High-temperature fatigue of cemented carbides under cyclic loads

Cemented carbides are mainly used as cutting tool materials. In technical application, they are subject to wear and to complex thermomechanical loadings. In the past the authors have shown that these materials exhibit a strong lifetime-limiting fatigue sensitivity at room temperature. Recent investigations focussed on the high-temperature behaviour of cemented carbides under different loading conditions and included studies of fatigue mechanisms in the microstructure. The investigated materials are a WC-6wt% Co hard metal and a Ti(C,N)-based cermet. For mechanical tests under static, monotonically increasing, and cyclic loads at different temperatures between 25 degrees C and 900 degrees C in air, special new apparatus were built. The characterization of the microstructure in the volume and on fracture surfaces were carried out by TEM, SEM and SAM. The results show strong fatigue effects in cemented carbides from 25 degrees C up to 900 degrees C. TEM investigations show temperature-dependent fatigue mechanisms: At low temperatures cobalt binder ligaments are subjected to a phase transformation from fee to hcp structure. At higher temperatures, oxidation of the binder ligaments near crack tips and brittle-ductile transitions of mixed carbides seem to play an important role in the fatigue behaviour. (C) 1999 Elsevier Science Ltd. All rights reserved.