ISSN 1061-3862, International Journal of Self-Propagating High-Temperature Synthesis, 2007, Vol. 16, No. 2, pp. 79–86. © Allerton Press, Inc., 2007.

Reaction Mechanism for SHS of MoSi2 from Mechanically Activated Powder Mixtures1 G. Cabouroa, S. Chevaliera, E. Gaffetb, A. S. Rogachevc, D. Vreld, N. Boudete, and F. Bernarda a LRRS–UMR

5613 CNRS/University of Burgundy, BP 47870, F-21078 Dijon, France 5060 CNRS/UTBM, Site de Sévenans, F-90010 Belfort, France c Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia d LIMHP UPR 1311 CNRS, Paris-Nord, 99 Av. J.-B. Clement, F-93430 Villetaneuse, France e Laboratoire de Cristallographie, CNRS, F-38042 Grenoble, France e-mail: [email protected] b NRG–UMR

Received January 9, 2007

Abstract—Controlling the Mechanically Activated Self-propagating High-temperature Synthesis (MASHS) process is exceedingly important for production of MoSi2-based materials with a desired microstructure. Consequently, it seemed essential to carry out Time-Resolved X-ray Diffraction (TRXRD) experiments using an X-ray synchrotron beam (DM2, ESRF Grenoble) coupled with IR thermographic measurements to monitor in situ the structural and thermal evolutions taking place during SHS. The versatility of this technique and new possibilities offered by the design of new sample holders have already been proved. In addition, this work clearly shows that this equipment is perfectly adapted to investigating phase transitions occurring during the MASHS process in the Mo–Si system. Keywords: nanoscale materials, mechanical milling, self-propagating high-temperature synthesis (SHS), phase transitions PACS numbers: 81.05.Je, 81.07.b, 81.20.b DOI: 10.3103/S1061386207020045 1

INTRODUCTION

SHS, the synthesis time is markedly shorter than that in case of powder metallurgy [7–9]. Until recently, it has been difficult to investigate SHS reactions by conventional methods in view of the high temperatures involved and high burning velocities. Indeed, the conventional methods did not allow determining the role of liquid phase, the effect of intermediate phases and transient species, and other parameters on the microstructure of end product. Earlier studies have demonstrated the influence of mechanical activation on the kinetics of the SHS process. Recently, realtime investigations of structural changes in situ using a synchrotron radiation shed new light on thermal evolution of the combustion area. A fast detection system— high speed detector and IR camera—made possible to monitor the course of phase transformations taking place in a high-temperature reactor.

Molybdenum disilicide (MoSi2) is a promising material for high-temperature applications. It has a high melting point (2030°C), high hardness, and good oxidation resistance compared to other refractory silicides and intermetallic compounds [1]. This material is being used mainly as furnace heating elements and coating materials for Mo and other refractory metals. Several techniques have been developed to produce nanostructured massive MoSi2, such as solid-state reactions [2], spray forming [3], and mechanical alloying [4, 5]. The large negative enthalpy of MoSi2 formation (∆Hf = −131.8 ± 8.4 kJ mol–1 [6]) can be expected to afford the SHS of this compound from Mo + Si powder mixtures, which was confirmed by several publications. In this technique, a compact sample is prepared by cold pressing of a stoichiometric powder mixture. Upon local heating at one side, the combustion front (heating rate 1000–3500 K s–1) begins to gradually spread over the entire sample leaving behind a target product. In case of 1 The

EXPERIMENTAL Mixtures of pure Mo powder (Sigma-Aldrich, 100 mesh, 99.9% pure) and Si (Sigma-Aldrich, 325 mesh, 99% pure) were co-milled in a Fritch planetary ball mill (vario-mill P4 Pulverisette) [20]. Si and

text was submitted by the authors in English.

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