Xingwei Liu

Orientation relationship in WC-Co composite nanoparticles synthesized by in situ reactions.

Using the nanoscale violet tungsten oxide as the tungsten source, the WC-Co composite powder was synthesized by the in situ reactions. The particle size of the WC-Co composite powder has a narrow distribution with the mean particle size below 100 nm, and the single composite particle has a nanocrystalline structure with a mean grain size smaller than 10 nm. The detailed characterizations of the nanoparticle microstructure reveal that the orientation relationship and coherence at the interfaces can form during the in situ reactions and further inherit in the consolidated cemented carbide bulk material. The favorable crystallographic characteristics of the WC-Co composite nanoparticles play a significant role in the enhancement of the mechanical properties of the prepared cemented carbide bulk material.

Preparation and mechanisms of cemented carbides with ultrahigh fracture strength

WC–Co cemented carbides were prepared by liquid-state sintering of in situ synthesized composite powders with a constant Co content but different carbon concentrations, and with different size scales of VC particles as grain-growth inhibitor. With an optimized carbon addition and doping with microscale VC particles, an ultrahigh fracture strength with a mean value above 5000 MPa was achieved for cemented carbides. By detailed crystallographic analysis of the configuration and interactions of the WC, Co and VC phases, the effects of VC particle size on the microstructure and mechanical properties of cemented carbides are identified. The mechanisms by which the fracture strength depends on the VC particle size contain the effects on the changes in Co binder distribution, atomic matching at the phase boundary and WC grain size. The dominant factors for ultrahigh fracture strength of cemented carbides are proposed.

In situ study of fracture behavior of ultrafine WC–Co cemented carbide

ABSTRACT The fracture behavior of ultrafine WC–Co cemented carbide was demonstrated by in situ tensile tests in a transmission electron microscope. Using two kinds of cemented carbide samples with and without previously introduced microcracks, the process from deformation till fracture was studied. Particularly, the development and interaction of dislocations within Co and WC, as well as the alternation of dislocation motion between the two phases, was demonstrated, and the features of crack propagation in cemented carbide were disclosed. The finding of the dislocation characteristics and its correlation with the fracture behavior proposed the mechanisms of the ultrahigh fracture strength of the cemented carbide. GRAPHICAL ABSTRACT IMPACT STATEMENT The fracture behavior of ultrafine WC–Co cemented carbide was for the first time directly observed by in situ TEM tensile tests. The correlation of dislocation motion and fracture mechanisms was proposed.

Reinforcement of tungsten carbide grains by nanoprecipitates in cemented carbides

In contrast to the conventional method that obtains a high fracture strength of tungsten carbide-cobalt (WC-Co) cemented carbides by reducing WC grain size to near-nano or nanoscale, a new approach has been developed to achieve ultrahigh fracture strength by strengthening the WC grains through precipitate reinforcement. The cemented carbides were prepared by liquid-state sintering the in situ synthesized WC-Co composite powders with a little excess carbon and pre-milled Cr3C2 particles having different size scales. It was found that the nanoscale dispersed particles precipitate in the WC grains, which mainly have a coherent or semi-coherent interface with the matrix. The pinning effect of the nanoparticles on the motion of dislocations within the WC grains was observed. The mechanisms for the precipitation of nanoparticles in the WC grains were discussed, based on which a new method to enhance the resistance against the transgranular fracture of cemented carbides was proposed.

Distribution of CSL grain boundaries in oriented cemented carbides

WC–Co cemented carbide bulk materials with anisotropic distribution of specific WC planes were prepared by spark plasma sintering (SPS) using different pre-sintering temperatures of 850 °C, 900 °C and 1000 °C at a constant pressure of 60 MPa. The coincidence site lattice (CSL) grain boundaries were characterized and the anisotropic distribution of the CSL boundaries was demonstrated. The combined effects of the composite powder and the SPS parameters on the formation of the anisotropic CSL boundary distribution were analyzed. The mechanisms were proposed such that the CSL grain boundary distribution can be tailored by adjusting the composition of the composite powder and the SPS processing parameters such as the pre-sintering temperature, sintering pressure and its working stage. The findings facilitate to obtain a beneficial CSL grain boundary distribution that enhances mechanical properties in certain directions of the cemented carbide.