Brittle-ductile behavior of single crystals of MoSi2

MoSi2 is a candidate material to be used in oxidizing environments at temperatures higher than the upper limit for Ni-base superalloys (1100–1200°C). Recently, single crystals of MoSi2 with orientations other than [001] have been found to exhibit plastic flow at temperatures as low as room temperature, while [001]-oriented crystals exhibit appreciable plastic deformation only above 900°C. Five slip systems, {110)〈111], {011)〈100], {010)〈100], {023)〈100] and {013)〈331], have been identified to be operative in MoSi2 single crystals, depending on crystal orientation. Only {013,〈331] has a non-zero Schmid factor for the exact [001] orientation. However, the critical resolved shear stress for the slip system strongly depends on crystal orientation, in particular near [001]. It drastically increases as the compression axis approaches to [001]. Thus, [001]-oriented crystals can hardly be deformed at low temperatures. First the results of such deformation experiments of single crystals of MoSi2 are summarized. Finally the results of our recent study of the behavior of Si vacancies and their clusters in MoSi2 single crystals by positron lifetime spectrometry are summarized. The recovery of electron irradiated specimens begins at 240 K and is completed at 800 K. Thus, Si vacancies in MoSi2 become mobile at relatively low temperatures. The implications of these results for the deformation of MoSi2 are briefly discussed.