On the fatigue crack growth behavior of WC–Co cemented carbides: kinetics description, microstructural effects and fatigue sensitivity

Abstract

The influence of microstructure and load ratio (R) on the fatigue crack growth (FCG) characteristics of WC–Co cemented carbides are studied. In doing so, five hardmetal grades with different combinations of binder content and carbide grain size are investigated. Attempting to rationalize microstructural effects, key two-phase parameters, i.e. binder thickness and carbide contiguity, are used. On the other hand, the effect of load ratio is evaluated from the FCG behavior measured under R values of 0.1, 0.4 and 0.7. Experimental results indicate that: (1) WC–Co cemented carbides are markedly sensitive to fatigue; and (2) their FCG rates exhibit an extremely large dependence on Kmax. Furthermore, both fatigue sensitivity and relative prevalence of Kmax over ΔK, as the controlling fatigue mechanics parameter, are found to be significantly dependent upon microstructure. As mean binder free path increases, predominance of static over cyclic failure modes diminishes and a transition from a ceramic-like FCG behavior to a metallic-like one occurs (conversely in relation to contiguity). Consequently, the trade-off between fracture toughness and FCG resistance becomes more pronounced with increasing binder content and carbide grain size. The observed behavior is attributed to the effective low ductility of the constrained binder and its compromising role as the toughening and fatigue-susceptible agent in hardmetals, the latter on the basis that cyclic loading degrades or inhibits toughening mechanisms operative under monotonic loading, i.e. crack bridging and constrained plastic stretching.