Physical and mechanical properties of single crystals of the T 2 phase in the Mo–Si–B system

Abstract

The T 2 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 T 2 phase with a composition of Mo–14Si–26B (at.%) using an optical floating zone method. Electrical resistivity of the T 2 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 ( α c/ α a) of the T 2 phase is approximately 1.4 at 500 °C which is significantly smaller than 2.2 for Mo 5Si 3. The bulk, shear and Young's moduli and the Debye temperature of the T 2 phase are slightly higher than those of Mo 5Si 3. 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.