EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

WC-Co hard metals are used in various fields, e.g. for wear protection or for metal, wood, or stone machining tools. In these applications, hard metals are exposed to different stress and temperature conditions that promote the formation of fatigue cracks and thus material failure. In this work, cyclic uniaxial tests were performed to investigate the shape changes of stress-strain hysteresis loops during the tests. Experiments were performed in a stress-controlled mode under a stress ratio R = $\sigma_{min}/\sigma_{max}$ of minus one at 700 °C and 800 °C in vacuum. For both temperatures the tests were performed at three different stress amplitudes $\sigma_{a}$ = 750, 1000 and 1500 MPa. The investigated specimens were made of a WC-Co hard metal with 10 wt. % Co-binder and an average WC grain size of 1.8 µm. The results show that the area and strain evolution of the stress-strain hysteresis loop increases with increasing number of load cycles for certain stress amplitudes and temperatures. Additionally, an increasing tensile-compression-strain asymmetry was observed with increasing number of load cycles. Changes in the microstructure, such as defect formation with increasing number of load cycles, are discussed using scanning electron microscopy on argon ion polished surfaces. The microstructure of one undeformed and two cyclically tested specimens up to $\sigma_{a}$ = 1000 MPa at 800 °C was analysed. The two cyclically tested specimens were subjected to up to 20 and 80 load cycles, respectively, without fracture. The results revealed that there is a clear correlation between the increase in the area of the stress-strain hysteresis loop and the tensile-compression-strain asymmetry and the formation of cavities and nanopores at WC/Co/WC and WC/Co interfaces.