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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

esulting MMCs were

esulting MMCs were checked using non-destructive computed tomography (CT) and only those with negligible porosity were tested. (a) (b) 177 Z 2 5 2 5 µm mm µm mm Figure 4.77 TOPC10IS infiltrated in a tool with a bottom plunger made of steel without chill: a) Overview macrograph. Light phase: alloy; darker phase: MMC. b) Detail view of the porous region. 4.10.2 Influence of reinforcement type on strength and elastic modulus The bending strengths of the MMCs were compared with that of the pure alloy IS. The stress- deflection curves of IS and the AOPC20IS, are presented in Figure 4.78. The pure alloy IS exhibited a linear behaviour up to a bending stress of 110 MPa which represents the change from elastic to plastic deformation. The test was finished at a deflection of 1300 µm, the latter being the maximum deflection possible in the test which was designed for brittle materials. In the stress-deflection curve presented in Figure 4.78 the maximum deflection corresponded to a stress of 230 MPa. In contrast to IS, the elastic portion of the curve for the AOPC20IS MMC was significantly steeper and was linear up to a stress of 130 MPa. Subsequently, the curve developed a steadily decreasing gradient up to the stress at fracture of 430 MPa, which corresponded to a deflection of 450 µm. X

Bending stress σ (MPa) / MPa 500 400 300 200 100 AOPC20IS 0 0 500 1000 1500 Deflection Deflection / µm (µm) Figure 4.78 Comparison of bending stress - deflection curves of the pure alloy (IS) and the MMC (AOPC20IS). As already indicated by the profile of the stress-deflection curves in Figure 4.78, the elastic moduli Edyn of the MMCs were significantly higher than the value of 69 GPa for IS which is presented in Figure 4.79 a). The Edyn values of the MMCs fabricated with PFA varied between 135 GPa for TOPC10IS and 148 GPa for AGPC15IS. In contrast, the MMCs fabricated with purchased preforms, AODY30IS and FATOIS, showed lower moduli of 119 GPa and 105 GPa, respectively. These values were still significantly higher than that of IS. The characteristic stress σ0 was based on the results of a minimum of 24 test bars. The σ0 values in combination with the corresponding Weibull moduli are presented in Figure 4.79 b). They were calculated using Equation 45 presented in section 3.5.5. As aforementioned, the IS samples did not break at the maximum deflection and this deflection was constant. Therefore the calculated σ0 value of IS does not represent the characteristic stress at fracture and it is higher than the value of 241 MPa. In contrast, as there was a deflection limit which led to smoothing of the maximum value, with 88 the m value is overestimated and not representative for an comparative study which was focussed in the present work. 178 IS

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