Experimental Test of Morphological Stability Theory for a Planar Interface During Rapid Solidification

Abstract

We report a parameter-free test of the theory predicting the critical solute concentration that destabilizes a planar solid-liquid interface in the high-velocity regime where nonequilibrium interface kinetics are important. Rapid solidification following pulsed laser melting was used to make metastable solid solutions of silicon-tin. Rutherford backscattering spectrometry and transmission electron microscopy were used to measure the breakdown concentration. Samples remained microsegregation free with near perfect crystallinity at tin concentrations up to 10 times the maximum equilibrium solubility and 100 times that predicted by linear stability theory with local interfacial equilibrium. These measurements, covering velocities from 1 to 10 m/s, agree with the predictions of linear stability theory when the latter incorporates a velocity-dependent partition coefficient and a thermodynamically consistent kinetic liquidus, and contains no adjustable parameters. We also report a systematic increase of the breakdown concentration with increasing deviation from steady-state conditions, which is not addressed by current stability theories, parametrized by the concentration gradient just prior to breakdown.