Brittle-ductile behavior of single crystals of MoSi 2

MoSi 2 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 MoSi 2 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 MoSi 2 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 MoSi 2 are summarized. Finally the results of our recent study of the behavior of Si vacancies and their clusters in MoSi 2 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 MoSi 2 become mobile at relatively low temperatures. The implications of these results for the deformation of MoSi 2 are briefly discussed.