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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

cast A356 (178) (12

cast A356 (178) (12 MPa·m 1/2 ). This is reasonable if other microstructural conditions were close to that of A356 mentioned by Chan et al. (176) . This may not be the case since the chemical composition is different: apart from Si, the A356 contained 0.097 wt.% Fe, whereas the infiltration alloy IS contained 0.76 wt.% Fe. Iron forms intermetallic phases which reduce the elongation and toughness (6) . Therefore IS may exhibit a KIC lower than 12 MPa·m 1/2 . However, the microstructure of IS made by DSQC is finer than that of the gravity cast alloys and so ultimately 12 MPa·m 1/2 was assumed. As aforementioned, the fracture toughness of a metal is generally determined using the CT method. In this work, the SEVNB method was used to characterise MMCs with a metal per cent by volume of 60-70%. Kounga-Njiwaa et al. (179) have compared the KIC of commercial lead zirconate titanate (PZT) ceramics measured using CT and SEVNB. They stated that the investigated PZT material, which was at the morphotropic phase boundary and doped with nickel and antimony, show a similar stress-strain behaviour to metals. Quasi-plasticity was found similar to that in the MMCs indicated by distinctive non-linear, quasi-plastic behaviour shown in the curve of AOPC20IS in Figure 4.78. Furthermore the authors found that for PZT the toughness calculated using the standard equations was 5% higher for the CT than in the SEVNB test. Therefore, the SEVNB method was rated as an applicable method for MMC characterisation which is comparable with KIC values tested with the CT method. 5.1.2 MMC properties in relation to the pure alloy Figure 5.1 presents the values of the relative MMC performance as a ratio of that of the MMC (PMMC) to that of IS (PIS). For the tribological properties, low wear and low friction coefficients are considered favourable and therefore reciprocal values of width of the wear path (ww) and the friction coefficient (f50m) were calculated and presented. 189

The MMCs showed similar wear with the exception of AODY30IS which had the largest metal and ceramic ligaments and showed no visible wear in the metal phase (Figure 4.83). This behaviour agrees with Clyne and Withers (31) who stated that large ceramic particles showed lower wear compared to finer ones at similar volume fractions. This also agrees with Long et al. (50) who found that the coarser the particles in a AlCu4Mg-SiC MMC, the lower the wear rate. Nevertheless, large particles embedded into finer ones, like the shot particle in FATOIS shown in Figure 4.84, seem to be detrimental to friction properties, indicated by the highest f50m of all MMCs tested (Figure 4.82). Ratio P MMC / P IS () 30 25 3 2 P = 1 IS 0 AOPC20IS AGPC15IS TOPC10IS MOPC20IS AODY30IS FATOIS 190 E dyn σ 0 m K IC 1/w w 1/f 50m Figure 5.1 Relative characteristic performance of the MMCs in relation to the pure alloy IS. Edyn: elastic modulus; σ0: characteristic strength and its Weibull modulus m; KIC: fracture toughness. Reciprocal value of the ratio for ww: width of wear path and f50m: friction coefficient. All MMCs had higher elastic moduli than the pure alloy IS. Hashin and Shtrikman (55) proposed a model (Equation 5) to calculate the upper and lower bounds (Ec,upper , Ec,lower ) of the composite´s elastic modulus Ec based on the moduli of the two constituents (E1, E2) and their volume fractions (Vf, 1-Vf), neglecting the morphology of the reinforcing phase. The moduli of the reinforcing phases listed in Tables 3.3 and 3.7 were taken from the

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