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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The strengthening in

The strengthening in MMCs is divided into two categories: direct and indirect strengthening (42) . In the direct mode, the applied load is transferred from the weaker matrix, across the matrix-reinforcement interface, to the typically stiffer reinforcement. Due to the lower aspect ratio of particulate materials, load transfer is not as efficient as in the case of continuous fibre reinforcement, but is still significant in providing strengthening (43) . Upon cooling from casting temperatures, dislocations form at the reinforcement/matrix interface due to the thermal mismatch and thermally-induced dislocation punching results in indirect strengthening of the matrix (44) . The effect of indirect strengthening is difficult to quantify. With an increase in the reinforcement volume fraction, higher elastic moduli, macroscopic yield and tensile strengths were observed, coupled with lower ductility. Ductility falls off rapidly such that, at relatively low reinforcement volume fractions of about 0.20, the elongation of most MMCs is below 5%. The lower ductility at higher volume fractions can be attributed to the earlier onset of void nucleation with increasing amount of reinforcement. It has been found (42) that tensile ductility increases as the particle size decreases which may be attributed to an increase in the ceramic particle strength with a decrease in particle size. Therefore the probability of strength-limiting flaws existing in the volume of the reinforcement decreases. Microplasticity in the composites has been attributed to stress concentration points in the matrix at the poles of round reinforcing particles and/or at sharp corners of irregular shaped reinforcing particles (45) . The initial micro-yielding stress decreases with increasing volume fraction, as the number of stress concentration points increases. The primary factors which determine the yield strength of the MMC are that of the matrix and the reinforcement volume fraction. Significant secondary factors include the matrix/ 15

einforcement interface and reinforcement shape. Yield strength data for randomly oriented fibre reinforced MMCs is rarely found in the published literature. Model predictions for strength are still under development since strength is a complex function of the composite microstructure (46) . Numerous models have been developed with the majority being the law of mixture, shear lag, Eshelby or dislocation type models (47) . The dislocation models can be further classified based on their selected contribution of Orowan strengthening, grain and substructure strengthening, quench hardening and work hardening (47) . There is still a lack of reliable predictive capability. Since the strength depends strongly on the matrix properties and nature of the reinforcement/matrix interface, methods which incorporate both load sharing and matrix strengthening approaches will be needed, as reported by Wu and Lavernia (48) . Long et al. (49) published experimental results for pure Al and Al alloy composites reinforced with continuous and chopped alumina fibres (Saffil - Saffil is a trademark of ICI Americas, Inc., Wilmington, DE). An extract of their results is listed in the mechanical properties summary in Table 2.1. It has been shown that, when using identical processing conditions, in general the maximum strength of continuously reinforced composites was higher than that of chopped fibre composites, even when the strength of the matrix alloy, which was pure aluminium for the continuous fibres and AlCu4MgAg for the chopped fibres, was lower in the higher strength material. This higher strength was a result of the higher volume fraction of the reinforcement in combination with the improved strengthening effect of continuous fibres. For reasons of direct comparison, an equal reinforcement volume fraction should be aimed for. However, in general for continuous fibres the lower limit in volume fraction was around 0.50 and the upper limit of chopped fibres was 0.30, excluding direct comparison of these two MMC material groups. 16

  • Page 1 and 2: Pressure Infiltration Behaviour and
  • Page 3 and 4: ABSTRACT In the pressure infiltrati
  • Page 5 and 6: CONTENTS 1. INTRODUCTION 1 2. LITER
  • Page 7 and 8: 4.8.3 Evaluation of infiltration be
  • Page 9 and 10: Symbol Meaning γRv surface energy
  • Page 11 and 12: Symbol Meaning TYS tensile yield st
  • Page 13 and 14: these materials are the detrimental
  • Page 15 and 16: 2. LITERATURE REVIEW 2.1. Materials
  • Page 17 and 18: changes in the oxide film chemistry
  • Page 19 and 20: or inside the bulk fluid only. Inte
  • Page 21 and 22: that are most effective in decreasi
  • Page 23 and 24: initiation stress of 25 %. Further,
  • Page 25: Beffort (36) suggested that even th
  • Page 29 and 30: It is interesting to note that, for
  • Page 31 and 32: 20 Table 2.1 Compilation of the mec
  • Page 33 and 34: General models to predict fracture
  • Page 35 and 36: with values observed by others for
  • Page 37 and 38: The work of adhesion characterises
  • Page 39 and 40: and vapour, is difficult to evaluat
  • Page 41 and 42: system Al-Al2O3 is 10 -49 Pa at 700
  • Page 43 and 44: In the Al-Cu system, although the p
  • Page 45 and 46: The heat of reaction ΔGr may be es
  • Page 47 and 48: al. (100) who found non-wetting beh
  • Page 49 and 50: capillary or threshold pressure has
  • Page 51 and 52: using constant gas pressure. Infilt
  • Page 53 and 54: The superficial velocity v0 in the
  • Page 55 and 56: The permeability K can be expressed
  • Page 57 and 58: 2.4. Preform fabrication Composites
  • Page 59 and 60: According to Kniewallner (51) even
  • Page 61 and 62: 2.4.3. Foamed preforms Another inte
  • Page 63 and 64: structure. This is shown schematica
  • Page 65 and 66: 2.5.1. Gas pressure infiltration (G
  • Page 67 and 68: MMCs infiltrated with an Al-9Mg or
  • Page 69 and 70: layer oxide films. The Weber number
  • Page 71 and 72: Long et al. (50) suggested that v0
  • Page 73 and 74: 3. EXPERIMENTAL PROCEDURE The influ
  • Page 75 and 76: sintered at 1550°C, which represen
  • Page 77 and 78:

