The hardness of cemented carbide is inversely proportional to the size of WC grains, and grain growth inhibitors are widely used to prepare cemented carbide with smaller grains. It is reported that the solute atoms of the grain growth inhibitor segregate at the WC-Co interface, resulting in the formation of a thin film-like two-dimensional interface phase. VC and Cr3C2 are the most effective inhibitors of grain growth in industry, and both doping can induce interfacial recombination.

Recently, researchers from Fuzhou University bent the fiber sheet of vanadium-doped WC-Co cemented carbide in situ and observed the crack propagation with transmission electron microscope. The atomic configurations of WC-Co interface before and after fracture were characterized and compared by AC-STEM, and the bond fracture path at atomic scale was proposed, and the fracture behavior observed in the experiment was verified by DFT calculation. Related papers were published in the journal of Materials Science and Technology.

Paper link:

https://doi . org/ 10. 10 16/j . jmst . 202 1.09 . 02 1

The transmission electron microscope image of figure 1 shows the in-situ TEM bending experiment of FIB thin plate. (a) the state before loading, (b) no crack found after continuous loading for 3 minutes, (c) notch caused by crack, and (d, e) crack propagation along the WC-Co interface. The crack in (f) deflects the adjacent WC particles.

Fig. 2AC-STEM describes the phase interface of WC-Co before fracture. (a, b) Ju Chi lines of b)Co bonded phase and WC grains, (c) high-resolution HAADF image of phase interface, (d-g) EDS diagram of phase interface, (h)EDS intensity line profile and (i) composition profile analysis.

Fig. 3 Systematic characterization of fracture WC-Co interface (a) HAADF image of cleavage plane on WC side, (b-e) EDS diagram of phase interface, (f)EDS intensity line profile and (g) composition profile analysis.

Fig. 4 (a) DFT optimized interface structure of vanadium-doped WC-Co interface. (b) Interfaces of b)WSEP layers are fractured to do work.

In this study, the atomic resolution observation of fracture surface is provided from the single bond interface of V-doped WC-Co, and a new understanding of failure mechanism is provided by first-principles calculation. The three atomic layers induced by segregation, together with the crystal lattice of WC particles, show high toughness. On the other hand, the non-lattice between the three-layer superlattice structure and the randomly oriented cobalt phase reduces the interlayer bonding strength, which is verified by theoretical calculation. Constructing WC-Co interface with * * lattice may be an effective method to improve the mechanical properties of cemented carbide.

Although this study reveals that WC-Co interface is the preferred fracture interface under mechanical load, it does not necessarily mean that WC/Co interface is one of the weakest key components in WC-Co composites. The current research aims to compare the strength of different interface layers on the V-doped WC-Co interface, which is common in commercial cemented carbide. In the future, systematic research is needed to compare the interfacial bonding strength of different adhesives.

In a word, the atomic structure and chemical properties of the fracture were studied by HAADF imaging and energy spectrum analysis after aberration correction. By comparing the WC/Co interface before and after fracture, it is found that the fcc-like three-layer structure containing V is fractured together with the adjacent Co phase. DFT calculation shows that the bonding characteristics and non-* lattice of the interface reduce the bonding strength between layers (article: Xiao Taiyang).