PHASE DISTRIBUTION AND ASSOCIATED MECHANICAL PROPERTY DISTRIBUTION IN SILICON-CARBIDE PARTICLE-REINFORCED ALUMINUM FABRICATED BY LIQUID-METAL INFILTRATION

The reaction between silicon carbide and aluminium to form silicon and Al4C3 in SiC particle-reinforced aluminium fabricated by liquid aluminium infiltration was most severe near the original interface between liquid aluminium and the SiC preform. This resulted in the highest concentration of Al4C3 and the lowest concentrations of silicon and SiC in the part of the composite near this interface. In particular, the silicon concentration was highest in the bottom centre of the composite when infiltration occurred from the top, because silicon diffused toward the surrounding aluminium melt before solidification. These non-uniform phase distributions, as measured by X-ray diffraction and differential scanning calorimetry, did not cause any non-uniform shear strength distribution. However, excessive reaction between SiC and aluminium, as observed for an infiltration (= mould = liquid metal) temperature of 780-degrees-C, caused the tensile strength to decrease. In the case where a steel mould was used during infiltration at 780-degrees-C, iron-containing precipitates, such as ternary Al-Fe-Si, were observed in the part of the composite within 5 mm from the above-mentioned interface; their formation was related to the silicon out-diffusion in the form of liquid Al-Si; they caused the shear strength to be lower in this part of the composite; larger such precipitates (up to 100 mum) were observed in the excess aluminium adjacent to the cast composite. For pure aluminium as the infiltrating metal, the optimum infiltration temperature for the highest tensile strength was 700-degrees-C. An infiltration temperature of 670-degrees-C resulted in incomplete infiltration, which was more severe when a steel mould rather than a graphite mould was used because of the higher thermal conductivity of the former.