The role of tungsten in the Co binder: Effects on WC grain size and hcp–fcc Co in the binder phase

• Binder phase polymorphism investigated within the two-phase field in WC-Co.
• Co polymorphism is affected more by carbon activity than carbide grain size.

Recent advances in microscopy techniques have made possible the quantitative examination of the Co binder phase in WC–Co cemented carbides [1]. The composition of the binder phase is of interest with respect to applications where fracture toughness and fatigue are particularly important. A previous work has observed that the hcp/fcc ratio can change with fatigue and applied stress [2].This work is part of a wider study examining the role of Co mean free path, mean WC grain diameter (dWC) and composition on the hcp–fcc content on the Co binder phase extracted from a wide range of WC–Co samples with an average dWC of 0.6 μm < dWC < 10 μm and Co content from 6 wt.% to 25 wt.%. This paper aims to study the effect of carbon composition on three 10 wt.% Co grades with target dWC 0.8 μm < dWC < 6 μm. The concentration of W in the Co was monitored by examining the binder magnetic saturation (BMS). Samples with a variety of Co Msat values through the WC–Co pseudo-binary phase field were studied. The effects on grain size, Co distribution, physical properties and hcp content with varying W content were examined. Experimental results show that lower BMS resulting from dissolved tungsten in the binder has a negative exponential correlation with hcp abundance for grades with dWC < 1.5 μm. A significant abundance of hcp was associated with a more diffuse large scale fcc–fcc structure of the binder phase, which has been associated with improved crack deflection [14].This work belongs to the Special Issue of the International Conference on the Science of Hard Materials (ICSHM10).