Phase constitution and mechanical properties of carbides in the Ta-C system

Carbides in the Ta-C system were prepared by hot-pressing mixed powders of TaC and Ta in C/Ta atomic ratios of TaCo.6, TaCo.7, TaCu.8, TaCo.9, and TaCi.o at 1800 °C for 2 h. Rietveld analysis was performed on all compositions, and it was found that TaCo.6 was a mixture of 21% hexagonal close packed (HCP) a-Ta2C and 79% rhombohedral ^ - T a ^ - x , TaCo.7 was a mixture of 44.6% rhombohedral ^ - T a ^ - x and 55.4% face centered cubic (FCC) TaCy , TaCo.s and TaCo.9 were approximately 100% FCC TaCy , and TaC 1.0 was approximately 100% TaC. The TaC and TaCy compositions, produced were single phase with C/Ta ratios essentially the same as the starting nominal compositions and densities exceeding 96.5% of theoretical. Grain size decreased with increasing carbon deficiency from 1.0 to 0.8, with a corresponding increase in Vickers hardness from 13.5 to 20.0 GPa. The flexural strength, however, decreased from 650 to 500 MPa due to a decrease in fracture toughness from 5.3 to 3.8 MPaVm. Fracture toughness measured by single-edge-precracked-beam (SEPB) technique was exceptionally high (12.7 ± 0.7 MPaVm) for the a-Ta2C and ^ - T a ^ . x composition with a high weight percent of the ^ - T a ^ . x phase (~83 wt %). This disconnect between the change in hardness and strength indicates that the toughened compositions may exhibit R-curve behavior. Fracture surfaces were highly faceted with multiple cleavage crack planes and steps for TaCo.6 and TaCo.7. It is possible that the cleavage planes correspond to the stacking faults between the FCC and the HCP stacking layers within the stacking sequence ABABCACABCBC of the ^ - T a ^ . x phase bridge cracks in the crack wake with accompanying variations in local carbon concentrations fluctuating between TaCy and a-Ta2C.