Haynes 282 is a gamma prime (𝛾′)-strengthened nickel base superalloy developed in 2005, exhibiting a good combination of high temperature properties and fabricability. Microstructural features such as 𝛾′ and carbides play an important role in deriving the mechanical properties of the alloy during heat treatment. As Haynes 282 is a relatively new alloy with insufficient literature availability, the present thesis is aimed at studying the evolution of microstructure for different heat treatment times and temperature with a special focus on 𝛾′ phase precipitation kinetics with different initial conditions for the material. The study is divided into two sections with objectives which are focused on the different ends to the heat-treatment time scales. The first objective of this study was to investigate γ' precipitation at short heat treatment times and develop Time-Temperature Precipitation (TTP) and Hardness (TTH) diagrams for Haynes 282 using a novel arc heat treatment. In this technique, a steady state temperature gradient, covering room temperature to liquidus, was created using stationary TIG arc on a disc mounted on a water-cooled chamber. Aged and solutionized samples were arc heat treated for 1.5 minutes, 30 minutes and 4 hours. The study was complemented with temperature modelling, thermodynamic calculations, and 𝛾′ precipitation simulation. A unique graded microstructure formed, consisting of dendritic region in fusion zone; dissolution area of all phases including MC carbides, grain boundary carbides, and 𝛾′; grain boundary carbide zone, 𝛾′ band; and base metal. 𝛾′ precipitate size increased with increasing time and temperature. 𝛾′ precipitation simulation model was developed, and it showed very good agreement with experimental results. Finally, the results were summarized in TTH and TTP diagrams. The second objective in this work was to study understand the coarsening behaviour of 𝛾′ phase with an initial pre-heat-treated GKN heat treatment using furnace heat treatment. Isothermal heat treatments for temperatures from 800°C to 1120°C and times from 30 seconds to 96 hours were performed. Morphological changes in 𝛾′ phase, particle size distribution, grain sizes and hardness on these isothermal heat-treated states are presented in this work. Additionally, A TC PRISMA precipitation model was evaluated to predict 𝛾′sizes and compare it with the measurements. It was concluded that complex initial microstructure, containing bimodal distribution of 𝛾′ precipitates, caused deviations between predicted and measured values, while the model, in the previous objective, predicted the sizes in close approximation to the experimental values. Therefore, further understanding and development of precipitation kinetics with the software should be done to achieve closer results to the experiment. / Haynes 282 är ett gamma prime (𝛾′) - förstärkt superlegering av nickelbas som utvecklades 2005 och uppvisar en god kombination av högtemperaturegenskaper och tygbarhet. Mikrostrukturella egenskaper såsom 𝛾′ och karbider spelar en viktig roll för att få de mekaniska egenskaperna hos legeringen under värmebehandling. Eftersom Haynes 282 är en relativt ny legering med otillräcklig litteraturtillgänglighet syftar den aktuella avhandlingen till att studera utvecklingen av mikrostruktur för olika värmebehandlingstider och temperatur med ett särskilt fokus på 𝛾′ fasutfällningskinetik med olika initiala förhållanden för materialet. Studien är uppdelad i två sektioner med mål som är inriktade på de olika ändarna på värmebehandlings tidsskalorna. Det första syftet med denna studie var att undersöka 𝛾′nederbörd vid korta värmebehandlingstider och utveckla Time-Temperature Precipitation (TTP) och Hardness (TTH) diagram för Haynes 282 med användning av en ny bågvärmebehandling. I denna teknik skapades en jämn temperaturgradient, som täcker rumstemperatur till liquidus, med användning av stationär TIG-båge på en skiva monterad på en vattenkyld kammare. Åldriga och lösningsbara prover bågvärmebehandlades under 1,5 minuter, 30 minuter och 4 timmar. Studien kompletterades med temperaturmodellering, termodynamiska beräkningar och 𝛾′utfällningssimulering. En unik graderad mikrostruktur bildad, bestående av dendritisk region i fusionszon; upplösningsområde för alla faser inklusive MC-karbider, korngränsande karbider och 𝛾′; korngränsen karbidzon, 𝛾′ band; och oädel metall. 𝛾′utfällningsstorlek ökade med ökande tid och temperatur. 𝛾′utfällningssimuleringsmodell utvecklades, och den visade mycket bra överensstämmelse med experimentella resultat. Slutligen sammanfattades resultaten i TTH- och TTP-diagram. Det andra syftet med detta arbete var att studera förstå det förgrovning beteendemönster hos 𝛾′ fasen med en initial förvärmebehandlad GKN-värmebehandling med ugnsvärmebehandling. Isotermiska värmebehandlingar för temperaturer från 800 ° C till 1120 ° C och gånger från 30 sekunder till 96 timmar utfördes. Morfologiska förändringar i 𝛾′fasen, partikelstorleksfördelning, kornstorlekar och hårdhet på dessa isotermiska värmebehandlade tillstånd presenteras i detta arbete. Dessutom utvärderades en TC PRISMA-nederbördsmodell för att förutsäga 𝛾′ storlekar och jämföra den med mätningarna. Det drogs slutsatsen att komplex initial mikrostruktur, innehållande bimodal fördelning av 𝛾′-fällningar, orsakade avvikelser mellan förutspådda och uppmätta värden, medan modellen i det tidigare målet förutspådde storleken i nära anpassning till experimentvärdena. Därför bör ytterligare förståelse och utveckling av utfällningskinetik med programvaran göras för att uppnå närmare resultat till experimentet.

