Allotropoic Strengthening and in situ Phase Transformations During High-Temperature Flexure of Bulk Tantalum Nitride

In situ phase transformation and change in the grain size were observed during high-temperature flexural tests performed on tantalum nitride bulks consolidated by spark plasma sintering. This study, for the first time, shows that tantalum nitride with cubic structure is the main phase at temperatures between 25 °C and 2000 °C, while previous studies suggested it should be hexagonal TaN. Cubic TaN phases with lattice parameters of a = 4.338 Å and a = 4.45 Å were observed below 1600 °C and above 1600 °C, respectively. The possibility of the formation of multiple TaN phases during the flexural tests significantly increases the high-temperature strength as values of greater than 500 MPa were observed at 1600 °C. This strength level is typical for the bulk TaC at room temperature.