Effect of thermosolutal convection on microstructure formation in the Pb-Bi peritectic system

摘要

Experimental studies have been conducted in the two-phase region of the Pb-Bi peritectic system to investigate the effect of thermosolutal convection on the banded microstructure. A systematic study is carried out by varying convection effects through the use of thin cylindrical samples of different diameters. A strong oscillatory convection is found in a 6.0-mm-diameter sample that produces a large treelike primary phase in the center of the sample that is surrounded by the peritectic phase matrix. The length of the treelike structure is found to decrease as the diameter of a sample is reduced to 0.8 mm. When the sample diameter is further reduced to 0.4 mm, laminar flow is present that gives rise to discrete bands of the two phases. The banded microstructure, however, is found to be transient and only the peritectic phase forms after a few bands. Composition variations in the banded structure are measured to determine the nucleation undercoolings for both phases and to characterize the banding cycle. The banding cycle is determined by the nucleation undercoolings and is independent of convection in the melt, but the banding window closely depends on convection. The presence of the transient banding process is analyzed by using a boundary layer model, and the number of transient bands is found to agree with the model for samples of different compositions and lengths.