PhD Thesis Defence: Mr. Kandula Muni Kumar (22/09/23)
Thesis title:
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
Faculty advisor(s):
Prof. Aloke Paul and Prof. Chattopadhyay K
When?
22nd September, 2023 (Friday), 11:30 AM (India Standard Time)
Where
K I Vasu Auditorium, Dept of Materials Engineering.
Abstract
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 with addition of Re, confirmed from high-resolution XRD. As a result, the morphological transition from cuboidal to rounded cornered cubes with Re addition is observed. Re solubility limit in the alloy is 3 at. %, and excess Re promoted the formation of TCP phases. Through APT analysis, it was found experimentally that there was no segregation of Re at the / interface. With the addition of Re, the 0.2% proof strength is retained up to 870°C with strength values greater than 650 MPa. However, absence of yield strength anomaly (YSA) with Re is observed.
To understand the coarsening kinetics behavior of precipitates in the alloys, the study is conducted by isothermal treatment at t= emperatures of 900, 950 and 1000C for various time periods. By studying the temporal evolution of the following parameters during coarsening, the interpretations are made, and the correlations are established among 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 energy for coarsening is estimated to be 260 and 240 kJ/mol, for addition of Re 2 and 3 at. %, respectively.
Pseudo-Binary diffusion couple approach is introduced to estimate the interdiffusion coefficients in multicomponent system, in which only two elements will participate in developing the composition profiles. The same approach is examined for alloy designing and validated for recently developed alloy system, Co-Ni-Al-Mo-Ta-Ti superalloy, in which 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 evaluate through diffusion couple method: morphological transition from cuboidal to spherical, precipitate volume fraction, solubility limit of the Cr for TCP phases, micro-hardness, elastic modulus, oxidation behavior (top oxide grain morphology and layer thickness).