Study On Damage Behavior And Anti-damage Microstructures Of 2E12 Aluminum Alloy Under Service Environments

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.