Mo5Si3 single crystals: physical properties and mechanical behavior

The materials processing, physical properties and mechanical behavior of an ultrahigh temperature structural silicide, Mo5Si3, have been studied. High purity single crystals of Mo5Si3 have been synthesized by both optical flo ating zone and Czochralski methods. The thermal and elastic properties of t he Mo5Si3 single crystals were experimentally measured. Results show that M o5Si3 has significant thermal expansion anisotropy along the a and c direct ions with alpha(c)/alpha(a) = 2.2. The single crystal elastic moduli of Mo5 Si3 indicate that it has less elastic anisotropy and lower shear moduli tha n transition metal disilicides. Tensile stresses of up to 1.8 GPa can devel op at grain boundaries after cooling from the melting point due to the ther mal expansion mismatch in Mo5Si3, causing grain boundary cracking during pr ocessing of polycrystals. Room temperature Vickers indentation tests on (10 0) and (001) planes have been performed with different indenter diagonal or ientations and the orientation dependence of hardness and fracture toughnes s of Mo5Si3 single crystals have been obtained. The corresponding deformati on and fracture modes have been revealed by microscopy studies. Finally, a comparison of Mo5Si3.with other high temperature structural silicides is di scussed. (C) 1999 Elsevier Science S.A. All rights reserved.