Poster Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Microstructure and composition of segregation layers at WC/Co interfaces in ultrafine-grained cemented carbides co-doped with Cr and V (#240)

Weiteng Guo 1 , Kai Li 1 , Yong Du 1
  1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China

WC–Co cemented carbides are widely used in machining tools due to their excellent hardness and wear resistance. Ultrafine-grained WC-Co cemented carbides with average WC grain sizes varying from 0.2 to 0.5 μm, exhibit higher mechanical properties such as hardness and wear resistance than traditional WC-Co cemented carbides. However, during their fabrication, smaller grain size of raw powders is required, which enhances the surface reactivity and produces a higher possibility of abnormal grain growth according to Ostwald-ripening mechanism. Several methods have been applied to retard this abnormal grain growth. The most widely applied one among them is the addition of transition metal carbides such as VC, Cr3C2, TiC, TaC, and NbC which act as grain growth inhibitors. VC-Cr3C2 is the most widely used grain growth inhibitor for ultrafine-grained cemented carbides. However, the mechanism for the high effectiveness of this inhibitor is still unclear. This work aims to clarify the inhibiting mechanism involving VC-Cr3C2 mixed inhibitor through investigations of a commercial ultrafine-grained WC-Co-VC-Cr3C2 cemented carbide in an objective-aberration-corrected transmission electron microscope. The interfacial microstructure and composition of the 1-nm-thick segregation layers on surfaces of WC grains were examined by means of high resolution transmission electron microscopy (HRTEM), high resolution energy dispersive X-ray (HREDX) spectroscopy and high angle annular dark field (HAADF) imaging. Continuous and coherent Cr-rich (Cr,W,V)C segregation layers was observed on prismatic facets for the first time [Orientation relationship with WC: (10-10)WC//(001)(Cr,W,V)C, [1-210]WC//[1-10](Cr,W,V)C], and the continuous segregation layers enveloping WC grains [i.e. (V,W,Cr)Cx on (0001)WC facets and (Cr,W,V)C on {10-10}WC facets] account for the high effectiveness of VC-Cr3C2 inhibitor. The grain-growth inhibiting mechanism revealed in this work will provide guidance for manufacture of ultrafine-grained cemented carbide and accelerate the optimization of this technology in return.

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  2. I. Sugiyama, Y. Mizumukai, T. Taniuchi, K. Okada et al., Three-dimensional morphology of (W,V)Cx in VC-doped WC–Co hard metals, Scripta Mater., 69 (2013) 473-476.