Redesigning heavy components with thinner components is one way to lower automotive weight and improve fuel efficiency. Therefore, replacing thick cast iron exhaust manifolds with thinner heat resistant stainless steel one is a prime example of this approach. Material for a thin exhaust manifold must tolerate cyclic thermal fatigue. In SUS 444, this characteristic is directly related to the influence of microstructure on high temperature strength and the stability of the microstructure at the high operating temperature range. The goal of this research is to identify the cause for the drastic difference in the stress-strain behavior between two potential manufacturer heat treatments that will serve as a simplified model case for high temperature cyclic fatigue.  Transmission electron microscopy (TEM) based microstructure analyses of samples which have been aged at 750"C for 100 hours and then hot-tensile tested at 750"C with a strain rate of   suggest continuous recrystallization as the mechanism responsible for the stable high temperature strength. The initial high temperature strength observed in the unaged sample was due to the precipitation of fine Laves phases which pinned down the motion of dislocations. As deformation progressed the strength increased until a critical precipitate size, volume fraction and dislocation density before Laves phases begun to rapidly coarsen and resulted in the abrupt decrease in strength. Microstructure evidence suggests the absence of precipitation strengthening effect in the aged samples could be a contributing factor to the decrease in peak strength between the aged samples and the unaged samples. / Master of Science

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At high temperatures, the properties of aqueous electrolyte systems differ markedly from those at 25°C. For mixed-electrolyte dilute solutions at high temperatures, the degree of ion-association is sufficiently large that the association equilibrium must be incorporated in any model describing the solutions. These association reactions usually do not occur to a measurable extent at room temperature. Oscarson and co-workers have designed a correlation model based on the excess Gibbs energy which can correlate log K and ∆H values as well as the heats of dilution for aqueous electrolyte systems as a function of temperature (T) and ionic strength (I) from 275 to 350°C. Use of calorimetric data to develop the model has been shown to be more accurate than using ∆dilH values from the variation of log K with temperature because one less differentiation with respect to temperature is required. In this study, the computer program developed by Oscarson and co-workers has been modified by incorporating the IAPWS-95 water equation of state and Archer and Wang's correlation of the dielectric constant of water into the excess Gibbs engergy model. The difference between the present work and prior work is that it uses a more accurate equation of state for water, a more accurate dielectric constant for water and the best equilibrium constants currently available. The properties of water play a very important role in the calculation of ∆dilH values, and the modified program developed here using improved water equations is shown to be superior to the previous one. The results of this modified model were tested by comparing the predicted heats of dilution with experimental measurements from Oscarson's work. These experimental data cover the range from 523.15 K to 623.15 K and 103 bar to 128 bar. The nominal concentrations of the solutions used for the ∆dilH experiments were 0.25, 0.5 and 1.0 m. Equilibrium constants K for Na2SO4 (aq), H2SO4 (aq), NaAc (aq), and HCl (aq) association were taken from conductivity values measured by Wood and co-workers using a flow conductance apparatus. These log K values were used to compare the predicted log K values from Oscarson's model and those from this modified model.

Maturity of high-temperature polymer-reinforced composites defer to conventionally expensive and intensive methods in both material and manufacturing aspects. Even traditional carbon-carbon, aerogel, and ceramic approaches are highly limited by difficult manufacturing techniques and are subject to sensitive handling throughout their processing and lifetime. Despite their utility in extreme environments, the high costs of existing high-temperature composites find limited practical applicability under high-performance applications. The development of continuous fiber-reinforced pyrolysis-derived carbon-matrix composites aim to circumvent the issues surrounding the manufacturing and handling of conventional high-temperature composites. Polymer matrix composites (PMCs) have a number of attractive properties including light weight, high stiffness-to-weight and strength-to-weight ratios, ease of installation on the field, potential lower system-level cost, high overall durability and less susceptibility to environmental deterioration than conventional materials. However, since PMCs contain the polymer matrix, their applications are limited to lower temperatures. In this study, a pyrolysis approach was used to convert the matrix material of phenolic resin into carbon-matrix to improve the mechanical and thermal properties of the composites. Composite material consisting of basalt fiber and phenolic resin was pyrolyzed to produce basalt-carbon composites through a novel method in which the pyrolysis promoted in-situ carbon nanotube growth to form “fuzzy fibers”. The carbon phenolic composites were pyrolyzed to produce carbon-carbon composites. Several types of composites are examined and compared, including conventional phenolic and carbon-matrix composites. Through Raman spectroscopy and scanning electron microscopy, the composition of materials are verified before testing. Investigation into the improvements from in-situ carbon growth was conducted with an open-flame oxyacetylene test (ASTM-E285), to establish high-temperature thermal behavior, in addition to mechanical testing by three-point bending (ASTM-D790), to evaluate the mechanical and thermal properties of the pyrolyzed composites.

