Study On Microstructural Evolution And Mechanical Behaviors Of Cast Al-2Li-2Cu Alloys

Aluminium-Lithium alloys have been considered as ideal structural materials for newly aircraft,aerospace and national defense applications owing to its low density,high specific strength/stiffness,high fatigue/corrosion resistance,and other exceptional properties.Significant weight reduction(10?20%)and stiffness enhancements(10?20%)could be achieved via replacing the traditional Al materials with Al-Li alloys.The previously published works focus mainly on the wrought Al-Li alloys,with which complex structures are difficult to be formed.It is noticeable that large-scale complex structures could only be prepared by cast Al-Li alloys.Moreover,it is a feasible approach to prepare novel lightweight structures.However,in the published domestic and foreign works,no commercial brand or material standard could be found.There is a relatively small body of literature that is concerned with cast Al-Li-Cu alloys,the strength and ductility of which are usually at a low level and cannot meet the practical need of engineering implementations.The effects of common micro-alloying elements on the micro structure and mechanical properties of cast Al-Li alloy have not been closely investigated,and currently adopted heat treatment regimes still require further optimization.The development of highly efficient approach to improve the performance and microstructural stability of cast Al-Li alloys has been the key difficulties in the current research and is urgent to be resolved.Hence,it is of highly significance to undertake in-depth investigations on cast Al-Li alloys with high performance.This work aim to offer deep insight into the influence of micro-alloying additions on the microstructural evolution,mechanical properties and microstructural stability of cast Al-Li-Cu alloys.Furthermore,designing of novel heat treatment regimes is also carried out to explore the cooperative regulation mechanism of alloying design and heat treatment on the performance of cast Al-Li alloys,contributing to designing of novel alloys with high performance and expansion of its application in large-scale and complex structural parts.In view of the current research status of cast Al-Li alloys,cast Al-2Li-2Cu-0.2Zr alloy has been chosen as the base alloy in present work.Comparative investigations are carried out to figure out the effects of several micro-alloying additions(Mg/Ag/Zn/Mn/Y/Sc)on the micro structure and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy.Influences of Mg and Sc contents on the alloy microstructure,competitive precipitation behavior,microstructural evolution and mechanical properties were investigated in detail.The main conclusions obtained in this dissertation are as follows:The role of minor Mg/Ag/Zn/Mn/Y addition on the microstructural evolution and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy are closely discussed.The largest enhancements of strength and ductility is obtained in the alloy doped with minor Mg,which is much higher than that derived from Ag or Zn addition.Zn addition is equivalent to the role of Ag in assisting T1 precipitation.Mn addition is capable of improving the alloy ductility via formation of Al₂₀Cu₂Mn₃ dispersoids,which will decrease the actual concentration of Cu atoms and thereby reduce the alloy strength.Y addition could strongly suppress the precipitation of T1 and ?' and coarsening of ?',contributing to substantial decrease of alloy strength in T6 temper.This work present the detailed microstructural evolution of Al-2Li-2Cu series alloys during heat treatment.Then the author try to illustrate the formation of composite precipitates(Al₃(Li,Zr)and ?'/?')and T1/?' interactions via establishing qualitative models.As the ageing proceeds,the Al₃Zr and ?' phases could serve as preferable nucleation sites for ?',leading to the interfacial energy decreased from (?),where (?).Besides,the formation of these composite precipitates could reduce the strain energy change associated with lattice misfit,contributing to improved precipitation of ?'.When a T1 phase comes through a?' phase,the interfacial energies between them will reach a new equilibrium state of(?),resulting in the offset or partial dissolution of ?'