Studying the effect of Re on the Co-Ni-Al-Ti-Nb-Cr superalloy’s coarsening kinetics and establishing the high-throughput diffusion couple approach for alloy design

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PhD; 2023; Materials Engineering

The studies are conducted to understand the effect of Re addition on microstructural evolution and coarsening kinetics of new Co-Ni-Al-Ti-Nb-Cr based superalloys. Lattice misfit is reduced by adding Re, confirmed from high-resolution XRD. As a result, the morphological transition from cuboidal to rounded cornered cubes is observed. Re solubility limit in the alloy is 3 at. % and excess Re promote the formation of TCP phases. Through APT analysis, it was found that there was no segregation of Re at the / interface. The Re addition helps retain the 0.2% proof strength up to 870°C with strength values greater than 650 MPa. However, the absence of yield strength anomaly (YSA) with Re is observed.
To understand the coarsening kinetics behavior of  precipitates in the alloys, isothermal heat treatment at temperatures of 900, 950 and 1000C for various times is conducted. By studying the temporal evolution of the following arameters during coarsening, the interpretations are made, and the correlations are established: precipitate size, PSD, lattice misfit, partitioning coefficients, morphological evolution, volume fraction, and micro-hardness. The rate constants (K) values for this class of alloys are comparable and better than many existing superalloys. Additionally, the activation energies for coarsening of the present alloys are estimated to be 260 and 240 kJ/mol, respectively, when 2 and 3 at. % Re are added. The pseudo-binary diffusion couple approach is introduced to estimate the interdiffusion coefficients in a multi-component system, in which only two elements will participate in developing the composition profiles. The same approach is examined for designing new alloys and validated for a recently developed superalloy system, Co-Ni-Al-Mo-Ta-Ti, where the effect of Cr is considered. The heat treatment conditions are established to transform the gradient of Cr in diffusion couple into corresponding microstructural evolution. The following changes can be evaluated using the diffusion couple method: morphological transition from cuboidal to spherical, evolution of precipitate volume fraction, the solubility limit of the Cr for the appearance of TCP phases, the evolution of micro-hardness and elastic modulus and the oxidation behavior (top oxide grain morphology and layer thickness).