Interfacial morphology undergoes a non-steady state evolution process before steady state. This process shows different features
under different solidification conditions. The development of
interfacial morphology of normal solidification (low growth rate) has been well studied by many researchers<1-3>. But up to now, no experimental research report on the morphological evolution in
sub-rapid solidification has been found in the open literature.In present paper, the directional solidification experiments have been carried out in a transparant carbon-tetrabromide system in-situ for studying the non-steady evolution of interfacial morphology in a wide range of growth rate. Fig 1 (a～d) show the non-steady evolution characteristics of primary spacing λ1 and tip radius R of the typical cell (a) and dcndrite (c) array at low growth rate and cell (b)and dcndritc (d) array at high groWth rate respectively. Symbol E in Fig.1 represents the model of elimination through competition and symbol S represents the model of tip splitting. Fig.2 demonstrates the dynamic evolution morphologics corresponding to Fig.1. The results show that: (1) at low growth rate, the spacing of normal cellular array is adjusted and dominated altCrnatcly by models E and S and both the cell spacing and tip radius appear to vary more or less irregularly (Fig.1 a); (2) in sub-rapid solidification, the adjustment is dominated by model S only over the whole of evolution process, and λ1 decreases monotonically with time (Fig. 1b); (3) at low growth rate, the evolving character of dendrite array is always adjUsted by model E over the whole process (Fig.Zc). which leads to monotonic ir-se of bland monotonic dccrcasc of R (Fig.lc); and (4) in sub-rapid solidification,when is higher than gs, the dendrite evolution is dominated by model S and violent tip-splitting causes a sharp decrease in λ1(Fig. 1d).The results show that the evolution features of sub-rapid solidification are quite different from those of normal solidification. The increase of primary spacing of dendritc array with time at low growth rate implies the development of side-branching instability. But at rapid growth rate the dcndrite primary spacing is decreased through tip-splitting, which indicates the development of iii) instability. It illustrates that the growth rate exerts a significant effect on the evolution ofinterfacial morphology.