.Thus spherical caps of ?' phases could be observed.Influences of Mg content on the competitive precipitation behavior and ambient mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy are investigated.The optimized addition of Mg content is determined.The formation and coarsening behavior of S'laths are detailedly discussed.Furthermore,this work also illustrates the effects of minor Mg addition on the coarsening or widening behavior of ?'-PFZs and quantitative model is established.The optimal Mg addition in cast Al-2Li-2Cu-0.2Zr alloy is 0.5 wt%.The relatively best combination of strength and ductility is obtained in 0.5Mg alloy ageing at 175? for 32 h(YS=366 MPa,UTS=443 MPa,EL=4.5%).Precipitation of ?'(Al2Cu)phase will be almost suppressed and S'(Al2CuMg)phase is consequently introduced due to Mg additions.As the Mg content increases,the S' precipitation is encouraged at the expense of T1 precipitation.The S' phase will grow preferentially as laths on specific matrix plane groups in[100]? direction,finally resulting in the formation of corrugated sheets on(110)planes.The uneven distribution of coarse S' sheets could strongly decrease the alloy strength and ductility.The growth of ?'-PFZs is assumed to follow a parabolic growth law expressed by (?).The ?'-PFZs growth rate constant Kp could be significantly decreased with 0.5%Mg addition,contributing to remarkable ductility enhancement in T6 state.The author further investigates the specific role of Sc content on the phase constitution,microstructures and mechanical properties of Al-2Li-2Cu-0.5Mg alloy.The existence of element Sc is confirmed during heat treatment.The optimized Sc addition is 0.2 wt%in Al-2Li-2Cu-0.5Mg-ySc-0.2Zr alloys.The relatively best combination of strength and ductility is obtained in 0.2Sc alloy ageing at 175? for 32 h(YS=408 MPa,UTS=480 MPa,EL=6.0%).No evidence for the presence of the ternary W(Al8-xCu4+xSc)phase is obtained in as-cast,as-quenched and aged Al-2Li-2Cu-0.5Mg-ySc alloys.It was found that the grain size of as-cast alloy was significantly reduced with the continuous increase of Sc addition,accompanied by the arising and increasing of cubic primary Al3(Sc,Zr)phases.Nanoscaled Al3(Sc,Zr)dispersoids will act as favorable nucleation sites for ?' during ageing,leading to the formation of core-double shell Al3(Li,Sc,Zr)composite precipitates.The addition of 0.2%Sc is also observed to refine the S' morphology and improve its number density.0.2%Sc addition is confirmed to enhance the precipitation of S' and T1 phases and contributes to the further retarded growth rate of ?'-PFZs,resulting in the improved alloy ductility.Investigations have been accomplished on the role of minor Mg and/or Sc addition on the micro structural stability and related mechanical property changes of Al-2Li-2Cu-0.2Zr alloys.The thermal stability differences of different precipitates are determined.This work provides the first comprehensive assessment of the coarsening behavior of ?' phase in cast Al-Li alloys.Quantitative model for ?' coarsening is established and the actual activation energy required for its coarsening is also obtained.It is found that,T1 and Al3(Li,M)composite precipitates own the highest thermal stability,which is higher than that of S' phase,and the thermal stability of ?'phase is lowest.The precipitation,growth and dissolution of ?' phase will be strongly accelerated as the ageing temperature increases.The growth of ?' size is assumed to follow a cubic root law(t1/3)with the ageing times.Independent minor Mg or Sc addition could both significantly reduce the growth rate constant of ?',and the relatively lowest ?' coarsening rate will be obtained in the alloy with combined Mg and Sc addition.Cast Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr alloy presents the highest micro structural stability during heat treatment.The temperature dependence of ?'coarsening rate constant could be expressed through the relation of (?).The actual activation energy Q for ?' coarsening in cast Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr alloy is estimated to be about 98.7 kJ/mol.Based on the above theoretical investigations,a novel lightweight cast Al-Li alloy with high performance is successfully developed,of which the chemical composition is Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr.Optimized heat treatment regimes is also obtained in this work(460? � 32 h+520? � 24 h).The best mechanical properties achieved in T6 alloy is as follows:YS=408 MPa,UTS=480 MPa,EL=6.0%.Cast Al-2Li-2Cu-0.5Mg-0.2Sc-0.2Zr alloy owns high micro structural stability.The density and elastic modulus of the alloy is evaluated to be approximate 2.57 g/cm3 and 79 GPa.