Effects Of Refractory Elements On Phase Separation Behavior In Model Ni-Al-Cr Based Superalloys

Ni-based superalloys are the current materials utilized for turbine blades of aerospace jet engines and land-based natural-gas fired power generators, due to their superior mechanical properties and corrosion, oxidation, and creep resistance at elevated temperatures. Their excellent high-temperature properties are derived from a unique microstructure: a high volume fraction of the ordered γ’(L12)-precipitates coherently embedded in a disordered γ(FCC)-matrix. Ternary Ni-Al-Cr systems are important as they represent the basis of commercial superalloys, which can contain as many as 12 or 13 microalloying elements, especially high melting point refractory elements such as W, Re, Ru, Nb or Ta. These refractory element atoms are distributed in both phases, playing various important roles. It is essential for both science and industry to obtain a better understanding of the roles played by refractory elements in the process of phase separation behavior in Ni-based superalloys.At Northwestern University, a broad systematic program have been implemented to study the effects of refractory elemental additions on model Ni-based superalloys. Based on the above research work, in the present research, the effects of refractory(R) elements(R = Re, Ru, W, or Ta) on the nanostructure, chemical compositions and γ/γ’ interfacial properties in Ni-Al-Cr based superalloys are studied, mainly utilizing both APT and First-principles calculations.The main findings and conclusions are summarized as follows:(1) The effects of both W and Re additions on the temporal evolution of nanostructurein a model Ni-Al-Cr based alloy are investigated. Based on the APT results, spheroidal γ’-precipitates are observed even in the as-quenched specimen. A significant fraction of the precipitates interconnected with other precipitates by necks at an aging times of 0 and 0.25 h, which indicates that the coarsening of the precipitates occurs as a result of the coagulation-coalescence mechanism. The volume fraction of the precipitates,achieves an equilibrium value at an aging time of 0.25 h. The average radius of the precipitates increasesand concomitantly the number densitydecreaseswith increasing time. For the aging times of 0.25 h and longer, the temporal exponents for coarsening for precipitate mean radius and number density are close to the predictions of UO model.(2)The effects of W, Re additions on the chemistryevolution are also investigated. The Ni concentrations barely change with increasing aging time in thematrix and precipitates.The Cr and Re concentrations in the matrix increase, while those of Al and W decrease with increasing aging time. Conversely, the concentrations of Al and W increase with increasing time in the precipitates, while those of Cr and Re decrease. W and Re are slow species, their partitioning behaviorbetween two phases evolves more slowly, which causes the composition in the quinary alloy equilibrates more slowly. The interfacial compositional widths in the model alloy have a tendency to decrease with increasing aging time. By 256 h, the relative significance of the interfacial region is close to zero, indicating that the interface is close to being a sharp interface.(3) Combining APT and microhardness measurements, the distributing characteristies of alloying elements in two phases and coherent interface are studied in several model Ni-Al-Cr based alloy. Rhenium and Ru atoms partition preferentially to the γ-matrix, while the W and Ta atoms partition to the γ’-precipitates. The above four elements all substitute on the Al-sublattice of the L12 structure in precipitates. With the refractory elemental additions, more Ni atoms stay in the matrix, and the partitioning of Al to the γ’-precipitates and Cr to the γ-matrix is enhanced. The effectiveness for the above two enhancement follow the same order. The added refractory elementsincrease microhardness as a result of the additional solid-solution strengthening and the concomitant increase in the γ’-precipitate volume fraction.(4) A deep and systematic study is implemented to explore the details and mechanisms of Ni retention excesses in Ni-Al-Cr based superalloys with refractory(R) elements(R = Re, Ru, W, or Ta), after long time aging. The spatial correlations and the binding conditions among R and S(S = Al, Cr) elements are presented, combining the experimental APT partial radial distribution function(RDF) results and first-principles calculations of the binding energy between R and S atoms. Strong attractivechemical binding energies between R-elements and solute(S) atoms are found in Ni-Al-Cr based alloys. The R-S binding energies cause changes in thecompositional diffusion fluxvectors in and out of γ’(L12)-precipitates, which result in larger solvent Ni retention excessesand wider interfacial compositionalwidthsat 256 h, when comparedwith the baseNi-Al-Cr alloy. Refractory elements are slow diffusers in nickel and the attractive R-Cr binding energiesdecelerate the solute diffusional fluxes, which results in a decrease of the Ni diffusivity, which in turn hinders the flux of Ni atoms away from the γ/γ’ interfaces.