The synthesis of NbSi 2 by mechanical alloying
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The synthesis of NbSi2 by mechanical alloying Taiping Lou, Guojiang Fan, Bingzhe Ding, and Zhuangqi Hu National Key Lab for RSA, Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China (Received 21 March 1996; accepted 31 July 1996)
The stoichiometric intermetallic compound NbSi2 has been synthesized by mechanical alloying (MA) elemental Nb and Si powders. The alloying process has been investigated by means of x-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was found that the formation of the Nb2 Si intermetallic compound occurs abruptly after 65 min of milling without any interruptions during the alloying process. However, short interruptions at a 5 min interval during ball milling result in a gradual reaction for the formation of the NbSi2 compound as well as a new metastable bcc structured solid solution. We conclude that the temperature rise during mechanical alloying plays an important role in initiating the abrupt reaction after an incubation milling time.
Mechanical alloying (MA) has been widely used to synthesize various kinds of metastable phases, such as extended solid solutions that are otherwise immiscible,1,2 amorphous alloys,3–5 nanostructured phases,6 and quasicrystalline phases.7 Mechanical alloying has been considered as a solid-state reaction, during which the alloying is accompanied by a gradual interdiffusion process. The kinetics during mechanical alloying is controlled by various kinds of defects (dislocations, grain boundaries, antiphase boundaries, etc.) created during the collision events. More recently, high-energy ball milling has been employed to prepare high temperature intermetallic compounds.8,9 The refractory metal silicide NbSi2 is receiving interest as a potential candidate material for replacing nickel and cobalt superalloys in high temperature applications.10 This metal silicide is difficult to produce by the conventional metallurgical technique due to its high melting temperature. Although some primary work on MA of NbSi2 has been done,11 the structural changes in NbSi2 during MA are insufficiently understood. In this paper, we used MA to directly synthesize the NbSi2 intermetallic compound. The solid-state reaction occurring between the elemental Nb and Si powders during mechanical alloying has been investigated by means of x-ray diffraction (XRD) patterns and differential scanning calorimetry (DSC). The reaction was found to occur by a form of self-propagating, hightemperature synthesis if the machine was not interrupted during the whole alloying process, and via a gradual diffusion process if interrupted at 5 min intervals. The pure elemental Nb (99.5%, 320 mesh) and Si (99.9%, 250 mesh) were mixed to give the desired average composition (NbSi2 ) and sealed under purified argon in a cylindrical steel vial together with the steel 1172
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J. Mater. Res., Vol. 12, No. 5, May 1997
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balls. The mechanical alloying was perform
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