Study On The Stability Of Phases And The First Principle In Nickel-Based Superalloys

In this paper,the stability and mechanical properties of the phases in nickel-based alloys are investigated by means of the first principle and special quasi-random structures method The parameters,total energy,heat of formation,binding energy,density of state,electron population,and relative mechanical parameters are calculated,by establishing the models of Nb,Fe,Mo doping in the γ’-Ni3Al,γ"-Ni3(Al,Nb),δ-Ni3Nb,and α-Cr phases,to investigate the effects of doping concentration on the stability and mechanical properties of the phases The properties and effect factors of γ"-Ni3Nb→δ-Ni3Nb phase transition are investigated by means of calculating the total energy,entropy,Gibbs free energy,phononic properties and other thermodynamic data of v"-Ni3Nb and δ-Ni3Nb phases.By establishing supercell modeling of the doped a-Cr phase,the total energy,electronic and mechanical properties of the models are calculated to investigate the effect of Fe,Mo doping on the mechanical properties of a-Cr phase.The main conclusions are given as followsThe calculated results indicate that the calculated data of using GGA-PW91 functional theory are similar to the experimental values.The process of γ’→γ" phase transition is related to Nb doping concentration of the models.The γ-Ni3Al phase in the system displays a good stability when the doping Nb concentration is lower than 15.625 at%,while the γ"-Ni3Nb phase in the system may be stably existed when the Nb concentration exceeds 18.75 at%.The stability of γ"-Ni3Al phase is slightly higher than that of γ’-Ni3Al phase when the Nb concentration is located in the range of 15.625-18.75 at%,and the γ’,γ" phases may be coexisted in the system to occur the γ’→γ" phase transition.Wherein,the close lattice parameters of two phases are thought to be a necessary condition for occurring γ’→γ" phase transition under coherent.The Nb atoms are precipitated from the y matrix,as the time of heat treatment prolongs,to promote the growth of the γ"-Ni3Nb phase and the decomposition of theγ’-Ni3Al phase,which is thought to be the main reasons for increasing the amount of γ"-Ni3Nb phase and decreasing the one of γ’-Ni3Al phase in the alloy.The calculated energy-volume curves,entropy and Gibbs free energy of γ",δ phases possess a similar feature,wherein the similar thermodynamic functions,such as Debye temperature,heat capacity,and Gruneisen constant,are attributed to the similar energy-volume curves.While the shared mechanical,kinetic and thermodynamic stability of γ",δphases is thought to be the main reason of the γ",δ phases being balanced coexisted.In particular,the first-order partial derivatives of the chemical potential of the γ",δ phases occurs the continuous change,while the second-order partial derivatives of the chemical potential of the ones(such as isothermal compressibility,heat capacity at constant pressure and linear expansion coefficient)occurs the sudden change,when the occurring γ"→δ phase transition under the isothermal or isobaric conditions,which is thought to be the thermodynamic basis ofγ"→δ being identified as the second-order phase transition.And the relationship of the y" andδ phases structure having a subgroup and parent group is determined,which is accord with the path of second-order phase transition.The calculated mechanical parameters of γ’,γ",δ andα-Cr phases indicate that all of the phases display the ductile feature,and the γ"-Ni3Nb phase have the higher bulk modulus and shear modulus,which exhibits the stronger coherent strain hardening effect Wherein,the γ" phase is a main strengthening phase in the alloy.followed by the δ and γ’ phases,compared to the δ phase,the γ" is a meta-stable phaseThe precipitated α-Cr phase in alloy contains Fe and Mo elements.By using first principle and establishing the supercell doping models,the single and mutual doping behavior of Fe,Mo atoms in the α-Cr phase was explored,which indicates that the Fe doping diminishes the lattice parameters of α-Cr phase,the Mo doping increases the lattice parameters of one.Moreover,the stability of Mo doping α-Cr phase is higher than the one of Fe dopingα-Cr phase.Wherein,Fe doping can strengthen the α-Cr phase,while the strengthening role of Mo doping α-Cr phase is weaker,and the Fe and Mo doped α-Cr phase has a certain strengthening effect in the alloy.