Phase stability and electronic structure of ScAl₃ and ZrAl₃ and of Sc-stabilized cubic ZrAl₃ precipitates

The structural stability and the electronic structure of ScAl₃ were studied using an all-electron, total-energy, local-density approach. The calculated results show that ScAl₃ in the L1₂ structure is energetically favored compared with the D0₂₂ structure by about 0.42 eV per formula unit. The calculated lattice constant (4.055 Å) is in fairly good agreement with experiment (4.10 Å). As a comparison, the calculated electronic and cohesive properties for ZrAl₃ in its metastable L1₂ and D0₂₂ phases are also presented. It is argued, on the basis of density-of-states results, that a cubic Zr_1-xSc_xAl₃ compound (and also Ti_1-xSc_xAl₃) might be a good candidate as a dispersed phase in the aluminum alloys for elevated temperature applications. To test this prediction, we determined the electronic structure and the stability of Sc-stabilized cubic (Zr_0.5Sc_0.5)Al₃ using the same total-energy approach. The calculated total energy for (Zr_0.5Sc_0.5)Al₃, which is about 0.24 eV per unit cell lower than the sum of the total energies of ZrAl₃ and ScAl₃, clearly indicates that cubic (Zr_0.5Sc_0.5)Al₃ is energetically favored compared with a mixture of its constituents. Finally, an analysis of the results indicates that the stability of the aluminides appears to be understood in the rigid-band sense in terms of the band filling of the bonding states.