The creep-deformation behavior of (Mo 0.85 Nb 0.15)Si 2 crystals that were composed of C40 and C11 b two-phases with an oriented lamellar microstructure was examined over 1200-1400 °C under compression. The creep behavior of the crystals varied greatly depending on the orientation of the loading axis with respect to the lamellar interfaces. The steady-state creep strain rate (SSCR) when the loading orientation was parallel to the lamellar interfaces (0°-orientation) was approximately 2 orders of magnitude lower than that when the loading orientation was inclined by 45° (45°-orientation). Creep tests were performed also with single crystals of C40-single-phase (Mo 0.85 Nb 0.15)Si 2 , and proved that the C40-phase effectively acts as a strengthening phase in the creep behavior of the C40/C11 b duplex-phase crystals. The variant-1-type C11 b phase grains, which have a loading orientation parallel to [001], were also found to act as an effective strengthening component. The SSCR of the 0°-orientation was succeeded to be further decreased by controlling the lamellar microstructure via the addition of Cr. The SSCR of the 1 at.%-Cr-added lamellar-structured crystal under 300 MPa in the 0°-orientation was approximately a third of that of the non-added ternary crystal.

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a b s t r a c tVariations in the fracture toughness of lamellar-structured C40/C11 b silicides as a function of the microstructure were examined by using Cr-, Zr-, and Ir-added (Mo 0.85 Nb 0.15 )Si 2 crystals. Fracture toughness variation showed strong orientation dependence, governed by the occurrence frequency of delamination and crack deflection. The refinement of the lamellar microstructure by Ir-addition led to an increase in fracture toughness when the lamella boundary was laid perpendicular to the loading axis, although it decreased at other orientations. The variation in fracture toughness when the alloyed crystal was loaded parallel to the lamellar boundary qualitatively showed good agreement with the predictions by firstprinciples calculations on the cohesive energy of the C40/C11 b interface.

A (Mo0.85Nb0.15)Si2 crystal with an oriented, lamellar, C40/C11b 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 C11b-phase grains that extend along a direction perpendicular to the lamellar interface in addition to the C40/C11b 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.

The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11b lamellar-structured (Mo0.85Nb0.15)Si2 silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11b duplex-phase crystals. The variant-1-type C11b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.