Effect of shape variations on the structure of directionally solidified Al-Al₃Ni composites

Abstract

The effect on structure of some of the possible changes in shape of directionally solidified Al-Al₃Ni eutectic composites has been studied for two growth conditions. The shape changes investigated included both contraction and divergence in cross-section of the grown part, as well as 90 deg bends in the center-line of the composites. The experimental results showed that contractions in the solidifying cross section do not seriously affect the growth of composites, apart from some coarsening of the structure near the surface. Fiber branching took place in the case of gradual divergence in the solidifying cross section with the fibers deviating from the general growth direction by an angle determined by the shape of equitemperature contours during solidification. Sharp changes in growth direction, 90 deg bend, gave rise to nucleation of new grains of considerable misorientation and hypoeutectic alloys nucleated primary aluminum phase before the eutectic structure was established. The relatively large under cooling needed for nucleation gave rise to high local growth rates in the 90 deg bend area. As the Al₃Ni fibers grow at right angles to equitemperature contours during solidification, it is concluded that control of the composite structure can be achieved by controlling the growth conditions and mold design. Molten alloy-graphite reactions resulted in the formation of aluminum carbides which were more extensive at higher temperatures and longer exposure times.