The Occurrence and Periodicity of Oscillating Peritectic Microstructures Developed during Directional Solidification

The layered microstructures that can form during plane-front directional solidification in peritectic systems were characterized quantitatively as a function of growth velocity using a Sn-Cd alloy. Layers were formed for an alloy composition outside of the two-phase peritectic region in the absence of longitudinal macrosegregation. The layers did not extend over the entire sample cross sections, so that the layered regions had a different composition than the alloy. Each of the two solids was found to be interconnected and continuous in three dimensions. The layer lengths and individual layer compositions did not vary with solidification distance. The average layer compositions were not a function of growth velocity and were approximately those at the peritectic temperature. This research was compared to the current model by Trivedi, which is based upon cyclic accumulation and depletion of solute in the liquid ahead of the interface linked to repeated nucleation events. The dependence of layer length on growth velocity predicted by the model was not obtained experimentally. The differences between results and predictions are related to the continuity of the two solids and the nonuniform cross-sectional composition in the Sn-Cd samples, which contradict assumptions of the model. A formation mechanism involving competitive lateral growth between the two solids at the solid-liquid interface would be more consistent with the current research