Morphological and structural evolutions of nonequilibrium titanium-nitride alloy powders produced by reactive ball milli
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Nonequilibrium titanium-nitride alloy powders have been fabricated by a high energetic ball mill under nitrogen gas flow at room temperature and characterized by means of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. Initial hep titanium is completely transformed to nonequilibrium-fcc Ti-N after 720 ks of the milling time. The fee Ti-N phase is stable at relatively low temperature and transforms at 855 K to Ti 2 N and 6' phases. At the final stage of milling, the particle- and grain-sizes of alloy powders are 1 mm and 5 nm, respectively, and the lattice parameter is 0.419 nm.
I. INTRODUCTION The mechanical alloying (MA) process is a typical energizing and quenching procedure. Ball-milling1 and rod-milling techniques2 have been applied for preparing many amorphous alloys.3"9 Apart from the amorphization reaction via the MA process, we have reported a novel technique called reactive ball mill (RBM) for preparing T M 2 - N (TM: Ti or Fe) alloy powders under nitrogen gas (N2) flow.10 So far, the RBM method has been accepted as a successful process for producing other metal-nitride alloy powders.11 In fact, metal nitrides12'13 possess unique properties which 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. Titanium nitrides have been prepared by activated reactive evaporation,14 the selfpropagating combustion method15 under high nitrogen pressure (105 atm) and high temperature (1500 K), and the plasma spray method under a nitrogen gas flow.16 In the present study, elemental titanium powders have been simply reacted with nitrogen gas to form nonequilibrium disordered Ti-N alloy powders containing 35 at. % N in a high energetic ball mill under nitrogen gas flow at room temperature, in contrast to the TiN compound over a relatively large region of nitrogen content (~28-50 at. % N) in the equilibrium phase diagram.17 We have utilized x-ray diffraction, scanning electron microscopy, and transmission electron microscopy to monitor the crystal-to-compound phase transition in TiN alloy powders. The thermal stability of the TiN alloy was investigated by differential scanning calorimetry. It is the goal of this work to provide a unique process for producing nonequilibrium Ti-N alloy powders by the RBM method. Further, the mechJ. Mater. Res., Vol. 7, No. 4, Apr 1992
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anism of synthesis of the Ti-N alloy via RBM will be discussed. II. EXPERIMENTAL PROCEDURES High purity elemental titanium powders (99.99%, 100 /7,m) and high purity nitrogen gas (H2O and O2 < 5 ppm) were used as starting materials. In the present study, we have used a high energetic vibrating ball mill (SUPER-MISUNI, NEV MA8, Nisshin-Giken, Japan) equipped with a rotary pump and a gas flow system. Titanium powders were sealed in a stainless st
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