DESIGN OF MODERN HIGH NB-CONTENT ,-,' NI-BASE SUPERALLOYS
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Superalloy design can prove to be a very complex and challenging task, as certain elemental additions can significantly improve properties when added in high contents, however, exceeding their solubility limits can promote the formation of undesired phases at grain boundaries as well as grain interiors, and can quickly deteriorate the high temperature structural integrity and stability of the alloy, resulting in a catastrophic failure. Precipitate phases, such as " and ⌘, appear similar and are often mistaken for one another, leading to the need for a better fundamental understanding of their formation, required for developing innovative new classes of Ni-base superalloys. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored and compositional relationships were developed to facilitate the design of !-!0-("/⌘) Ni-base superalloys. The e↵ect of increasing Nb alloying additions on the formation and long term phase stability of topologically close packed (TCP) phases was studied. Elevated levels of Nb can result in increased matrix supersaturation and promote the precipitation of ⌘-Ni6AlNb along the grain boundaries in powder processed, polycrystalline Ni-base superalloys, while reduced Nb levels favored the precipitation of blocky Cr and Mo rich $ phase precipitates along the grain boundary. Evaluation of the thermodynamic stability of these two phases using Thermo-Calc showed that while $ phase predictions are fairly accurate, predictions of the ⌘ phase are limited. In addition, atom probe tomography (APT) was used to quantitatively assess grain boundary phase compositions and local segregation along the grain boundary before and after a 1000 hour thermal exposure at 800 "C. The complex network of $ phase precipitates that formed upon the thermal exposure and the characteristic interfacial segregation profiles were studied. In addition, elemental boron was observed to segregate to the grain boundary and phase interfaces, but did not form borides, due to the relatively low concentration of B atoms, resulting from a higher B concentration in the matrix. APT studies were also performed on MC carbides of the alloys and the formation kinetics and morphological differences between NbC and Hf doped NbC were explained using density functional theory (DFT) calculations of the formation energies of different facets of the MC carbide. Detailed electron microscopy and APT techniques were then used to systematically quantify the chemical and morphological instabilities that occur during aging of polycrystalline !-!0 Ni-base superalloys containing elevated levels of refractory alloying additions. The morphological changes and splitting phenomenon associated with the secondary !0 precipitates were related to the discrete chemical compositions of the secondary and tertiary !0 along with the phase compositions of the ! matrix and the ! precipitates that form within the secondary !0 particles. Compositional phase inhomogeneities led to the precipitation of finely dispersed tertiary !0 particles within the ! matrix and secondary ! particles within the secondary !0 precipitates, which, along with surface grooving of the secondary !0 particles, contributed to the inverse coarsening or splitting of the precipitates during aging. As recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys, understanding of elemental partitioning to each phase is essential and was studied via APT. Compositions of the constituent phases were measured in four high Nb-content !-!0 Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used to predict the solid solution strength behavior of the four alloys. The di↵erences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work. Finally, creep behavior of high Nb-content !-!0 Ni-Based superalloys was related to the formation of secondary phases mainly at grain boundaries. As secondary phases form, their brittle nature leads to crack formation, which can propagate under the tensile load and lead to premature failure of the alloy.