MICROSTRUCTURE AND FRACTURE OF SIC-PARTICULATE-REINFORCED CAST A356 ALUMINUM-ALLOY COMPOSITES

A microstructural analysis of local microfracture of cast A356 Al-SiCp composites fabricated by permanent mold re-casting and squeeze-castin g methods was made. Notch fracture toughness tests were conducted on t hese composites to identify critical fracture parameters using a stres s-modified critical-strain criterion. The composite microstructure sho ws continuous networks of densely populated SiC and eutectic Si partic les along the intercellular regions. Squeeze casting produces a more h omogeneous structure and larger spacing of brittle particles and incre ases the tensile ductility and fracture toughness, while strength leve ls are almost identical to the re-casting case. The calculated values of the microstructurally characteristic distance l for the re-cast an d squeeze-cast composites are about 40 mu m, which is comparable to th e average sizes of the intercellular network. However, the reference c ritical strain <(epsilon)over bar>(0) for squeeze casting is larger t han that for re-casting, showing a trend to higher ductility and fract ure toughness.