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-nitrid e (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 mea ns of x-ray diffraction, differential thermal analysis, transmission e lectron microscopy, and chemical analysis. After 720 ks of milling, th e initial bcc-Nb is completely transformed to nonequilibrium-fcc-NbN c ontaining about 50 at. % N. The lattice parameter of the end product f cc-NbN was calculated to be 0.433 nm. In addition, the particle and gr ain sizes of the completely reacted alloy powders are 3 mu m and 5 nm, respectively. This transforms to a mixture of hcp-Nb4N3 and hcp-NbN a t the high temperatures, as high as 1150 K.