Abstract
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.
Highlights
There is a strong demand for the development of a novel ultrahigh-temperature (UHT) structural material as alternatives to the Ni-based superalloys, which can withstand the use above 1400 °C without requiring cooling
For deformation in the 0° orientation at 1400 °C, the yield stress does not show marked differences, the creep deformation behavior was significantly varied by the addition of the alloying elements
The addition of alloying elements all decreased the minimum creep strain rate (MCR) compared to that for the nonadded crystal, the magnitude of the decrease was different in each crystal
Summary
In the C40 structure have two-fold and six-fold rotational symmetries, respectively. As a result, three C11b-phase variants - variant 1 (V1), variant 2 (V2), and variant 3 (V3) – produced from the C40 single crystals are denoted as follows: V1: (0001)C40 //(110)C11b , [1210]C40 //[110]C11b , [1010]C40 //[001]C11b (1). To clarify the origin of the improvement in the creep resistance and its controlling factors in the CrIr-added crystals with the cross-lamellar microstructure, the temperature and applied stress dependencies of the creep deformation behavior were examined at the 0° and 45° orientations. The values of n and Q for the CrIr-added crystal with the cross-lamellar microstructure are relatively close to those measured for nonadded (Mo0.85Nb0.15)Si2 ternary crystals in both the 0° and 45° orientations This suggests that the mechanisms controlling the creep deformation behavior themselves do not greatly vary for the CrIr-added crystals compared to those for the nonadded crystal. A large improvement of the creep resistance was achieved in the 0° orientation
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