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A circumferential slot In-line Virtual Impactor (IVI) has been designed using Computational Fluid Dynamics (CFD) simulation tools and experimentally characterized using monodispersed liquid aerosols to validate simulation results. The base design, IVI-100, has an application as a pre-separator for sampling inlets, where the device scalps large particles from the aerosol size distribution. The IVI-100 samples air in at 111 L/min and deliver the fine aerosol fraction in a 100 L/min flow and provide a cutpoint particle size of 10 µm, with a pressure drop of 45 Pa. An inverted dual cone configuration encased inside a tube provides the IVI-100 with a characteristic circumferential slot of width 0.254 mm (0.100 inches) and a slot length of 239 mm (9.42 inches) at the critical zone. The upper cone causes the flow to accelerate to an average throat velocity of 3.15 m/s, while the lower cone directs the major flow toward the exit port and minimizes recirculation zones that could cause flow instabilities in the major flow region. The cutpoint Stokes number is 0.73; however, the cutpoint can be adjusted by changing the geometrical spacing between the acceleration nozzle exit plane and a flow divider. Good agreement is obtained between numerically predicted and experimentally observed performance. An aerosol size selective inlet for bioaerosol and other air sampling applications using an upgraded prototype of IVI-100, mounted inside a BSI-100 inlet shell was tested in an aerosol wind tunnel over a speed range of 2 – 24 km/hr. The BSI-IVI-100 inlet has a cutpoint of 11 µm aerodynamic diameter and delivers the fine fraction at 100 L/m. The geometric standard deviation of the fractionation curve is 1.51 and the performance is not affected by wind speeds. An IVI-350, which is an adaptation of the IVI to be used as a powder fractionator, was designed based on computational simulations, and provides a cutpoint of 3 µm AD, while operating in a total flow rate of 350 L/min. Four Identical IVI -350 units will be operated in parallel to fractionate aerosolized powders in a 1400 L/min flow. An optimized inlet, with a contoured tear-drop shaped insert provides uniform flow to four identical IVI units and prevents powder accumulation in the system entrance.

The effects of mechanical alloying milling time and carbon concentration on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mechanical alloying and powder metallurgy methods were used to prepare the samples. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% pre-milled graphite were milled in a SPEX mill with tungsten milling media for up to 100h. The milled powders were then cold-compacted and pressure-less sintered between 900°C and 1200°C for 1h and 5h followed by furnace cooling. Milled powders and sintered samples were characterized using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, scanning and transmission electron microscopes. Density and micro-hardness were measured. The milled powders and sintered samples were studied as follows: In the milled powders, the formation of Fe_3 C was observed through Mossbauer spectroscopy after 5h of milling and its presence increased with milling time and carbon concentration. The particle size of the milled powders decreased and tended to become more equi-axed after 100h of milling. Micro-hardness of the milled powders drastically increased with milling time as well as carbon concentration. A DSC endothermic peak around 600°C was detected in all milled powders, and its transformation temperature decreased with milling time. In the literature, no explanation was found. In this work, this peak was found to be due to the formation of Fe_3 C phase. A DSC exothermic peak around 300°C was observed in powders milled for 5h and longer; its transformation temperature decreased with milling time. This peak was due to the recrystallization and/or recovery α-Fe and growth of Fe_3 C . In the sintered samples, almost 100% of pearlitic structure was observed in sintered samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time, contrary to what was found in the literature. The decrease in pearlite occurred at the same time as an increase in graphite-rich areas. With milling, carbon tended to form graphite instead of Fe_3 C. Longer milling time facilitated the nucleation of graphite during sintering. High mount of graphite-rich areas were observed in sintered samples prepared from powders milled for 40h and 100h. Nanoparticles of Fe_3 C were observed in a ferrite matrix and the graphite-rich areas in samples prepared from powders milled for 40h and 100h. Micro-hardness of the sintered samples decreased with milling time as Fe_3 C decreased. The green density of compacted milled powders decreased with milling time and the carbon concentration that affected the density of sintered samples. / Ph. D. / The effects of milling time and carbon composition of the alloy on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% nano graphite were milled, pressed and the sintered between 900°C and 1200°C for 1h and 5h. Milled powders and sintered samples were characterized. Density and hardness were measured. The milled powders and sintered samples were studied as follows: In the milled powders, the formation of iron carbide was observed through Mossbauer spectroscopy after 5h of milling and its amount increased with milling time and carbon composition of the alloy. The particle size of the milled powders decreased with milling time. Hardness of milled powders increased with milling time as well as carbon composition of the alloy. In the bulk samples, almost 100% of pearlitic structure was observed in samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time. The decrease in pearlite occurred at the same time as an increase in graphite with milling time. High mount of graphite areas were observed in samples prepared from powders milled for 40h and 100h. Hardness of the sintered samples decreased with milling time as iron carbide (hard phase) decreased. The density of bulk samples decreased with milling time and the carbon composition.

