High temperature deformation of MoSi2 single crystals with the C11b structure

Outstanding compressive creep strength in Cr/Ir-codoped (Mo0.85Nb0.15)Si2 crystals with the unique cross-lamellar microstructure

A (Mo_0.85Nb_0.15)Si₂ crystal with an oriented, lamellar, C40/C11_b two-phase microstructure is a promising ultrahigh-temperature (UHT) structural material, but its low room-temperature fracture toughness and low high-temperature strength prevent its practical application. As a possibility to overcome these problems, we first found a development of unique “cross-lamellar microstructure”, by the cooping of Cr and Ir. The cross-lamellar microstructure consists of a rod-like C11_b-phase grains that extend along a direction perpendicular to the lamellar interface in addition to the C40/C11_b fine lamellae. In this study, the effectiveness of the cross-lamellar microstructure for improving the high-temperature creep deformation property, being the most essential for UHT materials, was examined by using the oriented crystals. The creep rate significantly reduced along a loading orientation parallel to the lamellar interface. Furthermore, the degradation in creep strength for other loading orientation that is not parallel to the lamellar interface, which has been a serious problem up to now, was also suppressed. The results demonstrated that the simultaneous improvement of high-temperature creep strength and room temperature fracture toughness can be first accomplished by the development of unique cross-lamellar microstructure, which opens a potential avenue for the development of novel UHT materials as alternatives to existing Ni-based superalloys.

Plastic deformation of directionally solidified ingots of binary and some ternary MoSi2/Mo5Si3 eutectic composites

The high-temperature mechanical properties of directionally solidified (DS) ingots of binary and some ternary MoSi₂/Mo₅Si₃ eutectic composites with a script lamellar structure have been investigated as a function of loading axis orientation and growth rate in a temperature range from 900 to 1500°C. These DS ingots are plastically deformed above 1000 and 1100 °C when the compression axis orientations are parallel to [1[Formula: see text]0]_MoSi2 (nearly parallel to the growth direction) and [001]_MoSi2, respectively. [1[Formula: see text]0]_MoSi2-oriented DS eutectic composites are strengthened so much by forming a script lamellar microstructure and they exhibit yield stress values several times higher than those of MoSi₂ single crystals of the corresponding orientation. The yield stress values increase with the decrease in the average thickness of MoSi₂ phase in the script lamellar structure, indicating that microstructure refinement is effective in obtaining better high-temperature strength of these DS eutectic composites. Among the four ternary alloying elements tested (V, Nb, Ta and W), Ta is found to be the most effective in obtaining higher yield strength at 1400 °C.

Interfacial Structures of MoSi2-Mo5Si3 Eutectic Alloys

Script lamellar microstructures which occur in certain intermetallic eutectic alloys, such as in the Mo-Si system, may provide desirable mechanical properties. Arc-melted specimens of MoSi2-Mo5Si3 eutectic alloys which exhibit the interlocking lamellar phases were examined in this study. We have observed, by conventional transmission electron microscopy (TEM), an orientation relationship between the MoSi2 and the Mo5Si3 phases, (110)[001]1-2 ‖ (330)[110]5-3, which is consistent with the crystallographic coordinate transformation matrix. High resolution transmission electron microscopy (HRTEM) discloses the interfacial dislocations to be of <100> and 1/2<111> types and the interfaces are regularly faceted. A shear fault which may be consisting of antiphase boundary (APB)-coupled 1/6<331> superpartial dislocations was observed in MoSi2 grain near the interface. Crack propagation paths suggest that the eutectic has a strong, low energy interface which is consistent with the observations of a low index orientation relationship between the two phases and the faceted interfacial structures.

Plasticity Enhancement of MoSi2 at Elevated Temperatures Through the Addition of TiC

Monolithic MoSi2 and MoSi2-TiC particulate composites with 10 vol % and 15 vol % TiC were tested in compression between 950°C and 1200°C. The MoSi2-TiC composites can be deformed plastically at lower temperatures than MoSi2 can before brittle fracture occurs. The composites exhibit much lower strain hardening rates and attain zero strain hardening rate at much lower strains than monolithic MoSi2. The differences between the composites and monolithic MoSi2 in plasticity and in strain hardening behavior is attributed to efficient dislocation generation into the matrix from sources at the MoSi2-TiC interfaces.

A Perspective on MoSi2 Based Composites

MoSi2 based composites represent an important new class of “high temperature structural silicides”, with significant potential for elevated temperature structural applications in the range of 1200–1600 °C in oxidizing and aggressive environments. The properties of MoSi2 which make it an attractive matrix for high temperature composites are described and the developmental history of these materials traced. Latest results on elevated temperature creep resistance, low temperature fracture toughness, and composite oxidation behavior are summarized. Important avenues for future MoSi2 based composite development are suggested.