Atomic and vacancy ordering in carbide ζ-Ta4C3−x (0.28⩽x⩽0.40) and phase equilibria in the Ta–C system

The structure of nonstoichiometric carbide phase ζ-Ta4C3−x formed in the tantalum–carbon (Ta–C) system is studied by X-ray and neutron powder diffraction and metallography. Investigated carbide ζ-TaC0.67 crystallizes in a trigonal (rhombohedral) space group image with cell parameters atr=1.0180(1) nm, αtr=17.64° (or ah=0.31216(2) nm, ch=3.0058(1) nm in hexagonal axes). The closely packed metal sublattice in carbide ζ-Ta4C3−x consists of alternating blocks where metal atoms are located in the same manner as on the FCC sublattice of the cubic carbide TaCy and the HCP sublattice of the hexagonal carbide Ta2Cy. This metal sublattice represents a transition sublattice between these FCC and HCP sublattices. An ordered distribution of atoms C and structural vacancies in carbide ζ-Ta4C3−x is revealed and the distribution function of atoms C is calculated for nonmetal sublattice sites, on which ordering takes place. It is shown that one long-range order parameter η describes the ordering of ζ-carbide and the η value in the investigated ζ-Ta4C3−x phase does not exceed 0.7. Carbide ζ-Ta4C3−x is stable in bulk and powdered states over a wide temperature interval of 300 to ∼2400 K and has a narrow homogeneity interval from TaC0.65 to TaC0.68. Microhardness of disordered and ordered tantalum carbide TaCy with the basic B1 structure and trigonal carbide ζ-Ta4C3−x is measured. The phase diagram of the Ta–C system is refined considering data obtained for the ζ-Ta4C3−x phase.