A Calculation Of The Alloying Effect On The Mechanical Properties Of B2-NiAl Intermetallic Compound

In this paper, a first-principles plane-wave method, a total energy CASTEP code based on Density Functional Theory is utilized to systemically investigate the geometrical and electronic structure of NiAl. The occupations of 3d transition metals and their influences on mechanical properties of NiAl are also studied.Firstly, in order to test the computational condition and parameters, several basic properties of NiAl were calculated, such as lattice parameters, band structure, density of states, electron density, formation energy, surface energy etc., a good agreement with experimental and other calculating results are found. And then the formation energies of point defect of NiAl were calculated, it is found that the formation energies of vacant and anti-site at Ni is lower than those at Al, which indicates the vacant and anti-site of Ni can emerge more easily.Secondly, site substitutions of 3d TM elements in NiAl intermetallic compound have been investigated. By calculating the formation energy of NiAl alloyed by X element, the site preference were investigated. The results show that the former and later TM elements with low valence electron numbers of Sc, V, Ti and Zn mainly substitute for the Al site in NiAl. The high valence electron numbers elements with un-filled d- shell, e.g. Mn, Fe, and Co, primarily substitute for the Ni site in NiAl, The elements with semi-filled or filled d-shell such as Cr and Cu can substitute for either the Ni site or the Al site in NiAl, but preferentially tend to the Ni site. And with the increase of valence electrons in the outer shells in 3d TM, the site preference to substitute for the Ni site increases before Mn and then decreases after Mn. From above results, a reasonable explanation has been given on the analysis basis of the states density of valence electrons of these 3d TM elements in NiAl.Thirdly, the geometrical and electronic structures of Ni₇Al₈X (X=Cr, Mn, Fe, Co and Cu) supercells were calculated. For elastic constants of pure B2-MA1 intermetallic compound, a good agreement with experimental and other theoretical calculation results can be seen. Several mechanical parameters such as elastic constant C₄₄, the Cauchy pressure (C₁₂-C₄₄), the Young modulus E, the shear modulus G , the bulk modulus B₀ and their ratio G/B₀, have been used to characterize and assess the effect of alloying element on the brittle or ductile behavior as well as the hardness of Ni₇Al₈X intermetallic compounds. The result shows that addition of Mn or Co, Fe makes the average atomic bonding strength of NiAl increase, whereas decrease for addition of Cr or Ni. Addition of Cr or Mn, Fe, Co, Cu can enhance the hardness of NiAl intermetallic compounds following order: NiAl < Ni₇Al₈Cu < Ni₇Al₈Cr <...