The low density and high relative strength of Mg alloys means they can offer engineering benefits over steels or Al alloys. However, the susceptibility of Mg alloys to corrosion has limited their exploitation and restricted their use to more benign environments. An Mg-Al intermetallic surface layer is a good candidate for a robust corrosion protection method. This work demonstrates their development by using a novel ionic liquid electroplating process to deposit Al on to Mg substrates that when heat treated diffuses to form discrete intermetallic layers. Examination of three Mg-Al-Zn alloys showed that the amount Mg-Al intermetallic phases in their microstructures was linked to the quantity of Al they contained. Subsequent self-corrosion measurements using electrochemical impedance spectroscopy demonstrated that their performance was connected to the amount of intermetallic present, and in particular the strength of the micro-galvanic couples generated between the anodic and cathodic phases. Measurements of the self-corrosion behaviour of manufactured samples of the Mg-Al intermetallics confirmed that they could provide significant improvements, but it was acknowledged that their noble nature compared to an Mg substrate would encourage galvanic corrosion if a surface layer was damaged. As such the galvanic activity of the Mg-Al-Zn alloys and Mg-Al intermetallics was compared against a pure Mg standard using zero resistance ammetry and the resistance box technique. Galvanic models of alloy self-corrosion and a damaged intermetallic surface layer were also used to assess the potential problem. These measurements demonstrated that the intermetallics could act as strong cathodes, but further discussion on the nature of the behaviour suggested means by which galvanic corrosion might self-limit or self-repair. The galvanic corrosion experiments also revealed how the combination of current flow and a solution saturated with Mg2+ ions could lead to the formation of a highly protective Mg(OH)2 film with promising characteristics.

Friction Stir Processing (FSP) is novel technique for localized modification of the surface layer of materials. FSP produces high local strains, strain rates and local temperatures that are 0.8 - 0.9 Tm, where Tm is the melting point. The processing enhances the microstructural and mechanical properties of materials through intense plastic deformation. This thesis examines the microstructure and tensile properties in FSPed Nickel Aluminum Propeller Bronze (NAB) as a function of position in the stir zone using a unique miniature tensile sample design. Test materials were single and multi-pass FSP runs from both 6 mm and 13 mm tools. Tensile ductility was observed to increase from 11 percent to more than 30 percent elongation to fracture at locations along the center of the stir zone. Yield and ultimate strengths also increased two-fold. These improved properties were associated with the formation of WidmanstaÌ tten [a] and fine, equiaxed [a] at peak temperatures of approximately 1000 [degrees] C in these locations. Some locations in the heat affected zone (HAZ) or thermomechanically affected zone (TMAZ) exhibited ductilities below that of as-cast material. Such regions had microstructures that contained a dark-etching constituent formed by cooling after being heated to approximately 800 [degrees] C. / US Navy (USN) author.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering / This study investigated the effect of microstructure on properties of selected Pt-based alloys. Six alloys of different compositions were analysed after heat treatment at 1500°C for 18 hours, followed by quenching in water; then annealed at 1100°C for 120 hours and air cooled, equivalent to a potential industrial specification. Microstructural characterisation utilised OM, SEM, AFM, TEM and EDX. Further characterisation was carried out using a nanoindentation hardness tester for nanohardness and elastic modulus measurements. The research focus was to characterize the different morphologies of γ′ ~Pt3Al precipitates during a single heat treatment, and to understand the nano-mechanical properties of the γ′ precipitates and γ (Pt) matrix, taking their proportions into account. In the present work, the samples were successfully etched, (which was not possible before) allowing optical microscopy and SEM to give much clearer microstructures than previously. The precipitate volume fractions were measured from SEM and AFM images, and agreed well. The γ′ volume fraction (expressed as percent) of nominal Pt78:Al11:Cr6:Ru5 (at.%) alloy was 51 ± 6% (SEM) and 57 ± 10% (AFM), while for nominal Pt85:Al7:Cr5:Ru3 (at.%) it was 45 ± 6% (SEM) and 48 ± 8% (AFM). A comparison of the γ′ volume fractions obtained from TEM showed that, compared to SEM, as the γ′ volume fraction observed with SEM increased, the γ′ volume fraction measured in TEM increased, although the TEM volume fraction results are believed to have considerable error due to TEM only revealing the microstructure of relatively small regions compared to SEM. Comparing with Pt-Al-Cr-Ni alternatives with γ′ volume fractions of 51-57%, the nominal Pt78:Al11:Cr6:Ru5 and nominal Pt85:Al7:Cr5:Ru3 (at.%) alloys have comparable γ′ volume fractions within, experimental error, and are considered as promising. From a microstructural viewpoint, these alloys were identified as the most promising. TEM revealed that at the specific heat treatment there were multiple size ranges of γ′ precipitates. The ~Pt3Al precipitate structure was found to be cubic L12, rather than tetragonal. The orientation relationship between the γ matrix and γ′ precipitates was found to be [114]M||[114]P; [001]M||[001]P; [103]M||[103]P. The nano-mechanical properties of the γ matrix and γ′ precipitates of Pt-Al-Cr-Ru alloys were investigated for the first time. At 2.5mN, it was possible to measure mechanical properties inside the individual γ′ precipitates and γ matrix channels, and in all six alloys the γ′ precipitates were the harder phase. The hardness of γ´, γ and the overall alloy was a function of the Pt content, and the hardness of the overall alloy was also a function of the Al content. The overall alloy hardness for nominal Pt85:Al7:Cr5:Ru3 (at.%) was 9.0 ± 0.3GPa and 9.2 ± 0.3GPa for nominal Pt78:Al11:Cr6:Ru5 (at.%). The new findings on image analysis showed that the precipitate volume fractions of nominal Pt78:Al11:Cr6:Ru5, nominal Pt85:Al7:Cr5:Ru3 and nominal Pt78:Al11:Cr8:Ru3 (at.%) were comparable to commercial nickel-based superalloys (NBSAs). TEM has shown that the precipitate morphology was similar to that of NBSAs, while nanoindentation studies indicated that the Pt-Al-Cr-Ru alloys’ overall, γ and γ phase nanohardnesses and elastic moduli were also similar to NBSAs. These results were encouraging, since the NBSAs already have commercial applications. Thus, more research efforts are encouraged on the Pt-Al-Cr-Ru alloys in order to further improve the properties of these alloys.

