Synthesis and properties of Mo₅Si₃ single crystals

The ultra-high temperature structural intermetallic Mo₅Si₃ has been studied for alloy processing, physical properties, and mechanical behavior. High purity single crystals of Mo₅Si₃ have been synthesized by both optical floating zone and Czochralski methods. Structural, thermal, and elastic properties of Mo₅Si₃ single crystals were measured by X-ray powder diffraction, thermal mechanical analysis, and resonant ultrasound spectroscopy, respectively. Results show that the thermal expansion of Mo₅Si₃, a tetragonal structure with I4/mcm symmetry, is strongly anisotropic along the a and c directions with α_c/α_a = 2.2. Single crystal elastic moduli of Mo₅Si₃ indicate that it has less elastic anisotropy and lower shear modulus than most transition metal disilicides. The impacts of these physical properties on alloy processing and mechanical behavior are discussed. Room temperature Vickers indentation tests on the (100) and (001) planes have been performed for different orientations of the indenter diagonal and the corresponding hardness, fracture toughness, and deformation behavior have been obtained as a function of the crystallography. Finally, the physical properties and mechanical behavior of Mo₅Si₃ are compared with those of other high temperature structural silicides, e.g. MoSi₂.