This master’s thesis work is a study of oxidation and degradation of nickel-base alloys at high temperatures. The materials studied are designed for use in critical gas turbine components such as turbine blades and vanes. Some of the alloys are used today, whereas others have not yet entered commercial application. In order to maximize the efficiency of gas turbines, there is an ambition to maximize the operating temperatures. There is therefore a demand for materials which can withstand the damage mechanisms active at high temperatures. Among these damage mechanisms are oxidation and microstructural degradation. To investigate the oxidation resistance of 7 different monocrystalline and polycrystalline alloys, samples have been exposed isothermally in still air at temperatures between 850 and 1000°C, for exposure times of up to 20000h. Two of the alloys were also exposed cyclically at 950°C. Oxidation during the heat treatment resulted in significant weight changes, which were measured after each cycle for cyclically exposed samples and after completed heat treatment for isothermally exposed samples. The weight change data was used to evaluate the relative oxidation resistance of the alloys. The ranking of the alloys with respect to oxidation resistance was generally in agreement with the oxidation resistance predicted by a simple consideration of the Cr and Al contents of the alloys. However, the single-crystal alloy PWA1483 displayed better oxidation resistance than predicted from its chemical composition. Metallographic analysis of the samples indicated that the oxide scales formed consisted of several different types of oxides. The oxide scales were mainly composed of Cr2O3 and Al2O3. Fragments of the oxide scales spalled off, primarily during cooling but also in some cases during the long-term heat treatments. Spalling of the oxide scale accelerated the oxidation process, since the ability of the oxide scale to impede diffusion decreased with its decrease in thickness. Oxidation caused depletion of Al and thereby local dissolution of the aluminum-rich γ′ particles, which are of vital importance to the mechanical properties of the material. A γ′ depleted zone thereby formed underneath the oxide scale. In this zone nitrides and needle-like particles, believed to be topologically close packed μ phase, precipitated during heat treatment. Recrystallization in the depletion zone was observed in some of the monocrystalline materials. MC carbides (M=metal) present in the virgin material decomposed during heat treatment and M23C6 carbides were formed. The γ′ particles coarsened during heat treatment, which resulted in decreased hardness. The hardness decreased with exposure temperature up to 950°C, as expected due to the increased coarsening rate. At 1000°C an unexpected increase in hardness was observed for all sample materials except one. A possible explanation for this hardness increase is redistribution of γ′, by dissolution of γ′ during heat treatment and reprecipitation during cooling as much finer particles. A fine dispersion of γ′ is expected to contribute more to the hardness than a corresponding volume of γ′ in the form of larger particles. For some of the sample series, clear correlations between hardness and γ′ particle size or exposition time were found. These relationships could potentially be used to estimate the exposure temperature of service-exposed material. A numerical model was implemented in Matlab to describe the process of oxide growth and spalling, cycle by cycle. The model was successfully adapted to experimental data from the cyclic oxidation measurements. The general applicability of the model to cyclic oxidation data at different temperatures and cycle frequencies was not investigated. At long times of cyclic exposure, the net weight loss of the samples could be well approximated as a linear function of the number of cycles. However, during the last few cycles the amount of oxide spalled in each cycle suddenly decreased. This change in spallation behavior was mainly observed for the samples cooled in air between every cycle and to a much smaller extent for the samples cooled in water. The proposed explanation is that spalling occurred preferentially at a weak subscale interface and that the spalling propensity decreased with decreasing area of this weak interface. The deviating results of the last few cycles were not included in the modeling of the cyclic oxidation process.

No description available.

