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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

esulted. This was the

esulted. This was the case for the Sint higher than 0.85 and 0.90 for TOPC10 and AOPC20, respectively. In contrast to the CP mode, accurate modelling could be achieved in the CF mode without variations in shape factor α, indicating a constant value for the entire infiltration period. It was demonstrated that Richards´ model was valid even in the dynamic CF infiltration mode. Here, and for the constant pressure infiltrations, good agreement was observed with the Preform 1D code, providing that all the relevant parameters were well characterised. It is possible to use this tool to optimize the process parameters in order to minimize residual porosity in the MMC and maintain the preform integrity. 235

6. CONCLUSIONS 1. An aqueous processing route and subsequent partial sintering can be used to produce preforms from commercial Al2O3, TiO2 and MgO powders with a minimum ceramic volume fraction (Vp) of 0.50 and open porosity between the grains (intragranular porosity). 2. To achieve a lower Vp of 0.30- 0.40, a pore forming additive (PFA) was incorporated into the slurry leading to a bimodal pore structure consisting of intragranular pores combined with larger cavities obtained by pyrolysis of the PFA during sintering. As a result, the target Vp could be reached by systematic variation of the processing parameters. 3. The lower limit of Vp for the cellulose pore former (PC) was 0.35, as below this value the preform was too weak to use to produce a MMC. In contrast, substitution of PC with carbon fibres (PF) allowed Vp to be lowered to 0.19. 4. A novel route for MgO preform and the subsequent MMC fabrication was investigated. The combination of MgO with the pore former PC gave an intermediate magnesium hydroxide during sintering. This offers the possibility for tailoring the preform specific surface area. The surface area achieved was from 0.7 to 5.7 m²/g. The latter value is higher than that of the starting powder. 5. By combining a conventional AlSi alloy matrix with the ceramic preform through melt infiltration, a new material resulted with significantly enhanced mechanical and tribological properties in comparison to monolithic alloys and Saffil fibre-reinforced MMCs. Preform MMCs made from conventional fine powders with Vp < 0.40 have not to the authors knowledge been previously reported in the literature. 6. MMCs with tensile strength and elastic modulus 1.9 times higher (456 MPa / 148 GPa) than that of the pure infiltration alloy (241 MPa / 69 GPa) have been achieved. Additionally, the wear resistance was more than twice that of the alloy, whereas fracture toughness KIC was only marginally lowered by the reinforcement. 236

  • Page 1 and 2:

    Pressure Infiltration Behaviour and

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    ABSTRACT In the pressure infiltrati

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    CONTENTS 1. INTRODUCTION 1 2. LITER

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    4.8.3 Evaluation of infiltration be

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    Symbol Meaning γRv surface energy

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    Symbol Meaning TYS tensile yield st

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    these materials are the detrimental

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    2. LITERATURE REVIEW 2.1. Materials

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    changes in the oxide film chemistry

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    or inside the bulk fluid only. Inte

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    that are most effective in decreasi

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    initiation stress of 25 %. Further,

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    Beffort (36) suggested that even th

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    einforcement interface and reinforc

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    It is interesting to note that, for

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    20 Table 2.1 Compilation of the mec

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    General models to predict fracture

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    with values observed by others for

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    The work of adhesion characterises

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    and vapour, is difficult to evaluat

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    system Al-Al2O3 is 10 -49 Pa at 700

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    In the Al-Cu system, although the p

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    The heat of reaction ΔGr may be es

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    al. (100) who found non-wetting beh

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    capillary or threshold pressure has

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    using constant gas pressure. Infilt

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    The superficial velocity v0 in the

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    The permeability K can be expressed

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    2.4. Preform fabrication Composites

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    According to Kniewallner (51) even

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    2.4.3. Foamed preforms Another inte

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    structure. This is shown schematica

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    2.5.1. Gas pressure infiltration (G

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    MMCs infiltrated with an Al-9Mg or

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    layer oxide films. The Weber number

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    Long et al. (50) suggested that v0

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    3. EXPERIMENTAL PROCEDURE The influ

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    sintered at 1550°C, which represen

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    using a AVT-Horn (Aalen, Germany) m

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    squares fit function within the MAP

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    areas, SsBET ,of the powders were m

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    with dimensions of 65 mm x 46 mm x

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    The preform sintering process was o

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    in the evaporation of mercury at lo

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    The compressive strength, σc , of

