Synthesis and phase transformation mechanism of Nb₂C carbide phases

In the present work, Niobium carbide samples were prepared through powder metallurgy route using spark plasma sintering technique. Some of these samples were heat treated at 900 °C up to 7 days. In order to investigate the phase transformation in Nb₂C carbide, the as-prepared and heat treated samples were characterized by X-ray diffraction, scanning electron microscopy and electron back scattered diffraction (EBSD) and transmission electron microscopy techniques. EBSD could index the same area of the sample in terms of any of the three allotropes of Nb₂C carbide phases (γ-Nb₂C, β-Nb₂C and α-Nb₂C) with good confidence index. From the EBSD patterns orientation relationships (OR) among γ, β and α-Nb₂C have been determined. Based on this OR when crystals of the three allotropes were superimposed, it has revealed that the basic Nb metal atom lattice (hcp lattice) in all the Nb₂C phases is same. The only difference exists in the carbides is the ordering of carbon atoms and vacancies in the octahedral positions of the hcp Nb metal atom lattice. Crystallographic analysis showed that for the transformation of γ-Nb₂C → β-Nb₂C → α-Nb₂C, large movement of Nb atoms is not required; but only by ordering of carbon atoms ensues the phase transformation. Literature shows that in the Nb-C system formation of the α-Nb₂C is not well established. Therefore, first principle calculations were carried out on these carbides. It revealed that the formation energy for α-Nb₂C is lower than the β and γ-Nb₂C carbides which indicate that the formation of α-Nb₂C is thermodynamically feasible.