Isolation of the Zeta Phase in the System Tantalum-Carbon

Abstract

ZETA phase in the system Ta-C was first reported by Lesser A and Brauer.' Rudy2 reported that the zeta phase is metastable and forms a t approximately 2400°C by precipitation froin substoichiometric monocarbide. Recently Zaplatynsky3 showed that the zeta phase is present in the "striated" zone observed metallographically in substoichiometric TaC and proposed that the zeta is a nonequilibrium phase formed under compressive stresses.Carbide laycr growth experiments confirmed that zeta is found as a second phase in substoichiornetric TaC. I t is proposed, however, that the zeta phase forms on cooling as a result of the decreasing solubility of carbon in the substoichiometric monoc:irbide with decreasing temperaturc. I n addition to the precipitated zeta in the monocarbide, a separate layer of singlephase zeta is found in all Ta-C laycr growvth espcrinieiits a t isotlicrmal orincaliiig tcrtipcraturcs from 1600" t o 220O"C, but not at IiiKlicr temperatures. The zeta phase was isolated by microdrilling from the scparate carbide layer, and X-ray analysis indiclatctl a single-pliasr zcta diffraction pattern. The stability of the zeta p11:1se ic indicated by its formation of a separatc carbide laycr at nnncaling temperatures and its persistence even after reniovol of comprcssivc stresses by microdrilling.High-purity tantalum metal slabs were packed in lampblack, carburizcd in a flowing argon atmosphere, and isothcrrnally annealed a t 1600" to 2800°C for times up to 200 hr a t the lower tempcraturcs. Autotnatic tcrnperature control held the relative temperature variation to f S°C . The accuracy of the temperature measurement is estimated a s f50'C. Chemical analyses before and after heat treatment are given in Table I. The carburized slabs werc prcparcd by standard metallographic methods. The etched slabs showed the typical TaC and Ta2C double layer on slabs carbutized at 2300' to 280OOC. A distinct additional layer, however, was found between Ta?C and TaC in slabs earburizccl for long times between 1600" and 2300°C. The additional layer was identified as the zeta phase. A typical microstructure of the carbide layers formed from 2300' to 'J800°C is shown in Fig. 1. The distinct boundaries betwccn layers indicate the thickness of the layer of carbide equilibrium phase formed during the isothcrmal annealing. I t is thought that the diffuse boundaries within the single-phase layer occur as n result of precipitation of second phases during cooling. Three distinct carbide layers were formed from 1600' t o 2200°C, as shown in Fig. 2. The thickness of the extra layer increased with annealing time. By alternately carburizing and decarburizing, the thickness of the estra layer was increased to morc than 10 mils.

Structure and properties of tantalum carbide crystals

“…This idea also receives some support from the work of Zaplatynsky [5] who found that the zeta-phase can be generated from tantalum carbide of the same composition, under compressive stresses. The phase transformation was not seen on indenting crystals decarburised above 2200 ~ C, which confirms a previous observation that the formation of the zeta-phase is temperature-sensitive [6]. Brizes has observed similar markings at indents in substoichiometric tantalum carbide and suggests that they are due to the precipitation of zeta-phase or Ta2C [18].…”

“…Storms [2] has suggested that the maximum melting point might lie at TaC0.s but he also points out that the actual values of composition and temperature are subject to some uncertainty due to the experimental difficulties in making the measurements [3]. A number of authors have reported a zeta-phase tantalum carbide [4][5][6] which might be a metastable compound [2,4,5]. Its composition has been determined as TaCo.75 [5] or TaC0.r4 [6], but it also appears to exist over a range of substoichiometry [7].…”

“…A number of authors have reported a zeta-phase tantalum carbide [4-6] which might be a metastable compound [2,4,5]. Its composition has been determined as TaCo.75 [5] or TaC0.r4 [6], but it also appears to exist over a range of substoichiometry [7]. Yvon and Parth6 [7] have recently shown that the structure of the zeta-phase consists of an arrangement of twelve close-packed metal atoms in a unit cell, with the carbon atoms in octahedral interstices.…”

A model for understanding the formation energies of nanolamellar phases in transition metal carbides and nitrides

“…The zeta phase has similar stability in the group IV nitrides as the epsilon phase. However, the zeta phase is thought to form in all the group VB carbides, but, as mentioned previously for ζ-Ta 4 C 3 , the nature of the stability of this phase is debated [15,16]. These phases are not known to be stable in other compounds to the author's knowledge.…”

Carbon diffusion in the group IVB and VB transition element monocarbides

“…It then follows that the analytical expression fcx-metal self-diffusion is very similar to that presented for carbon: o where N ., Is the thermal equilibrium metal vacancy concentration; d^ , the metal atom diameter; and d , the diameter of the gap for the metal diffusion atomic jump. A comparig son of the values of the terms in Equations (9) and (10) ; (14) ,09) Equation (9) may also be applied to the estimation of (1) diffusion of Interstitial impurities in the monocarbides, and (2) self-diffusion of interstitlals in other Bl structure interstitial compounds such as mononitrides, etc. When the concentration of impurities such as O, N, B, and Si in the Bl structure monocarbides is small, their effect cxi the lattice parameter and coefficient of thermal expansion may either be neglected or measured in order to substitute the values of a and a (T) into Equation (9).…”