高性能AlCuLi(Mg)铝合金中复杂共存共生相的析出强化与演化规律研究

AlCuLi(Mg) alloys are very important light-weight structural material for aero-space industry. The excellent properties and performance of the alloys, which can be optimized by thermal processes, depend on over 10 different types precipitates that competitively co-exist in the microstructure of the alloys, and on the evolution pathways of these precipitates. Most of these precipitates are known in the study of other aluminum alloys such as AlCu, AlLi and AlCuMg alloys. Nonetheless, addition of the key alloying element Li in the alloys not only profoundly influence the numbers and distributions of the final T1, σ and S phases upon thermal ageing, but also can change the relative stabilities, the precipitation paths and the structures of various early – stage precipitates, often leading to different precipitate variants and composite precipitates to form and the precipitation pathways to change in the aged alloys. The in-depth mechanisms by which Li-atoms influence the property–structure–process relations of the alloys have not yet been well uncovered thus far. It has long been difficult to characterize the structures of these very fine precipitates and to carry out in-situ observations of the precipitates evolution at the atomic scale, which is actually a bottle-neck to well understand the alloys. This project aims to make a breakout at this technical bottle-neck, using advanced atomic-resolution imaging techniques in association with property measurements and process optimizations for a few targeted typical alloys. Quantitative imaging, in-situ imaging and 3D-imaging in atomic-resolution transmission electron microscopy combined with first principles energy calculations shall be performed intensively to determine the structure of these precipitates and to closely follow their evolution in real time upon ageing, in order to better understand the property–structure–process relations of the alloys.

摘要：AlCuLi(Mg)合金是航空航天工业中极其重要的轻量化材料。该合金系优异的综合性能取决于其微观结构中十余种共存共生析出相的演化路径和竞争生存状态。这些强化相中的大多数在其它铝合金，如AlCu，AlLi等中已熟知。但关键元素Li的加入不但影响了该合金时效终态T1、σ、S相的数量和分布，而且改变各种早期相的析出方式、形貌和相对稳定性，导致析出相异常变体的形成和析出演化路径的改变。Li元素对该合金性能、结构、工艺及其关系的影响机制是尚未完全解决的关键科学问题。过去很难实现在原子分辨率水平上对细小析出相和Li原子的分布进行精准的和原位的测量表征。本项目针对上述技术瓶颈，拟利用先进原子成像电镜平台技术，结合块体材料的工艺调控和性能表征以及第一性原理计算等手段，对典型目标合金的微观结构展开原位和准原位以及三维的测量表征研究，较彻底阐明该合金系微观结构的演变规律，理解其综合性能与结构、工艺的关系

AlCuLi（Mg）铝合金具有高弹性模量、低密度、高强度和良好韧性，因而广泛应用于航空航天领域中的结构材料。该合金优异综合性能取决于其微观结构中十余种共存共生析出相的演化路径和竞争生存状态。Li元素对该合金性能、结构、工艺及其关系的影响机制是尚未完全解决的关键科学问题。本项目利用先进电镜技术、三维原子探针分析、力学性能测量和第一原理计算等研究手段，测定了目标铝锂合金不同时效阶段的析出相的形貌、结构、界面以及成份；探明了不同成分的目标合金中纳米析出相的演变规律，尤其是GPB区、S相、θ'相、Li-GPB区变体、Li修饰的类S相、δ'/GP/δ'、δ'/θ'/δ'复合相和T1相等析出相的竞争析出机制和演变规律。结果表明：（1）Li元素的添加可以改变Al-Cu-Mg合金析出相的结构和析出序列，促使δ'、δ'/GP/δ'、δ'/θ'/δ'共生相、T1相、Li-GPB区变体、类S相等含Li析出相的形成，进而改善合金的比强度、比模量、断裂韧性等综合性能；（2）δ'/GP/δ'和δ'/θ'/δ'复合相界面结构相同，界面上的Li与Cu原子均处于次近邻关系，这种特殊的界面关系抑制了δ'/θ'/δ'复合相的增厚；（3）自然时效形成的大量均匀弥散细小δ'相在后续180℃时效过程有利于T1相异质形核，促使晶内T1相弥散、均匀分布，增加了材料的强度；（4）引入预变形促进T1相和连续复合相析出，抑制δ'/θ'/δ相和晶内Li-GPB区形成，而预变形前预时效形成的弥散细小GPB区、GP、δ'/GP/δ'相有利于T1相、S相、δ'/θ'/δ'复合相均匀析出，粗大的先析δ'/GP/δ'相则抑制T1相形成；（5）AlCuLi（Mg）铝合金具有优良的耐腐蚀性能，但易出现局部腐蚀，尤其是平行于{111}Al的晶体学腐蚀条带，对具有GPB区的铝锂合金而言，在纳米级别上{111}腐蚀条带是由{100}台阶构成，{100}台阶形成与沿<100>生长的GPB区有关；（6）析出的类型和分布影响铝锂合金的腐蚀行为，T1相增多会增加合金的腐蚀，当T1相在晶界偏析增加晶界腐蚀；总之，本项目结合实验观察和理论分析开发出可以改善高强AlCuLi（Mg）铝合金性能的工艺，加深了人们对各类析出相在原子尺度、纳米尺度的微观结构与其演变规律及其与性能和工艺关系的理解。