EXPERIMENTAL TEST OF MORPHOLOGICAL STABILITY THEORY FOR A PLANAR INTERFACE DURING RAPID SOLIDIFICATION

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 a re important. Rapid solidification following pulsed laser melting was used to make metastable solid solutions of silicon-tin. Rutherford bac kscattering spectrometry and transmission electron microscopy were use d to measure the breakdown concentration. Samples remained microsegreg ation free with near perfect crystallinity at tin concentrations up to 10 times the maximum equilibrium solubility and 100 times that predic ted by linear stability theory with local interfacial equilibrium. The se 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 consi stent kinetic liquidus, and contains no adjustable parameters. We also report a systematic increase of the breakdown concentration with incr easing deviation from steady-state conditions, which is not addressed by current stability theories, parametrized by the concentration gradi ent just prior to breakdown. [S0163-1829(98)07117-3].