Journal Articles

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Author: search.filters.author.Udaya Bhat, K.

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    Welding Techniques for High Entropy Alloys: Processes, Properties, Characterization, and Challenges
    (MDPI, 2022) John, M.; Diaz, O.; Esparza, A.; Fliegler, A.; Ocenosak, D.; van Dorn, C.; Udaya Bhat, K.; L Menezes, P.L.
    High entropy alloys (HEAs) are the outstanding innovations in materials science and engineering in the early 21st century. HEAs consist of multiple elements with equiatomic or near equiatomic compositions, which exhibit superior mechanical properties, such as wear resistance, fatigue resistance, and corrosion resistance. HEAs are primarily used in structural and functional applications; hence, appropriate welding processes are essential to enhancing the performances and service lives of HEA components. Herein, a comprehensive overview of current state-of-art-of welding techniques for HEAs is elucidated. More specifically, the article discusses the fusion-based welding techniques, such as gas tungsten arc welding (GTAW) and laser beam welding (LBW), and solid-state welding techniques, such as friction stir welding (FSW) and explosive welding (EB), for a broad category of HEAs. In addition, the microstructural features and mechanical properties of HEAs welded using different techniques were explained for a broad spectrum of HEAs. Finally, this review discusses potential challenges in the welding of HEAs. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
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    Effect of heat treatment on structure and properties of multilayer zn-ni alloy coatings
    (International Association of Physical Chemists, 2013) Rao, V.R.; Hegde, A.C.; Udaya Bhat, K.
    Composition modulated multilayer alloy (CMMA) coatings of Zn-Ni were electrodeposited galvanostatically on mild steel (MS) for enhanced corrosion protection using single bath technique. Successive layers of Zn-Ni alloys, having alternately different composition were obtained in nanometer scale by making the cathode current to cycle between two values, called cyclic cathode current densities (CCCD’s). The coatings configuration, in terms of compositions and thicknesses were optimized, and their corrosion performances were evaluated in 5 % NaCl by electrochemical methods. The corrosion rates (CR)’s of multilayer alloy coatings were found to decrease drastically (35 times) with increase in number of layers (only up to 300 layers), compared to monolayer alloy deposited from the same bath. Surface study was carried with SEM, while XRD was used to determine metal lattice parameters, texture and phase composition of the coatings. The effect of heat treatment on surface morphology, thickness, hardness and corrosion behaviour of multilayer Zn-Ni alloy coatings were studied. The significant structural modification due to heat treatment is not accompanied by any decrease in corrosion rate. This effect is related to the formation of a less disordered lattice for multilayer Zn-Ni alloy coatings. © 2013 by the authors; licensee IAPC, Zagreb, Croatia.
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    Formation of microstructural features in hot-dip aluminized AISI 321 stainless steel
    (University of Science and Technology Beijing, 2018) Huilgol, P.; Rajendra Udupa, K.; Udaya Bhat, K.
    Hot-dip aluminizing (HDA) is a proven surface coating technique for improving the oxidation and corrosion resistance of ferrous substrates. Although extensive studies on the HDA of plain carbon steels have been reported, studies on the HDA of stainless steels are limited. Because of the technological importance of stainless steels in high-temperature applications, studies of their microstructural development during HDA are needed. In the present investigation, the HDA of AISI 321 stainless steel was carried out in a pure Al bath. The microstructural features of the coating were studied using scanning electron microscopy and transmission electron microscopy. These studies revealed that the coating consists of two regions: an Al top coat and an aluminide layer at the interface between the steel and Al. The Al top coat was found to consist of intermetallic phases such as Al7Cr and Al3Fe dispersed in an Al matrix. Twinning was observed in both the Al7Cr and the Al3Fe phases. Furthermore, the aluminide layer comprised a mixture of nanocrystalline Fe2Al5, Al7Cr, and Al. Details of the microstructural features are presented, and their formation mechanisms are discussed. © 2018, University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature.
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    Microstructure Evolution in Cast Al-Zn-Mg Alloys Processed by Equal Channel Angular Pressing
    (Springer New York LLC barbara.b.bertram@gsk.com, 2018) Manjunath, G.K.; Udaya Bhat, K.; Preetham Kumar, G.V.
