Al₂O₃ nanoparticle reinforced heterogeneous CrCoNi-matrix composites with improved strength-ductility synergy

Novel Al₂O₃ nanoparticle-reinforced heterogeneous CrCoNi-matrix composites (CrCoNi–Al₂O_3P) with 10–30 wt% of coarse grain (CG) matrix were prepared by multi-step mechanical milling plus hot pressing sintering. The relationships between microstructure and mechanical properties of the composites were studied in detail. The results reveal that the heterogeneous structure in CrCoNi–Al₂O_3P composites is composed of CG region and ultrafine grain (UFG) region with uniform distribution of Al₂O₃ nanoparticles. With increasing content of CG matrix in the nanocomposite, the hardness first decreases but then increases, and the increase of hardness may be due to the formation of carbides. Compared with homogeneous CrCoNi–Al₂O_3P composite, the compressive fracture strain of heterogenous composite increases by 53.2% at most, while sacrificing a small amount of strength, among which the 20 wt% CG heterogeneous composite exhibits an excellent strength-ductility synergy. For the mechanisms of strengthening and toughening, hetero-deformation induced (HDI) hardening makes the heterogeneous composites almost as strong as the homogeneous composite. Meanwhile, the CrCoNi matrix with low stacking fault energy can be deformed by dislocation slipping and twinning at room temperature, which improves the plasticity of the composite. Furthermore, the heterogeneous structure with soft and hard domains increases the work hardening ability of the composite.