Rare earth oxides with trivalent lattice dopants have been of great interest to researchers in the recent years due to its potential applications in catalysis and high temperature protective coatings. The ability to store oxygen in rare earths is the basis for catalysis because of the ability to change valence states which causes the presence of intrinsic oxygen vacancies in the crystal lattice. Although, several doped-rare earth oxide systems in micron scale have been investigated, the doping effect in cerium oxide nanoparticles with well characterized particle size has not been studied. The doping of ceria at that small size can be very beneficial to further improve its catalytic properties and alter the high temperature phases in alloy systems. Cost effective room temperature chemical methods are used in the current work to synthesize uniformly distributed undoped and doped (dopants: La, Nd, Sm, Gd, Y and Yb) rare earth oxide nanoparticles. In the present study, the variation of the properties in nanocrystalline ceria (NC) synthesized by microemulsion method is studied as a function of dopant size and its concentration. To further understand, the role of dopant (cation) size on the oxygen vacancy concentration, doped nanocrystalline oxide powders were analyzed by Raman Spectroscopy, X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). XRD studies showed that lattice parameter change in nanocrystalline oxide by doping trivalent rare earth elements is largely depending on size of trivalent ions. It showed that by doping larger cations (Gd3+ and Y3+) compare to Ce3+ causes lattice expansion where as smaller cations (Yb3+) leads to lattice contraction. It also showed that the lattice expansion or contraction is directly proportional to dopant concentration. The results of Raman Spectroscopy showed that the correlation length decreases resulting in increase in oxygen vacancies for larger trivalent dopants (Sm3+, Gd3+ and Y3+). However, the correlation length increases resulting in decrease in oxygen vacancies for smaller trivalent dopants (Yb3+) compare to nanocrystalline ceria. These nanostructured oxides are further applied to develop high temperature oxidation resistance coatings for austenitic steels. The present study investigates the role of oxygen vacancies in the performance of high temperature oxidation resistance as a function of various trivalent dopants and dopant concentration. NC and La3+ doped nanocrystalline ceria (LDN) particles were coated on AISI 304 stainless steels (SS) and exposed to 1243K in dry air for longer duration and subjected to cycling. The results are further compared with that of micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from the SEM cross-sectional studies. XRD analysis showed the presence of chromia in both NC and 20 LDN samples which is absent in uncoated steels. From SIMS depth profiles, Fe, Ni depletion zones are observed in presence of LDN coated sample indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the early formation of protective chromia layer is discussed and compared to its micron counterpart. This study helps in understanding the role of oxygen vacancies to protect austenitic stainless steel at high temperature and confirms the oxygen inward diffusion rather cation outward diffusion in rare earth oxide coatings. It also gives an idea to identify the type of dopant and its concentration in nanocrystalline cerium oxide which supplies the critical oxygen partial pressure required at high temperature to form primarily impervious chromia layer.

