Microstructural effects on the fatigue crack growth behavior of γ-phase containing WC-Co cemented carbides: Mechanics, mechanisms and fatigue sensitivity

The partial substitution of tungsten carbide by cubic refractory ones (¿-phase) represents an accessibility-driven approach for the microstructural design of hard ceramic-metal composites, offering an alternative to WC-Co hardmetals by reducing dependence on tungsten as a critical raw material. However, successful implementation of this approach requires knowledge and deeper understanding of microstructural effects on mechanical integrity, beyond simple hardness – indentation fracture toughness correlations, for these ¿-phase containing- WC-Co cemented carbides. In this study, a systematic and detailed investigation addressing the influence of ¿-phase carbides – as third phase – on crack growth resistance of WC-Co hardmetals, under monotonic and cyclic loading, is conducted. Materials studied include two ¿-phase containing grades with submicron and fine grain sizes, as well as two reference WC-Co systems with matching microstructural features. Fatigue crack growth behavior and fracture toughness are assessed by testing through-thickness pre-cracked specimens. The mechanical study is complemented by an extensive characterization of cracking paths and fractographic features. Independent of microstructural assemblage, crack propagation under variable loading is found to be dominated by static failure modes rather than pure cyclic ones. Meanwhile, quantitative analysis of crack-microstructure interactions reveals an increased frequency of transgranular cracking through the ¿-phase carbides in both submicron- and fine-grained grades. This is more pronounced in the former, significantly reducing the relative prominence of binder-related crack paths. Hence, despite exhibiting higher crack growth rates, the submicron-grained three-phase cemented carbide is found to have a lower fatigue sensitivity relative to its reference counterpart. This behavior reflects a microstructure-dependent trade-off and should be interpreted within a tailored application framework. However, such behavior is not kept as microstructure gets coarser, because this yields higher and lower proportions of ductile binder fracture and transgranular crack paths within ¿-phase carbides, respectively. Nanoindentation measurements revealed significant differences in hardness and modulus between WC, ¿-phase and binder regions, further validating the observed failure micromechanisms. The experimental findings and their corresponding analysis underscore the critical influence of microstructural assemblage — particularly the contiguity and distribution of the ¿-phase carbides — in controlling fracture and fatigue behavior in multielement cemented carbide systems.