PhD Thesis Colloquium: Ms. Sudeepta Mukherjee (16/01/25)
Thesis title:
Towards the development of high-strength, low-density Ni-based superalloys for gas turbine engine (GTE) applications
Faculty advisor(s):
Prof. Satyam Suwas and Prof. Surendra Kumar Makineni
When?
16th January, 2025 (Thursday), 10:00 AM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Ni-based superalloys represent a pioneering class of high-temperature materials essential for applications in gas turbines that power airplanes, steam turbines used for generating electricity, and catalytic reactors that operate in extreme conditions with high temperatures, stresses, and reactive gases. These alloys are distinguished by their exceptional combination of high-temperature strength, oxidation, and creep resistance, making them indispensable for advanced engineering applications. These superalloys derive their performance from forming Ni3(Al, Ti) γ′ precipitates with an ordered face-centered-cubic (fcc) based L12 structure embedded coherently in the fcc γ matrix. These precipitates provide outstanding thermal stability and mechanical strength during service.
However, the reliance of conventional superalloys on expensive, scarce, and heavy refractory elements such as Re, Ru, Ta, W, etc., and their nearing operational limits necessitates the development of innovative alternatives. This thesis attempts to address these challenges by designing Ni-based superalloys with low-density and cheaper refractory elements with superior or equivalent mechanical properties to conventional superalloys. First, a combination of 1) CALPHAD-based thermodynamic simulation, 2) electronic structure parameter-based Bo−Md approach, and 3) negative mixing enthalpy strategy was employed to reach a base composition of Ni-11Cr-9Al-8Ti alloy. Secondly, systematic variation of Co and Fe in the base alloy was carried out to explore the optimization of composition based on an improved combination of physical and mechanical properties, high-temperature microstructural stability via coarsening kinetics, and oxidation resistance. The designed alloy was set to Ni-30Co-11Cr-9Al-8Ti (Co30), having a low density of 7.5 g/cc with better coarsening resistance than other alloys but with poor room and high-temperature 0.2% yield strength. Thirdly, adding 2at.%V (Co30V2) led to an exceptional increase in 0.2% yield strength up to ≈ 1.1 GPa maintained at 670℃ and 770℃ with even better resistance to coarsening, oxidation and without affecting the γ’ solvus temperature. Atomic-scale compositional analysis reveals vanadium partitions to the γ matrix and interestingly segregates at the γ/γ′ interface. During high-temperature deformation, this behavior shifts dramatically, with vanadium exhibiting a reversal in partitioning to the γ’ and a reduction in composition at the interface. It is proposed that the redistribution of vanadium into the γ′ phase under stress is treated as a nano-diffusion process, where the coupled effects of thermodynamic gradients and applied stress drive solute migration. A comparative study of the deformation microstructure of Co30 and Co30V2 alloys after compression at 670℃ reveals the operation of different mechanisms.
In Co30 alloy, the microstructure shows extensive shearing of γ’ precipitates by the formation of superlattice-intrinsic-stacking-faults (SISFs) and superlattice-extrinsic-stacking-faults (SESFs) inside the γ.’ However, in Co30V2 alloy, the microstructure shows a high degree of dislocation pileups at the γ/γ’ interfaces forming dislocation networks and are unable to shear via SISF/SESF formation. Instead, in a few γ’ precipitates, shearing occurred via the formation of anti-phase-boundaries (APBs). Interestingly, these APBs are found to be enriched in V, Ti and Cr, indicating the operative of solute-drag during the shearing of γ’ precipitates. This difference in the deformation behavior might be attributed to three factors: V segregation at γ/γ’ 1) increases the interface stability by reducing the interfacial energy leading to pinning of matrix dislocations during loading and 2) stabilization of dislocation network by reducing the dislocation strain energy. 3) The reversal of V partitioning to γ’ during high-temperature deformation can lead to an increase in SISF/SESF planar fault energies. Hence, from the above three contributions, the alloy requires higher stress for shearing the γ’ precipitates via SISF/SESF formation to the extent such that the applied stress can reach the values that are enough for γ’ shearing via the formation of higher energy anti-phase-boundaries (APBs) instead of SISFs/SESFs. This makes the Co30V2 alloy stronger than the Co30 alloy and even commercially used Ni-based CMSX-4 alloy at high temperatures up to 770℃. Moreover, the Co30V2 alloy doesn’t contain any of the heavier and more expensive transition metals, making them relatively lighter (7.5 gm/cc) and cheaper with a higher specific 0.2% yield strength of 145 MPa.cc/gm at 770℃ as compared to commercially used Ni-based superalloys (MAR-M-247: 108 MPa.cc/gm, WASPALOY: 82.9 MPa.cc/gm and CMSX-4: 113 MPa.cc/gm).
On the microstructural stability of Co30V2 alloy at high temperatures, Vanadium addition significantly influences the coarsening kinetics of γ′ precipitates by reducing the γ/γ′ interfacial energy by segregating at the interfaces and creates a diffusion barrier that slows the coarsening process. Additionally, from coarsening data, vanadium addition potentially promotes a transition from matrix-diffusion-controlled coarsening to trans-interface diffusion-controlled coarsening, aligning with the TIDC model. The segregation of Vanadium minimizes solute fluxes across the interface, reducing the likelihood of rapid coarsening and ensuring consistent elemental partitioning between the γ and γ′ phases. By mitigating interfacial energy and slowing coarsening kinetics, Vanadium ensures the long-term thermal stability and mechanical performance of Co30V2, making it a robust candidate for high-temperature applications in energy and aerospace systems. With immense potential for further development and scalability, the alloy aligns seamlessly with sustainable alloy development goals, paving the way for next-generation materials capable of meeting the most demanding engineering challenges.