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Cooling rate effect on entrapment of short alumina fibres into aluminium alloy

Abstract Directional solidification effects on the entrapment of short alumina fibres (50 μm length and 3–5 μm in diameter) in an aluminium alloy 356 to make a composite, were studied. It was observed that most of alumina fibres of small diameter (3.16–3.25 μm) were entrapped in a region nearer to the water-cooled copper chill of the ingots where the cooling rate was high. Fibres with progressively larger diameter, up to 3.18 μm, were preferentially entrapped further away, inregions of reduced cooling rate. It was observed that solidification took place around the alumina fibres, engulfing them in enlarged dendrite arm spaces. Knoop hardness increased with cooling rate, and was 40% higher in the alumina–metal matrix than in the unreinforced aluminium matrix.