Effect of Cooling Rate on the Formation and Morphology of (W,V)Cx in VC-doped WC-Co Cemented Carbide

doi:10.1007/s40195-016-0501-3

Abstract

Abstract:

The grain growth retardation mechanism and the effect of cooling rate on VC-doped WC-Co cemented carbides were investigated in this work. WC-30Co and WC-30Co-VC were prepared by powder metallurgy, liquid-phase sintering at 1400 °C and followed by water quenching (>150 °C/s) or furnace cooling (~0.083 °C/s). Based on the results of electron probe microanalysis (EPMA), we found that WC concentration in the Co binder was independent of VC doping during liquid-phase sintering, hence barely contributing to the retardation of WC grain growth. In contrast, the (W,V)C_x phase formed at the WC/Co interfaces played a major role in retarding WC grain growth during liquid-phase sintering. The effect of cooling rate on the morphology of (W,V)C_x was revealed by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive spectroscopy (EDS). In the water-quenched WC-30Co-VC, (W,V)C_x precipitates were found as thin layers at the WC/Co interfaces. In contrast, both thin layers of similar thickness and nanoparticles of (W,V)C_x were observed in the furnace-cooled counterpart. These observations listed above suggested that thin (W,V)C_x layers were stable structures effectively suppressing the growth of WC grains and their thickness remained independent of the cooling rate. The (W,V)C_x nanoparticles, however, may be inhibited through rapid cooling, ensuring the VC-doped WC-Co cemented carbides desired toughness.

Key words: WC-Co, VC doping, (W,V)C_x, Water-quenched, Furnace-cooled

Xiao-Ou Yi,Xiang Huang,Chang-Bin Liu,Dan-Qing Yi,Yong Jiang,Bin Wang,Hui-Qun Liu,Li-Yong Chen. Effect of Cooling Rate on the Formation and Morphology of (W,V)C_x in VC-doped WC-Co Cemented Carbide[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(2): 146-155.

Figures/Tables 9

Fig. 1 Illustration of microstructural features and the position of EPMA probe on the Co phase in the water-quenched WC-30Co alloy (SEM, 15 kV)

Table 1 EPMA results of Co binder phase in four alloys sintered at 1400 °C

Alloy	W (wt%)	Co (wt%)	C (wt%)	V (wt%)
WC-30Co WQ	24.89 ± 0.35	74.26 ± 0.76	0.41 ± 0.07
WC-30Co-VC WQ	24.13 ± 1.22	74.29 ± 1.26	0.58 ± 0.15	1.00 ± 0.11
WC-30Co FC	21.25 ± 0.74	78.07 ± 0.62	0.34 ± 0.09
WC-30Co-VC FC	18.09 ± 1.26	79.53 ± 1.05	0.50 ± 0.16	1.13 ± 0.21

Fig. 2 TEM micrographs of a WC-30Co, b WC-30Co-VC alloys, sintered at 1400 °C for 2 h and followed by water quenching. The WC/Co interfaces in Fig. 2b were facetted with fine multi-steps, shown in white circled regions

Fig. 3 WC/Co interface in water-quenched WC-30Co alloy: a TEM micrograph of a WC crystal observed along the \( 1 {\bar{\text{2}}\text{10}} \) direction, b HRTEM image of the WC(0001)/Co interface, as labelled in white arrow in Fig. 3a

Fig. 4 WC/Co interface in water-quenched WC-30Co-VC alloy: a TEM micrograph of a WC grain surrounded by Co pools. The white arrowsb, c indicate the magnified area shown in Fig. 4b and c, respectively; b HRTEM image of the WC(0001)/Co interface, c HRTEM image of the WC(\( 1 0 {\bar{\text{1}}\text{0}} \))/Co interface

Fig. 5 WC/Co interface in furnace-cooled WC-30Co-VC alloy: a TEM micrograph of a WC grain, b HRTEM image of the WC(0001)/Co interface as indicated in Fig. 5a with a white arrow. The black dots represent columns of W atoms in WC, while the white dots represent columns of W atoms in the (W,V)Cx phase

Fig. 6 Precipitation behaviour of (W,V)Cx in furnace-cooled WC-30Co-VC alloy: a TEM micrograph illustrating the hemispherical geometry of (W,V)Cx at the intersecting corners of (0001)WC and (\( 1 0 {\bar{\text{1}}\text{0}} \))WC habit planes in a WC grain, as shown in the white circled region, b HRTEM image of this region, featuring a preferred pile-up of (W,V)Cx on (0001)WC to (\( 1 0 {\bar{\text{1}}\text{0}} \))WC. The stacking of (W,V)Cx on (0001)WC was featured of small atomic-scale steps so as to relieve misfit strain, as shown in the white boxed region

Fig. 7 Formation of nanoscale (W,V)Cx particles in furnace-cooled WC-30Co-VC: a TEM image of a 50 nm (W,V)Cx particle located at the triple point of WC grains 1, 2 and the Co binding phase, b EDS spectrum of this (W,V)Cx particle, from which the concentration of W was found ~8at%, c, d demonstrate the orientation relations for WC1-(W,V)Cx-Co (black box region in Fig. 7a) and WC1-(W,V)Cx-WC2 (white box region in Fig. 8a), respectively

Fig. 8 Schematic diagram of step profiles at the corner of two WC grains in WC-30Co-VC: a as-sintered; b as-cooled

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