    In the present work, microstructure development and enhancement in the microhardness of Al-Zn-Mg alloys (with 5, 10, and 15% zinc) during equal channel angular pressing (ECAP) were investigated. Dendritic morphology was observed in the cast condition of all three alloys, and precipitates were situated along the inter-dendritic regions. After homogenization, precipitates in the inter-dendritic regions were uniformly distributed in the aluminum matrix and grain boundaries were developed. After 4 passes in route BC, large reduction in the grain size was observed. X-ray diffractometry showed that MgZn2 precipitate was developed in the ECAP-processed samples. Increase in the intensity of MgZn2 peaks was observed when the quantity of zinc is increased in the material. Also, changes in the intensity of XRD peaks were observed in ECAP-processed samples due to shear deformation. After ECAP, substantial increase in the microhardness was perceived. After four passes, microhardness increased to 109, 67, and 58% from the initial condition in A1, A2, and A3 alloys, respectively. Also, improvement in the microhardness was also observed when the quantity of zinc is increased in the material. © 2017, Springer Science+Business Media, LLC, part of Springer Nature and ASM International.
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    Effect of Retrogression Heat Treatment Time on Microstructure and Mechanical Properties of AA7010
    (Springer New York LLC barbara.b.bertram@gsk.com, 2018) Nandana, M.S.; Udaya Bhat, K.; Manjunatha, C.M.
    The effect of retrogression time during retrogression and re-aging (RRA) treatment of AA7010 is evaluated by performing tensile tests and characterizing the microchemistry of the grain boundary precipitates (GBPs) using transmission electron microscope coupled with the energy-dispersive spectroscopy. Retrogression time is evaluated so that the ultimate tensile strength of the RRA-treated sample is equal to that of the T6-treated sample and the grain boundary microstructure similar to that of the over-aged (T7451) condition. The investigation reveals that the sample retrogressed at 200 °C for 20 min has UTS of 586 MPa which is equivalent to that of the T6 sample and 11.5% higher than that of the T7451 condition. The fracture toughness of the RRA-treated sample was 41 MPa?m. Microstructure of the RRA-treated sample is similar to T7451, along the grain boundaries and in the grain interior similar to that of the T6-treated sample. Energy-dispersive spectroscopy confirmed the increment of Cu content on the GBP’s with increase in the retrogression time, which is expected to improve the stress corrosion cracking resistance of the alloy. © 2018, ASM International.
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    Insights into formation of gradient nanostructured (GNS) layer and deformation induced martensite in AISI 316 stainless steel subjected to severe shot peening
    (Elsevier B.V., 2018) Jayalakshmi, M.; Huilgol, P.; Badekai Ramachandra, B.R.; Udaya Bhat, K.
    Severe peening is a well-accepted top-down approach to engender surface nanocrystallization in austenitic stainless steels. In the present study, AISI 316 grade austenitic stainless steel is subjected to severe peening through air blast shot peening technique. Study is aimed at analyzing the microstructural features of the peened layer and deformation induced martensite through transmission electron microscopy technique. Gradient nanostructured (GNS) layer formed as a result of high strain rate, multi-directional deformation during severe peening found to extend to about 500 ?m from the surface. Nucleation of deformation induced martensite is not limited to shear band intersections as affirmed by the published literature related to severe peening. It is observed to nucleate at multiple locations in the austenite matrix. Martensite units thus formed, coalesce with each other to form continuous layer of lath martensite layer at about 15–20 ?m from the surface. Upon further deformation, lath morphology transforms to dislocation cell-type; resulting in fine martensite crystallites at the topmost layer of the peened surface. © 2018 Elsevier B.V.
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    Microstructure and Wear Performance of ECAP Processed Cast Al–Zn–Mg Alloys
    (Springer, 2018) Manjunath, G.K.; Udaya Bhat, K.; Preetham Kumar, G.V.; Ramesh, M.R.
    In the present investigation, wear performance of equal channel angular pressing (ECAP) processed cast Al–Zn–Mg alloys under dry sliding wear conditions was studied against a steel disc. Initially, Al–Zn–Mg alloys (with 5, 10, 15% zinc and 2% magnesium) were ECAP processed. After ECAP, grain size was reduced and enhancement in the hardness was observed. Wear resistance of the alloys increased after ECAP processing. Wear resistance of the alloys also increased when the quantity of the zinc was increased in the alloys. But, wear resistance of all three alloys decreased with increase in the load and the sliding speed. Coefficient of friction of the alloys decreased after ECAP processing. Coefficient of friction of the alloys also decreased when the quantity of the zinc was increased in the alloys. Coefficient of friction of all three alloys increased with increase in the load and the sliding speed. Irrespective of the alloy composition and applied load, worn surfaces of the cast and homogenized samples were composed of plastic deformation, scratches and micro-ploughing. On the other hand, in ECAP processed samples, morphology of the worn surfaces depended on the applied load. Abrasive wear is the main wear mechanism perceived in cast and homogenized samples at all loads. While in ECAP processed samples, the wear mechanism shifted from adhesive and oxidation wear to abrasive wear with increase in the load. Formation of oxide layers on the surface of the sample increased with increase in the ECAP passes. In ECAP processed samples, transfer of iron content from the disc to the sample surface was identified. © 2018, The Indian Institute of Metals - IIM.
