1. Ph.D Theses
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Item Studies on Mechanical Properties and Corrosion Behavior of Ze41 Magnesium Alloy Subjected to Equal Channel Angular Pressing(2022) Sekar, Prithivirajan; S, Narendranath; Desai, VijayAn exceptional combination of properties such as high specific strength, good damping capacity and abundant availability confirm magnesium alloys as a potential candidate for applications in automobile, biomedical, aerospace as well as electronics industries. However, the limited ductility of Magnesium and its alloys as well as their relatively lower corrosion resistance still remains a hindrance to extend their applications. Equal channel angular pressing (ECAP) has immense potential to attain remarkable grain refinement thereby improving mechanical properties such as yield strength, ultimate tensile strength and percentage of elongation of Mg alloys. In the present work, ZE41 Mg alloy is subjected to two step ECAP. The microstructure and mechanical properties of ZE41 Mg alloy before and after two step ECAP are investigated. Further, the corrosion and galvanic corrosion behaviour of ZE41 samples are evaluated in 0 M, 0.1 M and 1 M NaCl solutions to mimic conditions encountered in automobile applications. Two step equal channel angular pressing carried out on as received ZE41 Mg alloy resulted in a remarkable grain refinement. As compared to grain size of 46μm in as received sample, refinement upto 2.5μm is achieved after 8th pass equal channel angular pressing (ECAP). The combined effect of crystallographic orientation and grain refinement is investigated by analysing the mechanical properties and corrosion behaviour of ZE41 Mg alloy using electron back scattered diffraction (EBSD). The first stage comprises of 1st, 2nd, 3rd and 4th passes at a processing temperature of 300 °C while the 5th, 6th, 7th and 8th passes are ECAPed at 275 °C in second stage. The mechanical properties of ZE41 Mg alloy 158 MPa yield tensile strength (YTS), 230 MPa ultimate tensile strength (UTS) and 7 % elongation in as received condition is enhanced to 236 MPa YTS, 295 MPa UTS and 19.76% respectively after first stage ECAP. The yield tensile strength deteriorated due to the effect of texture predominating grain refinement during the second stage ECAP. The corrosion resistance of ZE41 Mg alloy is significantly enhanced by ECAP and is inferred from electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation results. The role of microstructure is minimal on corrosion behaviour of ZE41 Mg alloy due to extra resistance when tested in 0M NaCl. However, the influence of grain refinement greatly influenced the improvement in corrosion resistance of ZE41 Mg alloy rather than crystallographic orientation observed from EBSD analysis. In contrast, the crystallographic orientation predominated the effect of grain refinement during ZE41 Mg alloy corrosion in chloride containing 0.1M and 1M NaCl solutions. From the observation of results it is found that equal channel angular pressing has the dual advantage of improving mechanical properties and corrosion resistance of ZE41 Mg alloy. Further, the galvanic corrosion behaviour of as received and ECAPed ZE41 Mg alloy coupled with Al7075 alloy is investigated using zero resistance ammeter (ZRA) in three different corrosive environments 0 M, 0.1 M and 1 M NaCl to mimic the conditions experienced in engineering applications. The mechanism of galvanic corrosion for ZE41 Mg alloy –Al7075 Aluminium alloy is explained. It is observed that a robust surface film containing composite layer of oxide/hydroxide of magnesium and aluminium is established in 0 M NaCl solution. However, only a single layer of magnesium oxide/ hydroxide is detected in chloride containing environments. ECAP improved the resistance to galvanic corrosion by 58% and 54% when compared with as received ZE41 Mg alloy in 0 M and 1 M NaCl solution respectively. In contrast, galvanic corrosion resistance decreased by 26% in 0.1 M NaCl after equal channel angular pressing while the as received samples evinced pits unfavourable to be used in engineering applications. ECAP is a promising method to combat galvanic corrosion encountered by ZE41 Magnesium alloy used in automobiles and components of military vehicles.Item Severe Plastic Deformation of Copper-Titanium Alloys Using Multi Axial Cryo-forging(National Institute of Technology Karnataka, Surathkal, 2020) S, Ramesh.; Nayaka, H Shivananda.Severe plastic deformation (SPD) is a technique where high strains are induced into the material to