The commercial polycrystalline superalloy RR1000 developed for turbine disc applications contains a large number of alloying elements. This complex alloy chemistry is required in order to produce appropriate microstructures and the required mechanical properties, such that the most important strengthener γʹ displays complex alloy chemistry. The broad aim of this project is to develop an approach to measuring the composition of γʹ precipitates at a broad range of length scales from nanometres to hundreds of nanometres, and subsequently develop a better understanding of the role of thermodynamics and diffusion kinetics on γʹ phase separation and precipitate growth. A solution of the absorption-corrected EDX spectroscopy to spherical particles was developed in our work, therefore enabling the quantitative analysis of precipitates' composition using an absorption-corrected Cliff-Lorimer approach. By performing this quantification, size-dependent precipitate compositional variations were obtained. Examination of this quantitative approach was compared to thermodynamic calculations of primary γ' precipitates possessing equilibrium compositions. Given the development of semi-quantitative compositional measurements for spherical γʹ precipitates and that cooling is one of the most common and critical regimes in physical metallurgy of Ni-based superalloys, this approach was then applied to study the local compositional variations that are induced in γ' precipitates when the alloy RR1000 undergoes different cooling rates. These measured compositions have been compared to detailed thermodynamic calculations and provide new experimental evidence of the importance of the dominant role of aluminium antisite diffusion in determining the low-temperature growth kinetics of fine-scale γ' precipitates. We have applied a similar analysis approach to study the compositional variations of γʹ cores within the class of secondary precipitates upon cyclic coarsening and reversal coarsening. It was shown that supersaturated Co in secondary γʹ exhibits an overall trend towards the equilibrium but Co content can significantly increase as γʹ coarsens. It was demonstrated that the limited elemental diffusivity in γ and γʹ compared to the observed coarsening rate in the coarsening regime results in the long-lasting Co supersaturation in γʹ and builds up elemental enhancements or depletions. These inhomogeneous elemental distributions produce compressive elastic constraints on large-scale secondary γʹ, therefore inducing morphological instability of these γʹ and causing the reversal coarsening. These results enable us to better understand the role that both thermodynamics and limited diffusion kinetics plays in controlling the complex microstructures of γ' precipitates.

L'alliage N19 est un superalliage base nickel, élaboré par métallurgie des poudres, qui a été développé récemment en vue d'une application pour disques de turbine aéronautique. L'objectif de cette étude est d'optimiser la microstructure de cet alliage en agissant sur les paramètres de traitements thermiques pour améliorer les propriétés mécaniques de l'alliage. Une bonne compréhension des relations entre les paramètres de traitements thermiques et la microstructure, d'une part, et, des relations entre la microstructure et les propriétés mécaniques, d'autre part, est donc nécessaire. De nombreux traitements thermiques ont été appliqués à l'alliage N19 pour évaluer l'effet de la température de mise en solution, des conditions de refroidissement et de la température de revenu sur la taille de grains, et sur la taille et la distribution des précipités gamma prime. L'observation des microstructures en microscopie électronique à balayage et en transmission a permis d'évaluer l'effet des différentes étapes du traitement thermique sur les caractéristiques microstructurales de l'alliage. L'effet de ces modifications microstructurales sur la vitesse de propagation de fissure en fatigue-fluage à 650°C a été étudié. Les résultats de ces essais de propagation de fissure en fatigue-fluage ont été analysés à l'aide d'essais de comportement en fatigue-relaxation. Une synthèse des différentes propriétés mécaniques de l'alliage en fonction des paramètres de traitements thermiques et des caractéristiques microstructurales a été proposée.

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No description available.

No description available.

