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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

and contributes to

and contributes to premature fatigue life, because of the increased propensity of particle cracking as the particle size decreases. Narrowing of the particle size range distribution also results in a higher fatigue failure, particularly when eliminating larger particles that are more prone to cracking (60) . Enhanced creep resistance was obtained with the higher aspect ratio whiskers than with particles, presumably due to more effective load transfer from the matrix to the high stiffness reinforcement (42) . The coefficient of thermal expansion (CTE) mismatch of metal and ceramic phases in the composites leads to thermal residual stresses and induces plastic deformation in the matrix, resulting in indirect strengthening (61) . The stresses and the strengthening in these composites were generated during the cooling after processing at a high temperature. Agrawal et al. (62) measured residual stresses in Al-Al2O3 composites with cellular ceramic structures built up of particles in the sub-micron size with a ceramic volume fraction of 0.29. A residual compressive stress of up to 100 MPa in the ceramic phase and the same order of tensile stress in the metal matrix were obtained. When regarding the ceramic phase, the compression stress is beneficial for preventing particle cracking when a tensile stress is applied to the composite. In contrast, the residual tension in the metal phase is detrimental to composite properties. Yielding of the aluminium alloy matrices is already reached at tensile stress levels of about 200 MPa at room temperature (63) . Hoffmann et al. (64) synthesized alumina which was toughened by interpenetration with aluminium. The metal ligament size was in the range of 0.12 to 1 µm. Due to their small ligament size, dislocation movement in the ligaments is expected to be limited. The high level of mechanical constraint induced by small ligaments tightly bonded to the ceramic is expected to raise the flow stress in the metal well above that of the bulk material which is consistent 23

with values observed by others for highly constrained ductile particles and thin films (65,66) . This high level of residual stresses has been confirmed experimentally. At the lower ligament size of 0.12 µm, a high residual stress of 1430 MPa were obtained whereas at a ligament size of 1 µm a lower stress level of 760 MPa was reported. On the other hand interfacial debonding occurred with a larger ligament size, as shown by Kohle et al. (67) . In their experiments, Al2O3 was toughened with Ni at a volume fraction of 0.20. The critical diameter of the Ni phase in the toughened Al2O3 was in the range of 1.3 to 8.5 µm. Knechtel et al. (68) used metal toughened ceramics to show that the interface of Al2O3-Al is significantly stronger than the Al2O3-Cu interface. The influence of metal ligament size on both fracture toughness and fracture strength was assessed. Different average ligament sizes were proposed to be represented by using the median pore diameter of a mercury intrusion porosimetry analysis which was in the range of 0.08 µm to 0.13 µm. Fracture toughness increased with increasing metal ligament size. SEM studies of the Al2O3-Al composite microstructure with an average ligament size below 0.10 µm clearly showed, that the small cavities could not be infiltrated even though a gas pressure of 15 MPa and relatively high processing temperature of 1110°C were used. From the standpoint of infiltration behaviour, large particulates are more favourable than smaller ones. Mitzumoto et al. (69) synthesized MMCs by a low pressure infiltration process. The pressure required to infiltrate preforms having the same volume fraction of particles increased from 0.05 MPa to 0.5 MPa with a decrease in the particle size from 100 µm to 20 µm. This indicates that, the infiltration is achieved with lower pressure at higher particle diameters. 24

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