Synthesis of MoSi₂–TiC nanocomposite powder via mechanical alloying and subsequent annealing

Abstract

MoSi₂–30 wt.% TiC nanocomposite powder was successfully synthesized by ball milling and following heat treatment. Effect of milling time and annealing temperature were investigated. The products synthesis and reactions progress were characterized by XRD. Morphology and microstructure of milled powders were monitored by SEM and TEM, respectively. Results showed that the synthesis of this composite begins after 10 h of milling and progresses gradually up to 30 h of milling. MoSi₂–TiC composite was completely synthesized after annealing of 30 h milled powder at 900 °C. On the basis of Reitveld refinement method, the mean grain size and microstrain of 13.2 nm and 0.44% were obtained, respectively for 30 h milled powder that is in consistent with TEM image. In the spite of grain growth and strain release, this nanocomposite powder maintained its nanostructure after annealing.

Introduction

MoSi₂ is a very promising intermetallic compound as a high-temperature structural material because of its much higher melting point of 2030 °C. At present, it is mainly used as a heating element of furnaces operating in air at temperatures as high as 1750 °C, but many studies indicated that materials based on MoSi₂ had a wide variety of high-temperature structural applications such as high-temperature heat exchanger, turbine aircraft engine hot-section components like blades, vanes, combustors and nozzles, automotive turbocharger rotors and valves [1], [2], [3], [4]. Its oxidation resistance at high temperatures is excellent, clearly the best of the silicides and nearly as good as silicon carbide SiC due to the formation of protective silicon dioxide SiO₂ layers.

However, MoSi₂ has the intrinsic disadvantages such as low fracture toughness at room temperature and poor tensile creep resistance at high temperature. Therefore, it is imperative to increase the room temperature fracture toughness and high-temperature tensile creep resistance of monolithic MoSi₂ by means of proper strengthening and toughening [5]. Fortunately, it is feasible to improve the high-temperature mechanical properties of monolithic MoSi₂ by the introduction of a suitable second phase such as SiC, Al₂O₃, Si₃N₄, CrSi₂, and TiC [6], [7], [8], [9], [10], [11], [12], [13], [14].

Another possible mechanism to improve mechanical properties is to prepare these materials in nanostructure. MoSi₂–TiC nanocomposite can be produced easily by direct mixing of its components [15], [16], [17], [18], [19]. But the resulting heterogeneous microstructure and high cost of the starting materials are two important setbacks of this method. Alternatively, MoSi₂–TiC nanocomposite powder can be synthesized through high-energy reactive milling of mixtures of Mo, Si, Ti and C powders. Mechanical alloying (MA) has been used for preparing thermally stable metallic glasses and amorphous alloys, nanocrystalline and nanocomposite materials, and refractory hard materials, carbides, and hydrides [20].

The aim of this work is in situ synthesis of MoSi₂–TiC nanocomposite powder by mechanical milling of its elemental powders. Formation of this composite will be studied by thermodynamic discussions and its microstructure will be investigated by Reitveld refinement method.