Competitive precipitation among grain interiors, grain boundaries and triple junctions in gradient polycrystalline Al–Cu alloys: Simulation and qualitative experimental support

Grain boundary engineering has emerged as an effective approach to tailoring microstructural evolution and precipitation behavior in age-hardenable aluminum alloys. However, the competitive mechanisms by which heterogeneous interfaces such as grain boundaries, triple junctions, and dislocations govern solute segregation and nucleation remain unclear. In this work, a structural phase-field crystal (PFC) model is used to construct a polycrystalline Al-Cu alloy with a grain-size gradient, enabling systematic investigation of Cu diffusion and precipitation behavior at heterogeneous interfaces, together with qualitative experimental observations that provide microstructural evidence. The results show that the grain-size gradient regulates precipitate distribution: in the fine-grained region, short grain boundaries promote solute enrichment at triplet junctions; whereas in the coarse-grained region, longer grain boundaries lead to more nucleation events and increase the density of grain boundary precipitates. Residual dislocations formed during grain coalescence also become favorable sites for intragranular precipitation. Cluster coalescence is observed at triple junctions, while dissolution occurs in solute-depleted regions. The calculations confirm that the nucleation energy is lowest at triple junctions, followed by grain boundaries and grain interiors, highlighting the dominant role of heterogeneous interfaces in precipitation regulation. Consistent with experimentally observed interface-dominated precipitation and localized clustering behavior, this study provides atomic-scale insight into interface-controlled precipitation mechanisms and offers theoretical guidance for microstructural design and aging optimization in aluminum alloys.