Precipitation sequence of η phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy during arti？cial aging
(1. State Key Laboratory of Solidi？cation Processing, Northwestern Polytechnical University,
Xi’an 710072, China;
2. Beijing Institute of Aeronautical Materials, Beijing 100095, China)
Xi’an 710072, China;
2. Beijing Institute of Aeronautical Materials, Beijing 100095, China)
Abstract: The precipitation sequence of η(MgZn2) phase along low-angle grain boundaries in Al-Zn-Mg-Cu alloy was investigated by examining samples aged at 135 °C for various times from 5 min to 6 h. High resolution transmission electron microscopy (HRTEM) observations and energy dispersive X-ray spectroscopy (EDX) analysis indicate that the precipitation sequence of η phase along low-angle grain boundaries should be supersaturated solid solution (SSS)→vacancy-rich clusters (VRC)→GP II zones→η′→η. Based on the theory of non-equilibrium grain boundary segregation(NGS) and non-equilibrium grain boundary co-segregation (NGCS), the excessive solute elements gradually segregate to the grain boundaries by the diffusion of the solute-vacancy complex during aging treatment. The grain boundary segregation plays an important role in the nucleation and growth of VRC, GP II zones, η′ phase as well as η phase.
Key words: Al-Zn-Mg-Cu alloy; aging; low-angle grain boundaries; grain boundary segregation;precipitation sequence