Recurring Instability of Cellular Growth in a Near Peritectic Transparent NPG-TRIS Alloy System

Abstract

In this study, a near peritectic transparent metal-like solidifying NPG-TRIS alloy was directionally solidified in a thin sample under process conditions which favor cellular growth. Dif-ferent to an identical horizontally processed sample, the vertically processed sample revealed a cel-lular array which was curved. This curving was attributed to a convection-induced higher alloy con-centration at the sample edges compared to the sample center. Surprisingly, it was found that the curved cellular array is inherently unstable as three times dendrite-like structures shot forwards. As origin of these rapid advancements two possibilities were discussed. Either a convection-induced variation of the solute boundary layer might be responsible. Or it can be explained by a more gener-ic approach which due to the curved form of the cellular array considers extended freedom for cel-lular branching in combination with a relative large deviation of the preferential crystal growth di-rection from the heat flow direction. For the discussed phenomena the presence of the peritectic phase within the intercellular spacing was found to be not of importance.