A combined experimental and analytical approach is used to study damage initiation and evolution in three-dimensional second phase particle fields. A three-dimensional formulation of a damage percolation model is developed to predict damage nucleation and propagation through random-clustered second phase particle fields. The proposed approach is capable of capturing the three-dimensional character of damage phenomena and the three stages of ductile fracture, namely void nucleation, growth, and coalescence, at the level of discrete particles. The experimental work focuses on the acquisition of second phase particle field data and measurement of damage development during plastic deformation. Two methods of acquisition of three-dimensional second phase particle fields are considered. The first method utilizes three-dimensional X-ray tomography for the acquisition of real microstructural data. The second method involves statistical stereological reconstruction of second phase particle fields from two orthogonal metallographic sections of the as-received material. The reconstruction method is also used to introduce parametric variation of key microstructural parameters to support a study of the effect of particle clustering and second phase constituent content on formability. An in situ tensile test with X-ray tomography is utilized to quantify material damage during deformation in terms of the number of nucleated voids and porosity. The results of this experiment are used for both the development of a clustering-sensitive nucleation criterion and the validation of the damage percolation predictions. The three-dimensional damage percolation model is developed based on the acquired second phase particle fields and the damage evolution characterization using the results of the in situ tensile test. Void nucleation, growth, and coalescence are modelled within the considered second phase particle field. The damage percolation model is coupled with a commercial finite element code, LS-DYNA. The damage percolation model is applied to simulate the in situ tensile test as well as to study bendability. In particular, the effect of second phase particle field parameters on formability is examined. The volume fraction of Fe-rich and Mg2Si particles is shown to be of critical importance in controlling the formability of aluminum alloy AA5182. This study of microstructural heterogeneity using the damage percolation model has resulted in a more fundamental understanding of the processes of material degradation during deformation in the presence of second phase particles. The results of the study indicate a significant effect of second phase content on formability and provide practical recommendations to improve material formability in future alloy designs.

A combined experimental and analytical approach is used to study damage initiation and evolution in three-dimensional second phase particle fields. A three-dimensional formulation of a damage percolation model is developed to predict damage nucleation and propagation through random-clustered second phase particle fields. The proposed approach is capable of capturing the three-dimensional character of damage phenomena and the three stages of ductile fracture, namely void nucleation, growth, and coalescence, at the level of discrete particles. The experimental work focuses on the acquisition of second phase particle field data and measurement of damage development during plastic deformation. Two methods of acquisition of three-dimensional second phase particle fields are considered. The first method utilizes three-dimensional X-ray tomography for the acquisition of real microstructural data. The second method involves statistical stereological reconstruction of second phase particle fields from two orthogonal metallographic sections of the as-received material. The reconstruction method is also used to introduce parametric variation of key microstructural parameters to support a study of the effect of particle clustering and second phase constituent content on formability. An in situ tensile test with X-ray tomography is utilized to quantify material damage during deformation in terms of the number of nucleated voids and porosity. The results of this experiment are used for both the development of a clustering-sensitive nucleation criterion and the validation of the damage percolation predictions. The three-dimensional damage percolation model is developed based on the acquired second phase particle fields and the damage evolution characterization using the results of the in situ tensile test. Void nucleation, growth, and coalescence are modelled within the considered second phase particle field. The damage percolation model is coupled with a commercial finite element code, LS-DYNA. The damage percolation model is applied to simulate the in situ tensile test as well as to study bendability. In particular, the effect of second phase particle field parameters on formability is examined. The volume fraction of Fe-rich and Mg2Si particles is shown to be of critical importance in controlling the formability of aluminum alloy AA5182. This study of microstructural heterogeneity using the damage percolation model has resulted in a more fundamental understanding of the processes of material degradation during deformation in the presence of second phase particles. The results of the study indicate a significant effect of second phase content on formability and provide practical recommendations to improve material formability in future alloy designs.

