We present details of an experimental study of crystallization adjacent to a cooled boundary from an aqueous solution of potassium nitrate and sodium nitrate. This transparent system is typical of many ternary melts that do not form solid solutions, including examples in igneous petrology and metallurgy. We have measured the rates of advance of the front of crystallization and the eutectic front, behind which the system is completely solid. From careful measurements of the concentration and temperature fields, we have been able to infer the location of an internal phase boundary: the cotectic front separating a region in which only one component of the ternary system forms crystals from a region in which two components form crystals. Our experiments were conducted under conditions in which fluid flow is minimal, so that rates of crystallization are determined principally by the diffusive transport of heat. We have confirmed that the thicknesses of the various regions all grow in proportion to the square root of time, as is expected of diffusion-limited growth, and have determined the constants of proportionality for a range of different initial concentrations and boundary temperatures. We have found evidence to suggest that there may be a significant nucleation delay in the secondary and tertiary crystallization. Our measurements of concentration provide much more information about the ternary phase diagram than has hitherto been available. † Lateral temperature gradients due to thermal leakage through the sidewalls of a mould may occur and cause some extraneous convection, but its influence is weak and will not be considered here.

Solidification as a first order phase transition is described in the Landau theory by the same equation as tricritical phenomena. Here, the solidification or melting temperature against pressure curve is modelled to end at a tricritical point. The model gives the phase transition temperature's dependence on pressure up to the quadratic term with a definite expression for the coefficients. This formula is expected to be generally valid for pure materials having melting curves with dT/dP approaching zero at very high P. Excellent experimental agreement is obtained for iron, the material having the most high pressure data which rather accurately determines the value of the coefficient defining the curvature. The geophysically interesting iron solidification temperatures at the Earth's core pressures are obtained. In addition, the general formulae for entropy change, latent heat and volume contraction in solidification are found and calculated for iron as functions of pressure and temperature.

We present measurements of the fluid velocity in a rotating Couette-Taylor system of aspect ratio L near 1. At small angular speed co of the inner cylinder, the system contains a symmetric vortex pair. As co increases, the vortex boundary moves off center, and a suitably defined order parameter \fj becomes nonzero. This bifurcation changes from forward to backward as L is increased. Results for t// agree quantitatively with the predictions of a Landau model for tricritical behavior.

We present the results of an experimental study on the solidification of aqueous solutions of potassium nitrate and sodium nitrate cooled from below. Upon cooling, two distinct mushy layers form, primary and cotectic, separated by an approximately planar horizontal interface. A density reversal between the two mushes causes the residual liquid in the upper, primary mush to be more buoyant than the melt overlying it, while the cotectic mush is compositionally stable. The unstable concentration gradient between the melt and primary mush causes convection that keeps the melt well-mixed and reduces the concentration gradient to zero after a finite time. At this point, the cotectic mush overtakes the primary mush and a transition from a convective regime to a diffusive regime occurs. Our measurements show that this transition is rapid and alters the growth rate of the single (cotectic) mush layer that remains. Concentration measurements taken from within the melt during convection and from within the mush during the diffusive regime show good agreement with the concentration evolution predicted by use of the equilibrium ternary phase diagram. We describe a global conservation model for solidification of a ternary alloy in this regime. Predictions from our model forced with empirical data for the heat and solute fluxes are in good agreement with the measured data for the interface positions of the two mushy layers. We also discuss how solid fractions vary with different melt concentrations in a non-convecting alloy and examine the influence of vertical solute transport in the convecting case. The identification of a density reversal in the solidification of a ternary alloy begins to address the complexities in solidification processes of multi-component alloys. IntroductionThe formation of solids by cooling a liquid melt is an integral part of many natural and industrial processes. Many of these solidification processes also generate fluid flows that are effective means of heat and mass transport and significantly influence the structure and growth rate of the solid phase. These fluid motions occur in liquid alloys owing to thermal convection caused by cooling the melt or to compositional convection caused by the removal of one or more components from the melt to form the solid phase. Preferential incorporation of components into the solid and rejection of others into the melt can also lead to constitutional supercooling, which † Present address:

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