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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Figure 2.13 shows an

Figure 2.13 shows an Al-Saffil MMC with a surface layer (S) of the binder on the Saffil fibre (F). The mechanical properties of the final preform depend upon the volume fraction of fibres and the amount of inorganic binder used. Binder levels vary between 1 and 10% by weight. The higher the binder contents the stiffer the final preform (118) . In contrast, a high binder content is detrimental to MMC properties. With increased binder content, the preform exhibits brittle behaviour which is not favourable during infiltration. Furthermore, in the MMC, excessive binder-metal reactions led to brittle phases (119) . M Figure 2.13 SEM microstructure of Saffil fibre MMC: pure Al matrix (M), fibres (F) , porosity (P) and silicate binder (S) surrounding the fibres (113) . The fibres in the preforms have an average diameter below 10 µm and a length of 200- 500 µm (30,119) . Due to the processing of the fibres, which is in general based on a sol-gel process, large particles in the range of 200-800 µm with low aspect ratio are present in the bulk material. The amount of the large ceramic particles, the so-called shot content, determines the quality of the final MMC. The lower the shot content, the higher the MMC quality and reliability. Large shot concentrations in loaded MMCs result in significant lowering of the fatigue properties due to crack initiation on the surface of the particles (120) . Thus low shot contents should be the intent. Starting from an initial shot weight content of 4 %. An intensive washing procedure can lead to a reduction to less than 0.01 % (119) . 47 P S F

According to Kniewallner (51) even at this low fraction, shots were observed in the microstructure as shown in Figure 2.14. Figure 2.14 Microstructure of a Saffil fibre reinforced Al alloy with an Al2O3 inclusion (51) . The fabrication route leads to an anisotropic fibre distribution where the fibres are predominantly orientated perpendicular to the pressing axis, leading to a two-dimensional random distribution. This anisotropic structure is also represented by different permeabilities in different directions. Mortensen et al. (112) proposed a model to estimate the Ks of fibre preforms in the different directions. For flow parallel to the fibre axis, it can be calculated as: 4 2 0. 427r ⎡ ⎤ ⎡ ⎛ ⎞⎤ sf 2Vsf ⎢ ⎜ Π K ⎥ ⋅ + − ⎟ s = ⎢1 − 1 0. 473 1 ⎥ Equation 31 V ⎢ ⎥ ⎢ ⎜ ⎟ sf ⎣ Π ⎦ ⎣ ⎝ 2Vsf ⎠⎥⎦ where rsf is the radius of the fibre and Vsf is its volume fraction. For flow perpendicular to the fibre axis, the Ks can be calculated as: 5 2 2 2 2r ⎡ 4 ⎤ sf Vsf K = ⎢1− ⎥ Equation 32 s⊥ 9Vsf ⎢⎣ Π ⎥⎦ 48

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