Effect of composition on the structure of directionally solidified AlNi and AlNiCu composites

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AlNi alloys with compositions ranging from 4 to 11 wt% Ni are directionally solidified at a growth rate of 3 cm/h and a temperature gradient of 130 K/cm. The structures obtained consist mainly of primary dendrites embedded in aligned eutectic except for compositions with from 5·8 to 6·06% Ni where complete couplede utectic growth is found. In the dendritic structures the area fraction of primary dendrites was measured and the interdendritic eutectic composition evaluated. This eutectic-like material exhibited compositions always richer in Ni than the equilibrium eutectic alloy. The inter-rod spacing of the interdendritic eutectic decreased with Ni content in the ‘eutectic’. The addition of small amounts of copper (1 to 2%) to the AlNi eutectic gave a cellular structure where the misorientation and irregularities of the Al 3Ni phase increased with increasing copper content.