PhD Thesis Defence: Ms. Sudeepta Mukherjee (11/09/26)
Thesis title:
Design and Development of Low-Density, High-Strength, Cost-Effective Ni-Based Superalloys
Faculty advisor(s):
Prof. Satyam Suwas, Prof. Surendra Kumar Makineni and Prof. B.S. Murty (IITH)
When?
11th September, 2026 (Friday), 11:00 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 under 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-centred-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, and W, together with 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 comparatively cheaper alloying 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, designated B. Secondly, systematic variation of Co and Fe in the base alloy was carried out to explore the optimisation of composition based on an improved combination of physical and mechanical properties, high-temperature microstructural stability via coarsening kinetics, and oxidation resistance. Co addition increased the γ′ volume fraction, refined the γ′ precipitate size, increased the γ′ solvus temperature, and improved the mechanical strength. In contrast, increasing Fe content progressively destabilised the γ′ phase, with the γ′ solvus temperature decreasing from 1250 °C for B–10Co to 1181 °C for B–10Co–5Fe and 1120 °C for B–10Co–11.5Fe. Fe addition also produced a non-monotonic influence on oxidation resistance. B–10Co–5Fe promoted the formation of relatively porous NiFe₂O₄/FeCr₂O₄-type spinels, facilitating oxygen ingress, whereas B–10Co–11.5Fe promoted early spinel saturation and the development and stabilisation of a denser Cr₂O₃-rich protective barrier at 1000 °C. Thus, the influence of Fe on oxidation resistance was strongly composition- and temperature-dependent.
The effect of Co-for-Ni substitution on microstructural stability was further investigated in the B–10Co, B–20Co, and B–30Co alloys. Although a reduction in γ′ solvus temperature and an increase in γ/γ′ lattice misfit would conventionally be expected to accelerate coarsening, increasing Co content produced the opposite behaviour. Despite an approximately 65 °C reduction in γ′ solvus temperature, the activation energy for γ′ coarsening increased from approximately 156 to 302 kJ mol⁻¹. This enhanced coarsening resistance was attributed to composition-dependent multicomponent solute transport across the γ/γ′ heterophase interface. The transport-resistance parameter increased from approximately 8.81 × 10¹⁵ to 1.27 × 10¹⁶ s m⁻² with increasing Co content, while the dominant transport resistance shifted from Ni and Cr in B–10Co to Ni, Co, and Cr in B–30Co. Based on the combined optimisation, the designed alloy was set to Ni–30Co–11Cr–9Al–8Ti, designated B–30Co, having a low density of approximately 7.5 g/cc and superior coarsening resistance compared with the other alloys, but with relatively poor room- and high-temperature 0.2% yield strength.
Thirdly, adding 2 at.% V to B–30Co produced B–30Co–2V and led to an exceptional increase in 0.2% yield strength up to approximately 1.1 GPa, maintained at 670 °C and 770 °C, with even better resistance to coarsening and oxidation and without significantly affecting the γ′ solvus temperature. Atomic-scale compositional analysis reveals that vanadium preferentially partitions to the γ matrix and, interestingly, segregates at the γ/γ′ interface. This segregation reduces the interfacial energy and creates a diffusion barrier to solute transport. During high-temperature deformation, this behaviour shifts dramatically, with vanadium exhibiting a reversal in partitioning towards the γ′ phase and a reduction in its concentration at the γ/γ′ interface. It is proposed that the redistribution of vanadium into the γ′ phase under stress can be treated as a nanoscale diffusion process, where the coupled effects of thermodynamic gradients, applied stress, point-defect-assisted transport, and pipe diffusion along dislocation cores drive solute migration. A comparative study of the deformation microstructures of B–30Co and B–30Co–2V alloys after compression at 670 °C reveals the operation of distinctly different mechanisms.
In the B–30Co alloy, the microstructure shows extensive shearing of γ′ precipitates through the formation of superlattice intrinsic stacking faults (SISFs) and superlattice extrinsic stacking faults (SESFs) inside the γ′ phase. However, in the B–30Co–2V alloy, the microstructure shows a high degree of dislocation accumulation at the γ/γ′ interfaces, forming stabilised dislocation networks, while the precipitates are largely unable to shear via SISF/SESF formation. Instead, in a limited number of γ′ precipitates, shearing occurred through the formation of antiphase boundaries (APBs). Interestingly, these APBs were found to be enriched in V, Ti, and Cr, indicating the operation of solute drag during the shearing of γ′ precipitates. This difference in deformation behaviour might be attributed to three factors associated with V segregation at the γ/γ′ interface: 1) it increases interface stability by reducing the interfacial energy, leading to accumulation and pinning of matrix dislocations during loading; 2) it stabilises the interfacial dislocation networks by reducing the associated dislocation strain energy; and 3) the reversal of V partitioning towards γ′ during high-temperature deformation can lead to an increase in SISF/SESF planar fault energies. Hence, from these three contributions, the alloy requires a higher stress for shearing the γ′ precipitates via SISF/SESF formation, to the extent that the applied stress can reach values sufficient for γ′ shearing via the formation of higher-energy antiphase boundaries instead of SISFs/SESFs.
This makes the B–30Co–2V alloy stronger than the B–30Co alloy and even the commercially used Ni-based CMSX-4 alloy at high temperatures up to 770 °C. Moreover, the B–30Co–2V alloy does not contain any of the heavier and more expensive refractory transition metals commonly used in commercial Ni-based superalloys, making it relatively lighter, with a density of approximately 7.5 g/cc, and cheaper, with a higher specific 0.2% yield strength of 145 MPa·cc/g at 770 °C compared with commercially used Ni-based superalloys: MAR-M-247, 108 MPa·cc/g; Waspaloy, 82.9 MPa·cc/g; and CMSX-4, 113 MPa·cc/g.
Regarding the microstructural stability of the B–30Co–2V alloy at high temperatures, vanadium addition significantly influences the coarsening kinetics of γ′ precipitates by reducing the γ/γ′ interfacial energy through segregation at the interfaces and creating a persistent diffusion barrier that slows the coarsening process. The apparent activation energy for γ′ coarsening increases from approximately 355 kJ mol⁻¹ in B–30Co to approximately 387 kJ mol⁻¹ in B–30Co–2V. The improvement also extends to creep resistance: at 800 °C and 600 MPa, B–30Co–2V exhibits a minimum creep strain rate of 2.89 × 10⁻⁷ s⁻¹ compared with 4.54 × 10⁻⁶ s⁻¹ for B–30Co, representing a reduction of more than one order of magnitude. The segregation of vanadium minimises 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 B–30Co–2V, 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.