http://localhost:8081/jspui/handle/123456789/35 Wed, 06 May 2026 02:32:49 GMT 2026-05-06T02:32:49Z http://localhost:8081/jspui/handle/123456789/20503 Title: Substitutional-Interstitial High Entropy Alloys Authors: Qadri, Syed Amuth-U-Rasool Abstract: Since the last two decades, a new alloying concept that involves the addition of multi principal elements in high concentrations has been introduced. These alloys, termed “high entropy” alloys, are under vast exploration due to the scientific and technological exploration possibilities in these new alloy systems. These alloys have shifted the alloy design strategy from a single element-based alloy system to a multi-element-based alloy system. Thus, offering a vast compositional space for the development of new alloy systems. Classically, high entropy alloys are defined as alloy systems with at least five major elements, each element content between 5 to 35 atomic percentage. Another definition of these alloys is based on configurational entropy, where alloys with configurational entropy, Sconf, larger than 1.61 R are categorised as “high entropy” alloys. Most of the investigations in these alloys show that Fe, Ni, Co and Cr are used with maximum frequency in these alloys. While the addition of interstitial elements such as Carbon, Oxygen, Nitrogen, Boron and Hydrogen into the interstitial sublattice is less explored. These interstitial elements can bring about additional configurational entropy to the alloy system. Also, the addition of these interstitial elements into these alloys brings about some additional interstitial strengthening effects, including some Transformation Induced plasticity (TRIP) and Twinning induced plasticity (TWIP) effects upon plastic deformation and compositional metastability effects such as spinodal decomposition and precipitation. As thorough investigations on the role of interstitials in HEAs are lacking, therefore it is of great interest to study High and Medium Entropy alloys based on “Substitutional- Interstitial” alloy design strategy. For solid solutions with both substitutional and interstitial elements present, a regular solution model with two sublattices are considered: substitutional and octahedral-interstitial. The configurational entropy “𝑆𝑐𝑜𝑛𝑓” of such solution is given by the following equation: � �𝑐𝑜𝑛𝑓= -R [{𝑎∑ 𝑌𝑀ln(𝑌𝑀) 𝑀 }+ {𝑐∑𝑌𝐼 𝐼 ln(𝑌𝐼)+(1−𝑌𝐼)ln(1−𝑌𝐼)}] where R is the Universal gas constant, 𝑌𝑀 is the fraction of substitutional sites occupied by M and � �𝐼 is the fraction of octahedral interstitial sites occupied by interstitial element, I. The relation between atomic fraction of elements and their occupied site fraction is given below � �𝑀= 𝑋𝑀 (1−𝑋𝐼) � �𝐼= 𝑎𝑋𝐼 𝑐(1−𝑋𝐼) ‘a’ and ‘c’ are the number of sites for the substitutional and octahedral-interstitial sublattices, respectively (for FCC, a = c = 1 and for BCC, a = 1;c = 3 ). XM and XI are the atomic fraction of substitutional and interstitial atoms, respectively. Due to the higher number of interstitial sites per substitutional site in BCC, the configurational entropy increase is higher for BCC alloyed with N as compared to FCC alloyed with N. Due to this additional entropy contribution from interstitial sublattice, one can alter the substitutional element composition to tune the properties without reducing the total configurational entropy of the alloy. This way a low or medium entropy alloy can be converted to high entropy alloy by introducing the interstitial N. Interstitials are at the heart of steels alloy design and heat treatment to bring in wide variety of properties. In HEA’s due to the differences in size and chemical activity of different elements the interstitial sites experience changes in size, shape and chemical surroundings making it as an interesting sublattice to understand the way interstitial solubility and ordering reactions emerge. Against the above background, this PhD thesis aims to understand the N alloying into medium entropy alloys by applying various nitriding treatments. The contents of this Ph. D thesis are organized as detailed below: First chapter of this thesis consists of an introduction to high entropy alloys with the presentation of literature information on the addition of interstitials into HEAs, Interstitial- Substitutional sublattice design approach, low-temperature nitriding of stainless steels and thermodynamics of salt bath and gaseous nitriding. At the end of this chapter, the objectives of this PhD thesis are detailed. Second chapter presents the research outcomes from the low temperature salt bath nitriding experiments on 316 L stainless steel powders. The N content taken up by the 316 L powders after different nitriding treatments is calculated based on the enhanced