Thermodynamic investigation of Guinier–Preston zone formation in the Al–Cu binary system

The thermodynamic mechanism of Guinier–Preston (GP) zone formation in the Al–Cu system was examined. The free energy of the FCC phase was initially calculated, considering the volume change due to the difference in atomic radii between Al and Cu. This free energy was analyzed based on the CALPHAD method, calculating the phase diagram of a metastable equilibrium between the ordered structure with a GP(II) structure and the disordered FCC phase. Furthermore, the time–temperature–transformation curve for spinodal decomposition was calculated using the Cahn–Hilliard equation. The results showed that below 200 K, a spinodal decomposition occurred between the disordered FCC and the GP(II) phase, and no decomposition occurred at higher temperatures. With the direct introduction of effective cluster interactions in Monte Carlo simulations, spontaneous and continuous transformation from GP(I) to GP(II) was observed at 200 K, but no such change was observed at 300 K.