Now showing 1 - 10 of 66
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    Hot corrosion-creep interaction in IN718 under simulated marine environment: Introducing strain-associated-time (SAT) plots for comprehensive understanding
    (01-09-2021)
    Mannava, Venkateswara Rao
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    Koundinya, N. T.B.N.
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    A., Sambasiva Rao
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    Paulose, Neeta
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    M., Kamaraj
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    Creep-rupture behaviour of IN718 with deposit of 87.5 wt.%Na2SO4+5 wt.%NaCl+7.5 wt.%NaVO3 (3SM) in the stress range of 550–850 MPa at 650 °C is investigated. 3SM shortens the creep-rupture time by ≈70–90 %. Comprehensive microstructural analysis suggests that oxygen-induced-dynamic- embrittlement, sulphidation followed by oxidation, and vanadic-hot corrosion operate during creep. Creep-hot corrosion interaction influence on rupture strain is explained by introducing strain-associated-time (SAT) and Δtε plots, for the first time. These plots accompanied by detailed microstructural examination demonstrate that deleterious influence of 3SM increases with decreasing stress. Further, SAT plots and EPMA analysis confirm that oxygen-induced-dynamic-embrittlement commences from ∼9% strain under 3SM at 550 MPa.
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    Plasma-Sprayed High Entropy Alloys: Microstructure and Properties of AlCoCrFeNi and MnCoCrFeNi
    (01-02-2015)
    Ang, Andrew Siao Ming
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    Berndt, Christopher C.
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    Sesso, Mitchell L.
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    Anupam, Ameey
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    S, Praveen
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    Murty, B. S.
    High entropy alloys (HEAs) represent a new class of materials that present novel phase structures and properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated that may be used as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimise the appropriate thermal spray parameters and spray deposits were prepared. After mechanical alloying, aluminum-based and manganese-based HEA powders revealed contrary prominences of BCC and FCC phases in their X-ray diffraction patterns. However, FCC phase was observed as the major phase present in both of the plasma-sprayed AlCoCrFeNi and MnCoCrFeNi coatings. There were also minor oxide peaks detected, which can be attributed to the high temperature processing. The measured porosity levels for AlCoCrFeNi and MnCoCrFeNi coatings were 9.5 ± 2.3 and 7.4 ± 1.3 pct, respectively. Three distinct phase contrasts, dark gray, light gray and white, were observed in the SEM images, with the white regions corresponding to retained multicomponent HEAs. The Vickers hardness (HV0.3kgf) was 4.13 ± 0.43 and 4.42 ± 0.60 GPa for AlCoCrFeNi and MnCoCrFeNi, respectively. Both type of HEAs coatings exhibited anisotropic mechanical behavior due to their lamellar, composite-type microstructure.
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    Unusual substructure evolution and post-dynamic recrystallization effects on flow softening mechanism in a γ′-free Co-base superalloy
    (01-08-2022)
    Karnati, Abhinav Kumar
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    Koundinya, N. T.B.N.
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    Nayak Majila, Anuradha
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    Fernando D, Chandru
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    Motivation for this study was the absence of any report of stacking fault energy (SFE) estimate and post-dynamic recrystallization (PDRX) effects at temperatures >1000 °C in γ′-free Co-base superalloys. Therefore, the substructural evolution during hot compression of Co-22Cr-22Ni-14W-2Fe-0.1C superalloy (H188) at temperatures (1050–1150 °C) and at 10−1 s−1 was studied. Unusual substructural features viz. extrinsic stacking faults, dense dislocation walls, microbands, and subgrains were observed with an increase in strain (0.01–0.70), even at 1100 °C. Denoised electron backscattered diffraction data revealed an enormous increase in overall recrystallized (PDRX+DRX) fraction from 30 to 75% with increase in temperature from 1050 to 1100 °C at ε=0.7. Comprehensive microstructural analyses suggest that continuous dynamic recrystallization is the dominant flow softening mechanism in low SFE (estimated to be 14 ± 2.0 mJ/m2) alloy H188. However, the PDRX effects triggered by the presence of higher stored energy and recrystallized nuclei contributing to a massive increase in recrystallized fraction at 1100 °C.
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    Effect of molybdenum and niobium on the phase formation and hardness of nanocrystalline CoCrFeNi high entropy alloys
    (01-01-2014)
    Praveen, S.
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    Murty, B. S.
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    In the present study, influence of molybdenum and niobium additions on phase formation during mechanical alloying and spark plasma sintering of CoCrFeNi high entropy alloy was studied. Major FCC and minor BCC phase were observed after mechanical alloying of CoCrFeNi. However, major FCC and sigma phase were observed after spark plasma sintering. A maximum relative density of 95% was obtained with the hardness of 570 HV in CoCrFeNi HEA. The phase formation behavior was not significantly affected by the addition of molybdenum or niobium. However, addition of Mo to CoCrFeNi increased the hardness from 570 HV to 620 HV, and the hardness increased to 710 HV with combined addition of molybdenum and niobium. After sintering, major FCC phase with crystallite size of 60-70 nm was observed in all the compositions. Further, the microstructure and hardness retention was observed in CoCrFeNiMo0.2 with annealing temperature up to 800°C.
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    Estimation of diffusivity from densification data obtained during spark plasma sintering
    (01-03-2019)
    Chawake, Niraj
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    Ghosh, Pradipta
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    Raman, Lavanya
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    Srivastav, Ajeet K.
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    Paul, Tanaji
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    Harimkar, Sandip P.