Aluminas have had some form of chemical and industrial use throughout history. For little over a century corundum (α-Al2O3) has been the most widely used and known of the aluminas. The emerging metastable aluminas, including the γ, δ, η, θ, κ, β, and χ polymorphs, have been growing in importance. In particular, γ-Al2O3 has received wide attention, with established use as a catalyst and catalyst support, and growing application in wear abrasives, structural composites, and as part of burner systems in miniature power supplies. It is also growing in importance as part of the feedstock for aluminium production in order to affect both the adsorption of hydrogen fluoride and the feedstock solubility in the electrolytic solution. However, much ambiguity surrounds the precise structure of γ-Al2O3. Without proper knowledge of the structure, understanding the properties, dynamics and applications will always be less than optimal. The aim of this research was to contribute towards settling this ambiguity. This work was achieved through extensive computer simulations of the structure, based on interatomic potentials with refinements of promising structures using density functional theory (DFT), and a wide range of supporting experiments. In addition to providing a more realistic representation of the structure, this research has also served to advance knowledge of the evolution of the structure with changing temperature and make new insights regarding the location of hydrogen in γ-Al2O3. / Both the molecular modelling and Rietveld refinements of neutron diffraction data showed that the traditional cubic spinel-based structure models, based on m Fd3 space group symmetry, do not accurately describe the defect structure of γ-Al2O3. A more accurate description of the structure was provided using supercells of the cubic and tetragonal unit cells with a significant number of cations on c symmetry positions. These c symmetry based structures exhibited diffraction patterns that were characteristic of γ-Al2O3. The first three chapters of this Thesis provide a review of the literature. Chapter One provides a general introduction, describing the uses and importance of the aluminas and the problems associated with determining the structure of γ-Al2O3. Chapter Two details the research that has been conducted on the structure of vi γ-Al2O3 historically. Chapter Three describes the major principles behind the computational methods employed in this research. In Chapter Four, the specific experimental and computational techniques used to investigate the structure of γ-Al2O3 are described. All preparation conditions and parameters used are provided. Chapter Five describes the methodology employed in computational and experimental research. The examination of the ~ 1.47 billion spinel-based structural possibilities of γ-Al2O3, described using supercells, and the selection of ~ 122,000 candidates for computer simulation, is detailed. This chapter also contains a case study of the structure of κ-Al2O3, used to investigate the applicability of applying interatomic potentials to solving complex structures, where many possibilities are involved, and to develop a systematic procedure of computational investigation that could be applied to γ-Al2O3. Chapters Six to Nine present and discuss the results from the experimental studies. / Preliminary heating trials, performed to determine the appropriate preparation conditions for obtaining a highly crystalline boehmite precursor and an appropriate calcination procedure for the systematic study of γ-Al2O3, were presented in Chapter Six. Chapter Seven details the investigation of the structure from a singletemperature case. Several known structural models were investigated, including the possibility of a dual-phase model and the inclusion of hydrogen in the structure. It was demonstrated that an accurate structural model cannot be achieved for γ-Al2O3 if the cations are restricted to spinel positions. It was also found that electron diffraction patterns, typical for γ-Al2O3, could be indexed according to the I41/amd space group, which is a maximal subgroup of m Fd3 . Two models were presented which describe the structure more accurately; Cubic-16c, which describes cubic γ-Al2O3 and Tetragonal-8c, which describes tetragonal γ-Al2O3. The latter model was found to be a better description for the γ-Al2O3 samples studied. Chapter Eight describes the evolution of the structure with changing calcination temperature. Tetragonal γ-Al2O3 was found to be present between 450 and 750 °C. The structure showed a reduction in the tetragonal distortion with increasing temperature but at no stage was cubic γ-Al2O3 obtained. Examination of the progress of cation migration indicates the reduction in the tetragonal nature is due to ordering within inter-skeletal oxygen layers of the unit cell, left over from the breakdown of the hydroxide layers of boehmite when the transformation to γ-Al2O3 occurred. Above 750 °C, δ-Al2O3 was not observed, but a new phase was identified and designated γ.-Al2O3. / The structure of this phase was determined to be a triple cell of γ-Al2O3 and is herein described using the 2 4m P space group. Chapter Nine investigates the presence of hydrogen in the structure of γ-Al2O3. It was concluded that γ-Al2O3 derived from highly crystalline boehmite has a relatively well ordered bulk crystalline structure which contains no interstitial hydrogen and that hydrogen-containing species are located at the surface and within amorphous regions, which are located in the vicinity of pores. Expectedly, the specific surface area was found to decrease with increasing calcination temperature. This trend occurred concurrently with an increase in the mean pore and crystallite size and a reduction in the amount of hydrogen-containing species within the structure. It was also demonstrated that γ-Al2O3 derived from highly crystalline boehmite has a significantly higher surface area than expected, attributed to the presence of nano-pores and closed porosity. The results from the computational study are presented and discussed in Chapter Ten. Optimisation of the spinel-based structural models showed that structures with some non-spinel site occupancy were more energetically favourable. However, none of the structural models exhibited a configuration close to those determined from the experimental studies. Nor did any of the theoretical structures yield a diffraction pattern that was characteristic of γ-Al2O3. This discrepancy between the simulated and real structures means that the spinel-based starting structure models are not close enough to the true structure of γ-Al2O3 to facilitate the derivation of its representative configuration. / Large numbers of structures demonstrate migration of cations to c symmetry positions, providing strong evidence that c symmetry positions are inherent in the structure. This supports the Cubic-16c and Tetragonal-8c structure models presented in Chapter Seven and suggests that these models are universal for crystalline γ-Al2O3. Optimisation of c symmetry based structures, with starting configurations based on the experimental findings, resulted in simulated diffraction patterns that were characteristic of γ-Al2O3.

No description available.