Study on Microstructure and Properties and Precipitation Behavior of Al-Cu-(Li) Alloy

Author:Fan Li

Supervisor:hao qi tang


Degree Year:2018





With the continuous development of the aerospace industry,the manufacturing of aerospace vehicles has become lighter,more comfortable,more long-life and more reliable than ever before.Cast Al-Cu alloy and the new Al-Cu-Li alloy is considered as the typical high strength aluminum alloy,possessing a series of advantages including low density,high strength,good processing performance,excellent welding performance and cost-effective,has been widely used as military,civilian aircraft,spacecraft structural materials.However,Al-Cu alloys is easy to produce defects and reduce the yield,which have limited its further development.The precipitation behavior of various precipitates in Al-Cu-Li alloy is still unknown and need more research on it.Therefor,how to improve the castability and ensuring the high strength and toughness of Al-Cu alloy is an urgent problem to be solved.In-depth and comprehensive understanding of the relationship between the microstructure and properties of Al-Cu-Li alloys and the precipitation process of the precipitates have important theoretical and practical significance for the design and development of the next generation of Al-Li alloys.In this research,Al-Cu alloy and Al-Cu-Li alloy with high strength and toughness that developed independently in China were selected as the research objects.The primary microstructure and element segregation behavior of Al-Cu alloy were analyzed firstly.The multi-scale interaction of RE on the microstructure and properties of Al-Cu alloy was discussed.Then Al-Cu alloy with non-dendritic structure was prepared by controlled diffusion solidification.The effects of controlled diffusion solidification on solidification microstructure and defects of Al-Cu alloy were discussed.Finally,the microstructral evolution during aging process of Al-Cu-Li alloy were studied by using transmission electron microscopy combined with atom probe tomography,to determine the precipitation sequence of hardening phase.The combinative hardening effect of the precipitations was analyzed and discussed.The precipitation behavior of T1,σ,S phase,and the effect of micro-alloying elements on their precipitation process were investigated.The main findings of this thesis are as follows:(1)It is found that the original as-cast microstructure of Al-Cu alloy mainly consists ofα-Al matrix,Al2Cu eutectic phase,primary Al3Zr and Al3Ti phases.Ageing microstructure mainly consists ofθ′phase,Al20Cu2Mn3 and Al3Zr dispersoids.Elemental segregation tends to occur during the solidification.Cu and Mn are easily segregated at the dendrite and grain boundaries,and Ti and V are easily segregated inside the grains.The segregation extent of alloying elements from strong to weak is as follows:Cu,Mn,V,Ti,Zr.It is revealed that the hardening and toughening mechanism of RE modified Al-Cu alloy:shortening the solidification interval of Al-Cu alloy,reducing the casting defects,refining the grains and eutectic structure,and promoting the precipitation ofθ′phase,improving the toughness of the alloy.RE can react with Ti and V to form Al20(Ti,V)RE phase in the grain,also can react with Cu at the dendritic and grain boundaries to form Al8Cu4RE phase.Excessive addition of RE leads to an increase in the number density of the rare earth containing compounds,which reduce the mechanical properties.The optimum amount of RE is0.3wt.%.(2)It is found that controlled diffusion solidification can significantly improve the solidification structure of Al-Cu alloy,such as reducing the degree of shrinkage,reducing shrinkage defects,reducing hot cracking tendency,reducing the segregation extent of Cu,Ti elements.Conventional gravity casting alloy mainly consist of dendrites,dendrite size decreased with temperature decreasing,while through controlled diffusion solidification the cast alloy can obtain a large number of spherical non-dendritic microstructure during solidification process.It reveals that the mechanism of refinement of Al-Cu alloy by controlled diffusion solidification:a large amount of nucleation particles formed and uniformly distributed in the melt when the alloy1 and the alloy2 are mixed under convection.The controlled diffusion solidification must satisfies the requirement as below:(i)the temperature of the two precursor alloy must be close to the respective liquidus temperature before mixing,(ii)the Gibbs free energy of the alloy1 and the alloye2 must be less than the Gibbs free energy of the target alloy at the liquidus temperature,(iii)the solid-liquid interface energy of the pure solid phase and the liquid phase must be less than the solid-liquid interface energy of the alloyed state.