This thesis is concerned with the accurate numerical simulation of localized deformation that can develop into necking and failure, induced by combined bending and tension in aluminum alloy shell structures. The study is motivated by the need to establish the onset and evolution of such failures in imploding underwater cylindrical aluminum alloy shell structures. However, failure under combined bending and tension is also of concern in sheet metal forming. Such localized zones of deformation are shown to develop under controlled conditions in specially designed crushing experiments of Al-6061-T6 cylindrical shells. In these experiments shells of finite length and radially constrained ends are crushed laterally by rigid punches. The crushing, which is conducted under displacement control, causes the shell to develop bending and stretching stresses that lead to arcs of localized wall thinning to appear near the radially constrained locations. The local wall thinning develops into depressions with a width of the order of the shell wall thickness. As crushing progresses the depressions deepen, increase their span, become neck-like and develop inclined failures. The crushing was terminated when the first of four such depressions ruptured. After unloading, the shell was sliced along the principal plane of crushing and the most deformed cross sections of the necks were measured using an optical microscope. The crushing experiments were simulated numerically using solid FE models. The material was modeled as a finitely deforming elastic-plastic solid that hardens isotropically using three constitutive models: the first is based on the von Mises yield function, the second on the non-quadratic isotropic Hosford yield function and the third on the anisotropic Yld04-3D yield function. The models were calibrated to the same stress-strain response and to data from a set of radial biaxial experiments conducted on the same alloy tubes. The overall structural response was reproduced well by all models. Apparently such global responses smear out local differences introduced by the shape of the yield function adopted. However, differences between the three constitutive models were observed in the evolution of localization in the depressions. For the von Mises yield function, the localized deformation was significantly milder than in the experiments. The isotropic Hosford yield function produced necks that were closer to the experimental ones, while Yld04-3D produced results that were very close to the measurements. Clearly, and in concert with other applications, the adoption of a non-quadratic yield function is necessary for reproduction of localization and other challenging deformation histories in Al alloys. The addition of anisotropy in such models improves further the predictions. The results also demonstrated that accurate simulation of the evolution of the depressions in the presence of normal contact stresses requires the use of solid elements. Localization is clearly a three-dimensional phenomenon and shell elements reproduce most of the structural response well, but not the depressions and their evolution that eventually cause failure. / text

A comprehensive mathematical model of the hot extrusion process for aluminum alloys has been developed and validated. The model is capable of predicting the material flow behaviour and microstructure evolution that occurs in aluminum alloy AA3003 during extrusion. The plasticity module was developed using a commercial finite element package, DEFORM, a transient Lagrangian model which couples the thermal and deformation phenomena and is able to predict the temperature, strain rate and strain distribution in the billet/extrudate at any position in the container and die. Validation of the model against industrial data indicated that it gave excellent predictions of the pressure and temperature history during extrusion. Material flow effects during extrusion such as surface cladding (a transverse weld defect) as one billet is fed in after another through the die were also well predicted. The results of the FEM model for material flow and thermomechanical history were post processed using MATLAB software to predict the grain deformation and stored energy in the extruded material as well as the thickness and extent of the transverse weld defect. Finally, the model predictions for microstructure and transverse weld were compared to microstructure observations. The stored energy or driving pressure for Static Recrystallization (SRX) and Geometric Dynamic Recrystallization (GDRX) and how they are influenced by extrusion parameters were investigated using the mathematical model and experimental measurements. The experimental measurements for grain thickness and microstructural features made using Electron Back Scattered Diffraction (EBSD) technique and optical microscope show good agreement with model predictions. The mathematical model was then used to assess the effect a change in die design would have on the flow behaviour of the material during extrusion and on the transverse weld that forms.

