基于晶体学取向的非均匀组织对新型铝锂合金局部腐蚀行为的影响

Since traditional corrosion theories could not explain properly the formation of stable localized corrosion in the new type aluminum lithium alloy, a new theory has been proposed that heterogeneous microstructure at grain scales is responsible for such localized corrosion, with consideration of two facts that, at low and intermediate strain, cold-worked microstructure of aluminum alloys strongly depends on crystallographic orientation and cold-worked microstructure of aluminum alloys affects their corrosion behavior. In order to prove the theory, the new type aluminum lithium alloy AA2099 which is subjected to solid solution treatment, controlled stretching and artificial ageing will be used to study the correlation between crystallographic orientation, the magnitude of plastic deformation, stored energy, relative amount of precipitates in individual grains and the corrosion behavior of the grains. The crystallographic orientation and the stored energy in individual grains will be determined using electron backscattered diffraction (EBSD) technology. It is expected that the mechanism for the formation of stable localized corrosion in the new type aluminum lithium alloy will be unveiled; the principles for microstructural control of the new type aluminum lithium alloy for improving corrosion resistance will be put forward; a theory system concerning the effect of heterogeneous microstructure at grain scales on localized corrosion in aluminum alloys will be constructed. The outcome of the project is directive for microstructural optimization and corrosion control of the new type aluminum lithium alloy and, could push forward the development of the aerospace industry which requires low energy consumption and high safety performance.

针对传统腐蚀理论不能合理解释新型铝锂合金中稳态局部腐蚀成因的问题，并根据"中低应变量下铝合金的冷变形组织强烈依赖晶粒取向"和"冷变形组织影响铝合金的耐腐蚀性能"两个研究事实，提出"晶粒尺度上的非均匀组织影响铝合金的局部腐蚀行为"的研究命题。围绕该命题，以新型铝锂合金AA2099为主要研究对象，以电子背散射衍射技术（EBSD）为主要研究手段，通过对固溶处理的合金进行控制拉伸及人工时效，研究在中低应变量下不同晶体学取向晶粒内部储存能或析出物的相对量与晶粒腐蚀行为之间的关系，揭示新型铝锂合金稳态局部腐蚀的形成机理，提出改善新型铝锂合金耐腐蚀性能的组织控制原则，构筑基于晶体学取向的非均组织对铝合金局部腐蚀行为影响的理论体系。研究成果对新型航空铝锂合金的组织优化和腐蚀控制有重要指导意义，对发展低能耗、高安全性的航空产业有一定的推动作用。

铝锂合金因在比强度、比刚度和损伤容限方面的优势而在飞上的使用量不断增加。然而，由于锂、铜等合金元素的添加，铝锂合金在腐蚀环境下有较强的局部腐蚀倾向。本项目采用扫描电镜、透射电镜、纳米扫描断层分析等技术研究了第三代新型铝锂合金AA2099中粗大第二相颗粒和时效强化相对合金局部腐蚀行为的影响，特别研究了晶体取向在T8态合金局部腐蚀过程中的作用。研究表明：合金中的Al-Fe-Mn-Cu第二相颗粒导致了合金浅表层的点蚀，同时因铜含量较高的颗粒中含有较高含量的锂元素而表现出更高的化学活性。通过比较不同热处理状态合金的腐蚀行为发现：T8态合金比其它状态合金表现出更高的局部腐蚀倾向，且这种局部腐蚀更容易在施密特因子较大的晶粒内部出现；而在晶界和位错上非均匀析出的T1 (Al2CuLi) 相的优先腐蚀是引起这种局部腐蚀的根本原因。考虑冷变形过程中的非均匀塑性变形以及T1相在晶粒内部的晶体缺陷上择优析出两个因素，我们提出了一个模型解释基于晶体取向的非均匀微观组织对铝锂合金局部腐蚀行为的影响。此外，我们还简要研究了T1相的腐蚀行为以及T1的分布对铝锂合金腐蚀行为的影响。基于上述研究结果，我们提出了改善铝锂合金耐蚀性能的微观组织控制原则。