(3)It is found that a large number of dispersed particles Al20Cu2Mn3 and Al3Zr are distributed in Al-Cu-Li alloy,which work together to ensure an elongated pancake-like non-recrystallized grain structure along the rolling direction after solid solution treatment and quenching.The microstructural evolution of Al-Cu-Li alloy aged at 165℃can be summarized as:SSS→GP zones+δ′→GP zones+δ′+T1+GPB zones→δ′+T1+θ′+S+σ→T1+S+σ.Ageing process involves several kinds of precipitation sequence,includingδ′,θ′,S,T1 andσphase.The experimental results of APT show that Li,Mg and Zn atoms prefer to segregate on theβ′phase,while the Mg and Zn atoms prefer to segregate on the T1 phase.The hardness curve of Al-Cu-Li alloy appeared regression phenomenon during early ageing stage.The main hardening phases of Al-Cu-Li alloy at peak aging stage are T1 phase andθ′phase,accompanied by a small amount ofδ’,S andσphase.The yield strength,tensile strength and elongation ais 570MPa,615MPa and 7.5%,respectively.The fracture mode is mainly intergranular fracture and transgranular fracture.The hardening mechanism of Al-Cu-Li alloy is revealed that the dispersed phases Al20Cu2Mn3 and Al3Zr,and the co-existence hardening precipitations like T1,θ′,δ′,S andσformed during ageing process play a comprehensive effect on the maniacal properties.(4)The precipitation behavior of T1 phase during ageing was revealed:(i)in the early aging stage(2h),the T1 phase preferentially nucleates at the tangles and kinks at the dislocation,which is the main way of early heterogeneous nucleation.The gradual precipitation of T1 phase at the dislocation coincides with the trend of hardness value.(ii)T1 phase is susceptible to nucleate at the coherent and semi-coherent Al3Zr interface,where the segregation of Mg-Zn clusters at the T1/Al3Zr interface.T1 phase can heterogeneous nucleates at the interface of Al20Cu2Mn3 phase with a preferential orientation of the variants.(iii)T1 phase tends to precipitate densely on the subgrain boundaries,possessing the smaller the angle between the T1 phase and the grain boundaries.The average thickness of the T1 phase precipitated in the subgrain boundary is smaller than that of the adjacent region T1.(vi)In the natural aging process of T351,the matrix is still in a high supersaturated state.T1 phase can nucleate at the GP zone and continue to grow in a manner of consuming the GP zone and theδ’phase.APT analysis show that the composition of T1 phase is about 50.6%Al,22.7%Cu,18.8%Li,5.5%Mg and 2.4%Zn(at.%).Mg and Zn are enriched in the T1 phase and segregates at the interface of T1/Al.In the solution quenching state,Mg-Cu and Mg-Zn atoms segregate,atomic clusters have been formed.The co-addition of microalloying elements Mg and Zn promoted the massive non-uniform nucleation of T1 phase and accelerated the aging process.The rapid formation of Mg-Zn and Mg-Cu clusters during early aging is an important factor affecting the heterogeneous nucleation of T1 phase.(5)Two variants ofσphase were found.The orientation relationship betweenσphase and Al is shown as:σⅠ,<100>σ//<100>Al,{100}σ//{100}Al;σⅡ,<001>σ//<110>Al,{100}σ//{100}Al.An ordered solute enrichment region that denoted as{110}All was found,which is different from the traditional GPB region structure.σⅠphase tends to nucleate on the T1 phase that enriched in Mg,Cu,Li and Zn,andσⅡphase tends to nucleate and grow through{110}All ordered clusters enriched in Mg,Cu,Li and vacancies.The S phase can nucleate at{110}All ordered clusters.In this research the precipitation sequence of S phase can be summarized as below:SSS→Clusters→GPB zones→S→GPB zones+{110}All ordered clusters→S The{110}All ordered clusters enriched with Mg-Cu-Li-vacancy can be used as the direct or indirect nucleation sites for the S phase and theσⅡphase.Whether or not{110}All ordered clusters transfer to S phase orσⅡphase is determined by the degree of ordering of Mg and Cu atoms and the compositional difference in such clusters.