produce fine-grained structural materials, thereby improving the wear resistance and corrosion resistance. There is an increase in scientific and industrial interest in the development of bulk ultra-fine-grained (UFG) alloys, intended for structural applications. UFG materials offer vastly improved mechanical and physical properties. They also exhibits superplastic properties at elevated temperatures. SPD is done using Equal Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Repetitive Corrugation and Straightening (RCS), Accumulative Roll Bonding (ARB) and Multi Axial Forging (MAF). In MAF, materials are forged repeatedly in a closed die along three orthogonal directions, sequentially. It allows processing of relatively ductile material, because it can be performed at cryogenic temperature. Literature review shows that by using MAF technique, grain refinement phenomena can be observed in some ferrous and non-ferrous metals. MAF is one of the simple and most effective methods of SPD to improve material properties. MAF is a process in which the workpiece is subjected to shear deformation and thus, severe plastic strain is induced into the material without any change in the cross-sectional dimension of the sample. Microstructure has major effect on mechanical properties. MAF process leads to ultrafine-grained microstructure in the material which may show superplastic deformation at low temperature and high strain rate. In FCC structured metals, grain refinement also leads to textural changes i.e. high strengthening at cryogenic condition deformation. Copper-Titanium (Cu-Ti) alloy is the nontoxic substitute for Cu-Be and it showed good mechanical and electrical properties and can be used for the production of high strength spring, corrosion-resistant elements, and electrical connections like contact, relay, gears and electrical wires. Hence, in the present study, three alloys of Cu-Ti, namely, Cu-1.5%Ti, Cu-3%Ti and Cu-4.5%Ti, have been processed by MAF. Microstructural evolution in different MAF cycles is studied and it is correlated to the mechanical properties observed. As UFG materials have much higher hardness, they are expected to have higher wear resistance. MAF processed material exhibits higher wear and corrosion resistance, than the asreceived material. Hence MAF processed samples find wider engineering applications.viii Literature review consists of features of various SPD Techniques, advantages, and limitations. MAF process, parameters which influence MAF process, advantages and applications of MAF processed Cu-Ti alloys are discussed in details. Works of different researchers on MAF processed Copper alloys, with respect to, mechanical properties, wear and corrosion behavior are reported. Motivation from literature survey and objectives of the present work are highlighted. Details of the experimentation performed, right from the process adopted for the development of the UFG Cu-Ti to their characterization, are given in chapter three. Microstructural analyses were performed using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Tensile tests were performed on both as-received and MAF processed samples. Dry sliding wear testing was performed using Pin on disc testing machine for both unprocessed and MAF processed samples. For the study of corrosion behavior, electrochemical polarization studies were performed and tofel extrapolation technique was used to obtain the corrosion rates. Chapter 4, Chapter 5 and Chapter 6, explain the results and discussion of various experiments carried out on three alloys Cu-1.5%Ti, Cu-3%Ti and Cu-4.5%Ti Microstructural characterization by OM, SEM, TEM, EBSD and XRD analysis has been discussed. Mechanical properties which includes hardness, tensile followed by fractography has been analyzed. Wear test with different loading conditions and sliding distances has been explained. Corrosion studies by electrochemical measurements test method has been highlighted.Item Effect of Equal Channel Angular Extrusion on Microstructure Mechanical Properties and Corrosion Behavior of Wrought AZ-Magnesium Alloys(National Institute of Technology Karnataka, Surathkal, 2020) Naik, Gajanan M.; S, Narendranath.Wrought magnesium alloys are lightest engineering material and it has quite special properties which lead to particular applications. In specific, their highest strength to weight ratio, good machinability and high damping capability makes magnesium alloys tremendously attractive in aerospace, electronics, marine and automobile industries. Indeed, Magnesium alloys have poor tensile