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    Microstructure and Mechanical Properties of Cast Al-5Zn-2Mg Alloy Subjected to Equal-Channel Angular Pressing
    (Springer New York LLC barbara.b.bertram@gsk.com, 2018) Manjunath, G.K.; Preetham Kumar, G.V.P.; Udaya Bhat, K.; Huilgol, P.
    In the present work, cast Al-5Zn-2Mg alloy was processed through equal-channel angular pressing (ECAP) in route BC up to four number of passes. Microstructure and mechanical properties were investigated on processed and unprocessed materials. In cast condition, the material was composed of dendritic structure. After homogenization treatment, large-sized grains were observed. After ECAP processing, significant grain refinement was observed. After ECAP processing, high-density dislocations and high degree of misorientation between the grains were observed. In cast material, rod-shaped precipitates were observed, while, after ECAP processing, spherical-shaped precipitates were observed. ECAP processing leads to a noticeable improvement in the mechanical properties of the material. After four passes, 122% improvement in the microhardness and 135% improvement in the ultimate tensile strength of the material were observed. After three passes, a slight decrease in the mechanical properties was observed. This is attributed to the dissolution of the metastable ?? phase, annihilation of dislocations, dynamic recrystallization and texturing during ECAP processing. Brittle fracture mode was observed in tensile testing cast and homogenized samples. After ECAP processing, fracture mode was changed into shear fracture mode. © 2018, ASM International.
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    SnO2 nanoparticles functionalized MoS2 nanosheets as the electrode material for supercapacitor applications
    (Institute of Physics Publishing helen.craven@iop.org, 2019) Prabukumar, C.; Mohamed, M.; Krishna Bhat, D.; Udaya Bhat, K.
    Tin oxide (SnO2) nanoparticles undergo the volume expansion during an electrochemical cycle. This volume expansion leads to discontinuities in the form of microcracks in the electrode material. The problem of charge transportation associated with this microcracking limits the application of SnO2 in the energy storage application such as supercapacitors. The present work approached to solve this problem by incorporating the MoS2 nanosheets along with the SnO2 nanoparticles. The SnO2 nanoparticles are functionalized onto the surface of the MoS2 nanosheets by the ligand exchange process. The MoS2 nanosheets act as the support material for the SnO2 nanoparticles. The electrode material prepared using SnO2 nanoparticles and nanocomposite of SnO2 functionalized MoS2 nanosheets are tested by cyclic voltammetry and galvanostatic charge-discharge measurements. The specific capacity of the MoS2-SnO2 nanocomposite is calculated to be 61.6 F g-1 which is 4.4 fold higher than that of bare SnO2 nanoparticles. The improvement in the electrochemical performance of SnO2 is attributed to the high surface area and the charge transportation provided by the MoS2 nanosheets. © 2019 IOP Publishing Ltd.
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    Characterization of DC Magnetron Sputtered Copper Thin Film on Aluminium Touch Surface
    (Springer, 2019) Augustin, A.; Rajendra Udupa, K.; Udaya Bhat, K.
    Hospital care-assisted infections introduce problems like prolonged hospital stay, additional financial burden and higher death rate. Since copper is registered by US Environmental Protection Agency as the only solid antimicrobial metal, it could be used in hospital touch surfaces. In the present work, copper has been deposited on the aluminium substrate by DC magnetron sputtering method with different target power. Prior to the coating, the substrate has undergone double zincation process. The coating has been characterized by XRD, SEM, TEM, scratch hardness test and microhardness test. From the TEM micrographs, the grain size has been found to decrease from 49 to 18 nm on increasing the deposition power from 50 to 150 W. Along with the decrease in the grain size, the mechanical properties like scratch hardness and microhardness of the coating have been increased. The preferred growth along [111] direction observed in XRD analysis is responsible for the increase in the hardness of the coating apart from the presence of the nano-grains. The SEM image of the coating shows nodular morphology which enhances the surface area. © 2019, The Indian Institute of Metals - IIM.