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    as the measured mean value 0.23. Th

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    for 90 s to ensure complete solidif

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    ottom punch surface. The temperatur

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    A graphic presentation of the relat

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    detected. This operation took appro

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    modulus Edyn of the unreinforced al

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    calculated using the methods outlin

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    Positive volume changes were predic

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    Figure 4.5 Droplet formation of the

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    with the metal alloy IM: examples a

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    As shown in Figure 4.9, apart from

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    4.3.2 Powder specific surface area

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    The particles of TO and MO were dis

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    oom temperature and 270°C, with a

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    obtain usable products when they we

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    strengths, whereas with 10 and 20 w

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    strength showed no significant diff

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    Relative change in dimension s x, s

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    (a) AOPC20 (b) AGPC15 2 µm (c) TOP

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    At higher magnification, Figure 4.2

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    intrusions started at 4 µm and end

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    As shown in Figure 4.27, the pore s

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    An overview of the specific values

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    1.71 to 1.98·10 6 m²/m³. The sim

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    logarithmic compression behaviour,

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    The volumetric stiffness Eiso of th

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    Figure 4.37 shows that the TOPC20 p

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    unhindered through the gap between

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    intrusions and the other areas were

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    4.8.1 Unreinforced matrix propertie

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    die, Tmelt,die , could not be recor

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    pressure was recorded as a function

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    the linear fits for AOPC20, TOPC20

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    4.8.6 Non destructive testing of MM

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    X-Y Y-Z Figure 4.51 Virtual cross-s

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    The metal filling the intragranular

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    the ceramic particles was not visib

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    etween the dark grey ceramic phases

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    The windows, one of which is marked

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    potential interfacial reactions, th

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    In order to determine the effect of

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    Infiltration depth L² L² (mm²) /

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    4.8.12 Microstructure of MMCs with

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    minor fraction of suboxides with hi

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    4.9. High pressure die casting infi

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    In the Y-Z plane section in Figure

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    4.9.2 Compression of preforms The c

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    Relative preform compression c pr (

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    decrease depended on the tooling us

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    Bending stress σ (MPa) / MPa 500 4

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    4.10.3 Influence of reinforcement t

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    Significant deformation developed i

  • Page 195 and 196: a) b) 2 50 2 50 µm µm 2 50 2 50
  • Page 197 and 198: 5. DISCUSSION First the properties
  • Page 199 and 200: The measured elastic modulus, Edyn
  • Page 201 and 202: The MMCs showed similar wear with t
  • Page 203 and 204: interfacial debonding: Peng et al.
  • Page 205 and 206: The area Sml was derived using data
  • Page 207 and 208: MMC. Due to the solidification shri
  • Page 209 and 210: measurements which resulted in a lo
  • Page 211 and 212: 5.1.5 Influence of reactions No rea
  • Page 213 and 214: 5.2. Preform pore formation The tar
  • Page 215 and 216: kinetics were reported to be rather
  • Page 217 and 218: The newly formed water vapour led t
  • Page 219 and 220: In order to achieve minimum porosit
  • Page 221 and 222: the present work. These pressures w
  • Page 223 and 224: indicated by zero values of the fre
  • Page 225 and 226: influence on the pO2,calc. The lowe
  • Page 227 and 228: during extended holding and acts as
  • Page 229 and 230: Compared to Hg, the Al melt may con
  • Page 231 and 232: preforms with IM, Figure 4.67. For
  • Page 233 and 234: preform compression, cpr , increase
  • Page 235 and 236: Specific Specific permeability Perm
  • Page 237 and 238: Permeability (m²) / m² 1x10 -12 1
  • Page 239 and 240: As the predominant fluid flow was a
  • Page 241 and 242: In the CP mode, the Preform 1D code
  • Page 243 and 244: Local Saturation saturation S () lo
  • Page 245: listed in Table 5.1 and 5.3 were us
  • Page 249 and 250: anged between 112 and 131° for the
  • Page 251 and 252: 8. REFERENCES 1. Altenpohl, D.: Alu
  • Page 253 and 254: 43. Davis, L.C. and Allison, J.E. :
  • Page 255 and 256: 85. Gennes, P.G. : “Wetting: Stat
  • Page 257 and 258: 127. Corbin, S.F., Lee, J. and Qiao
  • Page 259: 171. Gmelin, L. : Handbook of Inorg
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