Focused Ion Beam Nanotomography of ruthenium-bearing nickel-base superalloys with focus on cast-microstructure and phase stability

The influence of rhenium and ruthenium on the multi component system nickel-base superalloy is manifold and complex. An experimental nickel-base superalloy containing rhenium and ruthenium within defined contents, named Astra, was used to investigate the influences of these two elements on the alloy system. The last stage solidification of nickel-base superalloys after Bridgman casting and the high temperature phase stability of these alloys, could be explored with the aid of focused ion beam nanotomography. FIB-nt therefore was introduced and realized at the chair of General Materials Properties of the University Erlangen-Nuremberg. Cast Astra alloys are like other nickel-base superalloys morphologically very inhomogeneous and affected by segregation. In the interdendritic region different structures with huge γ' precipitates are formed. These inhomogeneities and remaining eutectics degrade the mechanical properties, witch makes an understanding of the subsiding processes at solidification of residual melt important for the casting process and the heat treatment. This is why the last stage solidification in the interdendritic region was analyzed. With the help of focused ion beam nanotomography, three different structures identified from 2-D sections could be assigned to one original 3-D structure. It was pointed out, that only the orientation of the plane of the 2-D cut influences the appearance in the 2-D section. The tomography information was used to explain the development during solidification and to create a model of last stage solidification. The interdendritic region is solidifying under the development of eutectic islands. The structure nucleates eutectically epitaxially at primary dendrite arms, with formation of fine γ/γ' precipitates. During solidification the γ' precipitates coarsen in a rod-like structure, and end up in large γ' precipitates. Simulations and other investigations could approve this model. First three dimensional investigations of cellular colony formation in nickel-base superalloys generated interesting information about the high temperature phase stability of the alloy. 3-D reconstructions showed preferred orientations of TCP precipitates. The reconstructions could furthermore explain how to interpret and analyze 2-D sections properly. Near to the original grain boundary many TCP nuclei are existent, for example. Only a few nuclei continue growing through the cellular colony. In addition different TCP morphologies in 2-D sections can be explained by the reconstructions. In the case of the Astra1-22 alloy a discontinuous coarsening could be proved instead of discontinuous precipitation. The observation of former unknown TCP precipitates with γ/γ' core material, discovered by reconstructions, confirmed the assumed diffusion controlled growth mechanism. First pictures of TCP needles growing within the grain and cellular colony at the same time were made by tomography. 3-D quantifications of TCP nuclei density and TCP volume content led to reasonable results compared with 2-D data, also.