lattice parameter of austenite and the known dependence between lattice parameter and N content for austenitic phase of stainless steels. Thereafter, the increase in the configurational entropy as a result of dissolved nitrogen is calculated using Hillert-Staffanson two sublattice model. Colossal levels of N solubility (up to 30 at. % N) was realized which drastically enhanced the configurational entropy of the system. Hardness of powders is increased from about 200 HV before nitriding to above 1500 HV after nitriding. The obtained results were discussed based on the thermodynamic calculations and a discussion on utilizing these nitrided powders to make high N stainless steels via sintering or additive manufacturing utilizing nitrided steel powders. Mon, 01 Apr 2024 00:00:00 GMT http://localhost:8081/jspui/handle/123456789/20503 2024-04-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/20502 Title: Synthesis and electrochemical characterization of Hard Carbon(HC) anode and HC/P2-Na2/3Ni1/3Mn2/3O2 full cell characteristics for Na-ion battery Authors: Verma, Bharat Abstract: Chapter 1 focuses on the need for cleaner and sustainable energy storage technologies. In the realm of energy, the global community faces two pivotal challenges: transitioning from fossil fuel-dependent energy sources to renewable alternatives for electricity generation, and shifting ground transportation from internal combustion engine-driven vehicles to electric propulsion systems. While considerable strides have been made in harnessing renewable energies from diverse sources such as wind and solar radiation, the development of cost-effective energy storage devices, such as batteries, to meet the rising energy demands lags significantly behind. With the rapid evolution of portable electronics and electric mobility, energy storage technologies have assumed a central position in research endeavors, playing a pivotal role in the effective utilization of green and clean energy resources. Among these technologies, Lithium ion Batteries (LIBs) have spearheaded a revolution in energy storage across portable electronics, grid storage, and Electric Vehicles (EVs), owing to their flexible, lightweight design, rechargeability, and high energy density. The fast kinetics of Li+ ions, facilitating rapid transfer of ions during charge-discharge cycles, renders LIBs a dependable and energy-efficient choice. Chapter 2 meticulously elucidates the intricacies of hard carbon, encompassing its structure, properties, and synthesis methods, emphasizing its significance in advancing battery technologies. Carbonaceous materials have historically played a pivotal role as negative electrode materials for alkali-ion batteries, owing to their abundance, low cost, and tunable structural properties. Within this discourse, various analytical techniques like (X-Ray diffraction, high resolution transmission electron microscopy, BET) used to evaluate the structural and morphological properties of hard carbon have been discussed in detail. Chapter 3 deals with the synthesis of a carbon-based anode material through the pyrolysis of tissue waste over temperatures ranging from 800 to 1400°C. Waste pyrolyzed at 1400°C exhibited the highest de-sodiation capacity of 350 mAhg⁻¹ at a rate of 20 mAg⁻¹, accompanied by commendable cyclic stability evidenced by a 70% capacity retention after 400 cycles at 100 mAg⁻¹. The investigation further delved into scrutinizing the morphology, structure, and surface chemistry of the cycled hard carbon electrodes using ex-situ Field Emission Scanning Electron Microscopy (FE-SEM), High-Resolution Transmission Electron Microscopy (HRTEM), Energy Dispersive X-ray Spectroscopy (EDX), and X-ray Photoelectron Spectroscopy (XPS). In i addition, the assessment of diffusion kinetics parameters was carried out via Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) measurements. In the Chapter 4, the focus is on synthesizing an anode material derived from Pistachio shells, an abundantly available waste material, through a pyrolysis process conducted within a temperature range of 900 to 1400°C. Remarkably, the material treated at the upper limit of this temperature range, specifically at 1400°C, demonstrates exceptional electrochemical performance, showing a reversible discharge capacity of 302 mAhg-1 and a plateau capacity of 223 mAhg-1 at 10 mAg-1. Notably, this material exhibits stable cyclic performance, retaining an impressive 80% of its capacity after 500 cycles at 200 mAg-1. To gain insights into the morphological, structural, and surface chemical characteristics of the cycled hard carbon anode materials, comprehensive analyses were conducted utilizing ex-situ field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. Furthermore, the study delves into the sodiation and desodiation mechanisms of the synthesized hard carbon employing operando Raman spectroscopy. Additionally, diffusion kinetic parameters were elucidated through Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry (CV) methods. In Chapter 5, we focus on the synthesis of carbonaceous anode material for sodium-ion batteries (SIBs) through the pyrolysis of sugarcane bagasse, a readily available biowaste. Sugarcane bagasse comprises carbon-rich compounds such as hemicellulose, lignin, and cellulose, which impede the graphitization process during pyrolysis. Consequently, the resulting material exhibits a highly disordered and porous structure, characterized by flake-like formations and increased interplanar spacing. These structural features are instrumental in facilitating faster sodium ion transport and enhancing sodium storage capabilities. The electrochemical performance of the synthesized hard carbon, particularly the sample pyrolyzed at 900°C (referred to as SW 900), demonstrates remarkable characteristics. Specifically, SW 900 exhibits a second-cycle discharge capacity of 300 mAhg-1 at 25 mAg-1, indicative of its high energy storage capacity. Even after undergoing 300 cycles at a higher current density of 100 mAg-1, SW 900 retains over 65% of its initial capacity, showcasing its robust cycling stability. In Chapter 6, we have developed a 3D porous carbon-based anode material by subjecting pea skin waste, in conjunction with an in-situ MgO/NaCl template, to pyrolysis at 750℃ (referred to as PSC-MN). The inclusion of MgO and NaCl serves a dual purpose, acting as mesopore and macropore templates, thereby facilitating the creation of a porous structure while concurrently reducing the impedance associated with the transfer of Na+ ions from the electrolyte to the ii micropores. Notably, the presence of MgO contributes to enhanced porosity and structural integrity. Furthermore, to explore the influence of MgO, we synthesized 3D porous carbon without MgO (referred to as PSC-N) and conducted an in-depth analysis of the structural and electrochemical properties of both PSC-MN and PSC-N. The precise control over porosity induced by the optimal presence of the NaCl template, coupled with a reduced BET surface area and the presence of mesopores, has culminated in the exceptional electrochemical performance of these materials. Specifically, PSC-MN exhibits a reversible charge capacity of 240 mAhg-1 at 20 mAg-1, with a notable 60% capacity retention after 250 cycles at 100mAg-1. To gain insights into the electrochemical kinetics of the PSC-MN and PSC-N samples, we employed Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry (CV) methods. Additionally, ex-situ scanning electron microscopy (SEM) and Raman spectroscopy were utilized to further elucidate the sodium storage mechanism during the charge-discharge process. Overall, our findings underscore the significance of tailored porosity control and template utilization in optimizing the electrochemical properties of carbon-based anode materials for SIBs. Chapter 7 highlights the conclusion of thesis. This underscores the effect of the carbonization temperature of all the four hard carbon precursors (with different morphologies) on the structural properties, surface chemistry and the electrochemical properties. Plateau capacity an important aspect of the electrochemical performance is compared for all the biowaste precursors and the role of morphology in determining the plateau capacity has also been highlighted in this section. Chapter 8 reveals the future scope of work like making hard carbon / metal oxide composites and evaluating their structural and electrochemical performance in half and full configuration for Na-ion battery. Mon, 01 Apr 2024 00:00:00 GMT http://localhost:8081/jspui/handle/123456789/20502 2024-04-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/20470 Title: DEVELOPMENT OF ELECTRICAL DISCHARGE MACHINABLE AND WEAR-RESISTANT SPARK PLASMA SINTERED SiC BASED COMPOSITES Authors: Rao, Chodisetti Surya Prakasa Abstract: Owing to the unique set of properties like high hardness, strength, chemical stability, low thermal expansion, and low density, silicon carbide (SiC) ceramics are preferred for wear-resistant components such as cutting tools, dies, nozzles, brake pads, nose caps, and leading edges of space vehicles, etc. However, applicability for SiC ceramics is limited because of the difficulty in sintering and shaping. The poor sintering characteristics of these covalent bonded and high melting point materials can be improved by sintering in optimized conditions with appropriate additives and reinforcement. It is to be