【作者】 杨胜；

【导师】 易丹青；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 飞机的寿命和可靠性要求航空铝合金具有优良的耐疲劳及腐蚀损伤性能,合金微观结构与合金耐损伤性能有着密切关系。本文利用光学显微镜、扫描电镜和透射电镜、疲劳试验机、蠕变试验机以及电化学综合测试仪等设备系统地研究了2E12合金在服役温度和腐蚀环境下的疲劳、蠕变及腐蚀损伤行为,分析和探讨了微观结构在合金耐损伤中的影响和作用机制,本论文的主要研究内容及结果如下:2E12合金具有良好的力学性能和疲劳性能,常规力学性能和抗疲劳裂纹扩展性能基本达到目前报道2524-T3合金相关性能要求。运用疲劳损伤谱的相关理论,并综合考虑裂纹闭合效应的影响,建立了2E12合金R=0.1和R=0.5条件下的疲劳损伤谱,并对合金的裂纹扩展行为进行预测。探讨了温度对合金疲劳寿命及断裂机制的影响,温度升高,合金的疲劳性能降低,低温条件下位错运动的阻力较大,位错运动主要是以滑移为主;室温和高温条件下,位错运动需要克服的能垒降低,位错发生缠绕和攀移,同时在晶界处出现位错堆积,导致晶界的变形程度增加,断口以穿晶断裂为主同时伴随局部沿晶断裂。研究了合金在100℃-150℃条件下的蠕变行为,合金具有较高的蠕变应力指数,考虑门槛应力的影响,采用位错攀移模型对合金的蠕变行为进行解释,门槛应力值随着温度的升高而降低,合金的蠕变行为符合不完全回复蠕变,为应力辅助下的位错管扩散机制。在NaCl溶液和EXCO溶液中,2E12合金的耐腐蚀性能要优于传统的2024铝合金,盐水溶液中合金的腐蚀以点蚀为主,剥落腐蚀试验中,合金腐蚀动力学过程包括点蚀的诱导形成,点蚀发展及严重点蚀向轻微剥蚀发展,利用交流阻抗和等效电路分析和解释了合金表面的界面状态和腐蚀的微观过程。合金中Al₂Cu和Al₇Cu₂（Fe,Mn）杂质相主要作为阴极。而S相腐蚀微观过程由三阶段组成,在浸泡初期S相表现为阳极相,合金中的主要组分特别是Mg原子发生去合金化;随Mg原子含量降低,Cu原子的相对含量升高,S相转变为阴极,周围基体发生阳极溶解;最终当S相与基体连接中断时,S相发生脱落。自然时效条件下,合金晶界和晶粒内部的元素分布较为均匀,晶界对合金腐蚀的影响较小。在潮湿气体和盐雾条件下的裂纹扩展速率要快于室温空气条件下的裂纹扩展速率,疲劳断口形貌呈现出更多的脆性裂纹扩展特征。在腐蚀介质条件下,腐蚀介质与铝反应生成活性氢原子向塑性区扩散,导致塑性区发生氢脆,同时腐蚀诱导的阳极溶解降低了合金的裂纹扩展性能。通过在电解质中加入添加剂,延长电解充氢时间,静态电解充氢后合金表现出一定的氢脆,电解充氢后2E12合金的强度并没有明显的变化,但伸长率降低,静态电解充氢14天,塑性损失为12%。拉应力对氢的扩散起到加速作用,合金在较短的时间内显示出氢脆的特征,合金断口仍显示为韧性断口特征,总体上2E12合金的氢脆敏感性较低。固溶处理促进了粗大过剩相的溶解,但由于2E12合金中Cu、Mg元素含量较高,即使采用长时间固溶处理,合金中仍有相当多数量的过剩相无法溶解。利用强化固溶处理,促进了过剩相的溶解,强化固溶2小时后,合金的稳态裂纹扩展速率降低了约40%。但在固溶处理过程中基体中的合金元素向包铝层发生明显的扩散,导致包铝层耐腐蚀性能降低,因此强化固溶处理在改善包铝合金性能上的作用是非常有限的,对于不包铝合金可以采用强化固溶提高疲劳性能。预变形提高了合金的强度,降低了合金的塑性,而腐蚀性能变化并不显著。按照航空标准生产大规格T3态板材比T4态合金屈服强度提高了15%,伸长率仅降低了2%,合金的疲劳裂纹扩展性能基本相同,腐蚀性能没有明显差别,总体上综合力学性能要优于T4态合金,通过预变形处理工艺改善合金性能是比较有效的手段。人工时效处理促进析出相的析出,合金的强度提高,经形变热处理可获得更为细小均匀的析出相,晶界处无析出带变窄,强度更高,但塑性降低。析出相的析出和晶界处无析出带的出现导致人工时效处理后合金的耐腐蚀性能显著降低,按照剥落腐蚀标准判断均达到严重腐蚀的程度。研究先低温后高温双级时效处理对合金疲劳裂纹扩展性能的影响,结果发现欠时效处理形成的GPB区+S″可以降低合金的疲劳裂纹扩展速率,提高了合金的疲劳裂纹扩展性能,欠时效处理导致合金的耐腐蚀性能有所降低,合金的腐蚀性能接近2024合金。更多还原

【Abstract】 The long life and reliability of aircraft require aeronautical aluminum alloy with good fatigue and corrosion resistance properties. The damage resistant properties of the alloys are related to the microstructures.The fatigue,creep and corrosion damage properties of 2E12 aluminum alloy,which was exposed to different temperature and corrosive environments,were tested by OM,SEM,TEM,fatigue testing machine,creep testing machine and electrochemical properties testing devices.The influence of microstructure and the corresponding mechanism were investigated.The main conclusions are as follows:2E12 alloy has good mechanical properties and fatigue crack propagation resistance,which are similar to the properties of 2524-T3 alloy reported in literature.The fatigue damage spectra of2E12 alloy was created at stress ratios of 0.5 and 0.1,and the crack propagation behavior was predicted according to the fatigue-damage theory with consideration of crack closure effect.The effect of temperature on fatigue life and fracture mechanism of 2E12 was explored,and the increase of temperature resulted in the decrease of fatigue properties.At low temperature,due to the high pinning force slip was the main pattern of dislocation movement.At room and elevated temperatures,the energy barrier to overcome for dislocation movement was decreased.Consequently,tangle and climb of dislocations occurred,and dislocations piled up at grain boundaries,which led to higher deformation degree of grain boundaries,giving rise to primarily transgranular fracture with some local intergranular cracks.2E12 alloy has high creep stress exponent according to the investigation of the creep behavior from 100℃to 150℃.The dislocation climb model was applied to explain the creep behavior considering the influence of stress threshold,the value of which decreased with temperature increasing.The creep mechanism,in accordance with