【作者】 宋丰轩；

【导师】 张新明；

【作者基本信息】 中南大学 ， 材料加工工程， 2014， 博士

【摘要】 摘要：7050铝合金厚板是重要的航空用结构材料,广泛用作飞机的机身框架、桁条及机翼壁板等承力部件。由于环境腐蚀介质和应力的作用不可避免,7050铝合金板在服役过程中发生剥落腐蚀和应力腐蚀开裂。针对这一问题,本文结合大飞机工程材料研制需要,通过浸蚀和电化学测试,采用光学显微镜、扫描电镜、透射电镜和扫描开尔文探针原子力显微镜观察分析,研究了7050铝合金板在含C1-1离子介质中的局部腐蚀行为,探求了固溶、淬火、预拉伸和双级时效对合金组织和局部腐蚀敏感性的影响规律以及厚板局部腐蚀的方向性,获得了以下结论：(1)合金局部腐蚀起源于粗大第二相,如残留的亚微米A12CuMg相(电极电势比铝基体低100～500mV)和初生A17Cu2Fe相(电极电势比铝基体高100-600mV)。孤立A12CuMg相随着腐蚀的进行极性发生转变,导致腐蚀扩展困难,一般不会形成微裂纹；而A12CuMg相与A17Cu2Fe相间强烈的电耦作用会使腐蚀加速并形成微裂纹。单级固溶温度升高粗大非平衡结晶A12CuMg相逐渐溶解,板材局部腐蚀敏感性降低；但高温固溶会导致合金中再结晶分数快速增加,降低板材的力学性能和腐蚀性能；最佳的单级固溶制度为470℃/2.5h。逐级升温强化固溶可使合金在保持高强度的同时,抗应力腐蚀性能提高；最佳的双级固溶制度为450℃/1.5h+480℃/0.5h；逐级固溶后板材的KISCC值最大,为17.8MPa·m1/2。(2)晶界区域η相覆盖比率决定腐蚀形貌。淬火过程中,随淬火转移时间延长,晶界和亚晶界区域η相覆盖比率依次增加,板材晶间腐蚀形貌由穿晶腐蚀逐渐向粗大网状结构、精细网状结构转变。过时效处理可促进Cu元素在晶界析出相中的均匀扩散,降低板材晶间腐蚀敏感性；但当Cu元素含量接近10at.%(达到平衡状态)后,继续增加时效程度,PFZ宽化严重反而会提高板材腐蚀敏感性。(3)预拉伸变形量宜控制在2%左右。预拉伸变形量增加,晶界析出相尺寸、相邻析出相间距和PFZ宽度先略有增加然后减小；晶界析出相分布由链状变得连续分布；合金抗应力腐蚀性能不断减小。(4)在先低温后高温的双级时效处理的高温阶段,随着第二级时效时间延长,合金硬度先增加后减小；合金腐蚀类型由沿晶腐蚀向穿晶腐蚀转变,使得合金失重腐蚀速率增加；但腐蚀电流密度和腐蚀裂纹扩展速率减小,合金耐蚀性能提高。最佳的第二级时效工艺为163℃/12h。(5)7050-T7451120mm铝合金厚板腐蚀性能具有方向性。从表层到中心层,板材抗剥落腐蚀性能先减小后略有增加。同一厚度层内不同方向样品的应力腐蚀敏感性由大到小的顺序为：S方向>L方向>T方向。这主要与晶粒的形貌有关：晶粒越狭长,合金腐蚀敏感性越高。更多还原

【Abstract】 Abstract:Aluminium alloy7050thick plate is an important aeronautic material, and has been widely used as the main structural components in aeroplanes like frames, longerons and wallboard, etc. However, the aluminium alloy7050thick plate in service is usually susceptible to exfoliation corrosion and stress corrosion cracking (SCC) due to the effect of aggressive medium and tensile stress. To improve the resistance to localized corrosion of the aluminium alloy7050thick plate and to meet the production need for the big airplane project, the corrosion behavior of the alloy7050in a solution containing Cl-1ion was studied using immersion tests, electrochemical tests and complementary techniques like optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning Kelvin probe force microscopy. Also, the influences of solution heat treatment, quenching, pre-stretching and two-step ageing on the microstructure and corrosion resistance, and the corrosion anisotropy of the thick plate were investigated. The main conclusions are as follows:(1)The localized corrosion initiated from coarse second phases in the aluminium alloy7050plate. For example, the residual submicron Al2CuMg phase had an anodic character and its Volta potential relative to the surrounding Al matrix was about-100～-500mV, while primary Al7Cu2Fe phase had a cathodic character and its Volta potential difference with respect to the matrix ranged between+100and+600mV; the nature of the isolated Al2CuMg phase changed with the increase of immersion time, which led to the decrease in rate of corrosion and little emergence of corrosion micro-cracks; clustering of the isolated Al2CuMg and Al7Cu2Fe phases which have different natures will accelerate corrosion attack and produce micro-cracks. With the increase of single-stage solution temperature, the volume fraction of the residual phases in the aluminium alloy7050plate reduced gradually resulting in low corrosion susceptibility, while the volume of the recrystallized grains increased with solution temperature which induced the decrease of both the strength and the corrosion resistance; the optimal solution process was470℃/2.5h for single-stage solution. Through multi-stage solution heat treatments, the aluminium alloy7050thick plate can obtain high strength and high resistance to localized corrosion; the optimal two-stage solution process was450℃/1.5h+480℃/0.5h; Step-to-step solution heat treatment can obtain the highest KISCC value (about17.8MPa-m1/2).(2)The corrosion morphologies of the aluminium alloy7050thick plate were determined by the coverage ratio of η-MgZn2precipitates at grain boundaries. The coverage ratio of η-MgZn2precipitates at grain boundaries and sub-grain boundaries increased one after another with prolonging quenching transfer time, leading to the corrosion morphologies of the aluminium alloy7050thick plate transformed gradually from intragranular corrosion to large intergranular corrosion (IGC) network and fine IGC network. Overageing treatment can promote the diffusion of copper element at grain boundary precipitates (GBPs) and hence effectively reduced the high sensitivity to IGC of the7050alloy. After the copper content in GBPs had reached about10at.%(an equilibrium state), however, the IGC susceptibility of the aluminium alloy7050thick plate will show a modest decrease with the increase of ageing degree.(3)The pre-stretching amount should be controlled in about2%. With the increase of pre-stretching deformation degree, the GBPs size, neighbour distance and PFZ width in the alloy7050plate first increased slightly and then decreased which led to change of the GBPs distribution from interrupted distribution to continuous distribution and the gradual decrease in SCC resistance.(4) At high temperature stage of two-step ageing (ageing treatment first at a relatively low temperature then at an elevated temperature), the hardness value of the alloy7050plate first increased and then declined with secondary ageing time. Meanwhile, the corrosion morphologies of the alloy7050plate changed from intergranular to intragranular with increase of ageing degree. This made the weight loss rate of the alloy increase, but the corrosion rate and propagation rate of cracks reduced with ageing degree. Overall, the corrosion resistance of the aluminium alloy plate was improved with the increase of ageing degree. The optimal secondary ageing process was163℃/12h.(5)The aluminium alloy7050-T7451plate with a thickness of120mm showed a high anisotropy of localized corrosion. The resistance to exfoliation corrosion of the alloy plate first reduced and then increased slightly from surface to center. For the same thickness layer of the alloy plate, the SCC susceptibility to different directions relative to the rolling direction was assessed by SSRT technique, which showed the susceptibility order, S direction> L direction> T direction. The result showed that the smaller the grain aspect ratio, the better the corrosion resistance to SCC was.更多还原