Eutectic Nucleation in 7xxx Series Aluminum Alloys from a Non-classical Viewpoint

The early stages of eutectic solidification in a copper-containing 7xxx series aluminum alloy (AA 7068 or AMS 4331) were studied using the two-thermocouple computer-aided thermal analysis (CATA) technique. A feature was detected on the cooling rate curve at the equilibrium solidus temperature of the alloy which persists until the peak of the subsequent final eutectic solidification. Detailed analysis of the temperature difference between the wall and the center of the thermal analysis sample, together with examination of the eutectic solidified on the walls of porosities and a study of the eutectic nucleation on the basis of the non-classical theory of adsorption heterogeneous nucleation, indicated how the feature can be related to the faceting of the atomic structure of the solid/liquid (S/L) interface. The solidification of the remnant liquid after the faceting transition at the equilibrium solidus point depends on the interfacial undercooling and proceeds via either primary phase re-nucleation or secondary phase nucleation by adsorption. The eutectic solidification is affected by the presence of the primary phase which acts like an adsorbent.