strength, ductility and corrosion resistance properties associated with other engineering materials like aluminium alloys, steels and super alloys etc. Therefore, many researchers worked on equal channel angular pressing of magnesium alloys to improve the mechanical properties and corrosion resistance. In this work, the effect of channel angles on material properties were investigated during equal channel angular pressing of AZ80/91 magnesium alloy using processing route-R at 523K, 598K and 663K processing temperature. Channel angles of 90⁰ and 110⁰, common corner angle of 30⁰ have been considered for the study. It has been revealed that the channel angle has a significant influence on deformation homogeneity, microhardness, ultimate tensile strength, ductility and corrosion behaviour of AZ80/91 magnesium alloys. Specifically, AZ80/91 Mg alloys processed through 90⁰ channel angle i.e die A is considered as optimal die parameter to improve above-said material properties. Investigation showing with reference to as-received AZ80 and AZ91 Mg alloy indicates 11 %, 14 % improvement of UTS and 69 %, 59 % enhancement in ductility after processing through 4P through die A (90º) at 598K respectively. Also, the corrosion rate reduces to 97 % and 99 % after processing the sample with 4P-ECAP die A (90º) at the same processing temperature for AZ80 and AZ91 Mg alloys respectively. This is mainly due to grain refinement and distribution of Mg17Al12 secondary phase during ECAP. Further, this work investigates the effect of annealing and aging treatment on microstructure and corrosion behaviour of as-received and ECAPed AZ80/91 Magnesium alloys. Here, annealing at 523K, 623K, and 723K were accomplished, meanwhile samples were cooled in the furnace after 6 h and 12 h of diffusion annealing treatment. In this study, samples were characterized by using optical microscopy (OM) and scanning electron microscopy (SEM) and electrochemical corrosion behavior of annealed AZ80/91 Mg alloy has beeninvestigated. With this, an attempt has been made to enhance the corrosion resistance of the AZ80/91 Mg alloy by changing its microstructure and re-distribution of secondary phase during annealing and aging treatment. It was found that corrosion rates are minimum at higher annealing temperature and aging time because of uniform distribution of secondary β-phases in Mg matrix, evidently shown in the microstructure of the heat-treated AZ80/91 Mg alloy. As a result, the annealing treatment at 723K for 12 h aging is desirable to enhance the corrosion resistance. Further enhancement of asreceived and ECAPed AZ80/91 Mg alloys were observed after High Velocity Oxy-Fuel (HVOF) coating of 316 stainless steel powder. Our results revealed that 316 stainless steel coating on ECAP-4P AZ80/91 Mg alloys were uniform and compact on substrate with a thickness of 80±5 µm. Furthermore, HVOF-coating process of 4P-ECAP significantly reduce corrosion rate at 3.5wt.% NaCl solution making it promising for industrial applications. The corrosion behaviour and effect of the ECAPed fine-grained magnesium alloy and coarse-grained as-received AZ80/91 Mg alloy was investigated in a 2.5wt.% NaCl, 3.5wt.% NaCl solution and Natural Sea Water (NSW) in order to explore the corrosion performance of ECAPed magnesium alloys in various environments. From, electrochemical corrosion experiments and surface morphology observations evidently shown that grain refinement exhibited improved corrosion resistance of the AZ80/91 alloy in all environments, also which shown a protective passive film on the surface to shield corrosionItem Corrosion Inhibition Studies of GA9 Magnesium Alloy in Chloride and Sulphate Media(National Institute of Technology Karnataka, Surathkal, 2020) Shetty, Sudarshana.; Shetty, A Nityananda.; Nayak, JagannathaThe corrosion behaviour of GA9 magnesium alloy in two different media, namely, sodium chloride and sodium sulphate in different concentrations and temperatures have been studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The effect of pH of the medium on the corrosion behaviour of GA9 magnesium alloy have also been studied in both the media. The results revealed a trend of higher corrosion rate associated with higher medium concentration, lower pH and higher temperature. The corrosion rate in the sodium chloride medium was higher than that in the sodium sulphate medium. Four different alkyl sulfonates namely sodium dodecylbenzenesulfonate (SDBS), sodium 4-n-octylbenzenesulfonate (SOBS), sodium 2,4-dimethylbenzenesulfonate (SDMBS) and sodium benzenesulfonate (SBS) were tested as corrosion inhibitors for GA9. The results pertaining to the corrosion inhibition studies of four inhibitors in two different media at different temperatures in the presence of varying concentrations of inhibitors are reported in the thesis. The inhibition efficiencies of all the four inhibitors decrease with the increase in temperature and increase in the concentration of the media. Activation parameters for the corrosion of the alloy and thermodynamic parameters for the adsorption of the inhibitors have been calculated and have been documented in the thesis. The sulfonates predominately physisorbed and adsorption was in accordance with Langmuir adsorption isotherm. The studied sulfonates were found to function as mixed type inhibitors. The sulfonates were more efficient at lower temperatures in both the media. Inhibition efficiency is in the order SDBS > SOBS >SDMBS > SBS. Proposed mechanism attributed the cathodic inhibition to the blockage of the reaction spots by chemisorbed sulphonates. The anodic inhibition resulted from the compaction of the porous film by precipitated magnesium sulfonates.Item Effect of Pulsed Laser Deposited Ceramic Coatings on Microhardness and Corrosion Behavior of Titanium, Ti6al4v and Inconel(National Institute of Technology Karnataka, Surathkal, 2013) C, Sujaya; Shashikala, H.D.Coatings are used to modify and increase the functionality of a bulk surface or substrate without modifying the bulk properties of the material. The present work aims at obtaining uniform adhesive coatings of alumina and silicon carbide on different substrates viz., titanium, Ti6Al4V and inconel by pulsed laser deposition technique using Q-switched Nd: YAG laser at low temperature. Processing parameters such as laser fluence, substrate target distance, substrate temperature and target density during deposition were standardized to get adhesive films. Coated films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, spectrophotometer, optical microscope, nanoindentation, surface roughness measurements using 3D optical profilometer, adhesion test. Microhardness and corrosion studies were carried on substrates and after coating. Composite microhardness of ceramic coated substrates was measured using Knoop indenter and its film hardness was separated from composite hardness using a mathematical model based on modified area-law of mixture. Then by including indentation size effect the film hardness was compared with values obtained using nanoindentation method. Composite hardness as well as film hardness of the ceramic coating was found to be higher compared to the substrates. Corrosion behavior of substrates after ceramic coating was studied using 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The Nyquist and the Bode plots obtained from the electrochemical impedance spectroscopy data are fitted by appropriate equivalent circuits. The pore resistance, the charge transfer resistance, the coating capacitance and the double layer capacitance of the coatings were obtained from the equivalent circuit. Alumina coated substrates showed more corrosion resistance than silicon carbide coated substrates. After the corrosion testing, the surface topography of the uncoated and the coated system were examined under scanning electron microscopy. Experimental results confirmed the possibility of using Nd: YAG laser for ceramic film deposition which improves the microhardness and corrosion resistance of the substrate considerablyItem Studies on Corrosion Inhibition of 18% Ni M 250 Grade Maraging Steel under Weld Aged Condition in Acidic Media(National Institute of Technology Karnataka, Surathkal, 2013) B. S., Sanatkumar; Shetty, A. Nityananda; Nayak, JagannathaThe corrosion behaviour of 18 % Ni M250 grade maraging steel under weld aged conditions in two different acid media, namely, hydrochloric acid and sulphuric acid in various concentrations and temperatures have been studied by Tafel polarization and electrochemical impedance spectroscopy techniques. The corrosion rate in the sulphuric acid medium was higher than in the hydrochloric acid medium. Five organic inhibitors were synthesized and characterized using spectral and elemental analysis. The five inhibitors were 1(2E)-1-(4-aminophenyl)-3-(2-thienyl) prop-2-en-1-one (ATPI), 2-(4-chlorophenyl)-2-oxoethyl benzoate (CPOB), 2-(4- bromophenyl)-2-oxoethyl- 4-chlorobenzoate (CPOM), (E)-1-(2,4-dinitrophenyl)-2-[1- (2-nitrophenyl) ethylidene] hydrazine (DNPH) and 5-diethylamino-2-{[2-(2,4- dinitrophenyl) hydrazin-1-ylidene]methyl} - phenol (DDPM). The results pertaining to the corrosion inhibition studies of five inhibitors in two different acid media at different temperatures in the presence of varying concentrations of inhibitors are reported in the thesis. Activation parameters for the corrosion of the alloy and thermodynamic parameters for the adsorption of the inhibitors have been calculated and the results have been analysed. The adsorption of first four inhibitors on the alloy was through both physisorption and chemisorption, with predominant physisorption in both the media. The mode of adsorption for the DDPM predominantly chemisorption in both the media. The adsorption of all the five inhibitors on alloy surfaces follows Langmuir adsorption isotherm. The inhibition efficiencies of first four inhibitors decrease with the increase in temperature, the inhibition efficiency of DDPM increases with the increase in temperature.Item Study on Corrosion Behavior and Corrosion Inhibition of Magnesium Alloy Ze41(National Institute of Technology Karnataka, Surathkal, 2014) K, Nandini; Shetty, A. NityanandaThe alloys of magnesium are in the spot-light lately. With applications that run the gamut from automobile parts to medical implants, this class of alloys truly deserve all the adulation coming their way. ZE41 is one such cast alloy, which is lighter than aluminium, can be cast or machined into variety of desired shapes and has good damping and shock absorption abilities. A very low resistance to corrosion puts the otherwise remarkable efficacy of ZE41 in jeopardy. Hence understanding the corrosion of ZE41 and developing the measures to combat the same are indispensable. In the present thesis the corrosion of ZE41 and its mitigation were analyzed using techniques like potentiodynamic polarization and AC impedance, along with SEM and EDX analyses. Sodium sulfate and its mixture with sodium chloride were chosen as two corrosive media. The blank corrosion studies were performed at various medium concentrations, pH and temperatures. The results revealed a trend of higher corrosion rate associated with higher medium concentration, lower pH and higher temperature. Five different long chain alkyl monocarboxylates namely stearate, palmitate, myristate, laurate and caprylate were synthesized and tested as corrosion inhibitors for ZE41. The calculated activation and thermodynamic parameters have been documented in the thesis. The carboxylates were predominantly physisorbed and adsorption was in accordance with Langmuir adsorption isotherm. The studied carboxylates were found to function as mixed-type inhibitors which predominantly suppressed anodic reaction. The carboxylates were more efficient at lower temperatures and in combined medium. At an optimum concentration the efficiencies of the carboxylates decreased in the order: stearate > palmitate > myristate > laurate > caprylate and this has been accredited to the reduction in aliphatic chain length. The proposed mechanism attributed the cathodic inhibition to the blockage of the reaction spots by chemisorbed carboxylates. The anodic inhibition resulted from the compaction of porous film by precipitated magnesium carboxylate salts.Item Studies on corrosion inhibition of 18% Ni M 250 grade maraging steel under welded condition in acidic media(National Institute of Technology Karnataka, Surathkal, 2014) Kumar, Pradeep; Shetty, A. NityanandaThe corrosion behaviour of 18% Ni M250 grade maraging steel under welded conditions in two different acid media, namely, hydrochloric acid and sulphuric acid in various concentrations and temperatures have been studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The corrosion rate in the sulphuric acid medium was higher than in the hydrochloric acid medium. The effect of corrosion inhibition on welded maraging steel was carried out by using five inhibitors, n a m e l y , 2,5-Bis (3,4,5-trimethoxy phenyl)-1,3,4-oxadiazole (BTPO), 1-Phenyl-4-(4-nitrophenyl) thiosemicarbazide (PNPT), 3,4,5-Trimethoxy benzoicacid(3,4,5-trimethoxy-benzylidene) hydrazide (TBTBH), 2-(5-Chloro-1Hbenzoimidazol-2-yl) phenol (CBP), 2-(4-Methoxy-phenyl)-benzo[d]imidazo[2,1-b] thiazole (MPBIT). The results pertaining to the corrosion inhibition studies of five inhibitors in two different acid media at different temperatures in the presence of varying concentrations of inhibitors are reported in the thesis. Activation parameters for the corrosion of the alloy and thermodynamic parameters for the adsorption of the inhibitors have been calculated and the results have been analysed. The adsorption of