noted that improved sinterability leads to refined microstructures and enhanced mechanical properties, which result into superior tribological performance. However, machining SiC ceramics into complex shapes is difficult due to their high hardness and brittleness. Therefore, it is understood that improved hardness is needed for superior tribological performance, but high hardness is detrimental for machining. The machining of hard and brittle SiC ceramic workpiece is possible if direct contact with the machining tool is avoided. In this regard, a non-conventional machining technique namely electrical discharge machining (EDM) is useful where the material removal occurs by electrical discharge. However, electrical discharge is possible only when the material exhibits minimum electrical conductivity of 10-2 (Ω·cm)-1. Being a poor/semiconductor, SiC ceramics are machinable by EDM only when reinforced with a conductive second phase. Metallic additives or reinforcements are not recommended because of the possible oxidation and degradation of strength and hardness. Among ceramic reinforcement materials, borides (TiB2, HfB2, etc.) and nitrides (ZrN, TiN, etc.) with high electrical conductivity, high melting point temperatures and high hardness are recommended for development of electrical discharge machinable SiC components for high temperature tribological applications. TiB2 with high strength, modulus and good electrical conductivity is suitable reinforcement to improve mechanical properties and electrical conductivity of SiC ceramics, and ZrN with high melting temperature and high electrical conductivity is suitable to improve electrical discharge machinability and high temperature wear resistance of SiC ceramics. However, literature on EDM and tribological performance of SiC-TiB2/ZrN composites is limited. This can be probably due to difficulty in sintering SiC-boride/nitride composites as higher temperatures and longer holding periods are needed because of poor diffusion characteristics of both matrix and reinforcement. Spark plasma sintering (SPS) is an advanced sintering technique with high rates of heating which can reduce sintering temperatures and shorten sintering time. Further, large control over sintering cycles in SPS helps in obtaining microstructures with homogenous and i connected network of second phase that can improve conductivity and subsequent EDM ability. In this context, an attempt is made to develop new electrical discharge machinable and wear resistant non-oxide composites of SiC with boride, nitride or boride-nitride reinforcement by liquid phase spark plasma sintering in the present thesis work. A total of three SiC-based composite systems: SiC-TiB2, SiC-ZrN and SiC-(ZrN-TiB2) are developed. AlN-Y2O3 additive system was used to facilitate liquid phase sintering. For each composite system, the effect of reinforcement type and content on densification, microstructure, mechanical properties, EDM and wear resistance is primarily discussed. The mechanisms of material removal in sliding wear contacts are elucidated. Firstly, SiC composites with (0 to 30vol%) TiB2 content and 10vol% AlN-Y2O3 additives were prepared by spark plasma sintering at the lowest temperature of 1800 ℃ for 10 min. Densification analysis emphasized improved density with increase in TiB2 content due to beneficial effect of nascent TiO2 presence in decreasing the melting point of liquid phase additives. The phase analysis and microstructure confirm restricted β→α phase transformation in the SiC matrix due to reduced sintering temperature and time. SiC-20vol% TiB2 composite exhibited a better combination of hardness (23.7 GPa) and fracture toughness (5.5 MPa.m0.5). Crack deflection and residual stress development are responsible for improved fracture toughness. For SiC-30vol% TiB2 composites, significant amount of conductive reinforcement, additive composition, and sintering in the nitrogen atmosphere resulted in high electrical conductivity (2.72 × 103 (Ω.cm)-1). The electrical discharge machining of sintered composites illustrates the potential of the sintered SiC-TiB2 composites in extending the application regime of conventional SiC-based ceramics. To assess tribological potential of sintered SiC-TiB2 composites, dry unlubricated sliding wear tests (unidirectional and reciprocating) against SiC ball were conducted at 5, 10 or 15N load in ambient conditions, and at 15N load and 600 ℃ (reciprocating wear). The friction and wear results are reported as a function of TiB2 content. The steady-state coefficient of friction increased with increase in TiB2 content, whereas wear rate decreased with increase in TiB2 addition. At room temperature, abrasion, fracture and grain pull-outs