Elastic Properties And Point Defects In MgNi₂ And Elastic Properties In RECu₉Mg₂?RE=Y,La,Yb? Ternary Alloys

As the hydrogen storage material of basis phase MgNi2 and strengthening phase RE-Cu-Mg,whose mechanical performance is a very significant basic properties,and dihexagonal C36 Laves model phase MgNi2 is the most important phase.Moreover,off-stoichiometry of Laves phases is a ubiquitous phenomenon,so it is also significant to study the the impact on the mechanical properties by the point defect.In the present work,elastic properties and point defects in C36 MgNi2 phase and elastic properties of RECu9Mg2(RE=Y,La,Yb)are investigated in detail,the main research contents are as follows:Firstly,elastic properties and point defects in dihexagonal C36 Laves model phase MgNi2 is conducted with using first-principles calculations within framework of density functional theory.The results show that thermodynamic stability of MgNi2 is well.The elastic anisotropy of bulk modulus in MgNi2 is the smallest.Employing the 96-atom supercell,ten kinds of point defects are studied in MgNi2 including five anti-site defects and five vacancy defects.Under both Ni-rich and Mg-rich condition,it is found that Mg anti-site Ni atoms at Wyckoff position 4f(MgNi3)is the most stable due to the lowest formation energy,indicating MgNi3 is the easiest to form.Further studying on the underlying mechanism of point defect formation in MgNi2,MgNi3 is the most favorable anti-site defect due to the strong charge density overlapping,which is correspond with the results of the formation energy.Secondly,elastic properties of RECu9Mg2(RE=Y,La,Yb)ternary alloys have been studied from first-principles calculations within generalized gradient approximation.The results show that thermodynamic stability is weaker from YCu9Mg2 to LaCu9Mg2 and YbCu9Mg2,and C11 is less than C33 for all three phases,indicating c-axis direction is harder to compress than a-axis directions.YCu9Mg2 phase shows the highest stiffness and the strongest resistance to volume/shear deformation due to the largest elastic modulus,so are the strongest directional bonding.From three-dimensional(3D)directional dependences,the elastic anisotropy has an increasing order:YCu9Mg2<LaCu9Mg2<YbCu9Mg2.The calculated DOS and the charge density difference on the(110)plane of RECu9Mg2(RE=Y,La,Yb)demonstrate that the RE-Cu and RE-Mg covalent bonding is stronger from YbCu9Mg2 to LaCu9Mg2 and YCu9Mg2,which reveals the underlying mechanism for the stability and elastic properties of RECu9Mg2(RE=Y,La,Yb)ternary alloys.