The atomic formation mechanism of GP zones in Al-Cu alloys: Insights from cluster expansion coupled with Monte Carlo simulation

Abstract

Guinier-Preston (GP) zones in Al-Cu alloys are noted for their precipitation-hardening effects and their critical role in elucidating the nanoscale organization of solute atoms. In this study, we employed the cluster expansion (CE) method combined with Monte Carlo (MC) simulations to investigate the formation and evolution of GP zones in Al-Cu alloys, particularly in the presence of vacancies and Mg. The CE model was trained on energies calculated by first-principles density functional theory (DFT), enabling subsequent MC simulations to explore clustering behavior of matrix-coherent Cu-rich structures. The simulations reproduced the formation of GPⅠ and GPⅡ zones, demonstrating that the model can capture atomic interactions responsible for Cu clustering. Crucially, the presence of vacancies promotes GP zone formation and facilitates the transition from GPⅠ to GPⅡ zones. Furthermore, Mg addition to Al-Cu alloys reduces the size of Cu-rich clusters while increasing shape diversity, and when combined with vacancies, leads to more complex structures consistent with experimentally observed Guinier-Preston-Bagaryatsky (GPB) zones. Subsequently, we studied GP zone decomposition during heating as a function of Cu concentration, revealing significantly improved agreement with experimental data compared to prior computational studies. These findings not only provide atomic-scale insights into GP zone formation mechanisms and the roles of vacancies and Mg, but also demonstrate the effectiveness of combining CE and MC approaches for studying nanoscale precipitation processes in Al-Cu and Al-Cu-Mg alloys.