are commonly observed on worn surfaces. Wear mechanisms changed from mechanical wear at room temperature to tribo oxidation at 600 °C. The formation of tribo oxide layer and load bearing roller debris formation resulted in lower wear at 600 °C compared to room temperature. At 600 ℃, the coefficient of friction and wear rate decreased. Though the dominant mechanical wear is observed at room ii temperature sliding, SiC-TiB2 composites exhibited lower wear rate in unidirectional sliding, compared to reciprocating sliding. This result is supported by the contribution of debris (different morphology) with respect to the sliding test configuration. In the second part, dense SiC-(10 to 30vol%) ZrN composites were developed using spark plasma sintering with 10vol% AlN-Y2O3 additive at 1800 °C for 5 min in nitrogen atmosphere. The ZrN addition significantly contributed towards enhancing densification during sintering of SiC composites by reducing melting point and viscosity of liquid phase with ZrO2 presence. While phase and microstructural analysis confirm major β-SiC and ZrN phase after sintering, hardness decreased due to relatively less hard ZrN phase addition and fracture toughness increased due to residual stress development and crack deflection at ZrN phase. Electrical conductivity increased with conductive ZrN addition. SiC-ZrN composites were successfully cut by using wire-EDM. The tribological potential of novel electrical discharge machinable SiC-ZrN composites at various conditions of reciprocating sliding wear contacts was investigated. Results show that friction is less affected by reinforcement content at room temperature (25 ℃), but significantly changed at high temperature (600 ℃). At least one order of lower wear rate obtained with an increase in temperature from 25 ℃ to 600 ℃. The formation of protective (ZrO2.SiO2) rich tribo layer is responsible for superior sliding wear resistance at 600 ℃. In the third part, dense SiC composites with (10 to 30vol%) hybrid (ZrN-TiB2) reinforcement were fabricated by liquid phase spark plasma sintering. The effect of co-addition of ZrN and TiB2 on densification, mechanical properties, and electrical conductivity of SiC ceramics was investigated. In addition to the retardation of β→α phase transformation in SiC, solid solutioning of reinforcement phases and in situ hBN formation are observed in sintered composites. The fracture toughness ranged from 4.1 to 4.6 MPa.m0.5, hardness from 19.5 to 20.3 GPa, and materials removal rate in wire EDM from 7.19 to 8.54 mm2/min with change in hybrid reinforcement content. In reciprocating sliding wear study, almost one order of magnitude reduction in wear rate observed for SiC-ZrN-TiB2 composites with increase in sliding test temperature from 25 ℃ to 600 ℃. The presence of soft liquid phase precipitates and in situ hBN phase resulted in severe fracture at room temperature, while the formation of a protective tribo oxide layer is attributed to lower wear rate at 600 °C compared to room temperature. Overall, the present research study demonstrated the potential of newly developed spark plasma sintered SiC composites with TiB2 and/or ZrN reinforcement for superior performance in electrical discharge machining and sliding wear conditions. The findings from the present study iii are important for the applications in structural, aerospace and defense fields where complex shaped wear resistant SiC ceramic components are needed. SiC composites with 20vol% reinforcement content exhibited better combination of mechanical properties, whereas SiC composites with 20 or 30vol% reinforcement content exhibited superior machining and wear performance compared to monolithic SiC. Mon, 01 Jul 2024 00:00:00 GMT http://localhost:8081/jspui/handle/123456789/20470 2024-07-01T00:00:00Z http://localhost:8081/jspui/handle/123456789/20428 Title: CORROSION BEHAVIOR OF HEAT-TREATED 7068 ALUMINUM ALLOY Authors: Ankur Abstract: 7xxx series (Al-Zn-Mg-Cu) aluminum alloys are high-strength precipitation hardening alloys. They have various applications in automobile and aircraft industries owing to the desired combination of their physico-mechanical properties. Additionally, their excellent workability, high specific strength, and higher heat and electric conductivity make them suitable materials for the fabrication of structural and industrial components. Moreover, compared to other light alloys, they are advantageous materials for the manufacturer due to the low fabrication costs, and well-developed fabrication techniques. However, high stress corrosion cracking (SCC) susceptibility can limit their use in various applications. Even though there has been much study on these alloys, researchers are still working