The mechanism of mechanical alloying of MoSi 2
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An incubation time exists for the formation of MoSi2 by mechanical alloying (MA). For the particular milling conditions employed, the bulk of MoSi2 formation occurs between 3 h and 12 min and 3 h and 13 min. This abrupt formation of MoSi2 during MA suggests that the reaction occurs by a form of self-propagating high-temperature synthesis (SHS), when a critical amount of stored energy (in the form of interfacial energy and cold work) is introduced by MA, and increases the adiabatic reaction temperature sufficiently to make the reaction self-sustaining.
I. INTRODUCTION MoSi2 is a brittle intermetallic/silicide ceramic with significant potential as the matrix phase of advanced high temperature structural composites1; it is refractory (mp 2020 °C), possesses good oxidation resistance, and has significant load-bearing ability at elevated temperatures.2"4 While MoSi2 is available as a nearstoichiometric powder from several sources, several reaction synthesis schemes, including combustion synthesis,5 the XD™ process,6 and mechanical alloying7 (MA), have all been suggested as a source of improved powders. An incubation time (a sharp increase in the rate of formation of the product phase at a specific time of milling) has been found in MA of Ta5Si3 (Ref. 8) and Nb 5 Si 3 (Refs. 8-11) intermetallics, which in the case of the former material involved an amorphous intermediate, as determined by x-ray analysis. The present work is concerned with the mechanism(s) involved in producing near-stoichiometric MoSi2 by MA, particularly the issue of possible incubation times.
In fact, MoSi2 did form quite abruptly. As shown in the XRD scans in Fig. 1, x-ray evidence for MoSi2 formation was lacking in powders milled for 3 h and 12 min [Fig. l(a)], whereas evidence for extensive MoSi2 formation was present from powders milled for 3 h and 13 min [Fig. l(b)]; using blended mixtures of Mo and MoSi 2 as standards, we estimated that only ~ 1 5 % unreacted Mo was present in the 3 h and 13 min sample. We refer, henceforth, to these as UR (unreacted) and R (reacted) powders. Unreacted Si was present in significant quantities in the UR samples, but the Si peaks were broadened so extensively by the milling that the peak intensities were attenuated by a factor of —15 or more. The major (111) and (220) Si peaks are easily visible in the XRD spectra of the UR powder, as shown in Fig. 2(a), but are not easily seen at the peak intensity scale used for Fig. l(b).
MoSi;
Mo and Si Spex Milled for 3 Hrs and 13 Min.
(103)
MoSi2 1
(110)
J. Mater. Res., Vol. 8, No. 6, Jun 1993 http://journals.cambridge.org
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