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    Eckert, Jürgen
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    Evaluation of the diffusion coefficient of metal powders was attempted by using the power-law creep model in conjunction with the isothermal densification kinetics during spark plasma sintering (SPS). The diffusion coefficients obtained from the densification data of elemental Fe, Ni and Al powders are found to be higher than those reported in the literature. The higher values of diffusivity can be attributed to electric current effects. Our analysis demonstrates that it is possible to evaluate diffusion coefficients from experimental SPS densification data.
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    Compaction and Pressureless Sintering Characteristics of Silicon and a Silicon Composite Containing a Multicomponent Molybdenum Alloy Reinforcement
    (01-05-2023)
    Samantaray, B. K.
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    Kumar, U.
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    Kumar, E. Nandha
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    Bartarya, G.
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    Gollapudi, S.
    The compaction and sintering characteristics of silicon and a silicon composite containing a multi-component Molybdenum alloy (Mo44Si26Ta5Zr5Fe3Co12Y5) as reinforcement is reported here. Compaction with different binders viz zinc stearate, stearic acid and poly-vinyl alcohol (PVA) revealed that only PVA is suitable for making crack free pellets. Furthermore, silicon particles with angular shape were found to develop cracks during compaction compared to those bearing spherical shape. The compaction characteristics of Si and Si composites were evaluated using the Heckel’s approach and analysis of data revealed that round particles ensure better die filling. Pressureless sintering of the silicon and silicon composite compacts was carried out at 1225 °C for a duration of 12 h and intriguingly the relative density of the sintered compacts was found to be lower than respective green compacts. This was attributed to poor sintering tendency of silicon and also to the loss of the volatile organic binder.
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    An innovative spraying setup to obtain uniform salt(s) mixture deposition to investigate hot corrosion
    (01-02-2016)
    Mannava, Venkateswararao
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    Swaminathan, A. Vignesh
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    A hot corrosion study via molten salt deposition and its interaction with creep/fatigue play a critical role in predicting the life of gas turbine engine components. To do systematic hot corrosion studies, deposition of molten salts on specimens should be uniform with good adherence. Thus, the present study describes an in-house developed spraying setup that produces uniform and reliable molten salt deposition in a repeatable fashion. The efficacy of the present method was illustrated by depositing 90 wt. % Na2SO4 + 5 wt. % NaCl + 5 wt. % NaV O3 salt mixture on hot corrosion coupons and on creep specimens, and also by comparing with other deposition methods.
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    Assessment of the post-dynamic recrystallization effects on the overall dynamic recrystallization kinetics in a Ni-base superalloy
    (05-01-2023)
    Koundinya, N. T.B.N.
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    Karnati, Abhinav Kumar
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    Sahadevan, Abhijith
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    Murty, S. V.S.Narayana
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    Understanding the role of microconstituents, viz., precipitates, and carbides, on dynamic recrystallization (DRX) and on post-dynamic recrystallization (PDRX) is essential to obtain a uniform microstructure with desirable grain size during hot deformation. Thus, the present study was taken up to address the role of carbides on DRX as well as on the PDRXcharacteristics of a Ni-base superalloy XH55 (Ni-17Cr-12Fe-9Mo-2 Nb-1.8Al-0.04 C). Uniaxial hot compression tests were conducted in the temperature range of 950–1100 °C and at a constant strain rate of 10−1 s−1 on the peak-aged specimens. The evolution of DRX without and with post-deformation hold for 15 s was correlated with the flow softening mechanisms across the tested temperature range. Besides, the PDRX fraction and its evolution with temperature was estimated from the electron backscattered diffraction data while using the denoising filters and a grain level misorientation parameter. Further, intermittent post-deformation microstructural characterization was carried out on the specimens deformed at 1050 °C to understand the role of carbides in the initial stages of DRX and PDRX. Overall, the present study illustrates that carbides have inhibited the DRX kinetics in the initial stage, i.e., at low strains (ε<0.40). Whereas, at higher strains (ε>0.40), the role of carbides is limited in inhibiting the growth of the recrystallized (DRX and PDRX) grains present in their vicinity.
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    Theoretical and experimental studies on thermal stability of nanocrystalline Mg–Mo alloy
    (01-12-2020)
    Rai, Nikhil
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    Samantaray, Bikash K.
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    Rajulapati, Koteswararao V.
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    Ravi, Rahul
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    Koundinya, N. T.B.N.
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    Gollapudi, Srikant
    This study is focused on the evaluation of the thermal stability of a nanocrystalline Mg alloy, a relatively less explored topic. Guided by Darling et al. model which advises choosing of alloying elements based on their enthalpy of mixing and elastic enthalpy with respect to the parent element, Mo was chosen for stabilizing the nc Mg structure. High energy ball milling experiments were conducted on powders of Mg and Mo to achieve a composition of Mg–2at%Mo. Characterization of the ball milled powders using XRD and TEM indicated a lack of mixing of Mg and Mo and the microstructure was found to bear a mix of Mg and Mo phases. Kissinger's analysis using DSC yielded the activation energy of grain growth in the ball milled material as 73 kJ/mol which was similar to that of pure Mg, indicating that Mo did not alter the grain growth kinetics of Mg. The poor grain size stability of the nanocrystalline Mg–2Mo composition was also observed during spark plasma sintering studies conducted at 673, 723 and 773 K. The obtained results were evaluated in the light of the Murdoch and Schuh model and this revealed that the Mg–Mo alloy is not expected to be thermally stable due to the tendency of Mo to exist as a separate phase. This analysis appears to suggest that the Murdoch and Schuh model is more suitable for identifying thermally stable nanocrystalline compositions compared to the Darling et al. model.