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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.

This research focuses on studying the microstructure of alloy 909, its susceptibility to oxidation at elevated temperatures (~700°C) and substrate coatings compatibility with high velocity oxy fuel (HVOF) sprayed oxidation resistance coatings. The characterization work involved in studying the microstructure of Incoloy 909 at three heat treated conditions namely solution treated condition (ST), commercially recommended solution heat treated and aged condition (STA), and solution treated and over aged condition (STOA) using optical microscopy, analytical scanning electron microscopy, and analytical transmission electron microscopy. The oxidation susceptibility were investigated at elevated temperatures of bare and coated alloy 909 substrates by subjecting test materials to isothermal and thermal cycle testing. The microstructure of alloy 909 in the ST condition showed only the presence of blocky Laves phase. The Laves phase in this alloy is a well known for its grain pinning effect that prevents excessive grain growth. In the STA condition, the microstructure revealed the presence of fine gamma prime, intergranular and intragranular Laves phase and occasionally gamma prime precipitates orienting in a platelet form ready to transition into the epsilon phase. In the STOA condition, the microstructure consisted of Laves phase in inter and intragranular locations, and a copious amount of Widmanstatten type epsilon phase. Incoloy 909 was observed to form oxide scales in both isothermal and cyclic thermal exposures, the oxide scale consisted of distinct outer and inner scales in the micrographs. The comparison base alloy (alloyl 718) used in this study surprisingly did not show any visible presence of oxide scale after 1000 hour exposure at ~700°C. Three coatings (CoNiCrAlY, 718 , and NiAl) were sprayed on alloy 909 and alloy 718 test coupons using the HVOF process to investigate the compatibility of the coatings with the substrate. The test results points out that all the coatings were compatible with 718 substrate and only one coating (NiAl) was found compatible with the Alloy 909 substrate, indicating that the coatings that are compatible with one substrate may not be compatible with another alloy within the same family of alloys.

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Aluminium alloys are essential to a variety of industry sectors, particularly transport, where they are used in the production of cars and aeroplanes. However, aluminium alloys are susceptible to degradation through corrosion which can compromise the integrity of components manufactured from this material. Therefore research into the means by which these alloys degrade is important. This thesis aims to understand how one of the more potentially damaging types of corrosion, known as pitting corrosion, occurs in the important aluminium alloy 2024-T3 (AA2024-T3). In order to study this phenomenon, this thesis first characterises the alloy microstructure in detail, particularly the type and distribution of intermetallic particles since these play an important role in corrosion processes. The microstructure was studied using an electron microprobe analysis of a 5 mm x 5 mm area of AA2024-T3 and some 80,000 particles were characterised. This investigation was one of the most comprehensive studies to date of any aluminium alloy. Of the particles studied, it was found that the major types included the S and θ phases and a number of compositions based around AlCuFeMn and AlCuFeMnSi. Depletion zones were an integral feature of the alloy microstructure. Pair correlation functions were used to determine the degree of clustering and it was found that there was both inter particle as well as intra particle clustering. Inter particle clustering was observed at length scales well beyond 50 µm. A detailed study of corrosion on AA2024-T3 was undertaken by examining the surface after corrosion over a time period spanning 2.5 minutes to 120 minutes. From this investigation, a hierarchy of the localised corrosion was observed as it was very apparent that particles of particular elemental compositions were more susceptible to attack much sooner than other compositions. Larger corrosion attack sites on the surface, which were called co-operative corrosion, were attributed to intermetallic clustering affects and changes in chemical composition such as Cu-enrichment. These results were used to develop a detailed model of the initiation of stable pitting corrosion in AA2024-T3, which will lead to a better understanding on how to prevent pitting attack on commercially important aluminium alloys. AA2024-T3 is rarely used in the polished state, for real world applications is it generally finished by mechanical or chemical processing. In the final part of this thesis, the influence of clusters on metal finishing was examined using a standard aluminium chemical deoxidiser. It was found that the etch rate of this deoxidiser increased dramatically with the increase in temperature. Under certain processing conditions only the intermetallic particles are etched out and these retain the history of the spatial distribution of the clustering of the intermetallic particles. This leaves a cluster of 'holes' which could trap metal finishing solution and lead to severe subsurface attack

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves 120-121).

