Research on primary dendritic spacing in unidirectional solidification of Al-4.5Cu single crystal

In unidirectional solidification of Al-4.5Cu alloy, history-dependent selection of the primary dendritic spacing is systematically investigated by the experimental method of step-variation in growth rate. A single crystal is formed in the sample before each experimental run, so that the influence of crystal boundary on the primary spacing is avoided. It is found that the primary dendritic spacing is remarkably history-dependent in the metallic alloy as in the transparent model alloy. The average primary spacing is dependent not only on the current growth conditions as described in the previous models, but also on the way those conditions were achieved remarkably. There exists a wide allowable range of primary spacing for given growth conditions. The lower limit obtained experimentally is compared with that calculated theoretically with the Hunt-Lü model, which shows excellent fit between them.