incomplete recovery creep,was the diffusion of dislocation tube with the help of stress.The corrosion resistance of 2E12 alloy was superior to that of 2024 in NaCl and EXCO solutions.Pitting was the main corrosion in NaCl solution.In EXCO solution,the corrosion kinetics consisted of formation of pitting,development of pitting and transition of serious pitting to slight exfoliation.EIS was used to explain the interface and micro-corrosion process.The Al₂Cu and Al₇Cu₂（Fe,Mn）in the alloy acted as cathode.The corrosion of S phase includes three stages:during the early stage S was anode,and the content of Mg decreased;S phase changed to be cathode as Mg decreased and Copper increased relatively,meanwhile the surrounding matrix dissolved as anode;finally,S phase disrupted due to disconnection between S phase and the matrix.Under natural aging,grain boundaries had little influence on corrosion behavior due to uniformly-distributed alloying elements in the grains and at grain boundaries.The crack propagation rate of the alloy in humid gas and salt spray is higher than that in the air at room temperature and fatigue fracture surface exhibits brittle characteristics.In corrosive medium,the active hydrogen atom generated by the reaction between corrosive medium and aluminum diffused to the plastic zones,leading to hydrogen embrittlement. Meanwhile the anode dissolution induced by corrosion mitigated the crack growth properties of the alloy.The hydrogen embrittlement susceptibility of the alloy was also studied by electrolytic hydrogen charge.The results showed that long time electrolytic hydrogen charge decreased the ductility,and the hydrogen charge of 14 days led to 12% loss in the ductility.Tensile stress accelerated the diffusion of hydrogen, and the alloy exhibited hydrogen embrittlement in a short time despite the tough characteristic of the fracture surface.In all,the hydrogen embrittlement sensitivity of 2E12 alloy was low.Solution heat treatment promoted the dissolution of coarse phases, but after long time solution there were still many undissolved particles due to high content of Cu and Mg in 2E12 alloy.Promotively -solutionizing induced more dissolution of constituents,decreased fatigue crack growth rate,but resulted in diffusion of alloying elements into the cladding layer.The diffusion of Cu decreased the corrosion resistance of the cladding layer.Therefore,promotively-solutionzing had limited improvement on cladding aluminum alloy,while improved the fatigue properties of aluminum alloy without claddingPre-deformation enhanced the strength but decreased the ductility of 2E12 alloy without significant effect on corrosion properties.The large-gauge T3-plates produced according to aviation standard has strength 15%higher and elongation 2%lower than T4 plates with similar fatigue crack propagation and corrosion resistance.In all,the properties of T3 alloy are superior to that of T4 alloy,and pre-deformation is an effective way to improve the properties of 2E12 aluminum alloy.Artificial aging led to precipitation of S’ phase and increased the strength.More uniform fine hardening precipitates with narrow grain boundary precipitate zone were obtained by thermo-mechanical treatment. Consequently,higher strength and lower ductility were obtained.The formation of grain boundary particles and precipitates-free zone led to a significant decrease of exfoliation corrosion resistance.The influence of first-low and then high temperature aging on fatigue crack growth was studied.It was found that GPB and S" in the under-aged alloy decreased fatigue crack growth rate,therefore improved the fatigue crack growth properties,but led to lower corrosion resistance,which was similar to that of 2024 aluminum alloy.更多还原