Selection of initial perturbation wavelength of a planar solid-liquid interface

A transparent model alloy, succinonitrile-0.93 wt. salol is unidirectionally solidified under the accurately controled experimental conditions to investigate the selection of initial perturbation wave-length of a planar interface. The experimental data are compared with the Mullins-Sekerka (M-S) theory and the Warren-Langer (W-L) model. The following results are obtained: (1) Some experimental points do not lie within the wavelength range predicted by the M-S theory when V_cms< V_e< 1.57 V_cms, where V_e is the pulling velocity, V_cms is the critical instability velocity of a planar interface given by M-S theory. When V_e≫1.57 V_cms, the experimental points are all within the wavelength range, but several times larger than the wavelength with the fastest amplitude developing speed predicted by the M-S theory. In short, the M-S theory cannot be used effectively to predict the selection of the initial perturbation wavelength of a planar interface. (2) The experimental results obtained in different alloys are all in good agreement with the W-L model, which proves that the initial perturbation of a planar interface is generated by the dramatic amplification of the ambient thermal micro-fluctuation with a characteristic wavelength.