all five inhibitors on the alloy was through both physisorption and chemisorption, with predominant physisorption in both the media. The adsorption of all the five inhibitors on alloy surfaces follows Langmuir adsorption isotherm. The inhibition efficiencies of all five inhibitors decrease with the increase in temperature and increase in concentration of acidic media.Item Severe Plastic Deformation of Magnesium Alloys by Equal Channel Angular Pressing(National Institute of Technology Karnataka, Surathkal, 2017) K. R, Gopi; Nayaka, H ShivanandaMagnesium and its alloys possess good mechanical properties like low density, good castability, high specific strength and low cost. Requirement of magnesium alloys is more towards industrial applications, particularly in aerospace and automotive industries. The main limitation is the poor room temperature formability, with limited number of slip systems, due to their hexagonal closed packed (HCP) structure. Formability at room temperature in Mg alloys can be improved by grain refinement, as fine-grained structure improves ductility. Severe plastic deformation (SPD) is a technique where high strains are induced into the material to produce fine grained structural material. They also have few limitations, like high wear rate and low corrosion resistance, which hinders the usage of magnesium in many applications. Wear rate and corrosion resistance can be improved by subjecting the material to SPD, thereby, increase the applications of magnesium alloys. Equal channel angular pressing (ECAP) is one of the simple and most effective methods of SPD to improve the material properties. ECAP is a process in which the workpiece is subjected to shear deformation and thus, severe plastic strain is induced into the material without any change in the cross-sectional dimension of the sample. Microstructure has major effect on mechanical properties. ECAP process leads to ultrafine-grained microstructure in the material which may show superplastic deformation at low temperature and high strain rate. In HCP structured metals, grain refinement also leads to textural changes i.e. high strengthening in some particular directions. Magnesium alloys are available in various systems – (a) Mg-Al system where aluminum addition improves the mechanical property as well as the castability. Addition of aluminum up to 6% and more makes the alloy age-hardenable. Manganese (Mn) addition plays a vital role in grain refinement for magnesium alloys. (b) Aluminum-manganese (AM) series magnesium alloys are widely used in manufacturing of various automobile components such as seat frames, instrument panels etc., due to their better damping, better toughness, impact absorption and elongation properties compared to aluminum-zinc (AZ) series alloys.In the present study, ECAP was performed on AM70, AM80 and AM90 alloy with varying percentage of aluminum and manganese. In spite of various applications of AM series magnesium alloy, limited work has been done to improve the physical properties of AM series magnesium cast alloys. We may further improve these properties by using ECAP, so that its application can be expanded in different areas of engineering. ECAP was carried out using hot die steel (HDS) die with channel angle (Φ) as 110° and outer arc of curvature (Ψ) as 20° using route BC. Unprocessed and ECAP processed samples were subjected to microstructural studies and tested for mechanical properties. Strength and hardness values showed increasing trend for the initial 2 passes of ECAP processing and then started decreasing with further increase in the number of ECAP passes, even though the grain size continued to decrease in all the successive ECAP passes. However, the strength and hardness values still remained quite high when compared to the initial condition (as-cast and homogenized). This behavior was found to be correlated with texture modification in the material as a result of ECAP processing. Wear and corrosion tests were conducted to study tribological and corrosion behavior of ECAP processed samples. Results showed reduction in wear mass loss for the ECAP processed samples. Coefficient of friction (COF) was studied for different loads and improvement in COF values was observed for ECAP processed samples compared to initial condition. Potentiodynamic polarization and electrochemical impedance spectroscopy test showed improvement in corrosion resistance of ECAP processed samples. Immersion test showed similar trend with increased corrosion resistance of ECAP processed samples with low hydrogen evolution.