ON PRIMARY DENDRITIC SPACING DURING UNIDIRECTIONAL SOLIDIFICATION

In-situ observations of directionally solidifying interface of typical transparent model alloys, succinonitrile-ethanol and succinonitrile-a cetone, have been performed on a Hunt-Jackson type temperature gradien t stage and a Bridgman apparatus, respectively. Experimental results s how that there exists a wide allowable range of primary spacings for b oth the two-dimensional and the three-dimensional dendritic arrays und er a given growth condition. The upper and lower limits of the allowab le range are very sharp at the same current growth condition, while th e average primary spacing is remarkably related to the history of vari ation of both temperature gradient and growth velocity. The upper limi t, lambda(max), and the lower one, lambda(min), of the allowable range as a function of growth velocity, V, obtained from the experiments of step-increment and step-decrement in growth velocity, can be generall y expressed as lambda(max) = AV(-b), lambda(min) = cV(-d), where a, b, c and d are constants for given alloy and temperature gradient. The l ower limit obtained experimentally by us is compared with that calcula ted theoretically with the Hunt-Lu model and the Warren-Langer model; it shows an excellent fit between experimental results and the Hunt-Lu model, and good agreement between experimental results and the Warren -Langer model at the high growth velocity. Copyright (C) 1996 Acta Met allurgica Inc.