Study On Localized Corrosion Behaviour And Microstructure Of Al-Cu-Li Alloys

As one of the most crucial lightweight structural materials used in aerospace industry,Al-Cu-Li alloys have many advantages including high strength,high toughness and low density.Due to the addition of Cu,Li and other elements,a variety of strengthening precipitates such as Al₂Cu Li（T₁）precipitate can be formed during the ageing process of Al-Cu-Li alloys,making this type of alloys show excellent mechanical properties.However,T₁ and other precipitates contain Cu,Li and other microalloying elements,thus their corrosion potential is different from that of the matrix,which causes Al-Cu-Li alloys to be prone to localized corrosion during service and adversely affects their corrosion resistance.Therefore,it is necessary to deeply study the localized corrosion behaviour of Al-Cu-Li alloys and its relationship with microstructure.In addition,due to the limitations of characterization techniques in the past,lots of nano-scale localized corrosion behaviour and corrosion mechanisms have not been thoroughly studied.Through standard intergranular corrosion experiment,quasi-in-situ immersion corrosion experiment and electrochemical corrosion experiment,combined with advanced micro and nano characterization techniques including scanning electron microscope,X-ray energy dispersive spectrometer,electron backscatter diffraction technique,transmission electron microscope and tilting series of three-dimensional electron tomography technique,multi-scale characterization of the corrosion morphology in three Al-Cu-Li alloys（Al-4.03Cu-0.91Li alloy with high Cu and low Li,Al-2.23Cu-0.96Li alloy with low Cu and low Li,and Al-3.14Cu-1.54Li alloy with medium Cu and medium Li）was performed to try to reveal the relationship between the localized corrosion behaviour and the microstructure of the alloys.By changing the chemical composition and thermal treatment process of Al-Cu-Li alloys,the localized corrosion behaviour of Al-Cu-Li alloys with different microstructure has been explored in this thesis.The main research contents are as follows:（1）By adjusting the elemental content and the ageing process of the Al-Cu-Li alloys to accurately tailor the type of the major strengthening precipitates within grains and the morphology,size and distribution of the grain-boundary precipitates,the influence of the microstructure on the localized corrosion behaviour and corrosion properties in the Al-Cu-Li alloys was systematically studied,revealing the relationship between the intergranular corrosion behaviour and grain-boundary precipitates as well as the precipitate-free zones of the alloys.According to the investigation results,the Al-2.23Cu-0.96Li alloy with low Cu and low Li has low hardness,consistent corrosion mode and good corrosion resistance because its main strengthening precipitates within grains are GPB（Guinier-Preston-Bagaryatsky）zones and grain-boundary precipitates are coarse and discontinuously distributed.In contrast,the Al-4.03Cu-0.91Li alloy with high Cu and low Li has high hardness,varying corrosion mode and poor corrosion resistance because its main strengthening precipitates are T₁ precipitates andδ′/θ′/δ′composite precipitates,and the microstructure at grain boundaries gets changing with the increasing ageing time.In addition,the intergranular corrosion behaviour of the Al-2.23Cu-0.96Li alloy with low Cu and low Li is not only related to the type,size and distribution of its grain-boundary precipitates,but also to the width of the precipitate-free zones.（2）The localized corrosion behaviour and its relationship with the microstructure in the Al-2.23Cu-0.96Li alloy with GPB zones as its main major strengthening precipitates were investigated in depth,and the corrosion morphologies of the alloy were finely characterized from micrometre scale and nanometre scale.The localized corrosion behaviours of the Al-2.23Cu-0.96Li alloy mainly are pitting corrosion,intergranular corrosion and crystallographic corrosion,where the pitting corrosion is associated with the Al₇Cu₂Fe particles,and the crystallographic corrosion is associated with the T₁ precipitates and GPB zones within grains.According to the characterization results,the crystallographic corrosion is made up of many step-like{100}_Al-oriented nano-scale sidewalls and propagates along the{111}_Al planes within grains.The formation of the nano-scale sidewalls is closely associated with the dissolution of<100>_Al-oriented GPB zones.（3）Through the introduction of natural aging pretreatment before artificial aging,the effects of one-step ageing（T6）and two-step ageing（natural ageing+artificial ageing）processes on the localized corrosion behaviour and microstructure of the Al-4.03Cu-0.91Li alloy with high Cu and low Li and Al-3.14Cu-1.54Li alloy with medium Cu and medium Li were comparatively studied,revealing that natural ageing pretreatment may weaken the corrosion resistance of Al-4.03Cu-0.91Li alloy but improve the corrosion resistance of Al-3.14Cu-1.54Li alloy.Al-4.03Cu-0.91Li alloy with high Cu and low Li may achieve sufficient precipitation strengthening effect after T6 peak ageing treatment and obtain higher hardness,meanwhile due to the discontinuously distributed coarse grain-boundary precipitates,its intergranular corrosion sensitivity is relatively low.The introduction of natural ageing pretreatment promotes the continuous distribution of the precipitates at grain boundaries in the alloy,resulting in the deterioration of the intergranular corrosion susceptibility and corrosion resistance of the two-step ageing Al-4.03Cu-0.91Li alloy.In contrast,Al-3.14Cu-1.54Li alloy with medium Cu and medium Li may not achieve sufficient precipitation strengthening effect after T6 peak ageing treatment,resulting in lower hardness of the alloy.At the same time,due to the continuous distribution of grain boundary precipitation phases,its intergranular corrosion susceptibility is relatively high.The introduction of natural ageing pretreatment may promote the precipitation of the intragranular T₁ precipitates in the Al-3.14Cu-1.54Li alloy,and at the same time interrupt the continuous distribution of the precipitates at grain boundaries,resulting in its increased hardness,reduced intergranular corrosion susceptibility,and enhanced corrosion resistance.（4）Through quasi-in-situ corrosion experiment and advanced characterization technologies,the corrosion morphologies of the Al-3.14Cu-1.54Li alloy were observed at the nanometre scale,and the corrosion mechanism of grain-boundary T₁ precipitates and intragranular T₁ precipitates was studies in detail.The results show that during the corrosion process,the slender plate-like T₁ precipitates that are continuously distributed at the grain boundaries preferentially dissolve completely compared to the surrounding Al matrix,forming the Cu films at the interface of the corrosion channels,and the coarse particle-like T₁ precipitates that are discontinuously distributed at the grain boundaries preferentially undergo the dealloying-driven selective dissolution compared to the surrounding Al matrix,forming the porous Cu-rich T₁-phase remnants in the corrosion channels.However,the nanoscale T₁ precipitates that are homogeneously distributed within the grains do not preferentially dissolve compared to the surrounding Al matrix,but can rapidly transform into the Cu-rich T₁-phase remnants at the early stage of the corrosion process,which prevents them from being attacked by the corrosion and thus maintains the plate-like morphology characteristic,at the same time,affects the intragranular corrosion morphology of the alloy at the nanoscale,making itself the sidewall of the corrosion channel parallel to the{111}_Alplane.In this thesis,by studying the localized corrosion behaviour and microstructure of Al-Cu-Li alloys in detail,the complex influence of precipitates on the localized corrosion behaviour of the alloys was clarified,the corrosion mechanism conversion of the T₁ precipitate was further revealed,and a clear relationship between macroscopic corrosion resistance with the microstructure of the grain-boundary region in the Al-Cu-Li alloys was established,to provide theoretical guidance for the preparation of Al-Cu-Li alloys with excellent comprehensive properties.