Study On The Dependence Of Fatigue Crack Propagation Behavior Of Al-xCu-1.5Li-X Alloy On The Cu Concentration And Microstructures

Al-Li alloys are widely used in the aerospace field due to their outstanding advantages of lower density,higher elastic modulus and excellent specific strength.Therefore,it is a hotspot for researchers to further develop and optimize the process of Al-Li alloys to obtain better mechanical properties.The reliability of most aviation alloy parts depends on their fatigue properties,especially the fatigue crack growth rates.How to balance the relationship between tensile and fatigue properties is very important for the safe application of Al-Li alloys,both of them are closely related to the grain structure and precipitated phase characteristics.At present,researchers have carried out many studies on Al-Li alloys in many fields.However,the microstructure evolution under different rolling and heat treatment processes as well as their influence on the fatigue crack growth performance（i.e.resistance to fatigue crack growth）need to be insightfully studied.Based on these problems,the microstructure evolution of three Al-x Cu-1.5Li-0.4Mg-0.4Zn-0.1Zr alloys with different Cu content（x=1.6,2.3,3.5 wt.%）under different rolling and heat treatment processes are studied in this thesis.The effects of microstructures on the fatigue crack growth performance of these alloys are discussed in detail,and the mechanism of grain microstructures and precipitates characteristics on the fatigue crack initiation and propagation is revealed.The main researches are as follows:（1）The deformation process changes the deformation stored energy of Alloy M（2.3%Cu）sheet after solid solution,which affects its grain structure and fatigue crack growth properties.There are a large number of subgrains in the hot-rolled（HR）alloy plates after solution treatment,and the subgrain boundaries prevent further crack propagation and release the stress of the plastic deformation zone at the crack tip,thereby reducing the fatigue crack growth rate.The recrystallization of cold-rolled（CR）alloy plate after solution treatment results in weak anisotropy in tensile and fatigue crack growth properties,while the fatigue crack growth rate of cold-rolled and solid solution（CRT）alloy plate is higher than that of hot rolled and solid solution（HRT）alloy under the same stress intensity factor.The fatigue resistance of HRT and CRT alloys along L-T orientation is higher than that of T-L orientation due to the grain boundary obstruction effect.（2）The Cu content affects the distribution of secondary phase particles and grain structure after solution treatment,which further influences the fatigue crack growth performances of alloys.The Alloy H（3.5%Cu）with high Cu content has a higher volume fraction of secondary phase particles and higher deformation stored energy,which results in the presence of subgrains with high density and small size in the Alloy H with HRT,and leads to the weak anisotropy in the fatigue crack growth.The fine recrystallized grains in Alloy H with CRT provide more opportunities for crack deflection,and thus reduce the fatigue crack growth rate.In addition,the fatigue crack growth threshold of Alloy H with CRT is lower than that of Alloy L（1.6%Cu）because the secondary phase particles are easy to induce microcracks.（3）The type and proportion of aging precipitates lead to the change of fatigue crack growth performance of Al-Li alloy.δ’and T₁ precipitates are the main precipitated phases of Alloy M with CRT after aging treatment at 150°C.At the early stage of aging,the shearableδ’and T₁ precipitates have a limited effect on the size and damage accumulation of reversible plastic zones（RPZ）at the crack tip,thus causing a lower fatigue crack growth rate of alloy.With the aging time,the quantity and size of non-shearable T₁ precipitates increase,which leads to the continuous accumulation of dislocation in the crack tip region,and promotes the uniform deformation in RPZ and the possibility of crack propagation along grain boundaries,further resulting in the decrease of plasticity and the increase of fatigue crack propagation rate.At the same time,the precipitation of T₁ precipitates at grain boundaries and the appearance of precipitation-free precipitation zones（PFZs）reduced the obstruction effect of grain boundaries,thus obstructing the strain transfer process and leading to the decrease of plasticity.（4）The precipitation behaviors of aging precipitates of Al-Li alloy are greatly affected by Cu content and aging temperature,which affects the strength and fatigue crack growth rate.Compared with Alloy M and H,Alloy L with low Cu content has the lowest strength and fatigue crack growth rate at different aging states due to itsδ’precipitates.With the increase of Cu content,the nucleation rate of T₁ precipitates and GP zone in Alloy H increases significantly,and T₁ phases become the main strengthening phase,which leads to the rapid increase of hardness and fatigue crack growth rate.With increasing aging temperature,the increasing diffusion rate of solute atoms causes the increased nucleation rate and growth rate of T₁,θ’andδ’precipitates.The number and size of T₁precipitates increase with the aging time,resulting in a significant decrease in the fatigue crack growth performance of the alloys with high Cu content.Therefore,Alloy L has better comprehensive properties after higher aging temperatures.(After aging at 170℃for 40 h,the tensile strength and elongation of Alloy L are 427.4 MPa and 9.7%,and the fatigue crack growth rate is about 1.29×10^-4mm/cycle atΔK=15 MPa·m^1/2).