Thermodynamic Investigation Of Phase Diagram Considering Size Effect And Its Application In The Al-Cu-Ag System

The thermal stability and dispersive distribution of nanoscale precipitates are usually the key to obtain the excellent mechanical properties for the age-strengthening alloys.The information of sizedependent phase diagram for nanomaterials could be demonstrated by the thermodynamic investigation of phase diagram considering size effect,which is important to design and develop the novel alloys.CALPHAD(CALculation of PHAse Diagrams)combines the computational simulation with the key experiments to design novel alloys,which can obviously cut down the research and development cycle.What’s more,CALPHAD approach is an important part in the ICME(Integrated Computational Materials Engineering)framework.The reasonable databases are the cornerstone and guarantee of the accurate computational simulation.Since the size dependence on the Gibbs energy is not considered in current multicomponent thermodynamic database,it would cause the deviation in the judgment of thermal stability,then be hard to obtain the nucleation driving force and the results of microstructure evolution simulation during the kinetic precipitation simulation accurately.This work aims to investigate the size-dependent thermodynamic database based on the ICME,then apply to the precipitate microstructure evolution of Al-Cu-Ag system during the aging process:(1)The first-principles calculation combined SQS(Special Quasirandom Structures)method was applied to systematically investigate the molar volume and surface energy of the stable and metastable phases in AlCu-Ag system.The endmember could be reduced and the thermophysical property would be the function of mole fraction by separating the description of thermophysical and thermodynamic model.The databases of molar volume and surface energy for Al-Cu-Ag system were constructed via CALPHAD approach.Considering the influence of excess molar volume and coupling the CALPHAD-type molar volume database,a surface energy calculation code,Be ST-Calc(Butler equation Surface Tension Calculation),was developed.Combined with the reported thermodynamic parameters and the constructed thermophysical database,the molar volume and surface energy of alloy were calculated,which the thermophysical database and the code were validated in comparison with the experimental data.(2)Based on the surface melting of pure metal elements,the effect of solid/liquid interface was taken into account during the melting process.Combined the thermodynamic principle of minimization for Gibbs energy,the calculation code,Cal Na Phad(Calculation of Nano Phase Diagram),was proposed and compiled.Coupling with the input parameters and literature data,the surface melting of macro-crystals and the melting behavior of nanocrystal for pure Al,Cu and Ag were predicted.The surface premelting and melting temperatures of macro-crystals were calculated,respectively.The equilibrium thickness of liquid surface layer would tend to the infinity with the increment of temperature.The corresponding melting behavior and size-dependent phase diagram of nanocrystal were predicted.And with the increment of size,the liquidus and solidus would increase and the liquidus would tend to the bulk melting point.Below the critical size,the solid would melt directly to the liquid while the solid would transform to the solid + liquid two-phase region then melt to the liquid with the particle radius larger than the critical size during the heating process.(3)The molecular dynamics simulation combined various criteria,including potential energy,mean square displacement,gyration radius and radial distribution function,was applied to investigate the melting behavior of Al-Ag and Al-Cu binary fcc＿A1 nanoparticles.And the melting temperature of nanoparticle would decrease with the decrease of size.Considering the solvi of Cu related to the metastable precipitates in Al-Cu binary system from the literature review and the formation enthalpy from the first-principles calculation,the thermodynamic descriptions of the metastable phases,θ" and θ’,were proposed by CALPHAD method.Based on the databases of molar volume and surface energy,the size-dependent thermodynamic description of Cu-Ag,Al-Ag and Al-Cu binary systems was developed by considering the available data from the molecular dynamics simulation and the reported experimental results.With the decrease of size,the size effect on Gibbs energy would gradually increase,and both liquidus and solidus would decrease based on the calculated results by the constructed thermodynamic databases.The eutectic points of L → fcc＿A1(Cu)+ fcc＿A1(Ag)in Cu-Ag system,L → hcp＿A3 + fcc＿A1in Al-Ag system and L → fcc＿A1 + θ in Al-Cu system would move to the Ag-rich,Al-rich and Cu-rich corner,respectively.(4)Based on the above binary thermodynamic databases and the parameters of molar volume and surface energy,the thermodynamic database of Al-Cu-Ag ternary system was established by considering the size effect.The composition of ternary alloy was selected and the corresponding heat treatment process was designed through the analysis of ternary vertical section.Subsequently the hardness and the microstructure evolution of precipitate at different aging time were investigated.Based on the constructed thermodynamic description,kinetic parameters,thermophysical database and key experimental data,the KWN(Kampmann-Wagner Numerical)model was applied to simulate the microstructure evolution of Al-Cu and Al-Cu-Ag system considering the precipitate morphology and aspect ratio.The mean length,number density,volume fraction,yield strength and hardness were predicted during the nucleation,growth and coarsening process.