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

Abstract TaCx (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 Fe7Ta3, Co2Ta and Ni3Ta were formed due to reactions of Fe, Co and Ni with Ta. The remaining TaC and Ta reacted to form hexagonal Ta2C, rhombohedral ζ-Ta4C3-z or cubic TaCy depending on the overall C:Ta ratios. The microstructure of the TaC0.5–20 mol% Fe, Co, Ni compositions (where C:Ta = 0.5) were composed of intermetallics and fine anisotropic Ta2C and ζ-Ta4C3-z grains. As the C:Ta ratio increased to 0.55–0.7, the ζ-Ta4C3-z and TaCy grains were enlarged in size and equiaxed in the TaC0.55-, TaC0.6-, and TaC0.7–20 mol% Fe, Co, Ni compositions. Addition of Ni instead of Fe and Co benefitted to refine microstructures of the composites. The TaC0.5–20 mol% Fe, Co, Ni compositions had good flexural strength (580-650 MPa) and fracture toughness (9.3–9.5 MPa·m1/2) due to growth of fine lamellar grains, despite the high Ta2C content.