Made available in DSpace on 2014-06-11T19:27:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-21Bitstream added on 2014-06-13T19:55:26Z : No. of bitstreams: 1 pereira_gmc_me_guara.pdf: 6912618 bytes, checksum: dd31e7ca80bfc105a171d6f3233f2537 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / O Brasil ainda não produz a liga Ni – Ti em escala comercial, importando grande parte dos produtos à base de Ni – Ti voltados à área médica. O grande custo envolvido na importação destes produtos estimula a tentativa de desenvolvimento nacional desta liga. Assim, esta pesquisa objetivou a caracterização de ligas Ni-Ti-Mo, Ni-Ti-Ta e Ni-Ti-Mo-Zr produzidas por meio de moagem de alta energia e subseqüente prensagem a quente. A caracterização dos produtos moídos e prensados a quente foi realizada por meio de microscopia eletrônica de varredura, difração de raios X, espectrometria por dispersão de energia. Os resultados indicaram que a formação dos compostos intermetálicos, como o Ni3Ti, NiTi e NiTi2, ocorreram nos estágios iniciais de moagem. Em todas as misturas constituídas de pós, os tamanhos de suas respectivas partículas foram aumentados, e a distância entre as lamelas foram reduzidas para longos períodos de moagem. Este fato ocorreu devido ao mecanismo de solda fria entre as partículas dúcteis, que foram totalmente revestidas em suas superfícies esféricas. O rendimento da mistura de Ni-Ti-Mo, Ni-Ti-Ta e Ni-Ti-Mo-Zr aumentou após a moagem molhada. A prensagem a quente contribuiu para a obtenção de estruturas homogêneas e a consolidação dos pós de Ni-Ti-Mo, Ni-Ti-Ta e Ni-Ti-Mo-Zr moídos. No caso das ligas Ni-45Ti-5Mo e Ni-40Ti-10Mo, o composto Ni-Ti foi identificado na estrutura de ambas as ligas, enquanto que as fases Ni3Ti e NiTi2 foram formadas nas microestruturas das ligas Ni-45Ti-5Mo e Ni-40Ti-10Mo. Estas fases intermetálicas dissolveram teores de molibdênio inferiores a 2,1 %-at. Além disso, foi encontrada a presença de precipitados ricos em molibdênio com teores (%-at) próximos de 16Ni-25Ti-59Mo.Não foi encontrada variação significativa dos valores médios de dureza Vickers relacionadas a estas ligas ternárias, após o aumento da... / The Brazil not yet product the Ni – Ti alloy commercially, importing the most products of the Ni-Ti of the medical area. The high cust to wrap up in the importation this products encourage the development national this alloy. In this way, this work aimed the characterization of Ni-Ti-Mo, Ni-Ti-Ta and Ni-Ti-Mo-Zr alloys produced by high-energy ball milling and subsequent hot pressing. The milled and hot-pressed materials were characterized by, scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry. Results indicated that the formation of intermetallic compounds such as Ni3Ti, NiTi and NiTi2 occurred at the initial milling stages. For all the powder mixtures, it was noted that the powder sizes were increased and the distance between lamellas were reduced for longer milling times. This fact occurred due to excessive cold welding mechanism between ductile particles, which were coated fully the vial and balls surfaces. The yield of Ni-Ti-Mo, Ni-Ti-Ta and Ni-Ti-Mo-Zr powders was increased after wet milling. The hot pressing of milled Ni-Ti-Mo, Ni-Ti-Ta and Ni-Ti-Mo-Zr powders contributed for obtainment of homogeneous structures containing amounts of pores lower than 1%. In Ni-45Ti-5Mo and Ni-40Ti-10Mo alloys, the NiTi compound was identified in structure of both the alloys while that the Ni3Ti and NiTi2 were formed in microstructures of the Ni-45Ti-5Mo and Ni-40Ti-10Mo, respectively. These intermetallic phases dissolved molibdenum contents lower than 2.1 at-%. Moreover, it was noted the presence of Mo-rich precipitates with contents close to 16Ni-25Ti-59Mo (at-%). No significant variation of average Vickers hardness values was noted in Ni-Ti-Mo alloys for increased Mo addition up to 10 at-%. In Ni-Ti-Ta alloys, the NiTi and Ni3Ti phases were formed in microstructures of the hot-pressed Ni-48Ti-2Ta and Ni-45Ti-5Ta alloys, while that the Ni3Ti phase was found in ... (Complete abstract click electronic access below)

