Faculty Publications
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Item Comparative hot corrosion performance of APS and Detonation sprayed CoCrAlY, NiCoCrAlY and NiCr coatings on T91 boiler steel(Elsevier Ltd, 2021) Sundaresan, C.; Rajasekaran, B.; Varalakshmi, S.; Santhy, K.; Rao, D.S.; Govindarajan, G.Hot corrosion performance of Atmospheric Plasma Spray (APS) and Detonation spray (DSC) CoCrAlY, NiCoCrAlY, and NiCr coatings on T91 steel were investigated at 650 °C for 100 cycles under Na2SO4-K2SO4-Fe2O3 mixed salt deposit in ambient air. The hot corrosion resistance of DSC coatings was found to be superior to their APS counterparts. Chromia and spinel oxides provided excellent corrosion resistance while no ?-Al2O3 was observed. DSC NiCr and APS NiCoCrAlY offered the most and the least corrosion resistance, respectively. DFT calculation was performed to validate the thermodynamic stability of each oxide and identify the precise oxide formation. © 2021 Elsevier LtdItem Understanding the initial stage oxidation and microstructural evolution of detonation sprayed NiCoCrAlY bond coat using in-situ high-temperature X-ray diffraction(Elsevier Ltd, 2022) Kala, V.; Santhy, K.; Govindarajan, G.; Rajasekaran, B.This study focuses on in-situ high-temperature X-ray diffraction (HT-XRD) investigation to understand the oxidation, microstructural evolution, and recrystallization of the NiCoCrAlY bond coat. HT-XRD analysis was carried out at a constant temperature of 1423 K for 3 h in a 10−4 Pa vacuum. The formation of α-Al2O3, Cr2O3, Co3O4, and NiCr2O4 oxides and microstructural evolution, such as homogenization of phases (γ and β) during in-situ HT-XRD, has been analyzed with FE-SEM (EDS), Density Functional Theory calculations and Rietveld refinement's support. β-(Ni,Co)Al phase depletion (by 77%) was found at 1423 K during the HT-XRD; however, no TGO formation was noticed. © 2022 Elsevier LtdItem Thermal expansion and microstructure evolution of atmospheric plasma sprayed NiCrAlY bond coat using in-situ high temperature X-ray diffraction(Elsevier B.V., 2023) Abhijith Vijay, V.; Santhy, K.; Govindarajan, G.; Rajasekaran, B.The paper focuses on in-situ high-temperature X-ray diffraction (HT-XRD) study on atmospheric plasma sprayed NiCrAlY coating. The sample was in-situ heated from 25 °C to 1150 °C in a controlled atmosphere (3 × 10−4 bar), and the corresponding X-ray diffraction patterns for different temperatures were recorded. The effect of temperature on crystallite size, lattice strain, and coefficient of linear thermal expansion was studied. Major phases identified are γ-Ni, γ’-Ni3Al, β-NiAl, and α-Cr. The formation of stable α-Al2O3 and spinel was found above 1000 °C. The transformation of β to γ’ and γ phase was observed as a function of temperature. The equilibrium phases and the thermal expansion of disordered Face Centered Cubic (FCC) and Body Centered Cubic (BCC) phases were predicted and supported by Thermo-Calc prediction for the stable temperature range. Results showed that the non-equilibrium microstructure produced by thermal spray process did not alter the thermal expansion behaviour. In-situ treatment resulted in microstructure and elemental homogenization. The thermal expansion and mechanism of phase evolution were discussed. © 2022 Elsevier B.V.Item Laser directed energy deposited Ti-48Al-2Cr-2Nb alloy: An investigation of high temperature oxidation behavior(Elsevier B.V., 2024) Gurugubelli, R.C.; Balla, V.K.; Rajasekaran, B.; Krishna, P.; Bontha, S.This study investigates high-temperature oxidation behavior and kinetics of Laser Directed Energy Deposited (LDED) Ti-48Al-2Cr-2Nb (Ti-48-2-2) alloy at 750 °C, 850 °C, and 950 °C, for 30, 60 and 100 h. Results reveal that the oxide-scale consists of alternating bands of TiO2 and Al2O3 and its stability is strongly dependent on the oxidation temperature and duration. At 850 °C and 950 °C, the oxide-scale delaminated following 100-h exposure. LDED Ti-48-2-2 exhibited an oxidation rate constant of 0.984 mg2 cm−4 h−1 at 850 °C (100 h) and 2.09 mg2 cm−4 h−1 at 950 °C (100 h), and an activation energy of 83.7 kJ mol−1 (850°–950 °C). LDED Ti-48-2-2 exhibited poor oxidation resistance compared to conventionally processed Ƴ-TiAl alloys. This can be attributed to the absence of N-rich layer and the typical nano-scale α2/γ banded lamellar microstructure observed in other processing routes. Post-process heat treatments can be utilized to obtain the desired microstructural features. © 2024 Elsevier B.V.Item Isothermal oxidation behavior of As-deposited and HIPed Ti-48Al-2Cr-2Nb alloy processed using Electron Beam Powder Bed Fusion(Elsevier Ltd, 2025) Gurugubelli, R.C.; Balla, V.K.; Rajasekaran, B.; Krishna, P.; Bontha, S.This work focuses on oxidation behavior of Electron Beam Powder Bed Fusion (EB-PBF) processed Ti-48Al-2Cr-2Nb at elevated temperatures. Two different sample conditions were considered: As-deposited (AD) and post-processed by hot isostatic pressing (HIPed). The oxidation studies were carried out at 750 °C, 850 °C, and 950 °C for 30, 60, and 100 h. The oxidized samples were analyzed for oxide layer growth and kinetics using Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDS), Raman Spectroscopy, and X-ray Diffraction (XRD) techniques. Results indicate that oxide layers are composed of alternative bands of TiO2 and Al2O3. These oxide layers spalled at 850 °C and 950 °C after an exposure of 100 h. The HIPed samples exhibited superior oxidation resistance when compared to AD samples, with an oxidation rate constant of 0.134 mg2 cm4 h?1 at 950 °C (100 h). The presence of homogenized microstructure with large nano-scale lamellar colonies aided in uniform oxide layer growth. EB-PBF samples exhibit fine fully lamellar microstructure due to the rapid heating and cooling cycles. Hence EB-PBF (AD and HIPed) samples exhibited better oxidation resistance when compared to conventionally processed Ti-48Al-2Cr-2Nb. © 2025 Elsevier B.V.Item On the merit of solute segregation and low angle grain boundary for thermal stability and thermal expansion of cold-sprayed CuCrZr(Elsevier Ltd, 2025) Abhijith Vijay; Sreerag, M.P.; Varalakshmi, S.; Santhy, K.; Singh, R.; Kondás, J.; Makineni, S.K.; Rajasekaran, B.The precipitation hardenable CuCrZr alloy is a potential alternative to copper for inner liners in rocket thrust engines. Cold spray manufacturing has been seen as a promising processing route to manufacture bulk additive structure of CuCrZr. This work reveals that the cold-sprayed as-deposited Cu-Cr-Zr alloy, in its inherent non-equilibrium state. It is highly stable up to 950 °C and exhibits lower thermal expansion than the equilibrium Cu-Cr-Zr alloy, deduced using HT-XRD and Thermo-Calc. Atomic-scale compositional and diffraction analysis using Atom Probe Tomography (APT) and Electron Backscatter Diffraction (EBSD) support the Zener pinning effect of Cr segregation near the grain boundaries, along with a large fraction of low-angle grain boundaries (LAGBs), that contribute to the high thermal stability and controlled thermal expansion of the deposit. Cold spray deposition naturally yields microstructural features that are conducive to high thermal stability and controlled thermal expansion, features which are comparable to the self-organized microstructures observed in segregation engineering (SE). © 2025 Elsevier B.V.