Cold Spray is a solid-state additive manufacturing process that uses metallic feedstock powders to create layers on a substrate through plastic deformation. This process can be used for the repair of mechanical parts in the aerospace industry as well as for structural applications. Aluminum alloy powders, including Al 6061, 7075, 2024, and 5056, are typically used in this process as feedstock material. Since this process takes place all in the solid state, the properties and microstructure of the initial feedstock powder directly influence the properties of the final consolidated Cold Spray part. Given this, it is important to fully understand the internal powder microstructure, specifically the secondary phases as a function of thermal treatment. This work focuses on the understanding of the internal microstructure of Al 6061, 7075, 2024, and 5056 through the use of light microscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron backscatter diffraction, and differential scanning calorimetry. Thermodynamic models were used to predict the phase stability in these powders and were calibrated using the experimental results to give a more complete understanding of the phase transformations during thermal processing.

No description available.

"High strength aluminum alloys are recently widely used in aircraft, automobile and construction industry fields. Typical T6 heat treatment process can be applied to improve the heat treatable aluminum alloy in order to facilitate the formation of prime strengthening precipitate phases. Critical steps in T6 heat treatment process include solution treatment, quenching and aging. Due to high thermal gradients in quenching process and aging process, large thermal stress will remain in the matrix and may bring unexpected deformation or distortion in further machining. Therefore, in order to predict the thermal stress effects, constitutive model and precipitate hardening model are needed to simulate the mechanical properties of alloy. In this dissertation, an optimized constitutive model, which is used to describe the mechanical behavior during quenching and intermediate period of quenching and aging process, was given based on constitutive models with Zenor-Holloman parameter. Modification for constitutive model is based on the microstructure model, which is developed for the quenching and aging processes. Quench factor analysis method was applied to describe the microstructure evolution and volume fraction of primary precipitate phases during quenching process. Some experimental phenomena are discussed and explained by precipitate distributions. Classical precipitate hardening models were reviewed and two models were selected for Al-Cu-Mn alloy aging treatment. Thermal growth model and Euler algorithm were used to improve the accuracy and the selected precipitate hardening models were validated by yield stress and microstructure observations of Al-Cu-Mn aging response experiments."