While the statistical mechanics of systems in thermal equilibrium is a well established discipline, nonequilibrium systems are fundamentally much less well understood, even though most natural phenomena fall into the latter category. In particular, there is as yet no nonequilibrium analog for the systematic formalism of Gibbs ensembles. Rather than deal with the difficult problem of general nonequilibrium systems, this study is restricted to the steady states of a simple model whose equilibrium properties are well known. The Ising lattice gas displays a number of surprising phenomena when driven into nonequilibrium steady states. This study extends previous work to a more general model with anisotropic interparticle interactions. Using Monte Carlo simulations, we obtain the phase diagram for the model, controlled by the driving field, temperature, and anisotropy parameter α. Under saturation drive, the shift in the transition temperature between ordered and disordered states can be either positive or negative, depending on α ≡ √(𝐽_∥/𝐽_⟂). The possible existence at large α of an additional phase ordered in only one direction is discussed. For finite drives, both first and second order transitions are observed. A novel technique for locating the first order transition line is presented. Some aspects of the phase diagram can be predicted by investigating the two-point correlation function to first order in a high temperature series expansion. However, the series expansion fails to predict even qualitatively the α-dependence of the critical temperature. / Master of Science

High strength boron steels are commonly used as structural reinforcements or energy absorbing systems in automobile applications due to their favourable strength to weight ratios. The high strength of these steels leads to several problems during forming such as poor formability, increased spring back, and tendency to work-harden. In view of these difficulties, high strength boron steels are usually formed by press hardening at elevated temperatures with a view to facilitate forming and simultaneous hardening by quenching of complex shaped parts.The high strength steel sheets are used with an Al–Si coating with a view e.g. to prevent scaling of components during hot-metal forming. The Al-Si coated high strength steel can lead to problems with galling (i.e. material transfer from the coated sheet to the tool surface) which have a negative influence on the quality of the produced parts as well as the process economy. The available results in the open literature pertaining to high temperature galling are very scarce. With this in view, the friction and wear behaviours of different tool steel coatings and different roughness lay directions sliding against Al–Si-coated high-strength steel at elevated temperatures have been investigated by using a high-temperature reciprocating friction and wear tester at temperature of 900 °C.The results have shown that parallel sliding with respect to the surface roughness lay reduces the severity of galling compared to perpendicular sliding. None of the coatings included in this study have shown beneficial effects in view of galling. The DLC coating experienced less galling compared to the AlCrN and TiAlN. Post polishing of the coated tool steel has resulted in more severe material transfer with higher and more unstable friction. / <p>Validerat; 20111022 (anonymous)</p>

Yes / Bond between substrate and overlay concretes is a key factor for the success of the repair method and significantly influences the structural performance of the repaired element. This study investigated the effect of fly ash and the surface preparation method on the bond strength of repaired concrete after exposure to high temperatures, that has not been comprehensively studied in the literature. For this purpose, overlay concretes containing 0, 5, 10, 15, and 20% fly ash as a replacement by weight of cement were cast on the original concrete surface prepared by four methods namely, as-cast, wire brushed, grooved and grooved-wire brushed. The bond strength of the interface between concrete substrate and overlay concrete was evaluated after exposure to 23, 200, 400, and 600oC temperatures for 1 hour. The results showed that partial replacement of cement by fly ash in the overlay concrete increased the bond strength of repaired concrete by up to 71%, depending on the amount of fly ash used, surface preparation method, and the temperature to which the sample was exposed. The maximum increase of bond strength was recorded for concrete containing 20% fly ash when the wire brushed preparation method was adopted at temperature of 200oC. However, surface preparation was the most influential parameter, achieving a bond strength gradual increase in order from as-cast, wire brushed, grooved to grooved-wire brushed methods. The results also showed that for most of the samples having similar surface preparation and the same percentage of fly ash, bond strength decreased with the increase of exposure to temperature; for example, for overlay concretes without fly ash, in as-cast and wire brushed surface preparation methods at temperatures of 400 and 600 oC, the bond strength has reached zero. On the other hand, for grooved and grooved-wire brushed surface preparation methods, the bond strength reduction was about 63%, when temperature increased from 23 to 600oC. / The full-text of this article will be released for public view at the end of the publisher embargo, 12 month from first publication.

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