Interface-dominated toughening mechanism in yttrium-modified Cr–Mo–V steel: Beyond grain refinement and inclusion modification

Enhancing the toughness of high-strength Cr–Mo–V tool steels without sacrificing hardness remains a persistent challenge. This study elucidates the toughening mechanism of rare earth yttrium in Cr–Mo–V tool steel. Despite having a coarser prior austenite grain size (102.1 μm vs 15.3 μm), the yttrium-containing steel (140Y) exhibited a 48.26 % higher impact energy than the yttrium-free one (0Y), deviating from the Hall-Petch relation. Microstructural analyses reveal that Y transforms Al–O inclusions into Y–Al–O complexes and suppresses continuous Cr-rich carbide precipitation along grain boundaries. Y segregation at defects reduces local tensile stress and carbon accumulation, enhancing boundary cohesion and lattice stability. Consequently, the fracture mode shifts from intergranular to quasi-cleavage with abundant ductile dimples. These results demonstrate that Y serves as a boundary-active element that strengthens interfaces and stabilizes lattice defects, establishing an interface-dominated toughening mechanism for developing high-strength steels with superior toughness and reliability.