Y₂O₃ nanoparticle reinforced H13 tool steel fabricated via electron beam melting: Variant selection, microstructural evolution, and strengthening mechanisms

H13 tool steel is widely applied in the die industry owing to its excellent thermal strength and wear resistance. However, components fabricated via electron beam melting (EBM) still suffer from unsatisfactory strength–ductility balance. In this study, yttrium oxide (Y₂O₃)/H13 composites were fabricated using EBM with 0.2 wt% Y₂O₃ nanoparticles added to optimize the microstructure and mechanical performance. The results showed that Y₂O₃ nanoparticles suppressed prior austenite grain (PAG) coarsening and promoted heterogeneous nucleation, resulting in finer and more equiaxed grains. The composite microstructure changed from an interwoven martensite–bainite structure to a continuous martensitic matrix with discrete bainitic islands, accompanied by higher martensite content. Furthermore, the refinement of PAGs improved variant selection, increasing the proportion of high-angle grain boundaries and improving crack resistance. Consequently, the Y₂O₃/H13 composite exhibited a yield strength of 1463 MPa, an ultimate tensile strength of 1927 MPa, and a total elongation of 11.8 %, outperforming EBM-fabricated H13 steel in both strength and ductility. Quantitative analysis revealed that the strength improvements primarily resulted from grain refinement strengthening provided by finer grains, dislocation strengthening associated with the high dislocation density, and dispersion strengthening introduced by the added Y₂O₃ nanoparticles. These findings emphasize the importance of ceramic particles in tailoring microstructure to overcome the strength–ductility tradeoff, providing a promising avenue for the development of high-performance advanced tool steel.