Mechanism of solid-gas reaction for formation of metastable niobium-nitride alloy powders by reactive ball milling
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K. Sumiyama, K. Aoki, T. Masumoto, and K. Suzuki Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980, Japan (Received 9 May 1993; accepted 8 July 1994)
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 N b - 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 fxra and 5 nm, respectively. This transforms to a mixture of hcp-Nb4N3 and hcp-NbN at the high temperatures, as high as 1150 K.
I. INTRODUCTION Mechanical alloying (MA) process via ball-milling1 and/or rod-milling2 techniques may be referred to the formation of many amorphous alloy powders3"17 through solid state amorphization reaction, SSAR.18 Apart from the amorphization for their subsequent beneficiation, El-Eskandarany et al. reported the first known novel technique for preparing metal nitride alloys such as Fe-N, 1 9 TiN,20 and AlTaN21 alloy powders by milling the elemental powders under a nitrogen gas (N2) flow using a method called reactive ball-milling, RBM. So far, the RBM method has been accepted as a promising technique for producing several metal-nitride alloy powders. • In fact, metal nitrides24'25 possess unique properties that are highly desirable for a variety of applications such as hard and refractory materials. The preparation of metal nitrides is relatively complicated because we have to apply high temperature and high gas pressure and/or addition of ammonia gas. In the present study, elemental niobium powders have been simply reacted with nitrogen gas in a highenergy ball-mill, under nitrogen gas flow, at room temperature. A new phase of nonequilibrium N b - N alloy powders containing nitrogen of about 50 at. % has been detected after 720 ks of the RBM time. For the purpose of the present work, we have utilized x-ray diffraction, scanning electron microscopy, and transmission electron microscopy to monitor the crystalto-compound phase transition in NbN alloy powders. J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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The thermal stability of N b - N alloy was investigated by differential thermal analysis. One goal of the present study is to continue the research progress of the solidgas reaction through the RBM technique. A second goal is to provide a simple process for producing N b - N alloy powders as a material of great interest in materials science. Finally, the present study has been undertaken in part to discuss the mechanism
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