Phase relations and substructure in single-crystal MoSi 2

Abstract

Single-crystal specimens of MoSi 2 were found to have a second phase present, identified as Mo 5Si 3 by X-ray diffraction and electron microprobe analyses. Isothermal annealing runs on bulk samples in vacuum at temperatures ranging from 1200 ° to 1600 °C for 24 h showed an increase in Mo 5Si 3 content with increasing annealing temperature. High-temperature X-ray diffraction on thin layers in vacuum showed an increase of Mo 5Si 3 content with a concomitant decrease of MoSi 2 up to 1550 °C, using Mo heaters. From 1550 ° to 1720 °C, a decrease in Mo 5Si 3 content was noted. Mass spectrometry up to 1700 °C in a vacuum of 10 −6 mm Hg did not reveal Mo-Si molecular species. Samples etched with 4 H 2 O: 1 HF: 2 HNO 3 showed the presence of individual and polygonized, chain-like, elliptical etch pits on the (001) plane. Rectangular, triangular, spiral, and hexahedral etch pits were found primarily on the (100) plane, the lattice indicating the pseudo-hexagonal character of MoSi 2 by analogy with WSi 2. Primary slip produced by hardness indentations at room temperature was found to be of the {100} < 001 > type, and secondary slip of the {110} < 001 > type was also determined for MoSi 2. The Knoop microhardness values at 100 g loads ranged from 870 to 1300 kg/mm 2 for MoSi 2. Mo 5Si 3 was found to have a microhardness value ranging from 1190 to 1330 kg/mm 2.