History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt-Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.