Anomalous precipitation behavior in T-phase strengthened Al-Mg-Zn(-Cu) alloys: Effects of aging temperatures and Cu contents

Abstract

The anomalous precipitation behavior, characterized by a decline in hardening rates with increasing aging temperature, was systematically investigated in Al-Mg-Zn(-Cu) crossover alloys. Hardness testing, tensile testing and transmission electron microscopy (TEM) observation were employed to elucidate the relationships among aging temperature, Cu content and the hardening behavior, as well as relevant microstructural evolution. The results indicate that this unique behavior is associated with the evolution of T phase, which is strongly influenced by aging temperatures and Cu contents. High aging temperatures are detrimental to T-phase nucleation, resulting in the formation of low-density precursors with distinctive substructure unit of T″ phase at the early stage of aging. So that insufficient nuclei are provided for the subsequent development of hardening phase. During the prolonged aging at elevated temperatures, T phase undergoes rapid coarsening, leading to a significant reduction in the hardening potential of the alloys. Such detrimental effects of high-temperature aging can be mitigated by Cu addition, which enhances the density of precursors for T-phase and improves the thermal resistance of precipitates during the later aging stages. These beneficial effects become more pronounced with increasing Cu content. Based on these findings, strategies for designing high-strength Al-Mg-Zn(-Cu) alloys were outlined, emphasizing the control of early precursors of T phase through optimized aging treatments.