Planar to cellular transition during directional solidification of Al-0.5 wt.% Cu

An experimental study on the alloy Al-0.5 wt.% Cu was performed to establish the behavior of the transition from a planar to cellular to planar during directional solidification. At first, it was verified that the critical velocity, V sub C , for growth runs at constant stable velocity agreed with the predictions of Mullins and Sekerka's model. At a variable increasing velocity was found a threshold, V sub C,exp exp + , which was approximately the same order as V sub C . At a decreasing interface velocity, the threshold found, V sub C,exp exp - , was < V sub C,exp exp + , so that a hysteresis behavior for the transition threshold was verified. This fact reveals that there is a characteristic subcritical behavior of the transition, such as was predicted by Cheveigne et al. in their theoretical analysis for dilute Al-Cu systems. The total behavior can be understood in the following way: when a planar interface is perturbed, the small perturbations hypothesis is valid, and the value found for this threshold is close to the predicted one using linear analysis. On the other hand, when a cellular state is present, the last assumption is not valid, and a non-linear expression is needed to predict the interface morphology. The dilute Al-Cu system behavior is thus a hysteretic one during the planar to cellular interface transition. The threshold variation range was > 0.1 V sub C .