For hypersonic flight to enable rapid global transport and allow routine space access thermal protection systems must be developed that can survive the extreme aerothermal heating and oxidation for extended periods of time. Ultra high temperature ceramics (UHTCs) are the only potential materials capable of surviving the extreme hypersonic environment however extensive research in processing science and their oxidation properties are required before engineering systems can be developed for flight vehicles. Investigating the role of oxides during processing of ultra high temperature ceramics shows they play a critical role in both synthesis of ceramic powders and during densification. During spark plasma sintering of UHTCs the oxides can result in the formation of vapor filled pores that limit densification. A low temperature heat treatment can remove the oxides responsible for forming the vapor pores and also results in a significant improvement of the densification through a particle surface physical modification. The surface modification breaks up the native continuous surface oxide increasing the surface energy of the powder and removing the oxide as a barrier to diffusion that must be overcome before densification can begin. During synthesis of UHTCs from sol-gel the B₂O₃ phase acts as the main structure of the gel limiting the transition metal oxide network. While heat treating to form diborides the transition metal oxide undergoes preferential reduction forming carbides that reduce B₂O₃ while at high temperature encourage particle growth and localized extreme coarsening. To form phase pure borides B₂O₃ is required in excessive quantities to limit residual carbides, however carbide reduction and grain growth are connected. When the UHTC systems of ZrB₂-SiC are exposed to oxidation, either as dense ceramics or coatings on Carbon-Carbon composites, at high temperatures they undergo a complex oxidation mechanism with simultaneous material transport, precipitation and evaporation of oxide species that forms a glass ceramic protective oxygen barrier on the surface. The composite effect observed between the oxides of ZrB₂-SiC enables them to survive extreme oxidizing environments where traditional SiC oxidation barrier coatings fail.

No description available.

