Onset of sidewise instability and cell–dendrite transition in directional solidification

The transition from cellular to dendritic microstructure in directional solidification is investigated in succinonitrile (SCN)–camphor alloys. This transition is found not to be sharp, but occurs locally over a range of velocities or thermal gradients, and the diffuseness of the transition is related to the existence of a range of primary spacing. Within the transition zone, critical cell spacing λcd is present where a cell just develops sidewise perturbations. An expression for the critical spacing for the onset of sidewise perturbation is obtained, and it is used to establish the conditions for the start and end of the transition. The results of the present study are then synthesized with those in the SCN–acetone system to incorporate the effect of system parameters on the onset of sidewise instability.