Ka-fai Ho. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Ka-fai Ho. / Acknowledgments --- p.i / Abstract --- p.ii / 摘要 --- p.iv / Figures Captions --- p.v / Tables Captions --- p.xii / Table of contents --- p.xiii / Chapter Chapter one --- Introduction --- p.1-1 / Chapter 1.1 --- Metal Matrix Composite --- p.1-1 / Chapter 1.1.1 --- Matrix Materials --- p.1-2 / Chapter 1.1.1.1 --- Aluminum --- p.1-2 / Chapter 1.1.1.2 --- Titanium --- p.1-3 / Chapter 1.1.2 --- Type of reinforcements --- p.1-3 / Chapter 1.2 --- Conventional Fabrication method --- p.1-4 / Chapter 1.2.1 --- Liquid Phase processing --- p.1-4 / Chapter 1.2.1.1 --- Slurry deposition --- p.1-4 / Chapter 1.2.1.2 --- Squeeze casing (Pressure infiltration) --- p.1-4 / Chapter 1.2.2 --- Solid Phase processing --- p.1-5 / Chapter 1.2.2.1 --- Diffusion bonding --- p.1-5 / Chapter 1.2.2.2 --- Powder Metallurgy (P/M) --- p.1-5 / Chapter 1.2.3 --- In-situ processing --- p.1-7 / Chapter 1.3 --- Sintering processing --- p.1-7 / Chapter 1.3.1 --- Pore structure --- p.1-8 / Chapter 1.3.2 --- Compression effect on sintering --- p.1-9 / References / Chapter Chapter Two --- Methodology and Instrumentation --- p.2-1 / Chapter 2.1 --- Al-Ti-C composites --- p.2-1 / Chapter 2.1.1 --- Introduction --- p.2-1 / Chapter 2.1.2 --- Aim and Motivation --- p.2-2 / Chapter 2.1.2.1 --- Compositions and Fabrications --- p.2-2 / Chapter 2.1.2.2 --- Testing --- p.2-3 / Chapter 2.1.3 --- The Flow of the Thesis --- p.2-3 / Chapter 2.2 --- Instrumentation --- p.2-4 / Chapter 2.2.1 --- Ball-milling machine --- p.2-4 / Chapter 2.2.2 --- High temperature furnace --- p.2-5 / Chapter 2.2.3 --- Arc-melting furnace --- p.2-5 / Chapter 2.2.4 --- Instron loading machine --- p.2-6 / Chapter 2.2.5 --- Density measurement --- p.2-6 / Chapter 2.2.6 --- Vickers' Hardness Tester --- p.2-8 / Chapter 2.2.7 --- X-ray diffraction analysis --- p.2-8 / Chapter 2.2.8 --- Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS) --- p.2-9 / References / Chapter Chapter Three --- Fabrication of Al-16Ti-C composites by Powder Metallurgy method --- p.3-1 / Chapter 3.1 --- Introduction --- p.3-1 / Chapter 3.2 --- Experiments --- p.3-1 / Chapter 3.2.1 --- Experiments on Pressing pressure --- p.3-1 / Chapter 3.2.2 --- Firing temperature and duration time --- p.3-2 / Chapter 3.3 --- Results --- p.3-2 / Chapter 3.3.1 --- Pressing pressure --- p.3-2 / Chapter 3.3.1.1 --- Relative Density --- p.3-2 / Chapter 3.3.1.2 --- Surface Porosity --- p.3-2 / Chapter 3.3.1.3 --- Microhardness --- p.3-3 / Chapter 3.3.1.4 --- X-ray diffraction analysis --- p.3-3 / Chapter 3.3.1.5 --- Microstructure --- p.3-3 / Chapter 3.3.2 --- Firing temperature and duration time --- p.3-4 / Chapter 3.3.2.1 --- Microhardness --- p.3-4 / Chapter 3.3.2.2 --- X-ray diffraction analysis --- p.3-4 / Chapter 3.3.2.3 --- Microstructure --- p.3-4 / Chapter 3.4 --- Discussion --- p.3-5 / Chapter 3.4.1 --- Pressing pressure --- p.3.5 / Chapter 3.4.2 --- Firing temperature and time duration --- p.3-6 / Chapter 3.5 --- Conclusions --- p.3-6 / References / Chapter Chapter Four --- Effects of the size of Aluminum powder on the properties of Al-16Ti-4C composites --- p.4-1 / Chapter 4.1 --- Introduction --- p.4-1 / Chapter 4.2 --- Experiments --- p.4-1 / Chapter 4.3 --- Results --- p.4-2 / Chapter 4.3.1 --- Relative density --- p.4-2 / Chapter 4.3.2 --- Microhardness --- p.4-3 / Chapter 4.3.3 --- Fracture Strength --- p.4-3 / Chapter 4.3.4 --- X-ray diffraction analysis --- p.4-3 / Chapter 4.3.5 --- Microstructure --- p.4-4 / Chapter 4.3.5.1 --- Microstructure of the surface --- p.4-4 / Chapter 4.3.5.2 --- Microstructure of the fracture surface --- p.4-4 / Chapter 4.4 --- Discussion --- p.4-5 / Chapter 4.4.1 --- Sintering procedure --- p.4-5 / Chapter 4.4.2 --- Fracture model --- p.4-6 / Chapter 4.4.3 --- X-ray diffraction analysis --- p.4-6 / Chapter 4.5 --- Conclusions --- p.4-7 / References / Chapter Chapter Five --- Effects of different sintering temperature on the properties of Al-16Ti-4C composites --- p.5-1 / Chapter 5.1 --- Introduction --- p.5-1 / Chapter 5.2 --- Experiments --- p.5-1 / Chapter 5.3 --- Results --- p.5-2 / Chapter 5.3.1 --- Relative density --- p.5-2 / Chapter 5.3.2 --- Microhardness --- p.5-2 / Chapter 5.3.3 --- Fracture Strength --- p.5-2 / Chapter 5.3.4 --- X-ray diffraction analysis --- p.5-2 / Chapter 5.3.5 --- Microstructure --- p.5-3 / Chapter 5.3.5.1 --- Surface microstructure --- p.5-3 / Chapter 5.3.5.2 --- Fracture surface microstructure --- p.5-3 / Chapter 5.4 --- Discussion --- p.5-3 / Chapter 5.4.1 --- Sintering procedure and microstructure --- p.5-3 / Chapter 5.4.2 --- Hardness and fracture strength --- p.5-4 / Chapter 5.4.3 --- Model of fracture --- p.5-5 / Chapter 5.5 --- Conclusions --- p.5-5 / Chapter Chapter Six --- Fabrication of TiC by Arc melting method --- p.6-1 / Chapter 6.1 --- Introduction --- p.6-1 / Chapter 6.2 --- Experiments --- p.6-2 / Chapter 6.3 --- Results --- p.6-2 / Chapter 6.3.1 --- X-ray diffraction analysis --- p.6-2 / Chapter 6.3.2 --- Microstructure --- p.6-2 / Chapter 6.4 --- Discussion --- p.6-2 / Chapter 6.4.1 --- Composition --- p.6-2 / Chapter 6.4.2 --- Sintering process --- p.6-3 / Chapter 6.5 --- Conclusions --- p.6-3 / References / Chapter Chapter Seven --- The Effects of the contents of Ti and C on the properties of Al-TiC and Al-Ti-C composites --- p.7-1 / Chapter 7.1 --- Introduction --- p.7-1 / Chapter 7.2 --- Experiments --- p.7-1 / Chapter 7.3 --- Results --- p.7-2 / Chapter 7.3.1 --- Relative density --- p.7-2 / Chapter 7.3.2 --- Microhardness --- p.7-2 / Chapter 7.3.3 --- Fracture Strength --- p.7-2 / Chapter 7.3.4 --- X-ray diffraction analysis --- p.7-3 / Chapter 7.3.5 --- Microstructure --- p.7-3 / Chapter 7.3.5.1 --- Surface microstructure --- p.7-3 / Chapter 7.3.5.2 --- Fracture surface microstructure --- p.7-4 / Chapter 7.4 --- Discussion --- p.7-4 / Chapter 7.4.1 --- Hardening effect --- p.7-4 / Chapter 7.4.2 --- Relationship between fracture strength and relative density --- p.7-4 / Chapter 7.4.3 --- Fracture model --- p.7-5 / Chapter 7.5 --- Conclusions --- p.7-5 / References / Chapter Chapter Eight --- Conclusions and Future Work --- p.8-1 / Chapter 8.1 --- Summary --- p.8-1 / Chapter 8.2 --- Future Work --- p.8-2 / References

Wreckage of ‘Carnauba’, a 1930s vintage Sikorsky S-38 aircraft, a beloved icon of SC Johnson's early history, was found on July 5, 2000, in seawater off of an Indonesian island of West Irian Jaya. The company decided to recover this aircraft from seawater, conserve it, and display it in its museum, as part of their rich heritage. The objective was to study the aluminum alloy used on the aircraft for its chemical and mechanical properties, suggest the corrosion mechanism of aluminum alloy 2024 in seawater, and recommend preservation methods for the same. Chemical analysis performed on the samples collected from the site revealed that copper was the primary alloying element. Copper is responsible for increasing the strength. However, copper is also the reason for pitting corrosion of the aluminum alloy, causing material loss and reducing the structural stability of the wreckage. Copper forms intermetallics with other elements, such as magnesium and aluminum, and is distributed in the aluminum matrix heterogeneously. In order to study the corrosion mechanism of aluminum alloy 2024, it was subjected to potentiodynamic tests in sodium chloride solution. In the presence of an electrolyte like seawater, the difference between the potentials of these intermetallics and the surrounding aluminum matrix creates a galvanic cell. The galvanic cells serve as sites for localized corrosion. Chloride ions are responsible for pitting of alloy 2024. A pitting potential of around -600mV