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) is in fairly good agreement with experiment (4.10). 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 Zr1-xScxAl3 compound (and also Ti1-xScxAl3) 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 (Zr0.5Sc0.5)Al3 using the same total-energy approach. The calculated total energy for (Zr0.5Sc0.5)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 (Zr0.5Sc0.5)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.