The Microscopic Phase-field Model And First-principle Study On The Precipitation Process And Properties Of Al₃Li Phase In Al-Li Alloy

Lightweight structural materials such as the Al-Li based alloys have been widely used in the aviation and aerospace field due to their excellent comprehensive performance,high strength,high stiffness,low density and fatigue crack resistance.These exceptional mechanical properties of Al-Li based alloys are due to the existence of metastable Al₃Li????precipitates,and the structure and properties of Al₃Li phase will have an important effect on the properties of Al-Li alloy.Pressure and point defects?intrinsic point defects and doping?can deteriorate or improve the properties of Al₃Li phase.In addition,the precipitation process of Al₃Li phase in Al-Li alloy and the interfacial properties of Al/Al₃Li can provide theoretical basis for experimental analysis and design.Therefore,the precipitation process of Al3Li,the interface properties of Al/Al₃Li and the changes of the properties of Al₃Li phases play an important role in the development of high performance Al-Li alloys.It is very difficult to explore the influence of these factors only by means of experiments.In this paper,the relevant research has been carried out based on the microscopic phase field theory and density functional theory.The specific research contents are as follows:?1?The effect of elastic strain energy on the precipitation process of binary Al-Li alloy was investigated based on the microscopic phase field model.The results show that the existence of elastic strain energy only changes the morphology of the ordered Al₃Li phase in instable region?Al-15.2at.%Li?and makes it have certain directionality,but it does not change the precipitation mechanism of the ordered Al3Li phase.There is an interface between ordered particles and disordered regions in intermediate region?Al-11.3at.%Li?with about 2-3atomic layer thickness,which indicates that the alloy follows the non-classical nucleation and growth mechanism.In addition,when the time step is more than 600,the elastic strain energy plays an important role in restraining the ordering of Al3Li during aging of Al-11.3at.%Li alloy.?2?The phase stability,elastic anisotropy,mechanical and thermodynamic properties of the Al-Li phases?Al₃Li?AlLi?Al₂Li₃?AlLi₂?Al₄Li₉?have been systematically investigated by first-principles methods.The Al₃Li?AlLi?Al₂Li₃ and Al₄Li₉ phases are thermodynamically stable,while the AlLi undergo phase transition when the pressure is bigger than 10 GPa.Almost all the C_ij(excluding C₁₄,C₁₅,C₂₅,C₃₅ and C₄₆)of Al-Li phases increase with the applied pressure.All the Al-Li phases tend to be brittle at 0 K and 0 GPa.The high pressures have beneficial effect improving the ductility of Al-Li phases.The variation trend of vibration entropy S,heat capacity C_v and C_p with temperature and pressure are basically the same.?3?The phase stability,elastic moduli,phonon velocity,Debye temperature and thermal conductivity of intrinsic point defects?vacancy and anti-site defects?of Al3Li at different concentrations are discussed based on the first principle methods.The dominant defects in Li-rich and Al-rich regions for off-stoichiometric Al₃Li phases are Li_Al and Al_Li,respectively.Compared with 3×3×3 supercell,the formation energy of LiAl in 2×2×2 supercell is negative,which means that Li_Al defect can form spontaneously in 2×2×2 supercell.The elastic properties of Al3Li phase are affected by the type and concentration of intrinsic point defects,but the effects are small.The velocity of longitudinal waves of all non-stoichiometric Al-Li phases are larger than those of transverse waves,and the anisotropy of sound velocity also reflects the elastic anisotropy of non-stoichiometric phases.The concentration of Al_Li and Li_Al are highest in Al-rich region and Li-rich region of non-stoichiometric Al₃Li phases,respectively,which are consistent with the calculation results of point defect formation energy.In AlLi-X systems,AlLi-Zn has the weakest ability to the resistance to volume change.?4?The elastic properties,hardness,anisotropy and Debye temperature of alloying elements X?X=Sc?Ti?Cu?Zn?Zr and Ag?doped Al₃Li were systematically investigated by first-principles methods.The Sc,Ti and Zr are preferentially occupying the Li sites in Al₃Li while the Ag,Cu and Zn prefer to occupy the Al sites.The values of bulk modulus B for AlLi-Zn decrease with the increase of doping concentration,while other AlLi-X phases increase with the increase of doping concentration.For Al Li-X?X=Sc,Ti and Zr?which tends to occupy the Li site,the increase of doping concentration is beneficial to the hardness of Al₃Li phase,while for Al Li-X?X=Cu,Zn and Ag?which tends to occupy the Al site,the doping concentration plays a completely opposite role.The shear anisotropy of AlLi-X phase is different,but the degree of deviation from 1 is not more than 0.2.Al₁₂Li₃Ti has the highest Debye temperature,while Al11Li4Ag has the lowest Debye temperature,which indicates that Al₁₂Li₃Ti has relatively strong interatomic binding force in AlLi-X systems.?5?Considering different terminations and stacking sequences,six Al?100?/Al₃Li?100?and three Al?111?/Al3Li?111?interface models were constructed.The adhesion work and interface energy of Al?100?/Al₃Li?100?and Al?111?/Al₃Li?111?are different in different stacking sequences.The adhesion work of the top-site in Al?111?/Al₃Li?111?is the smallest,indicating that the bonding strength of the interface is the lowest.In the Al?100?/Al₃Li?100?interface models,the change of adhesion work of Al-and Al+Li-terminals with stacking sequence is similar.The order of adhesion work from large to small is:center-site>bridge-site>top-site.It is difficult for Al to nucleate at top-site of Al?111?/Al₃Li?111?due to the high interface energy of this model.The interface energy of the center-site in Al?100?/Al₃Li?100?is the lowest,which makes it easier for Al to form nucleus on the center-site Al₃Li?100?.