    using a AVT-Horn (Aalen, Germany) m

  • Page 79 and 80:

    squares fit function within the MAP

  • Page 81 and 82:

    areas, SsBET ,of the powders were m

  • Page 83 and 84:

    with dimensions of 65 mm x 46 mm x

  • Page 85 and 86:

    The preform sintering process was o

  • Page 87 and 88:

    in the evaporation of mercury at lo

  • Page 89 and 90:

    The compressive strength, σc , of

  • Page 91 and 92:

    as the measured mean value 0.23. Th

  • Page 93 and 94:

    for 90 s to ensure complete solidif

  • Page 95 and 96:

    ottom punch surface. The temperatur

  • Page 97 and 98:

    A graphic presentation of the relat

  • Page 99 and 100:

    detected. This operation took appro

  • Page 101 and 102:

    modulus Edyn of the unreinforced al

  • Page 103 and 104:

    calculated using the methods outlin

  • Page 105 and 106:

    Positive volume changes were predic

  • Page 107 and 108:

    Figure 4.5 Droplet formation of the

  • Page 109 and 110:

    with the metal alloy IM: examples a

  • Page 111 and 112:

    As shown in Figure 4.9, apart from

  • Page 113 and 114:

    4.3.2 Powder specific surface area

  • Page 115 and 116:

    The particles of TO and MO were dis

  • Page 117 and 118:

    oom temperature and 270°C, with a

  • Page 119 and 120:

    obtain usable products when they we

  • Page 121 and 122:

    strengths, whereas with 10 and 20 w

  • Page 123 and 124:

    strength showed no significant diff

  • Page 125 and 126:

    Relative change in dimension s x, s

  • Page 127 and 128:

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

  • Page 129 and 130:

    At higher magnification, Figure 4.2

  • Page 131 and 132:

    intrusions started at 4 µm and end

  • Page 133 and 134:

    As shown in Figure 4.27, the pore s

  • Page 135 and 136:

    An overview of the specific values

  • Page 137 and 138:

    1.71 to 1.98·10 6 m²/m³. The sim

  • Page 139 and 140:

    logarithmic compression behaviour,

  • Page 141 and 142:

    The volumetric stiffness Eiso of th

  • Page 143 and 144:

    Figure 4.37 shows that the TOPC20 p

  • Page 145 and 146:

    unhindered through the gap between

  • Page 147 and 148:

    intrusions and the other areas were

  • Page 149 and 150:

    4.8.1 Unreinforced matrix propertie

  • Page 151 and 152:

    die, Tmelt,die , could not be recor

  • Page 153 and 154:

    pressure was recorded as a function

  • Page 155 and 156:

    the linear fits for AOPC20, TOPC20

  • Page 157 and 158:

    4.8.6 Non destructive testing of MM

  • Page 159 and 160:

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

  • Page 161 and 162:

    The metal filling the intragranular

  • Page 163 and 164:

    the ceramic particles was not visib

  • Page 165 and 166:

    etween the dark grey ceramic phases

  • Page 167 and 168:

    The windows, one of which is marked

  • Page 169 and 170:

    potential interfacial reactions, th

  • Page 171 and 172:

    In order to determine the effect of

  • Page 173 and 174:

    Infiltration depth L² L² (mm²) /

  • Page 175 and 176:

    4.8.12 Microstructure of MMCs with

  • Page 177 and 178:

    minor fraction of suboxides with hi

  • Page 179 and 180:

    4.9. High pressure die casting infi

  • Page 181 and 182:

    In the Y-Z plane section in Figure

  • Page 183 and 184:

    4.9.2 Compression of preforms The c

  • Page 185 and 186:

    Relative preform compression c pr (

  • Page 187 and 188:

    decrease depended on the tooling us

  • Page 189 and 190:

    Bending stress σ (MPa) / MPa 500 4

  • Page 191 and 192:

    4.10.3 Influence of reinforcement t

  • Page 193 and 194:

    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 and 246:

    listed in Table 5.1 and 5.3 were us

  • Page 247 and 248:

    6. CONCLUSIONS 1. An aqueous proces

  • 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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