Mechanism of solid-gas reaction for formation of metastable niobium-nitride alloy powders by reactive ball milling

We report on the formation of a new metastable phase of niobium-nitride (Nb-N) alloy powders. This nonequilibrium phase has been synthesized by milling elemental niobium (Nb) powders under purified nitrogen gas flow using a high energy ball mill at room temperature. The progress of the solid-gas reaction for the Nb-N system has been followed by means of x-ray diffraction, differential thermal analysis, transmission electron microscopy, and chemical analysis. After 720 ks of milling, the initial bcc-Nb is completely transformed to nonequilibrium-fcc-NbN containing about 50 at. % N. The lattice parameter of the end product fcc-NbN was calculated to be 0.433 nm. In addition, the particle and grain sizes of the completely reacted alloy powders are 3 μm and 5 nm, respectively. This transforms to a mixture of hcp-Nb_4N_3 and hcp-NbN at the high temperatures, as high as 1150 K.