Precipitate free zones (PFZs) near grain boundaries generally soften alloys. The quenching rate after solution treatment is an important factor influencing the width of PFZs in Al-Mg-Si-Cu alloy. This study explored the effects of high quenching rates on the grain boundary microstructures and mechanical properties of an Al-Mg-Si-Cu alloy. Samples of various thickness were quenched in water at room temperature and in ethylene glycol at −40 °C, respectively. The results showed that the rapidly quenched samples at −40 °C exhibited better comprehensive mechanical properties than the water-quenched samples. Transmission electron microscopy studies revealed the rapidly quenched samples had wider PFZs, shorter intragranular precipitates, and larger grain boundary precipitates (GBPs) than water-quenched samples. It is proposed that when the quenching rate exceeds the critical cooling rate, e.g., in water quenching or rapid quenching, the formation of PFZs is controlled by the solute depletion mechanism rather than the vacancy depletion mechanism. The nucleation and growth of GBPs thus lead to the depletion of solute atoms, resulting in wider PFZs rather than thinner PFZs according to previous knowledge. This research provides valuable insights into the application of rapid quenching technology for modifying alloys’ microstructures and properties.

Machine learning is widely used to accelerate materials design, but its performance remains limited in small-data scenarios, reducing effectiveness in guiding novel materials development. Here, we developed a novel representation transfer framework based on the Calculation of Phase Diagrams (CALPHAD) method to address the challenge. By integrating CALPHAD with feature engineering, the thermodynamics-informed descriptors related to strength and ductility were constructed and selected. These descriptors capture the coupled effects of composition and temperature on material properties, offering strong physical interpretability and enabling a knowledge transfer from well-studied 2xxx, 6xxx and 7xxx series aluminum alloys to the underexplored Al–Mg–Zn alloys. Subsequently, by coupling high-throughput CALPHAD calculations with the NSGA-II algorithm, the strength–ductility Pareto front is efficiently identified to guide alloy design. Experimental validation confirmed that the two designed alloys achieved ultimate tensile strengths of 472 ± 7 MPa and 569 ± 12 MPa, with elongations of 23.5 ± 0.5% and 14.9 ± 0.3%, respectively, demonstrating improved strength–ductility synergy in the Al–Mg–Zn system. This framework enhances model generalization and interpretability in small-data scenarios, offering a versatile strategy for rapid discovery of high-performance materials across diverse systems.

Cu additions notably strengthen Al-Mg-Si and Al-Si-Mg alloys due to the dense precipitation of quaternary nano precipitates during ageing. However, the chemical evolution and mechanical behaviors of the quaternary micro-scale Q constituent phase occurring in cast and homogenized states have rarely been studied. Meanwhile, there exists a type of AlCuMgSi cluster in the cast state, which has been regarded as Q particles. The accurate identification of phase constituents is the basis for the future design of alloys with better performance. In our work, this type of cluster was revealed to consist of α-Al, θ-Al₂Cu, Q, and Si phases through micro-to-atomic scale studies using scanning and transmission electron microscopes. The skeleton of the dendrite was θ phase. The second phases in the dendritic eutectic cluster dissolved quickly during a 4 h homogenization at 550 °C. The Q phase was found to effectively absorb the Fe impurities during casting and homogenization. As a result, the formation of other harmful Fe-rich intermetallics was suppressed. These Q constituent particles were observed to break into separate pieces in an intermediately brittle manner when compressed in situ in a scanning electron microscope. These findings provide insights into the thermodynamic modeling of the Al-Mg-Si-Cu system and alloy design.