Phase stability and microstructural formations in the niobium carbides

The phase stability and microstructural formations for a series of hot isostatically pressed (HIP) NbCx compositions was investigated. Based on the composition, single-phase B1 (Fm3¯m) NbC or (P3¯1m) β−Nb2C equiaxed grains were observed as well as β-Nb2C lath precipitation within NbC. Particular discussion is given to the complexities of X-ray and electron diffraction for proper identification for the various poly-types of Nb2C that have been computationally predicted. The phase transformation pathways for these microstructures was revealed by a NbC-Nb diffusion couple from the HIP powders. The migration of vacancies towards the NbC side resulted in a refinement of the NbC grains whereas carbon's migration into the Nb formed an uniform and distinct reaction front with a mixture of β-Nb2C and Nb grains. This latter microstructure suggests carbon's reaction through the matrix grain is as significant in the conversion process as is the precipitation of β-Nb2C at the grain boundaries.