High-temperature deformation behavior of coarse- and fine-grained MoSi2 with different silica contents

Abstract

The elevated-temperature deformation behavior of polycrystalline molybdenum disilicide (MoSi 2 ), in the range of 1000 °C to 1350 °C at the strain rates of 10 Ϫ3 , 5 ϫ 10 Ϫ4 , or 10 Ϫ4 s Ϫ1 , 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) MoSi 2 samples, processed by hot pressing an elemental Mo ϩ Si powder mixture and having a grain size of 5 mm and oxygen content of 0.06 wt pct, have been compared with those of samples prepared by hot pressing of commercial-grade Starck MoSi 2 powder, with a grain size of 27 mm and oxygen content of 0.89 wt pct. While the fine-grained RHP MoSi 2 samples have shown higher yield strength at relatively lower temperatures and higher strain rates, the coarse-grained Starck MoSi 2 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 MoSi 2 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 MoSi 2 . Serrated plastic flow has also been observed in a large number of samples, mostly when deformed at specific combinations of strain rates and temperatures.

Effect of microstructural parameters and Al alloying on creep behavior, threshold stress and activation volumes of molybdenum disilicides

“…grain boundaries remain as sinks for vacancies and dislocations, and are able to relax the internal stresses. During compression tests [10] by using strain rates of 10 23 to 10 24 s 21 , it was observed that the yield strengths of RHP MoSi 2 tend to be higher than those of Starck MoSi 2 at lower temperatures or higher strain rates, while the trend is reversed with increasing temperature and decreasing strain rate (Fig. 6).…”

“…Fig. 7 shows the yield strengths of polycrystalline MoSi 2 , with 5 and 27 mm grain sizes, compression tested at strain rates of 10 23 and 10 24 s 21 [10] and MoSi 2 -5.5Al alloy [11] and MoSi 2 -9Al alloy [20], subjected to compression at strain rate of 10 23 s 21 . The yield strengths and the critically resolved shear stresses (CRSS) for {110}k111l slip in single crystals of MoSi 2 and Mo(Si 0.97 Al 0.03 ) 2 are also shown in Fig.…”

“…Another important factor strongly affecting the creep strain rate of MoSi 2 is the presence of silica particles (SiO 2 ) [4 -7,10,11]. During high temperature deformation, the SiO 2 particles at the grain boundaries flow to form intergranular film, which slides or cracks [6,7,10]. A high volume fraction of SiO 2 and reduction in grain size, both enhance creep rates; but it is of interest to examine how the two are interrelated.…”

Effect of Al addition on the microstructure and low-temperature reactivity to oxygen of pre-formed MoSi2

“…Despite these interesting properties, several important limitations have limited the use of MoSi 2 in practical applications. Indeed, MoSi 2 is characterized by (i) low fracture toughness at temperature below 1000°C [4], (ii) low strength and creep resistance at elevated temperature [5,6], (iii) structural disintegration at moderate temperature (between 400 and 600°C) due to poor oxidation resistance in this temperature range [7][8][9][10][11], which is known as ''pest'' oxidation, and (iv) spalling of protective SiO 2 scales under thermal cycling [12].…”

Microstructures and Mechanical Properties of Hot-Pressed -Matrix Composites Reinforced with SiC and Particles

“…However, as in the case of many such compounds, the current concern about these materials focuses on their low fracture toughness below the ductile-brittle transition temperature [5,6]. To improve the mechanical properties of these materials, the addition of a second phase to form composites [7][8][9][10] has been found to be effective. Silicon nitride (Si 3 N 4 ) has a high thermal shock resistance, due to its low thermal expansion coefficient and good resistance to oxidation when compared to other structural materials [11][12][13].…”