Nowadays, field assisted sintering technology (FAST), such as spark plasma sintering (SPS) and electric-discharge compaction (EDC) is widely used due to shortening sintering time and enhancing sintering kinetics. However, the underlying mechanism of FAST is not well understood hitherto. In this work, nanocrystalline WC-Co powders were consolidated by EDC to produce ultrafine WC-Co cemented carbides, and the microstructure and mechanical properties of the as-sintered samples were also investigated. The migration of cobalt in sintering was evaluated in EDC of nanocrystalline WC-Co powders and coarse counterpart combination. EDC of W-Ni-Fe powders and subsequent annealing was also conducted. On the basis of compaction of W-Ni-Fe powders and WC-Co powders by EDC, powder compaction modeling and theory of liquid phase sintering, the densification mechanism in EDC was explored. The main results are summarized as follows:
(1) Ultrafine WC-10Co cemented carbides were fabricated by EDC of WC-Co powder synthesized by spray conversion process (SCP). A grain size as small as 120 nm could be achieved with hardness of 22 GPa and fracture toughness of 9.79 MPa.m1/2. The grain growth was constrained due to the short holding time in EDC. A high density of defect, such as stacking faults, was found in WC grains and the lattice distortion also occurred. It is suggested that the high strain rate and instantaneous stress induced the defect formation during densification.
(2) With EDC of nanocrystalline WC-Co powders and coarse-grained counterpart, graded WC-Co cemented carbides with ultrafine WC-Co cemented carbides and coarse counterpart combination which offered high hardness and excellent fracture toughness were achieved. Cobalt migration which always occurred in graded WC-Co cemented carbides during liquid phase sintering process induced by difference in WC grain size was constrained due to short holding time.
(3) EDC and subsequent annealing of W-Ni-Fe alloys was conducted. Inhomogeneous distribution of matrix phase and super saturated solid solution were formed in EDC processing. When annealed at 1400C, the matrix pool which may enhance the initiation of adiabatic shear deformation was observed. When annealing temperature up to 1450C, with the formation of liquid phase, no matrix pool was found due to redistribution of the matrix phase. The tungsten precipitated from the matrix phase during annealing, which leaded to formation of matrix pool and also in part contributed to grain growth at solid state.
(4) Consolidation of cemented carbide and tungsten heavy alloys was conducted under varying current densities to explore the effect of liquid phase on densification in EDC. The densification in EDC occurred only when liquid phase formed, and relative density increased with the increase of liquid phase volume. In the case of WC-11Co powders, the faceted grain evolution occurred but the grain growth was hardly observed. Furthermore, the depth of liquid penetration of Fe in WC-Co compact agreed well with that caused by particle rearrangement processing. These results suggest that the densification in EDC is mainly induced by particle rearrangement with liquid phase.
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