Physical and mechanical properties of single crystals of the T2 phase in the Mo–Si–B system

The T2 phase in the Mo–Si–B system is a refractory molybdenum borosilicide with great potential for ultra-high temperature structural applications. We recently succeeded in growing high purity single crystals of the T2 phase with a composition of Mo–14Si–26B (at.%) using an optical floating zone method. Electrical resistivity of the T2 phase is isotropic with respect to the a and c axes and it monotonously increases with temperature from ∼40 μΩcm at room temperature to ∼80 μΩcm at 800 °C. The anisotropy ratio of thermal expansion coefficient (αc/αa) of the T2 phase is approximately 1.4 at 500 °C which is significantly smaller than 2.2 for Mo5Si3. The bulk, shear and Youngʹs moduli and the Debye temperature of the T2 phase are slightly higher than those of Mo5Si3. Slip on [001]{100} was observed on [021] oriented specimens both compressed and crept at 1500 °C. However, slip on 〈100〉(001), which has been reported to be operative, was not observed. [001] dislocations are mostly straight and long, and tend to align along their edge orientation. The preference for slip on [001]{100} is interpreted on the basis of the crystal structure. The steady-state creep rate is low under the creep conditions. It is, for example, 3.1×10−8 s−1 at 1500 °C and 432 MPa. The activation energy and stress exponent for creep was found to be ∼740 kJ/mol and 6.8, respectively.