First-Principles Study Of The Structure And Physical Properties Of Nickel-Aluminum Multi-Entropy Alloys

Ni-based superalloys have been widely used in the manufacture of aeroengine hot end components owing to their excellent mechanical properties,oxidation and creep resistances.Medium-high entropy alloys（M-HEAs）have potential applications in many fields because they exhibit high hardness,high strength and good corrosion resistances.At present,research on nickel-aluminum multi-entropy alloys is mainly the experimental investigation about design and preparation,microstructure and mechanical properties of alloys.However,these studies focus on the macro scale.The electron-atomic scale is urgently needed to explain the mechanism of its dominant mechanical properties and the dependence of thermodynamic properties with temperature.Therefore,it is necessary to further study the structure and physical properties of nickel-aluminum multi-entropy alloys at the nanoscale.In this paper,the first-principles method based on density functional theory（DFT）is used to study the structure and elastic mechanical properties of nickel-aluminum binary/ternary ordered intermetallic compounds.And the structure,stability and elastic properties of nickel-aluminum binary,quaternary and quaternary disordered solid solution alloys are investigated.Moreover,thermodynamic properties of ordered/disordered alloys at finite temperatures are studied by combining with thermodynamic Debye model.The dominant mechanism of mechanical properties and the thermodynamic response of nickel-aluminum multi-entropy alloys are explained from the electron-atomic scale.It can provide theoretical guidance for the design of new Ni-based superalloys and M-HEAs,and contribute to the material genome initiative.The main research contents are concluded as follows:（1）The effects of pressure on the structure,elastic and thermodynamic properties of Ni-Al binary ordered intermetallic compounds（Ni₃Al,Ni₅Al₃,NiAl,Ni₂Al₃ and NiAl₃）are investigated.The five Ni-Al intermetallic compounds are mechanically stable in 0⁵0GPa.The volume change resistance of Ni-Al alloys can be improved by increasing properly pressure and Ni content.The ductility of Ni₃Al,Ni₅Al₃ and NiAl increases with increasing pressure.The hardness of Ni₅Al₃,NiAl and Ni₂Al₃ increases with the increase of pressure.The anisotropy of the five compounds also increases with pressure going up.At the same temperature,the bulk modulus and debye temperature of the five compounds increase with the increase of pressure,while the linear thermal expansion coefficient and heat capacity decrease with increasing pressure.And explain its stability and physical nature from the perspective of electronic structure.（2）The enthalpy,elastic and thermodynamic properties of paramagnetic Heusler compounds Ni₂XAl（X=Sc,Ti,V）are studied,and the effects of different pressure on its properties are investigated.Paramagnetic Ni₂XAl is mechanically stable from 0 to 50 GPa and exhibits ductility and anisotropy.The elastic modulus,hardness and ductility of Ni₂XAl increase with increasing pressure.The effect of pressure on elastic modulus and hardness decreases with increasing atomic number X（Sc,Ti,V）,while the impact on ductility and anisotropy increases with increasing atomic number X.The volume change resistance and thermal conductivity of Ni₂XAl decrease with increasing temperature,while the constant pressure heat capacity and thermal expansion coefficient increase with the increase of temperature.However,the effect of pressure on these thermodynamic properties is opposite to the temperature.It also understands its physical nature from valence electron orbital contributions and atomic bonding.（3）The structure,stability,magnetic properties,tetragonal distortion,thermodynamics and electronic structure of ferromagnetic Heusler compounds Ni₂XAl（X=Co,Cr,Fe）are investigated.Cubic Ni₂CrAl is mechanically stable at 0 GPa,while cubic Ni₂FeAl and Ni₂CoAl are mechanically unstable.Ni₂CrAl,which is the most stable in L2₁ phase,has no shape memory effect.However,it may lead to martensitic transformation by adjusting the stoichiometric ratio of Ni₂CrAl.Ni₂FeAl and Ni₂CoAl have austenite-to-martensite transformation existence by applying external magnetic field and temperature,i.e.,they have shape memory effect.The volume,Debye temperature and thermal expansion coefficient of ferromagnetic Ni₂XAl decrease as increasing atomic number X,while the constant volume heat capacity increases with the increase of X.The electronic density of states is analyzed to clarify the physical nature of magnetism and phase transition of Ni₂XAl.（4）The stability,elasticity and thermodynamics of the disordered BCC and FCC Ni_1-xAl_x（x=0.25,0.50,0.75）binary low-entropy alloys are studied,which is compared with ordered L2₁ Ni₃Al and B2 NiAl phases.Ni_1-xAl_x are mechanically stable,and FCC Ni_1-xAl_x is more stable than BCC Ni_1-xAl_x.Disordered Ni_1-xAl_x transform into ordered phase,it is most likely to precipitate the L2₁ Ni₃Al phase.The resistance to volume change,shear deformation and elastic deformation of Ni_1-xAl_x decrease with increasing Al content.The lattice constants and thermal expansion coefficient of Ni_1-xAl_x increase with the increasing Al content,nevertheless it is decreasing for bulk modulus and constant volume heat capacity.The thermal expansion coefficient,vibrational heat capacity and vibrational entropy of disordered Ni_1-xAl_x are larger than those of ordered L2₁ Ni₃Al and B2 NiAl phases.The density of states is analyzed to clarify the physical nature of the stability,mechanical and thermodynamic properties of Ni_1-xAl_x.（5）The special quasi-random structures（SQSs）technique are applied to model AlCrFeNi medium entropy alloys.The effect of BCC and FCC structures with ferromagnetic（FM）and nonmagnetic（NM）states on phase stability,elastic and thermodynamic properties of AlCrFeNi are investigated.FM BCC AlCrFeNi is the most stable and has the greatest anisotropy.FM AlCrFeNi is more stable than NM AlCrFeNi,but the latter is more resistance to volume change,shear and elastic deformation than the former.FM BCC AlCrFeNi has the largest constant volume heat capacity and vibrational entropy in 0¹200K,and its Debye temperature is the lowest.Electronic entropy of AlCrFeNi strongly depends on its crystal structure and magnetic states.Electronic entropy cannot be negligible although its contribution to total entropy is small.The electronic structure is analyzed to reveal the physical nature of the influence of crystal structure and magnetic states on physical properties for AlCrFeNi.（6）The formation rule,phase stability,elasticity and thermodynamics of AlCoCrFeNi high entropy alloy are studied,which is compared with AlCrFeNi medium entropy alloys.The formation enthalpy of AlCoCrFeNi is lower than that of AlCrFeNi.Its entropy and Helmholtz free energy are lower than that of AlCrFeNi.The resistance to volume change,shear and elastic deformation of AlCoCrFeNi are stronger than that of AlCrFeNi.The anisotropy of AlCoCrFeNi is smaller than that of AlCrFeNi.It clarifies that the addition of Co is beneficial to improve the stability and elasticity,and reduce the anisotropy of the alloys.The lattice constants,thermal expansion coefficient,constant volume heat capacity,electronic heat capacity,entropy,vibrational entropy and electronic entropy of AlCoCrFeNi increase with the increase of temperature.However,its Helmholtz free energies,vibrational Helmholtz free energies and electronic Helmholtz free energies decrease with increasing temperature.In addition,electronic structure of AlCoCrFeNi is studied to explain its bonding and physical origin.