PRIMARY SPACING SELECTION IN DIRECTIONALLY SOLIDIFIED ALLOYS

The stability of primary spacing in cellular and dendritic arrays has been examined through critical directional solidification studies in the succinonitrile-acetone system. The existence of a range of primary spacing has been established for steady-state growth under given experimental conditions, and the stability of primary spacing within this range has been confirmed through velocity change experiments. Different mechanisms that are operative in the unstable regime of primary spacing have been quantitatively examined. Besides the mechanisms of cell or dendrite creation or elimination, a new mechanism for spacing changes was also identified in which the adjustment of primary spacing occurred through the lateral motion of cells or dendrites. A theoretical model, based on the mass balance constraint, is developed to correlate primary spacing with processing conditions, and the results are compared with the experimental data.