Integrated Computational Materials, Process and Product Engineering – IC-MPPE 2022

Hardmetals are used as tool materials in various areas such as metal machining or metal forming, where they are exposed to high temperatures and high loads. Hardmetals have good high-temperature properties, but these properties are influenced by the microstructure and operating condition. Therefore, knowledge of limits for their failure-proof application are important for the design of hardmetal tools. The aim of the current work was the experimental determination of these limits and the connection of them with microstructural changes in WC-Co hardmetals at 700 °C and 800 °C. Six WC-Co hardmetal grades were tested under uniaxial cyclic compression at a stress ratio of R = $\sigma_{min}/\sigma_{max}$ = -$\infty$ in a vacuum. The investigated grades differ in their WC grain size (0.4 μm to 2.0 μm) and Co-content (6 wt.% to 12 wt.%). Limit stresses are determined by examining the influence of increasing stress ranges and their effect on the (residual) strain evolution at zero applied load as a function of microstructure. At low stress ranges, the residual strain was observed to stabilize after a characteristic number of load cycles. Advancing strain ratcheting occurred above a critical stress range (limit stress), above which strain stabilization does not occur any more. By scanning electron microscopy, the formation of microdefects, such as cavities and nanopores, at phase triple points, WC/WC grain boundaries, and WC/Co interfaces were observed to be promoted by advancing strain ratcheting. Due to the increase in microdefect density in the microstructure with increasing stress range, statements about early failure are possible based on the compressive stress-strain data and the course of the residual strain with increasing number of cycles. The limit stresses determined are therefore regarded as limits for the failure-proof use of tools and components.