Dissolution and melting of secondary Al2Cu phase particles in an AlCu alloy

When up-quenching an Al-4.2 wt pct Cu alloy which has been equilibrated at 450 °C to a temperature at or above the eutectic temperature of 547 °C, liquid drops are found to form. Inside the matrix grains they have a globular shape, while they have a lenticular shape with a low dihedral angle when formed on grain boundaries. It is demonstrated metallographically that drops are formed by the melting of the Al₂Cu phase particles together with the surroundingα matrix to form a liquid of chemical composition around the eutectic composition. On prolonged annealing, the drops are dissolved in theα matrix. The kinetics of this dissolution reaction of the drops, as well as the dissolution of Al₂Cu phase particles at a temperature below 547 °C, is studied in some detail and compared with a simple mathematical model. Also, the thermodynamics of the melting and dissolution reactions are discussed by means of a free-energy diagram.