Driving force for G. P. zone formation in an Al-Cu alloy

Abstract

The increase in free energy accompanying the formation of G. P. zone in an Al-2at% Cu alloy was discussed in terms of the changes in binding enthalpy, configurational entropy and strain energy. As far as the driving force for precipitation is derived from the decrease in binding enthalpy of the system, segregation of copper atoms to (100) planes in the aluminum lattice against the decrease in configurational entropy, can not take place at temperatures higher than about-100°C. Since the change in free energy shows a monotonous increase according to the increase in the radius of G. P. zone, the preferential formation of platelet-type precipitates does not occur at a normal aging temperature. The amounts of strain energy around G. P. zones and solute copper atoms were evaluated by a finite element method, and it is shown that the strain energy of copper atom is largest in a solid solution. While the strain energy around G. P. zones increases approximately in proportion to their radii, a strain energy of copper atom or a total strain energy of the system decreases progressively with respect to the growth of platelets. Therefore, the G. P. zone in an Al-Cu alloy is formed mainly through the relaxation of strain energy around solute copper atoms.