L'objectif de cette étude est de comprendre l'influence du silicium sur la microstructure d'alliages base TiAl. En effet, de faibles additions de silicium peuvent améliorer la tenue à chaud de ces intermétalliques. Nous montrons que le silicium a tendance à ségréger fortement durant la solidification, à l'échelle microscopique, provoquant l'apparition de siliciures primaires dans les zones interdendritiques. Après étude de cette ségrégation, nous avons procédé à des traitements thermiques d'homogénéisation afin d'étudier quantitativement les transformations solide-solide ayant lieu dans ces alliages. Ainsi, nous avons pu observer la précipitation de siliciures aux interfaces gamma/alpha2 dans des structures lamellaires homogènes. Mais, la structure lamellaire tend à se modifier en même temps que les siliciures germent et croissent. Les liens entre ces deux transformations simultanées sont mis en évidence expérimentalement, avant d'être modélisés à partir d'hypothèses simples. La réalisation d'essais mécaniques sur des microstructures contrôlées permet, en guise de conclusion, de donner des tendances quant à l'influence du silicium sur le comportement à chaud des alliages TiAl / The goal of this study is the understanding of the influence of silicon on the microstructure of TiAl-based alloys. Small additions of silicon are actually known to improve the heat resistance of these intermetallics. It is shown here that silicon segregates strongly at the microscopic scale during solidification, leading to the apparition of primary silicides in the interdendritic regions. After a study of this segregation, homogenization heat treatments were performed in order to focus on a quantitative study of the solid-solid transformations occuring in these alloys. Thus, silicide precipitation was observed at the gamma/alpha2 interfaces in homogeneous lamellar structures. However, the lamellar structure undergoes its own evolution as the silicides nucleate and grow. The links between these simultaneous transformations are shown by our experimental results, then modelled through simple considerations. As a conclusion, mechanical tests on controlled microstructures give some trends on the influence of silicon on the high temperature mechanical properties of TiAl alloys

No description available.

The incorporation of waste tyre rubber into thermoplastics to develop a class of polymer composites with both elastomeric and thermoplastic behaviour has gained a lot of attention and is becoming one of the most straightforward and preferred options to achieve the valorisation of waste tyres. In view of the unique properties rubber possesses and the rapid expansion and versatile application of wood plastic composites (WPC) materials, the inclusion of tyre rubber as raw material into WPC to develop an entirely new generation of WPC, namely rubber-wood-plastic composites (RubWPC), was presumed to be another highly promising solution to turn waste tyres into value-added materials. This research starts with the interfacial optimisation of Rubber-PE composites and WPC by the use of maleated and silane coupling agents, aiming at addressing their poor constituent compatibility and interfacial bonding, thus enabling the optimal design of RubWPC. Chemical, physical and mechanical bonding scenarios of both untreated and treated composites were revealed by conducting ATR-FTIR, NMR, SEM and FM analyses. The contribution of the optimised interface to the bulk mechanical property of the composites were assessed by carrying out DMA and tensile property analysis. The influence of the coupling agent treatments on the in situ mechanical property of WPC was first determined by nanoindentation analysis, which led to a thorough understanding of the interfacial characteristics and the correlation between in situ and bulk mechanical properties. This research focuses on the novel formulation of RubWPC and the understanding of bonding mechanism. Chemical bonding and interface structure studies revealed that interdiffusion, molecular attractions, chemical reactions, and mechanical interlocking were mutually responsible for the enhancement of the interfacial adhesion and bonding of the coupling agent treated RubWPC. The improved interface gave rise to the increase of bulk mechanical properties, while the continuous addition of rubber particle exerted an opposite influence on the property of RubWPC. The composite with optimised interface possessed superior nanomechanical properties due to the resin penetration into cell lumens and vessels and the reaction between cell walls and coupling agents.

Cement has been selected for wastes immobilization as a simple, low temperature and low cost process for decades. The mechanical and immobilization properties of cement are mainly decided by cement hydration process, especially in the first 24 hours. Previous methods for studying the cement hydration are those include isothermal calorimetry, continuous monitoring of chemical shrinkage, in situ quantitative X-ray diffraction, nuclear magnetic resonance spectroscopy (NMR), quasi-elastic neutron scattering (QENS) and small angle neutron scattering (SANS). Few available in-situ imaging methods were successfully used for net rate study of cement hydration. In this Ph.D. research, innovative imaging techniques such as X-ray computed tomography (XCT) combined with 2D SEM-BSD analysis were combined to study the microstructure and phase change of cement or cement & SrCl2 mixture during hydration. Digital Volume Correlation (DVC) and Digital Image Correlation (DIC) were applied to study the chemical volume shrinkageand drying shrinkage of cement samples during hydration. The effects of SrCl2 simulating the radioactive nuclide from nuclear waste on cement hydration were studied by XRD and ICP-AES techniques. These studies verified that the hydration net rate could be characterised by XCT imaging techniques and the volume shrinkage of cement or cement& SrCl2 mixture during hydration could be characterised by the DVC and DIC techniques.