| Description |
Tantalum carbides are commonly processed by hot-pressing, canned hot-isostaticpressing, or spark-plasma sintering because of their high melting temperatures and low diffusivities. This study reports processing of dense Z-Ta4C3-x by reaction sintering of a Ta and TaC powder mixture (C/Ta atomic ratio = 0.66). Z-Ta4C3-x is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face-centered-cubic (fcc) and hexagonal-close-packed (hcp) 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 PTaHx, 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: (a) decomposition of P-TaHx to Ta, (b) diffusion of C from y-TaC to Ta leading to the formation of a-Ta2Cy' with the kinetics described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation with an exponent, n = 0.5, and an activation energy of 221 kJ/mole, (c) equilibration of a-Ta2Cy' and Y-TaC078 phases, and (d) formation of Z-Ta4C256 from the equilibrated a-Ta2C and y-TaC0 78 phases with the kinetics characterized by a higher JMAK exponent (n « 3) and higher activation energy (1089 kJ/mole). The microstructure showed evidence of nucleation and growth of the Z-Ta4C2.56 phase in both the a-Ta2C and y-TaC0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane. A hot-pressed and hot-isostatic-pressed (HIPed) material (C/Ta atomic ratio = 0.66), having formed 95 w% Z-phase, attained a fracture toughness of 15.6 ± 0.5 MPaVm and a fracture strength of 508 ± 97 MPa, while a pressureless sintered and HIPed counterpart, having formed 89 w% Z-phase and 11 w% y-TaC0 78, attained a fracture toughness of 13.7 ± 0.3 MPaVm and a fracture strength of 679 ± 56 MPa. All Z-phase containing materials showed rising R-curves. The high fracture toughness and rising Rcurve were attributed to ligament bridging across the crack face. The ligaments, called lamella, were formed as a result of weak cleavage planes in the basal plane of the QTa4C3-x crystal. |
