Liquid migration and peritectic transformation: Investigation on their contributions to growth of peritectic phase in interrupted directional solidification

摘要

The microstructure of peritectic alloys of important applications determines the final properties of them. The growth of peritectic β phase which greatly influences the structure of peritectic systems is divided into three different types: peritectic reaction at T_P: L + α → β, then solid-state peritectic transformation α → β and direct precipitation L → β. In addition to transparent organic systems, peritectic transformation was always analyzed using interrupted directional solidification during which the sample is kept stationary after experiencing a distance of directional growth. Then, the growth of β phase through peritectic transformation is accompanied with the migration of the a/|3 interface towards a phase below T_P. However, it is found in Sn-Ni peritectic system (L + Ni_3Sn_2 → Ni_3Sn_4) that the migration rate of Ni_3Sn_2/Ni_3Sn_4 interface is much larger than that of peritectic transformation. Further examination shows that this interface migration can also be induced by the diffusion-controlled liquid migration initiating at the Ni_3Sn_2/Ni_3Sn_4 interface. Based on the model describing the liquid migration process, the migration of the Ni_3Sn_2/Ni_3Sn_4 interface through liquid migration is predicted, which matches well with the experimental measurement. This shows that the diffusion-controlled liquid migration is dominant during the growth of β phase. Thus, the application of interrupted directional solidification should be further examined.