1. Ph.D Theses
Permanent URI for this collectionhttps://idr.nitk.ac.in/handle/1/11
Browse
4 results
Search Results
Item Development and Characterization of Biomedical Porous Ti - Nb - Ag Alloy through Powder Metallurgy Method(National Institute of Technology Karnataka, Surathkal, 2021) J, Shivaram M.; Arya, Shashi Bhushan.; Nayak, Jagannatha.One of the major concerns in biomedical implants is the mismatch in the elastic modulus of the implant material and the bones leading to stress shield effect. The present investigation focuses on the development of low elastic modulus, porous Ti−Nb−Ag alloy through powder metallurgy (PM) space holder method. Elemental powders of Ti, Nb and Ag with varying amounts were mixed with the powders of space holder (NH4HCO3). These powders are blended using ball milling for 1h, 5h, 10 h, 15 h, and 20 h. The powders were compacted by applying a load of 500 MPa. These compacts were initially calcinated at 200ºC for 2 h to remove the space holder and then finally sintered at 1200ºC for 3 h under ultrahigh vacuum sintering furnace. Microstructure of the porous alloys exhibited micropores, macropores and interconnected pore structures. It was found that with increasing ball milling time, the porosity and pore size decreased while the mechanical properties and electrochemical corrosion properties [in simulated body fluid (SBF)] were improved. XRD results indicated formation of small amount martensite phase and intermetallic compound of Ti2Ag along with the α and β phases. Role of Nb was studied with various Nb content (x = 25, 30 and 35 wt%) in Porous Ti−xNb−5Ag alloys. Increase in Nb content led to decrease in porosity, reduction in both the elastic modulus and compression strength but improved corrosion resistance in SBF. Samples with different porosity levels (22% to 68%) with pore size ranging from 98 μm to 130 μm were fabricated by varying the amount of space holder. Increase in porosity further leads to the reduction in the compression strength, elastic modulus and also corrosion resistance in SBF. Tribocorrosion behaviour of porous Ti−20Nb−5Ag alloys were evaluated in SBF solution by applying various loads (0 N, 1N, 5N, 10N). The results indicate that increasing the applied loads lead to a material degradation and corrosion. The porous Ti−20Nb−5Ag samples are alkali-heat treated using 5 M NaOH, to aid the hydroxyapatite formation in SBF. Alkali treated samples were immersed in SBF for 7, 14 and 21 days at 37 ºC to examine the hydroxyapatite formation. The Ca/P ratio confirmed the formation of adequate hydroxyapatite coating. Further, the electrochemical corrosion test was conducted on hydroxyapatite coated porous alloy in SBF. The hydroxyapatite coated porous alloy after 21 days of immersion in SBF shows excellent corrosion resistance. The cytotoxicity test was conducted on the porous Ti−20Nb−5Ag alloy using MG-63 human osteoblast cells by incubating for 1, 4, and 7 days. The results indicated excellent cell growth and proliferation on porous alloy surface. Cytotoxicity test confirms that developed porous sample has non-toxic in nature and highly suitable for implant application.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 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.