was observed when sodium chloride was used as an electrolyte. The average corrosion rate measured for wrought aluminum alloys was around 0.05 mm/year. The thesis provides guidelines or recommendations for the procedure to be followed in recovering aircraft from seawater, and retain it in its as found condition. Recommendations about various measurements like pH, dissolved oxygen, salinity, pressure, temperature, and velocity need to be taken and the visual assessment needs to be done before the aircraft is hauled from the seawater were specified. After the aircraft has been recovered, recommendations for handling, cleaning, and prevention of corrosion using coatings such as carnauba wax and inhibitors such as chromates, have been stated.

Friction Stir Welding(FSW) experiments are conducted using 6061-T6 aluminum as specimens. The temperatures at different distances from the center of the joint are measured. Curve fitting analyses are used to predict the temperature distribution and calculate the central temperature of the joint, proceeding by measuring temperature. A second order curve is found to better fit the experiment values by the least square method.

<p>The development of highly formable aluminum alloy sheets is of great interest to the automotive industry, because they provide a lightweight alternative to steel sheet for structural panels. Finding ways to improve the formability of Al alloys is the main subject of the present investigation. This issue is tackled from two angles. First, a possibility of fabricating a two-phase material containing newly discovered ductile intermetallic compounds is considered. The Al-La-Mg system is thermodynamically optimized accompanied with a differential thermal analysis (DTA) experiment to validate the optimization results. A new approach is introduced to deal with the incompatibility of phase models in binary Al-La and La-Mg systems. This approach is successfully applied to the Laves and B2 phases in the binary La-Mg system. A utilization of the thermodynamic description of the Al-La-Mg system to model solidification at low and high cooling rates shows that it is impossible to fabricate such a two-phase material by casting.</p> <p>Second, the effect of small additions of cerium and lanthanum on Fe-bearing intermetallics in a wrought heat-treatable Al alloy is examined. Fe-containing intermetallics are known to deteriorate the formability of Al alloys by acting as void nucleation sites. It is found that in alloys containing 0.1-0.2 wt. % of lanthanum, the fraction of less harmful Chinese script particles is pronouncedly higher than that in the reference alloy. In addition to this advantage, much smaller grains are seen in the alloy with 0.2 wt. % La. Despite similarities between La and Ce, the latter metal neither modifies the microstructure nor noticeably affects the gram size. Hot rolling and solutionizing nullifies the beneficial effect of small La additions resulting in no improvement in the formability of the alloy.</p> <p>In order to understand how lanthanum affects the phase portrait of the alloy, a socalled direct thermal analysis experiment is performed. Solidification paths are derived for slowly cooled alloys by coupling the results of this investigation with microstructural observations. The likelihood of two modification mechanisms is speculated using these solidification paths.</p> / Thesis / Doctor of Philosophy (PhD)