to improve them by taking into account the alloying elements and heat treatment process, their effects on mechanical properties, SCC behavior, and the manufacturing process in order to widen their scope of applications. The peak aged temper condition (T6, T651) of 7xxx series alloys shows better strength, while it exhibits high susceptibility to stress corrosion cracking along with other localized forms of corrosion like pitting and exfoliation corrosion. To prevent these detrimental consequences, over-ageing heat treatment (T76, T7651) is practiced although it decreases the strength by up to 10-15%. Therefore, to overcome these drawbacks and in order to achieve a combination of better strength along with superior corrosion resistance, three-stage heat treatment process which is known as retrogression and re-ageing (RRA) treatment came into existence. The final morphology, size, and distribution of precipitates during RRA process are very different as compared to aforementioned conventional heat-treatment processes. Amongst the new 7xxx series alloys, high zinc (>7 wt%) containing 7068 alloy is one of the highest strength alloys and is projected to be useful for various applications in the aerospace and automotive industry. However, its response to T6 and different RRA treatment parameters in terms of microstructural evolution, GBP chemistry, and PFZ characteristics is unknown. Further, correlation of microstructural features resulting from different T6 and RRA procedures with its mechanical and corrosion behavior is yet to be ascertained. This is especially important since over-aging condition that is normally recommended for optimized corrosion performance of other 7xxx series alloys, does not ensure adequate corrosion performance in high zinc alloys. Therefore, present dissertation work deals with determining optimized T6 and RRA process parameters in order to obtain a combination of superior corrosion resistance and better strength. Furthermore, the passive film breakdown mechanism in borate buffer solution and metastable pitting behavior are studied. Various techniques like electron microscopy, differential scanning calorimetry (DSC), potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis are employed in order to analyze the effect of heat treatment process parameters on the corrosion behavior. Microstructural features of high zinc 7068 aluminum alloy resulting from different ageing treatments during T6 heat treatment process are correlated with mechanical properties, localized corrosion behaviour, and SCC susceptibility. Localized attack manifested in dissolution of second phase precipitates which occurs from selective leaching of magnesium and aluminum. The micro-galvanic effect of iron-containing precipitates on the matrix dissolution is less as compared to Al2MgCu phase. 7068 alloy exhibited comparable SCC resistance in both peak aged and over-aged conditions, which is attributed to the absence of precipitate free zones (PFZ) in the former. Besides, Peak and overaged show discontinuous grain boundary precipitates and fewer Al2MgCu/α-Al micro-galvanic couples. Thus, over-ageing process that reduces strength, is not necessary for satisfactory SCC performance in this alloy. Mechanical and corrosion behavior of high-strength, high-zinc (>7 wt%) containing 7068 aluminum alloy is investigated after employing different RRA treatments. The effect of pre-aging conditions on the distribution of copper, zinc, and magnesium, the volume fraction of η′ phase, and the width of PFZ have been investigated. Microstructural and compositional features are correlated with hardness, uniform corrosion, SCC, and intergranular corrosion resistance. A combination of two opposite effects, that is, the presence of nobler, high-copper containing grain boundary precipitates and micro-galvanic effect of PFZ along with the distribution of alloying elements, that is, Cu, Zn, and Mg govern the electrochemical behavior of RRA treated 7068 alloy. Specimens that were pre-aged at 100 °C and retrogression treated at 180 °C exhibited retarded reduction reaction kinetics in polarization studies, and provided better SCC resistance. Besides the microchemistry of grain boundary precipitates, matrix precipitate size affected the oxygen reduction kinetics. An increase in copper content of discontinuous boundary precipitates did not monotonously lead to improved SCC resistance. Thus, optimum pre-aging and RRA conditions are identified for this high-zinc 7xxx series alloy, and it is found that specimen pre-aged at lower temperature exhibited higher strength and better corrosion resistance. Mon, 01 Jan 2024 00:00:00 GMT http://localhost:8081/jspui/handle/123456789/20428 2024-01-01T00:00:00Z