracture with increasing YB4 content, suggesting a reduction in strain-hardening capacity. The contributions of various strengthening mechanisms were quantified using the summation of strengthening and modified Clyne models, revealing that strengthening due to load bearing is dominant across all composites. Insights gained from these results are important to strategize the design of boride-based metal-matrix composites with enhanced strength–ductility synergy for structural applications.