The marine biofouling and corrosion of Kunifer 10 (10% Ni, 1. 5% Fe, copper alloy), was studied using a combination of analytical techniques including x-ray photoelectron spectroscopy (XPS), energy dispersive x-ray analysis (EDXA) and electron microscopy. Particular emphasis was placed on the very early stages of fouling, although the study continued until a climax community of macrofouling organisms had become established. Dissolved organic matter (DOM), was successfully extracted from seawater by ultrafiltration and chloroform-emulsion separation. The collected materials were partially characterised using fourier transform infrared spectroscopy (FTIR) and standard chemical analyses. Extracted materials were found to be mainly carbohydrate in character, with lesser quantities of protein. The FTIR investigations indicated considerable seasonal variation in the extracted DOM. XPS proved to be a suitable technique to investigate the development of marine organic and inorganic films that form on the alloy. Adsorbed organic macromolecules exhibited a characteristic spectral "fingerprint". The effect of elevated DOM on the corrosion behaviour of the Kunifer 10 alloy was investigated, using DC and AC impedance electrochemistry. Studies indicated that a temporary loss of passivation occurred in the presence of DOM under transient oxygen conditions. The development of organic layers on the alloy was shown to influence subsequent stages of biofouling, Kunifer 10 coated with elevated levels' of organic extracts was less likely to become fouled by microorganisms, although certain extracts appeared to stimulate the settlement of marine protozoans. However, such effects were short lived due to the continual formation of bio/corrosion layers. An unusual form of corrosion, not previously documented, is reported and an explanation for the exfoliation of the bio/corrosion films on Kunifer 10 is suggested. A range of cupronickel-iron alloys were assessed for their short-term marine biofouling/corrosion performance. Of the alloys tested Kunifer 10 showed optimum resistance. The study includes a literature review on marine biofouling and corrosion.

The structure of Fe[x] - Pd[1-x] (0.85 > x > 0.5) and Fe[x] - Pt[1-x] (0.85 > x > 0.3) thin film alloys co-deposited from the vapour phase over substrate temperatures (T[s]) ranging over 200-700°C have been examined at room temperature, using X-Ray, Microprobe, TEM and SEM techniques. The sequence of phases developed as T[s] is changed have been explained in terms of three factors: a) The structure of the initial phase, which in turn is a function of the T[o] temperature where DeltaG[bcc-fcc]=0 b) Diffusion at the designated temperature, which depends on both T[s] and the rate of deposition. c) Further transformations which occur on cooling through critical ordering or martensite start temperatures. Based on the co-deposited thin film results, a model is proposed which facilitates the determination of alloy phase diagrams. A direct determination of T[o] temperature in certain composition ranges is also possible through vapour deposition techniques. Preliminary calculation of T[o] confirms the experimental results and the role played by magnetic contributions. The combined experimental data for Fe-Pt alloys confirm the earlier assumption of a eutectoid transformation at about 17at% Pt at around 600°C and provides evidence of the efficacy of thin films for the rapid assessment of low temperature equilibria.

The corrosion fatigue crack propagation behaviour of a high strength low alloy steel, N-A-XTRA 70, in a synthetic sea water solution was tested using S. E. N. specimens subjected to a loading frequency of 0.1 Hz and a load ratio of 0.6. In order to simulate the conditions encountered by a thumbnail type crack several specimens from each of the microstructural types tested, namely parent plate, heat affected zone and heat treated material, had their crack sides covered by transparent plastic covers. Severe overprotection and slight underprotection conditions were produced using cathodic protection potentials of -1400, -1300, -1200 and -700 mV (S. C. E. ). The Paris relationship da/dN = CLKm was found to be a useful tool in describing the crack propagation rate data. Results obtained, presented in the form of plots of log da/dN against log AK, show that for parent plate, H. A. Z. and heat treated material, covering the crack sides of specimens produces enhanced corrosion fatigue crack propagation rates, at cathodic protection potentials of -1400 and -1300 mV (S. C. E. ), when compared to non covered specimens. This trend was also true for H. A. Z. specimens at a potential of -700 mV (S. C. E. ). For parent plate specimens, however, covering the crack sides at a potential of -700 mV (S. C. E. ) produced reduced crack propagation rates over non covered specimens. It is believed restriced oxygen access may account for these results. Plots of the Paris exponent m and constant C for the three microstructures tested produced three lines of the form m= alnC +b where a and b were found to be dependent upon material parameters. Comparison of results with BS 4360: 50D revealed that N-A-XTRA 70 exhibited superior fatigue performance when tested in air but behaved worse under conditions of free corrosion.