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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

AODY30IS showed the

AODY30IS showed the lowest σ0 value of all materials (58 MPa). It was attributed to the coarse ceramic structure which fractured without influence of the metal phase. The hybridic reinforced FATOIS showed the lowest m value of 6. Apart from AODY30IS, all MMCs showed higher strength than the pure alloy IS. The alumina-reinforced MMCs, AOPC20IS and AGPC15IS, showed the highest bending strengths of 443 MPa and 456 MPa respectively. The Weibull moduli of both were around 30 combined with around 80% higher strength compared to the Al-Si alloy. The MMCs reinforced with magnesia (MOPC20IS), titania (TOPC10IS) and the hybrid reinforced MMC (FATOIS) showed bending strength values in the range of 345 to 380 MPa which were about 80 to 100 MPa lower than that of the alumina-reinforced MMCs. The Weibull modulus of 6 for FATOIS was the lowest of all MMCs investigated. Bending strength stress σ σ/ MPa (MPa) Elastic b Elastic modulus E E(GPa) dyn / GPa 200 150 100 50 0 500 400 300 200 100 0 (a) (b) 69 241 145 30 443 148 28 456 135 18 380 IS AOPC20IS AGPC15IS TOPC10IS MOPC20IS AODY30IS FATOIS Figure 4.79 Mechanical properties of unreinforced Al-Si alloy IS and the MMCs: a) elastic modulus b) bending strength and Weibull moduli m (circles). 179 143 22 345 119 14 58 105 363 6 50 40 30 20 10 0 Weibull modulus m m/ MPa

4.10.3 Influence of reinforcement type on fracture toughness The fracture toughness of the MMCs was measured in bending using beam-shaped samples with a V-notch as described in 3.5.5. The mean fracture toughness KIC of the MMCs varied between 7.2 and 9.5 MPa·m 1/2 except that of AODY30IS which was significantly lower, as shown in Figure 4.80. The value of the latter had to be estimated, as the samples broke prior to the end of the preloading phase of the test. The maximum preloading stress corresponded to a KIC of 3.0 MPa·m 1/2 and therefore KIC of AODY30IS was below this value. In the unreinforced Al-Si alloy (IS), the required notch geometry could not be obtained using the chosen notching device, as this was designed for brittle materials but produced a U-shaped notch in the pure alloy. The general geometry criterion, where the notch root should have a radius of less than 50 µm to minimize the influence of notch radius on KIC, could not be achieved. Therefore the KIC values could not be evaluated for the pure alloy, whereas the MMCs had a satisfactory notch geometry. Fracture toughness K (MPa m IC 1/2 Fracture toughness KIc / MPa m ) 1/2 10 8 6 4 2 0 n.a. 9.5 8.9 IS AOPC20IS AGPC15IS TOPC10IS AODY30IS FATOIS Figure 4.80 Fracture toughness KIC of the MMCs. 180 7.4 < 3 7.2

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