There is a need for simulation methodologies for multiphase three-dimensional microstructures that can be used in numerical simulations of material behavior or in exact computation of effective properties using microstructural correlation functions. Specifically, the methodology must be able to generate verifiably realistic microstructures, with complex morphology accurately represented. Striving to address that need, the research presented here develops a general microstructure simulation toolbox for multiphase two- and three-dimensional microstructures consisting of one connected phase and one or more particulate phases. Previous work by other researchers has found successful solutions to a variety of special cases of the general problem, but most of them are intended for binary microstructures, and nearly all simulate only two-dimensional microstructures. The toolbox presented here attempts to exceed those limitations. Its framework is a Metropolis stochastic-optimization routine running a simulated-anneal schedule, with particle position coordinates defining the configuration space and a range of forms available for the ﾓenergyﾔ? function. The toolbox allows several parameterizations of the microstructure, supplying all elementary properties (phase volume fractions, mean sizes, etc.) and some non-elementary properties (distributions of elementary properties, properties relating to inter-phase distances and morphology) of microstructures as possible parameters. The toolbox is able, as one special case, to simulate realistic microstructures of uniaxially compacted mixtures of elemental Al-Ti-B powders and achieve basic microstructure-processing correlation. Statistical tests involving microstructural correlation functions bear out the realism. The toolbox is also able to generate virtual microstructures for the same system, for use in the design of experiments (which are in fact high-strain-rate impact simulations), and for evaluating hypotheses involving achievable material properties. The Al-Ti-B powder compacts are potential advanced energetic materials that, when subjected to high-strain-rate impact (which may or may not constitute shock compression), explosively release heat by anaerobic reaction according as certain incompletely understood conditions are met or not. The study of those conditions and the mechanism of reaction initiation (carried out by a collaborator) is the specific application that the simulations in this work cater to. To ensure realistic morphology in simulated Al-Ti-B microstructures, this work included reconstruction (carried out by montage serial sectioning) of large three-dimensional volumes of Al-Ti and Al-B binary compacts for two sets of powders that yielded actual 3 D Ti and B particle images. Accordingly, advancement of the experimental technique of montage serial sectioning and a quantitative characterization of the real powder microstructures also formed part of this research. While only examples from Al-Ti-B powders are used throughout this work, it is clear that the methods will apply to other similar systems.

Al93Fe3Cr2Nb2 (at.%) nanoquasicrystalline alloys have been shown to have the potential to push the applications of aluminium alloys to more elevated temperatures, by maintaining a high strength. They also have more thermally stable microstructures than previous nanoquasicrystalline alloys from similar systems (the most studied of which is Al93Fe3Cr2Ti2 (at.%)). Al93Fe3Cr2Nb2 (at.%) alloys have never previously been produced in samples on a scale larger than melt-spun ribbon. This study examines the production parameters of bulk nanoquasicrystalline Al-Fe-Cr-Nb alloys. Firstly an attempt was made to reduce the melting temperatures of thermally stable nanoquasicrystalline alloys through additional alloying. The melting processes of binary, ternary, quaternary and quinary nanoquasicrystalline alloys was analysed though DTA, with endothermic reactions up to 1034oC observed. Rapidly solidified Al-Fe-Cr-Nb alloys were then produced in kilogram quantities through gas atomisation at an industrial scale. The smallest atomised powder particles contained fine scale microstructures consisting of nano-scale quasicrystals embedded in an aluminium matrix. As the cooling rate of the powder particles decreased new phases, including the theta phase (Al13(Fe,Cr)2-4) and Al3Nb were produced. 0-25μm, 25-50μm and 50-75μm (diameter) size fractions of atomised powder were each consolidated through extrusion to produce nanoquasicrystalline Al-Fe-Cr-Nb bars. Composite bars of the nanoquasicrystalline alloy mixed with 10(vol.)% and 20(vol.)% pure aluminium were also produced. The consolidation of the nanoquasicrystalline atomised powders through extrusion led to precipitation of intermetallics including (Al13(Fe,Cr)2-4) and Al3Nb, particularly in the smallest powder size fractions with the most metastable microstructures. Finally the effects of the atomisation and extrusion conditions on the microstructure and its mechanical properties were studied. Improved strength, coupled with reduced ductility was observed with decreases in the elemental aluminium composition of the Al-Fe-Cr-Nb bars and the powder size fraction they were produced from. There was however improvements in toughness of the extruded composite bars, over the nanoquasicrystalline alloy bars.

