Comparison between resistivity changes and Monte Carlo simulation for GP zone formation in Al-Cu base ternary alloys

The formation behavior of GP zones has not been fully understood because experimental detection of their nucleation and growth is rather difficult especially in the early stage of aging. Monte Carlo simulation is a most powerful method to examine such microscopic events occurring in alloys during phase transformations. The present work aims to investigate the effects of various additional elements to an Al-Cu alloy from both viewpoints of the nucleation and growth behavior of GP zones. Comparisons between the quantitative kinetics determined by the electrical resistivity changes and the microstructural evolution obtained from the simulation provide useful information on both the macroscopic transformation behavior and the detailed formation mechanism of Cu clusters. As is well known so far, addition of Zn or Ag exerts no marked influence on the GP zone formation, whereas addition of Sn or Zr markedly retards the growth rate of GP zones. As for a small addition of Mg, however, slightly complicated effects are revealed; i.e. a characteristic nucleation behavior and a suppressed growth of GP zones. This is well explained in terms of both the extremely increased number density of Mg/Cu/Vacancy complexes and the pronounced decrease in free-vacancies available for Cu diffusion due to the preferential vacancy trapping by Mg atoms. Such situations are directly observed in atom configurations during the simulation.