Abstract: The 8021 aluminum alloy is widely used in lithium battery packaging due to its excellent elongation. In this alloy, conventional casting is typically employed. However, coarse β-Al5FeSi phases inevitably form in conventional cast alloys, resulting in low elongation. Addition Cr can significantly suppress the segregation of Fe and Si and β-Al5FeSi phases, converting the original plate-like structures into fine platelets or particles. Such microstructual evolution further improves the mechanical properties of the alloy. To further improve the tensile strength and elongation of 8021 aluminum alloy, this work systematically investigates the effects of Cr addition on the morphological evolution of the second phase and the corresponding mechanical properties. The morphology, lattice structure, and phase composition of the second phase in the alloy are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The experimental results show that adding an appropriate amount of Cr greatly changes the morphology of the second phase in 8021 aluminum alloy. The contents of β-Al5FeSi and θ-Al3Fe phases decrease obviously, while a new Al95Fe4Cr phase with an icosahedral quasicrystalline structure precipitates. This newly formed phase can effectively improve the plasticity and other mechanical properties of the alloy. Fracture analysis of Cr-modified samples reveals that the dimples on tensile fracture surfaces become more numerous and deeper. These features are typical of ductile fracture, which further confirms that Cr improves the toughness of the alloy by promoting plastic deformation. This study clarifies the microscopic mechanism of synergistically improving strength and plasticity via Cr-induced evolution of the second phase. It also provides a new strategy for composition design and microstructure control of high-performance aluminum alloys.

Key words: 8021 aluminum alloy, elongation, the second phase, Al95Fe4Cr phase, quasicrystal

摘要： 为了提高8021铝合金的抗拉强度和伸长率，本工作系统研究了Cr添加对8021铝合金第二相形貌演变及力学性能的调控机制。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)及X射线衍射仪(XRD)，分别对8021铝合金第二相形貌、晶格结构和物相结构进行表征。实验结果表明，向8021铝合金中添加适量Cr后，第二相形貌发生显著变化，β-Al5FeSi相和θ-Al3Fe相含量显著减少，同时析出具有二十面体准晶结构的Al95Fe4Cr相，该相可有效提高合金塑性等力学性能。经Cr改性的合金试样断口韧窝增多且加深，呈现典型的韧性断裂特征，进一步证实Cr可通过促进塑性变形提升合金韧性的作用机制。本工作揭示了通过Cr微合金化调控第二相演化、实现铝合金强塑性协同优化的微观机制，为开发综合性能优良的铝合金提供了成分设计与组织控制新策略。

关键词: 8021铝合金, 伸长率, 第二相, Al95Fe4Cr相, 准晶体