Al-Mg alloys are widely used as important engineering structural materials in aerospace engineering, transportation systems, and structural constructions due to their low density, high specific strength, corrosion resistance, welding capability, fatigue strength, and cost-effectiveness. However, the conventional Al-Mg alloys can no longer fully satisfy the demands of practical production due to difficulties caused by many defects. The high strength of Al-Mg alloys as non-heat treatment precipitation-strengthened alloys is achieved primarily by solid solution strengthening along with work hardening rather than precipitation strengthening. Therefore, severe plastic deformation (SPD) techniques can be often used to produce ultrafine-grained structures to fabricate ultra-high strength aluminum alloys. However, this approach often achieves the strengthening of material at the cost of reduced ductility. This paper comprehensively summarizes the various approaches of ultrafine/nanocrystalline materials for enhancing their plasticity, elaborates on the creation of a bimodal microstructure within the alloy, and discusses the formation of a nanotwin microstructure within the alloy and the incorporation of dispersed nanoparticles. The mechanisms underlying both the strengthening and toughening during large plastic deformation in aluminum alloys are summarized, and the future research direction of high-performance ultrafine crystalline and nanocrystalline Al-Mg aluminum alloys is prospected.

The present study concentrates on the role and underlying mechanisms of in situ crystallization (employed for nanocrystal formation) in influencing the solidification microstructure and properties of aluminum alloys. By systematically analyzing the effects on α-Al refinement, silicon phase modification, and secondary phase control, as well as exploring the impact on room-temperature mechanical properties, high-temperature deformation behavior, and fatigue performance, this work reveals the potential physical mechanisms of improving mechanical properties by providing nucleation sites and inhibiting grain growth, such as fine-grain strengthening and dispersion strengthening. Moreover, stabilization of the second phase optimizes high-temperature deformation behavior, and a reduction in stress concentration improves fatigue performance. Compared with traditional microstructure control methods, in situ crystallization can achieve deeper grain refinement from micron to nanometer scale, ensuring high uniformity of grain distribution and showing good compatibility with existing processes. By defining the regulation of in situ crystallization on the microstructure and properties of aluminum alloy, the existing research provides a feasible material solution for high stress, high temperature, and high reliability. Its core significance lies in breaking through the performance bottlenecks of traditional modification technology, such as unstable refining effect, element segregation, and so on. The co-promotion of “strength–plasticity–stability” of aluminum alloys and the consideration of process compatibility and cost controllability lay a theoretical and technical foundation for the industrialization of high-performance aluminum alloys.