In-situ characterization of Mo-rich NiAl-(Cr,Mo) composites by synchrotron diffraction during directional solidification
During the past decades, materials based on the intermetallic phase NiAl have attracted interest due to its higher melting temperature, thermal conductivity, lower density and excellent high-temperature oxidation resistance as compared to Ni-based superalloys . However, the monolithic intermetallic B2-phase suffers from low creep resistance and insufficient strength at elevated temperatures as well as from poor fracture toughness at room temperature . These disadvantages can be compensated by alloying NiAl with refractory metals such as Mo and/or Cr . A composite formes with NiAl matrix and embedded reinforcing phase being a solid solution of the refractory metals. Most investigations were focused on directionally solidified alloys since it is known that an aligned microstructure improves the creep behavior with respect to load direction .
So far, investigations in terms of morphology and texture were based only on post process analysis. In agreement with literature, we found a lattice parameter difference at room temperature in NiAl-34Cr of 0.3 % only. In contrast, NiAl-10Mo exhibits a significant difference of 7.3 % in lattice parameter. Nevertheless, both binary eutectic alloys solidify with the same crystallographic relationship and preferred orientation with respect to the growth direction even though reorientation for strain energy reduction might be expected. In order to characterize the effect of the misfit strain on microstructural evolution further phase formation studies were focused on nucleation (texture selection) and growth (orientation-selective growth) at the solidification front. Therefore, a series of Mo-rich NiAl-(Cr,Mo), NiAl-10Mo and NiAl-34Cr alloys were directionally solidified and investigated in-situ. The resulting diffraction patterns at the solidification front for Mo-rich NiAl-(Cr,Mo) alloys were compared with the pseudo-binary alloys. Furthermore, we report on lattice parameter evolution near the solidus temperature as well as the occurrence of preferred orientation and crystallographic relationship in this system.
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