Faculty Publications
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Item Thermal expansion of Crofer 22 APU steel used for SOFC interconnect using in-situ high temperature X-ray diffraction(Elsevier Ltd, 2023) Manjunath, N.; Santhy, K.; Rajasekaran, B.Crofer 22 APU is ferritic stainless steel extensively used as metallic interconnect material in Solid Oxide Fuel Cell (SOFC) applications. The interconnects are exposed to both oxidizing and reducing atmospheres at high temperatures. As SOFCs are operated above 700 °C, understanding the thermal expansion behavior of the interconnect material with other components (anode, cathode, electrolyte) of the fuel cells is essential. Metallic interconnects should have a matchable thermal expansion to other ceramic materials such as anode, cathode, and solid electrolyte used in SOFCs. The present study evaluates the thermal expansion of Crofer 22 APU steel from 25 to 950 °C in a controlled atmosphere (10-4 mbar pressure) using in-situ high-temperature X-ray diffraction (XRD). The XRD patterns were analyzed using the ‘High Score Plus Software’ attached to the system, and the phases were identified using the standard Crystallographic Open Database (COD). The coefficient of thermal expansion (CTE) was determined based on the change in lattice parameter/peak shift to a lower 2θ value as a function of temperature. The normal XRD data showed no oxide formation on the Crofer steel after heating until 950 °C in in-situ high-temperature conditions. The peak shift to the lower 2θ degree observed in the XRD data was due to the relaxation of residual stress upon heating. The isothermal section and phase fraction of Crofer 22 APU alloys are analyzed with the help of thermo-calc with the iron database of TCFE7. The Fe-rich bcc phase was found to be stable up to high temperatures. The major phases are the Fe-rich bcc, Cr-rich BCC, and sigma phase in the solid state. The minor phases are FCC, M3P, TiC, Laves, and Ti4C2S2. The calculated lattice parameter of the Fe-rich BCC phase matches with the experimentally calculated data using XRD. The thermal expansion of Crofer 22 APU was found to be 11.9181 × 10-6 /°C at 950 °C. The in-situ high-temperature XRD technique has been an effective methodology for determining the thermal expansion behavior of the as-received Crofer steel. © 2023Item 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 Effect of thermal expansion on the high temperature wear resistance of Ni-20%Cr detonation spray coating on IN718 substrate(Elsevier B.V., 2023) Purushotham, N.; Parthasarathi, N.L.; Babu, P.S.; Govindarajan, G.; Rajasekaran, B.The temperature-dependent materials properties on the dry sliding wear resistance of the detonation sprayed Ni-20%Cr coating have been studied. In-situ high-temperature X-ray diffraction (HT-XRD) was used to investigate high-temperature properties such as stress relieving, recrystallization, and thermal expansion. The dry sliding wear test was performed by using a ball-on-disc tribometer by sliding velocities (0.1 m/s), varying loads (6 N and 10 N), and temperatures (25 °C and 850 °C) against alumina (Al2O3) ball. The phase evolution, thermal expansion, crystallite size, and lattice strain were determined by the Williamson-Hall method. Field emission scanning electron microscopy and a non-contact optical profilometer was used to characterize the wear scar and calculate the wear rate. The wear test results demonstrated that the as-deposited coatings coefficient of friction (CoF) and wear rate (ω) continuously decreased as the temperature increased. The primary wear mechanism changed from abrasive and surface fatigue to adhesive and oxidative wear. The impact of stress relieving, recrystallization, and forming a composite tribolayer (Cr2O3, NiO) at elevated temperatures reduced the friction and enhanced the wear resistance. The effect of stress relieving, recrystallization, thermal expansion, and oxidation on the wear resistance of the coating has been discussed with a suitable mechanism. © 2023 Elsevier B.V.Item Effect of Build Orientation on Anisotropy in Tensile Behavior of Laser Powder Bed Fusion Fabricated SS316L(Springer, 2024) Thanumoorthy, R.S.; Chaurasia, J.K.; Anil Kumar, V.A.; Pradeep, P.I.; Balan, A.A.S.; Rajasekaran, B.; Sahu, A.; Bontha, S.In the present study, Stainless steel 316L (SS316L) cylindrical specimens were fabricated at two different build orientations and two different laser powers using Laser powder bed fusion process (LPBF). Microstructural characterization such as XRD, SEM, EBSD analysis and tensile testing were carried out on fabricated specimens in stress relieved condition to understand the anisotropic behavior of LPBF printed specimens. Horizontally oriented specimens showed higher tensile strength when compared to vertically oriented specimens for both laser powers. XRD and EBSD phase maps did not reveal the presence of any secondary phases. However, build orientation and laser power affected the crystallite size of the samples. Bimodal grain structure comprising coarse columnar grains and fine equiaxed grains were observed from the micrographs. With variation in build orientation, there was a significant change in the average grain size of the specimens. High dislocation density was observed in horizontally oriented samples built at low laser power because of dislocation annihilation that can occur at high temperatures. However, EBSD analysis revealed random weak crystallographic texture which does not vary significantly with laser power or build orientation. Variation in grain size, grain morphology, sub-grain features and dislocation density are the reasons for the anisotropic tensile behavior observed in LPBF printed SS316L coupons in stress relieved condition. © ASM International 2023.