Hydrogen embrittlement in Al-Zn-Mg-Cu alloys is suggested to originate from debonding of the η phase interface. Previous studies have shown that intragranular T phase precipitation, facilitated by increased Mg content, contributes to the mitigation of quasi-cleavage fracture. However, the role of T phase precipitation on the grain boundary in suppressing intergranular fracture remains unclear. In this study, in-situ observational techniques were used to examine the relationship between grain boundary precipitates and hydrogen-induce intergranular cracking. Obtained results showed that while the T phase precipitates in the matrix of Mg-enhanced alloy, the η phase predominates on grain boundaries, which lead intergranular fracture. The presence of numerous voids at intergranular crack tips suggests that void nucleation along grain boundaries and subsequent coalescence is the primary mechanism of crack propagation. The observed void formation at η phase interfaces is consistent with first-principles calculations and supports the concept that intergranular fracture originates from debonding at η phase interfaces.

This study systematically investigated the solidification microstructures of Al-Si-Fe and Al-Si-Mn ternary alloys solidified at various cooling rates ranging from 0.3 to 145°C・s^‒1. The thermodynamic calculations assessed three alloy compositions of Al-5%Si-1.5%Fe, Al-8%Si-1%Fe, and Al-5%Si-1.5%Mn exhibiting two-phase eutectic reactions of α-Al and intermetallics (β-Al₅FeSi, α_h-Al₇Fe₂Si or α_c-Al₁₅Mn₃Si₂) phases in solidification. The needle-shaped β phase was formed in the Al-Si-Fe ternary alloy. The microstructural characterizations revealed that the α_c phase (rather than α_h phase) and Si phase were dominantly formed in eutectic reactions. The α_c phase with Chinese-script morphology was formed in the Al-5%Si-1.5%Mn ternary alloy. The dendritic α-Al phase was dominantly formed as the primary solidified phase, and the intermetallic phases were subsequently solidified at lower temperatures, which was independent of the cooling rate in solidification. The observed microstructures can provide insights into the solidification sequences, which were consistent with exothermal reactions detected by differential scanning calorimetry measurements at controlled cooling rates in solidification. The solidification sequence may not follow the calculated liquidus projections and its associated Scheil solidification simulations. The difference between experiments and calculations would be associated with the thermodynamic stability of the constitute phases below the solidus temperatures of Al-Si-Fe and Al-Si-Mn ternary systems.

While plate-like Guinier-Preston (GP) zones are formed during aging process in Al-Cu alloys, spherical nanocluster formation occurs in the early stage of aging in Al-Mg-Si alloys. Unlike well-known GP (I) zone in Al-Cu, there is no specific configurations within the nanocluster. However, the solute concentration and local configuration should play decisive role in subsequent formation of precipitates. In the present study, the first-principles calculations were performed to investigate the factors determining the stable shape during the formation process of GP zones and clusters in Al-Cu and Al-Mg-Si alloys. As a result of formation energy calculation of three-body bonds, the Cu-Cu-Cu triplet with the bond angle of 90° was the most stable. Monte Carlo simulations with newly developed machine-learning potential were then performed, and consequently the segregation of Cu atoms with the bond angle of 90° was observed more frequently. In contrast, three-body triplet with the bond angle of 60°was most stable without any specific directional anisotropy in Al-Mg-Si alloy, resulting in the formation of spherical nanoclusters. These results suggest that the intrinsic feature of the stability of local bonding dominates the shape of GP zones and nanoclusters, in which planar- or spherical-like cluster is formed.