Ni-based superalloys remain state-of-the-art materials for use in the high-temperature, corrosive environments experienced by turbine blades in gas turbine engines used for propulsion and energy generation. Increasing the operating temperatures of turbine engines can yield increased engine efficiencies. However, appreciably higher operational temperatures can exceed the capabilities of Ni-based superalloys. Consequently, interest exists to develop high-melting refractory complex concentrated alloys (RCCAs) with the potential to surpass the hightemperature property limitations of Ni-based alloys. RCCAs are multi-principal element alloys, often comprising 5 or more elements in equal or near equal amounts. Conventional solidificationbased processing methods (e.g., arc melting) of RCCAs tend to yield coarse-grained samples with a large degree of microsegregation, often requiring long subsequent homogenization annealing times. Additionally, the large differences in melting temperatures of component elements can further complicate solidification-based fabrication of RCCAs. <div>Herein, the feasibility of a new chemical synthesis, powder metallurgy route for generating fine-grained, homogenous RCCAs is demonstrated. This is achieved by first employing the Pechini method, which is a well-developed process for generating fine-grained, oxide powder mixtures. The fine oxide powder mixture is then reduced at a low temperature (600°C-770 ºC) to yield fine-grained metal alloy powder. Hot pressing of the metallic powder is then used to achieve dense, fine-grained metallic alloys. While this process is demonstrated for generating fine-grained, high-melting MoW and MoWCr alloys, this method can be readily extended to generate other finegrained RCCA compositions, including those unachievable by solidification-based processing methods.</div>

Studies of mechanics and deformation in metal cutting operations have been largely limited to steady-state processes assuming constant forces and shear strain of cutting. However, ‘transient’ or varying deformation conditions are frequently encountered in manufacturing processes, when one or more processing parameters vary during the progress of the cut. Such conditions impose a lower overall strain on the resulting chip and affect the cutting forces and energies. In this study, the transient deformation characteristics are studied through the analysis of chip attributes (hardness and shape change) in a periodic cutting technique, Modulation Assisted Machining (MAM). In MAM, a sinusoidal modulation is superimposed on the tool feed, resulting in periodic engagement between the tool and workpiece. Deformation is confined to a specific volume of material and is also transient due to varying local conditions, manifesting an inhomogeneous and lower shear strain compared to steady-state cutting. A wide variety of deformation conditions from near steady-state to completely transient was achieved through the control of modulation frequency, which determines the contact length in each cutting cycle. Particles produced at lower frequencies exhibit increased hardness, consistent with the deformation more approaching steady state. Micro-indentation tests performed on each particle tracked the local variations in hardness along the length of cut, which agreed well with the non-uniform shape change observed on the cross-section of the particles. Microstructural examination of the chips made with and without modulation helped further describe the different deformation modes acting under periodic and continuous cutting conditions. MAM is also a valuable technique for metal powder processing. Individual chip particles are produced during each modulation cycle with controllable shape and size, and composition identical to that of the workpiece. Advantages of the process include a significant reduction in the specific energy of production, zero compositional variance and a tight distribution of particle sizes compared to atomization. Implications of scaling up the process for large-scale production and the possible applications of the metal particles made with MAM are highlighted.

Agglomerated milk powders are produced to give improved properties such as flowability, dispersibility, reduced dustiness and decreased bulk density. A key function of these powders is to dissolve "instantly" upon addition to water and because of this they are also called "instant milk powders". They are produced by agglomerating the undersized fines that are returned to the top of the spray drier with milk concentrate droplet spray. Interaction occurs in a collision zone, often with multiple sprays and fines return lines. Agglomeration can be a difficult process to control and operators find it hard to fine tune the process to produce specific powder properties. This work aimed to understand the effects of key droplet and fines properties on the extent of agglomeration to allow a mechanistic understanding of the process. Three scales of spray drier were investigated in this study with different rates of evaporation; a small scale drier (0.5 - 7 kg water h-1), a pilot scale drier (80 kg water h-1) and a range of commercial production scale driers (4 - 15 000 kg water h-1). A survey of operators of commercial scale driers showed that control of instant milk powder production to influence bulk density is highly intuitive. Fines recycle rates were expected to be important in control of agglomeration processes and were estimated on a specific plant by using the pressure drop measured in the fines