Phase selection and microstructure evolution play important roles on processing and property
control of the materials with peritectic reactions. Many interesting microstructures, such as peritectic coupled
growth (PCG), discrete banding, island banding (IB) and oscillatory tree-like structure were observed in directionally
solidified peritectic alloys. In recent years, a number of theoretical models have been developed for the phase
selection. For example, it was believed that peritectic coupled growth is initiated by island banding
and is similar to eutectic coupled growth, discrete banding is shaped by nucleation and lateral spreading of one
phase onto the other phase, and island banding is formed when the nucleation rate of one phase on the other phase
growth interface is higher than a critical value, and etc.. But these models were developed under the assumption
that the growth of the phases are controlled by diffusion only, whereas the actual experimental studies were
mainly carried out in bulk metallic samples with the presence of convection. Although it has been believed that
the convection effects on microstructure evolution are very important and could not be neglected, studies were
seldom carried out in diffusive regime. In this paper, directional solidification of Fe-4.2Ni hypo-peritectic alloy
in diffusive regime was carried out in a Bridgman furnace by placing the samples in 1 mm inner diameter thin
tubes. The experimental results show that various microstructures and phase selections are obtained in the
directionally solidified thin samples at the pulling rates of 15 and 35 $\mu$m/s. Tree-like structure,
peritectic coupled growth and the steadily solidified structure of two phases separated growth are formed at the
pulling rate of 15 $\mu$m/s, and the microstructures evolution is: single phase δ→tree-like
structure→γ and PCG→separately growing δ/γ. At the pulling rate of
35 μm/s, the main microstructure is γ-IB, and the microstructures evolution is: δ→tree-like structure→γ-IB in the center and PCG at the edge→δ in the center
and γ-IB at the edge. As the pulling rate increases from 15 to 35 μm/s, the morphology of tree-like
structure formed by the competitive growth between the nucleated γ-phase and the parent δ-phase
becomes complicated. It is found that the pulling rate plays an important role in both the formation and evolution
of various microstructures.