Processing of Dense ζ- Ta4 C3− x by Reaction Sintering of Ta and TaC Powder Mixture.

Tantalum carbides are commonly processed by hot pressing, canned hot-isostatic-pressing, or spark plasma sintering because of their high melting temperatures and low diffusivities. This paper reports processing of dense ζ- Ta₄ C_{3− x} by reaction sintering of a Ta and TaC powder mixture ( C/ Ta atomic ratio = 0.66). ζ- Ta₄ C_{3− x} is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face-centered-cubic and hexagonal-close-packed Ta stacking sequences interrupted by stacking faults and missing carbon layers. This structure leads to easy cleaving on the basal planes and high fracture toughness. A key step in processing is the hydrogenation of the Ta powder to produce β- TaH _x, a hard and brittle phase that enables efficient comminution during milling and production of small, equiaxed Ta particles that can be packed to high green density with the TaC powder. Studies of phase evolution by quantitative X-ray diffraction during sintering revealed several intermediate reactions: (1) decomposition of β- TaH _x to Ta; (2) diffusion of C from γ- TaC to Ta leading to the formation of α- Ta₂ C _y' with the kinetics described by the Avrami equation with an exponent, n = 0.5, and an activation energy of 219 kJ/mole; (3) equilibration of α- Ta₂ C _y' and γ- TaC_0.78 phases; and (4) formation of ζ- Ta₄ C_2.56 from the equilibrated α- Ta₂ C and γ- TaC_0.78 phases with the kinetics characterized by a higher Avrami exponent ( n ≈ 3) and higher activation energy (1007 kJ/mole). The sintered material contained ~0.86 weight fraction ζ- Ta₄ C_2.56 and ~0.14 weight fraction γ- TaC_0.78 phases. The microstructure showed evidence of nucleation and growth of the ζ- Ta₄ C_2.56 phase in both the α- Ta₂ C and γ- TaC_0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane. [ABSTRACT FROM AUTHOR]