First-principles Calculation Of The Effect Of Alloying On Properties Of NiAl Intermetallic Compounds

As an aeronautical high temperature structural material,NiAl alloy has excellent properties such as high strength,high melting point,wide range of chemical composition change,and good corrosion resistance.However,NiAl exhibits brittleness at room temperature,which limits its practical application range.At the same time,with the development of multi-element alloying and the limitation of experimental conditions,it is still very important to explore the influence of alloying elements on the microstructure and properties of NiAl alloy,and the study on the improvement of plastic toughness and intrinsic properties of NiAl alloy needs to be further discovered.Alloying is an effective method to improve the properties of materials,and combined with the first principles,the various properties of NiAl intermetallic compounds can be studied from the atomic structure level.Therefore,in this paper,density functional theory is used to study the effects of Zr,Hf,Nb,Ta,Cr,Mo,w,Re,Ru,Rh,Os,Ir,etc.on the crystal structure,electronic structure,mechanical properties and thermodynamics of NiAl alloy.The analysis of lattice constant,cell volume,electronic density of States,energy band,atomic charge population,bond population,elastic modulus,bulk modulus,shear modulus,toughness and brittleness,entropy,enthalpy,free energy,Debye temperature,heat capacity and other characterization parameters show that the addition of alloy elements is beneficial to improve the plastic toughness of NiAl alloy,and the main research results are summarized as follows:(1)The addition of alloying elements changed the crystal structure of NiAl.The addition of A-type and C-type alloy elements and B-type Mo,W,Re and other alloy elements increase the lattice parameters and cell volume of NiAl,and the cell expands.In B-type alloy elements,the addition of Cr makes the lattice parameters smaller.(2)By comparing and analyzing the density of state and energy band diagrams of various alloying structures,it is shown that the electronic density of state of the alloy structure is mainly distributed in the energy range of-10?20e V,and the density of states are composed of the d orbital electrons of Ni,the s and p orbital electrons of Al and the hybridization of alloying elements.With the addition of alloy elements,the distribution of local electronic density of states appears on the left and far side of Fermi level,which enhances the ionic bond characteristics of alloy phase.(3)Through the analysis of bond population and atomic charge population,it is found that after alloying,the bond population numbers of Ni-M bond and Al-M bond are both negative and have strong ionicity.Among them,Nb,Mo and Ru has the smallest bond population and the strongest ionicity.(4)According the analysis of the first-principles mechanical stability criterion,the plastic toughness criterion G/B value and the Cauchy pressure value(C₁₂-C₄₄),the alloy phase structures meet the mechanical stability after adding alloy elements,and compared with the experimental value of NiAl alloy,the Cauchy pressure value can be increased to 2.52 times as much as that of the original alloy,and the modulus ratio G/B can be reduced by 41%as much as that of the original alloy,which indicates that the addition of alloy elements can effectively improve the plastic toughness of NiAl alloy.(5)By analyzing the changes of enthalpy,entropy,free energy,Debye temperature and heat capacity of alloy phase structure in the temperature range of 0?1700K,it is found that entropy and enthalpy of alloy phase structure meet the law of increasing with the increase of temperature,and the increasing trend of entropy with temperature is greater than that of enthalpy,and the free energy decreases with the increase of temperature.When the temperature is less than 20K and between 35k and 200k,the Debye temperature increases with the increase of temperature,which follows the quantum law and T³law of Debye model.At the same time,when the temperature is lower than 400K,the heat capacity increases sharply with the increase of temperature.when the temperature is higher than 400K,the change of heat capacity follows Dulong-Petit law,which is gradually approaches a saturation value of 95.03cal/cell.k.