Structure-structure-property relationships in eutectic alloys of the NiAl-Cr-Mo system

Due to the improved creep resistance at high temperatures as well as higher fracture toughness at room temperature compared to monolithic NiAl, two-phase NiAl-(Cr,Mo) alloys are the most promising candidates for high-temperature applications, among other eutectic NiAl-X alloys. Directionally solidified fibrous NiAl-34Cr and NiAl-10Mo as well as lamellar Cr-rich NiAl-(Cr,Mo) alloys were characterized in detail in literature with respect to the mechanical and oxidative properties. However, the orientation relationship and preferred orientations have been often neglected and only adopted from literature or cross-checked on the basis of small investigated regions. This caused inconsistent information in the literature. In addition, the eutectic trough in the quasi-ternary NiAl-Cr-Mo system has been calculated and published recently, allowing new eutectic alloys to be considered. Therefore, the aim of the present work is to obtain a detailed characterization of microstructural evolution of the already established and new eutectic NiAl-(Cr,Mo) alloys in order to fully describe the eutectic trough. With this purpose, bridging the available process steps were carried out in the as-cast state and the as-directionally solidified state on different length scales and at different temperatures ranging from room to liquidus temperature. For the investigations at liquidus temperature and during directional solidification, a device was used which was specially designed for in-situ observation by using synchrotron radiation. Apart from alloys with fibrous and lamellar morphology, alloys with mixed morphology were also observed in the as-cast state. For mixed morphologies, in-situ X-ray diffraction experiments accompanied by EDX measurements revealed the formation of a gradient in composition for the solid solution. Thermodynamic calculation predicted an extended solidification interval and a miscibility gap at lower temperature, which both can account for the formation of a compositional gradient. In order to evaluate which is the crucial region for the microstructure evolution, Scheil-simulations were compared with experimental investigations during rapid cooling. In addition, long-term heat treatments were carried out. It could be excluded that the predicted miscibility gap causes the inhomogeneous solid solution. This forms by segregation effects starting just below the liquidus temperature. Texture analysis performed directly at the solidification front utilizing the in-situ setup, demonstrated that the eutectic alloys tend to crystallize with a preferential orientation. Both,〈001〉and〈111〉directions parallel to growth direction were observed with the established cube-on-cube orientation relationship of the two phases. For the second detected orientation relationship of the phases, which can be described by a 60° rotation about the〈111〉direction, only the〈111〉direction parallel to growth direction solidifies. In order to describe the influence of the lattice mismatch on the resulting preferred orientation, the lattice parameter evolution of both phases was analyzed for all process stages. The dependencies were discussed as a function of alloy composition as well as a function of growth velocity and with respect to the growth direction. The transition of the preferred crystallographic orientations causes no change in morphology. For the resulting morphology, the orientation relationship is crucial.