A boundary layer model was used to investigate the convection effects on phase and microstructure selection in directionally solidified peritectic alloy. Due to the convection effects, the steady-state compositions of one phase at interface corresponding to an initial composition reduce, which causes its steady-state point moves upward along its solidus line and the compositional range is not consistent with the band cycle in banding. A criterion of critical interface temperature was put forward to determine whether a phase entered steady-state growth or not. Furtherly by equivalent transformation, the equivalent solidus lines and subsequent equivalent phase diagram were derived for peritectic solidification with convection. Using this equivalent phase diagram, a phase and microstructure selection map is built for a peritectic alloy with convection effect, which shows that the compositional range for banding reduces, and moves to the hyperperitectic region, and also the coupled growth region of both solids comparing with purely-diffusive limit. The predicted map for directionally solidified Pb-Bi alloy agrees well with its experimental observations.
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