return line. A model based on pressure drop along a pneumatic pipeline under-predicted the experimental values for pressure drop due to solids, which means a calibration curve should be generated for each specific drier. Fines recycle rates were predicted to be significantly higher at 95 to 130 % of production rates compared to those expected by operators of 50%. Experimental measurements agreed with existing models for the effect of temperature on the density and viscosity of milk concentrates. Experimental results showed that the surface tensions of concentrated milks were within the same range as literature values for standard milks below 60°C, but were significantly higher for milk above 60°C. This is thought to be linked to the mechanism of skin formation due to disulphide cross linking at high temperatures and concentrations. Powder properties were also established for selected products produced on the commercial scale driers. These powders were then used in experiments on the two smaller driers. Because collision frequency depends on the velocity and droplet size of sprays; these properties were measured for the small scale drier and estimated, where possible, for the pilot and commercial driers. The small scale agglomerating spray drier was configured to alter droplet and particle properties when interacting a vertical fines particle curtain with a horizontal spray sheet. An extensive design and improvement process was carried out to ensure the system consistently delivered these streams in a controllable manner. The processes of collision and adhesion occur very quickly inside the spray drier. In order to assess the extent of agglomeration that has occurred, the feed streams must be compared to the final product stream. An ideal way to do this is to use an agglomeration index which compares the particle size distributions of the feed (fines recycle and spray streams) and the particle size distribution of the product stream (the agglomerated powder). The index described changes between these steams across the particle size distribution and is called an agglomeration efficiency, ξg. However, it was found that the presence of fines in the product of the one-pass design obscured the agglomerates formed. The agglomeration efficiency, ξg, was modified to become ξh which subtracted the fines stream from the agglomerated product distribution. In this way ξh models industrial operation where the fines are recycled, by effectively just comparing the spray and product streams entering and leaving the process. The small scale drier was used for an experimental study on natural and forced agglomeration, where the drier was operated with spray only, then with spray and fines. For natural agglomeration, SEM images of the product powder indicated that little agglomeration occurred between spray droplets. The product yield was unacceptably low (~ 40%) due to adhesion of spray droplets to the drying chamber wall opposing the horizontal spray. When the fines curtain was introduced in the forced agglomeration experiments, product yield increased above 50% because the fines acted as collectors for the spray droplets. However, the agglomeration performance of the modified spray drier was lower than expected. The equipment design was then optimised by considering three key issues; fines dispersion, droplet dispersion and stickiness, and agglomerate breakdown. Final experiments studied agglomeration at low fines to spray mass flux ratios and showed that increasing the fines size had a positive effect on agglomeration efficiency,ξh. The agglomeration study at pilot scale identified the effect of key variables, total solids, concentrate and fines flow rate, and fines size on the agglomeration efficiency. A dimensionless flux approach was used to explain the experimental results. The fines to spray mass flux ratio and the projected area flux ratio (at constant concentrate flow rate) were found to be the most suitable to represent the physical processes during agglomeration. Experimental results showed that a higher dimensionless flux resulted in more agglomeration and as well as small fines size and atomising low solids concentrate. The critical Stokes number highlighted the importance of particle size and collision velocity on the outcome of the collision as well as the importance of stickiness on adherence following the collision. A statistical analysis established a relational model for predicting the agglomeration efficiency based on fines size, total solids and the fines to spray mass flux ratio. This thesis has gained insight into agglomeration processes during spray drying and knowledge about how to define the extent of agglomeration. Practical findings from this research can have a significant impact on successful spray drying operation for instant powders. There are some practical steps to be taken industrially to promote the control of agglomerating spray driers. The first step is to measure and control the flow of fines recycled to the top of the spray drier. The next step is to validate the findings at industrial scale and link the agglomeration index to the bulk powder properties. However, there are many challenges that remain to be tackled in the area of milk powder agglomeration. Milk powder agglomeration at the top of the spray drier is a complex process involving many different variables. A more detailed study of the micro processes that occur during agglomeration will give increased understanding of the relationships between key operating variables and agglomerate properties.