High-temperature deformation behavior of coarse- and fine-grained MoSi(2) with different silica contents

The elevated-temperature deformation behavior of polycrystalline molybdenum disilicide (MoSi2), in the range of 1000 °C to 1350 °C at the strain rates of 10-3, 5 × 10-4, or 10-4 s-1, has been studied. The yield strength, post-yield flow behavior comprising strain hardening and serrations, as well as some of the deformation microstructures of reaction-hot-pressed (RHP) MoSi2 samples, processed by hot pressing an elemental Mo + Si powder mixture and having a grain size of 5 (mu)m and oxygen content of 0.06 wt pct, have been compared with those of samples prepared by hot pressing of commercial-grade Starck MoSi2 powder, with a grain size of 27 (mu)m and oxygen content of 0.89 wt pct. While the fine-grained RHP MoSi2 samples have shown higher yield strength at relatively lower temperatures and higher strain rates, the coarse-grained Starck MoSi2 has a higher yield at decreasing strain rates and higher temperatures. The work-hardening or softening characteristics are dependent on grain size, temperature, and strain rate. Enhanced dislocation activity and dynamic recovery, accomplished by arrangement of dislocations in low-angle boundaries, characterize the deformation behavior of fine-grained RHP MoSi2 at a temperature of 1200 °C and above and are responsible for increased uniform plastic strain with increasing temperature. The silica content appears to be less effective in degrading the high-temperature yield strength if the grain size is coarse, but leads to plastic-flow localization and strain softening in Starck MoSi2. Serrated plastic flow has also been observed in a large number of samples, mostly when deformed at specific combinations of strain rates and temperatures.