Journal of Alloys and Compounds, 205 (1994) 27-34 JALCOM 959 Nanocrystalline MoSi 2 phase formation induced by mechanically activated annealing E. Gaffet and N. Malhouroux-Gaffet ISITEM/CNRS, 'Far from Equilibrium Phase Transitions' Group, La Chantrerie, CP 3023, F-44087, Nantes Cedex 03 (France) Based on X-ray diffraction investigations, the nanocrystalline MoSi z phase formation induced by mechanically activated annealing is reported. The effective parameters of such a new powder processing method combining short duration mechanical alloying and low temperature isothermal annealing, Le. shock energy and frequency, as weIl as the annealing temperature have been investigated. Recent research in intermetallic compounds has been stimulated by the advent of new processing techniques. Intermetallic compounds such as aluminides and sili- cides are being extensively evaluated for high tem- perature structural applications. For applications in the range 800-1000 cC, current Ni base superalloys are being used. Although these alloys require cooling during operational periods, they have excellent oxidation and mechanical (strength, toughness and creep) properties in addition to being cost effective. The drawback of the superalloys is that they are high density materials. Recently a few alu- minides (such as Ni 3 Al and Ti 3 Al) have been considered as replacements for conventional superaUoys because of their room temperature ductility and lighter weight. However, these aluminides have poor oxidation resis- tance above 650°C and require coatings. At higher temperatures, between 1000 and 1600 cC, primary can- didates are Si base ceramic material. Although such materials have excellent oxidation resistance and lower density, their development is considered to be of higher risk because of their brittleness over the entire tem- perature range. Thus alternative candidates are under consideration, these are based on aluminide (NiAI, NbAl 3 , TaA1 3 ) and silicide (MoSi 2 , TisSi 3 ) matrix com- posites. Their lower density, higher melting points and high thermal conductivities make them attractive for high temperature engine applications. The aluminides are brittle at room temperature, have low strengths at the required high temperature and lack long-time ox- idation resistance above 1200 cc. A new class of silicides 0925-8388/94/$07.00 © 1994 Elsevier Sequoia. Ali rights reserved SSDf 0925-8388(93)00959-3 designed around MoSi z provides an alternative to struc- tural ceramics (for a detailed review, see Vasudevan and Petrovic [1]). Molybdenum disilicide (MoSi z ), with density of 6.24 g cm -3, melting point of about 2020 °C and excellent high temperature oxidation resistance (up to 1700 0c) [2], is being considered for structural uses at temper- atmes as high as 1250 °C for turbine engine applications. At these temperatures, the strength and oxidation re- sistances of this refractory disilicide phase is better than that of most metals and ceramic-based composite materials. The MoSi 2 phase is one of the three well- defined compounds that may be found in the equilibrium phase diagram between Si and Mo. Such a phase diagram indicates the formation of three well-defined compounds corresponding closely to the fonnulae Mo 3 Si (cubic phase with a lattice parameter equal to 0.489 nm, melting through a peritectic decomposition at 2117 0C), MosSi 3 (tetragonal phase with the following lattice parameters: a = 0.962 nm and c = 0.480 nm, congruent melting point at 2195 0c) and MoSi z . The MoSi z com- pounds exhibit two structures hereafter referred ta as the LT and HT phases corresponding to the low tem- perature phase and to the high tempe rature phase. The LT phase is body-centred tetragonal (crystal struc- ture: CllB type) with space group 14/mmm and lattice parameters a=O.321 nm and c=0.785 nm. The HT phase is an hexagonal phase with the following lattice parameters, a=0.460 nm and c=0.655 nm. The tem- perature of the LT to HT phase transition is 1850 cC. MoSi2 has been synthesized by various methods such as conventional arc-melting and casting, powder pressing and sintering (typically 1200 °C for 2 h as reported by