Investigation of microstructure and mechanical properties at low and high temperatures of WC–6 wt% Co

• Properties of WC–6 wt% Co with grain sizes at submicrometer ranges are studied.
• Grain size and size distribution play a critical role in high temperature properties.
• Compressive stresses at 600 and 800 °C increase with the room temperature hardness.
• Fracture strains at 800 °C are not related to the room temperature toughness.
• Small grain sizes (~ 0.20 μm) are beneficial to properties at both 600 and 800 °C.

The correlations of the microstructure, room temperature hardness and toughness, and high temperature compressive stress and strain of a series of WC–6 wt% Co materials with grain sizes at sub-micrometer ranges (< 0.30 μm) have been investigated. It is found that the average size and size distribution of WC grains play a critical role in both room temperature and high temperature (600 and 800 °C) properties. Furthermore, the maximum compressive stresses at 600 and 800 °C increase with increasing the room temperature hardness and decreasing grain sizes. However, the fracture strain in compression at 600 and 800 °C cannot be predicted from the room temperature toughness. This absence of a relationship is attributed to the facts that the fracture strain of compressive tests at high temperatures is dictated by significant plastic flow of the Co phase, whereas the toughness at room temperature is controlled by crack propagation with little plasticity. In general, small grain sizes (at ~ 0.20 μm) with or without a bi-modal grain size distribution are beneficial to the maximum compressive stress and fracture strain at 600 and 800 °C. In contrast, grain sizes at ~ 0.27 μm with a bi-modal grain size distribution are good for room temperature toughness, but offer inferior high temperature compressive stresses and fracture stains. Thus, the design of the microstructure at a fixed Co composition depends on the intended applications.