<p>A study was undertaken to achieve a better understanding of the key microstructure-performance relationships involved with the intergranular corrosion and exfoliation corrosion of Cu-lean AA7003 alloy extrusions, as a function of the heat-treated condition. The heat treatments of interest in this study include the naturally-aged T4 condition, representing the as-extruded condition, an artificially-aged T6 condition, representing a post-weld stress-relief condition, and an artificially-aged automotive paint-bake cycle condition. The influence of heat treatment on the resultant microstructure is characterized using light optical microscopy, coupled with image analysis, and electron (scanning & transmission) microscopy, coupled with energy dispersive spectroscopy. The influence of heat treatment on the corrosion behaviour is characterized using anodic polarization measurements and ASTM standardized testing to evaluate the susceptibility resistance to intergranular corrosion (ASTM G110) and exfoliation corrosion (ASTM G34).</p> <p>The cross-sectional (LT-ST & L-ST) microstructures of all three heat treatments consist of a fibrous, non-recrystallized grain structure in the interior, and a coarse recrystallized grain structure at the exterior surface. Both grain structures are slightly elongated along L-direction. The grain size distribution and grain aspect ratio distribution is weakly dependant on the heat treatment applied, and on the orientation plane. Among the two artificial aging, the T6 (post-weld stress-relief) condition has the higher micro-hardness (yield strength), as it has higher density (volume fraction) of the strengthening MgZn₂-type precipitates (η, η′ and their GP zones) within the Al matrix grains.</p> <p>Anodic polarization measurements show a more negative corrosion potential (E_corr) for the two artificially aging heat-treated conditions. The shift is believed to be due to the micro-galvanic cell activity established between the more noble Al matrix grains and the more active strengthening MgZn₂-type precipitates within the Al matrix grains, which have a significantly increased surface area (volume fraction) in the artificially-aged condition. A similar, single breakdown potential (E_b) corresponding to a pitting potential (E_pit) is observed, regardless of the heat-treated condition. The similar potential is believed to be due to localized breakdown of the passive film at the periphery of coarse second phase intermetallic particles (Al₃Fe), which remain unaffected by artificial aging.</p> <p>Of the three heat-treated conditions studied, the T6 condition exhibits the lowest susceptibility to both intergranular corrosion and exfoliation corrosion. The lower susceptibility is believed to be due to the lack of any Cu enrichment in across the grain boundary region (either in the solute depted zone or in the generic Mg(Zn,CuAl)₂ grain boundary precipitates). This lack of enrichment is believed to produce a smaller micro-galvanic cell activity across the grain boundary region, as compared to that produced when Cu is enriched across the grain boundary region, particularly in the Solute depted zone (SDZ).</p> <p><br /></p> / Master of Applied Science (MASc)

<p> The effect of strain hardening rate and material strength on the development of surface roughness in AA6111 was investigated. No measurable change in the rate of roughening, or in the surface morphology was observed due to altering the strain hardening rate by using different test temperatures. Changing the material strength and strain hardening rate by altering the precipitation state also gave no significant change in either roughening rate or morphology with respect to strain. The development of surface roughness is also independent of strain history. Samples subjected to an intermediate polish after 20% true strain resumed roughening at the same rate regardless of amount of previous tensile strain. The development of surface roughness is dependent on only the strain level to which the sheet is deformed. The surface morphology seems to be controlled by the combination and distribution of texture components on the surface. The rate of roughening is grain size dependent and the surface grain size may provide a key to controlling roughening. </p> / Thesis / Master of Applied Science (MASc)