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.

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