Effects of 20 mol% Fe, Co, Ni additions on phase formation, microstructure and mechanical properties of TaCx (x = 0.5, 0.55, 0.6, 0.7) ceramics

TaC x (x = 0.5, 0.55, 0.6, 0.7) ceramics with 20 mol% Fe, Co or Ni addition were prepared by hot pressing TaC and Ta powder mixtures at 1800 °C. X-ray diffraction, scanning electron microscopy, transmission electron microscopy , high resolution transmission electron microscopy , X-ray energy dispersive spectroscopy and selected area electron diffraction were used to investigate the constituent phases and microstructures of the composites. Vickers hardness , flexural strength and fracture toughness were measured. Intermetallic compounds Fe 7 Ta 3 , Co 2 Ta and Ni 3 Ta were formed due to reactions of Fe, Co and Ni with Ta. The remaining TaC and Ta reacted to form hexagonal Ta 2 C, rhombohedral ζ-Ta 4 C 3-z or cubic TaC y depending on the overall C:Ta ratios. The microstructure of the TaC 0.5 –20 mol% Fe, Co, Ni compositions (where C:Ta = 0.5) were composed of intermetallics and fine anisotropic Ta 2 C and ζ-Ta 4 C 3-z grains. As the C:Ta ratio increased to 0.55–0.7, the ζ-Ta 4 C 3-z and TaC y grains were enlarged in size and equiaxed in the TaC 0.55 -, TaC 0.6 -, and TaC 0.7 –20 mol% Fe, Co, Ni compositions. Addition of Ni instead of Fe and Co benefitted to refine microstructures of the composites. The TaC 0.5 –20 mol% Fe, Co, Ni compositions had good flexural strength (580-650 MPa) and fracture toughness (9.3–9.5 MPa·m 1/2 ) due to growth of fine lamellar grains, despite the high Ta 2 C content.