First-principles Study On Elastic Properties, Brittle/Ductile Transition, Mechanical Properties Of NiAl-based Alloys

Due to its good oxidation resistance properties, high strength, high meltingtemperature, high thermal conductivity and low density, transition metal aluminumcompounds NiAl, FeAl and CoAl have been considerable attention as a very promisingstructure materials for high temperature applications. Nevertheless, poor ductility atroom temperature and low hardness at high temperature limits their applications.Experimental investigations show that the mechanical properties are influenced bysome factors such as impurity content, heat treatment, and constitutional defects.Alloys element Cr, Mn, V, Fe, Ti, Mo, Dy, Re and Ga have been demonstrated to bebeneficial to improve the room-temperature ductility of NiAl. The mechanicalproperties of materials are related to the elastic constants, including the second-, third-or even high-order constants. In the linear theory of elasticity, the second-orderconstants is sufficient to describe the elastic stress-strain response in solids. However,The linear theory of elasticity also have exhibited boundedness in explaininganharmonic properties of materials, the linear theory of elasticity must be extended tothe nonlinear elasticity theory. In this paper, the nonlinear elasticity theory andfirst-principles total-energy calculations are used to study the nonlinear elasticity ofNiAl, FeAl and CoAl to study the effect of points defect (Ni vacancy, Ni antisite, Ptand Cr substitution) on the mechanical and electronic properties of NiAl-alloys. Themain work and results involve:(1) The nonlinear elasticity of NiAl, FeAl and CoAl.The second-and third-order elastic constants of the B2-type NiAl, FeAl and CoAlhave been calculated based on density functional theory, in combination with themethod of homogeneous deformation, they are obtained from fitting the polynomial tothe calculated energy-strain relation. Lattice constants and the second-order constantsare in good agreement with the previous results. Comparing the results of nonlinearfitting with linear fitting, it shows the linear approximation is not sufficient for strainlarger than3%and must consider nonlinear elasticity. There are significant differencesfor NiAl between calculated third-order elastic constants and previous theoreticalvalues, especiallyC111. However, there is lack of experimental and theoretical valuesfor FeAl and CoAl. Then, we investigate the elastic properties of NiAl, FeAl and CoAlunder different pressure. The elastic constantsC ij, Bulk modulus B, Shear modulusG{100}andG{110}as a function of pressure are presented. The calculationsof B andG{100}are in good agreement with the data obtained from Fu et al., who predictthe results by Vinet equation.(2) The effect of Ni vacancy, Ni antisite Al, Pt in Ni sublattice and Cr in Alsublatticeon mechanical and electronic properties of NiAl.Firstly, effects of Ni vacancy, Ni antisite, Cr and Pt on second-and third-orderelastic constants of the B2NiAl have been investigated using the first-principlesmethods. The calculated lattices of NiAl (pure,with Ni antisite and Cr substitution) areslightly larger than the results obtained by Parlinski et al.. The third-order elasticconstants of NiAl (pure, with Ni vacancy and Pt substitution) are agree well with thatof Jiang et al..C111of NiAl-based alloys are in good agreement with the valuesdetermined by experiments with-11~-12C11. Secondly, study the effects of Ni vacancy,Ni antisite, Pt and Cr on mechanical properties of the B2NiAl. The values of bulkmodulus B, shear modulus G, poisson’s, anisotropy A, Pugh G/B, CauchypressurePc and Vickers hardnessH vof NiAl-based alloys under high pressure can beobtained from second-and third-order elastic constants at P=0. The brittle/ductiletransition map based on Pugh ratio G/B and Cauchy pressurePc shows that Ni antisite,Pt, Cr and higher pressure can improve the ductility of NiAl, respectively. Ni vacancyand lower pressure can enhance the Vickers hardnessH vof NiAl, the prediction is inaccordance with the results report by Lazar et al., who use Rice criterion to proposeMo and in particular Cr are promising candidates for improving ductility of NiAl-Xalloys, by Lü et al., who predict Cr and Ni antisite can improve the ductility of NiAlalloy by means of the criterion of ductility, antiphase boundary energy and peierlsstress, and by Fu et al., who predict pressure can improve the ductility of NiAl. Finally,the density of states (DOS) and the charge density difference are also used to analysisthe effects of of vacancy, Ni antisite, Cr and Pt on the electronic properties of NiAl. Incomparison with Ni-Al bond of NiAl, Ni vacancy in NiAl possesses stronger Ni-Albond, while Pt substitutes Ni in NiAl possesses weaker. The bonding (Ni-Ni/Cr-Ni) ofNiAl with Ni antisite and Cr atom show more metallic feature when compared NiAl.The less directional covalent bonding (Ni-Al) and more metallic bonding (Ni-Ni/Cr-Ni)may lead to the enhancement of ductility. This is consistent with the brittle/ductiletransition map.