A pre-melting phenomenon in sodium—potassium alloys

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Abstract
A study of melting in potassium-rich sodium-potassium alloys has been made by direct observation using the technique of transmission ultra-violet microscopy. In this way it has been possible to observe pre-melting of grain boundaries and certain other pre-determined sites in the solid metal. At high rates of melting a microstructure of diluted alloy and molten strips of metal is observed ahead of the main advancing melting front. This is explained in terms of an instability arising from the local diffusion of sodium from the solid into the liquid phase, resulting in a form of ‘constitutional superheating'. There is an analogy between this and constitutional supercooling which occurs in solidification and leads to the well-known cellular structure of the solid/liquid interface. However, unlike this latter effect, in constitutional superheating the instability does not occur in the immediate vicinity of the interface but only in the solid metal ahead of it. This can be explained if the solidus temperature distribution ahead of the advancing interface is significantly different in form from that of the liquidus in the solidifying melt. If the instability occurs generally in the melting of binary alloys it could lead to a significant change in the present concept of rate-controlling processes in melting.