Phase separation tendency in the as-solidified Zr<SUB>3</SUB>Al-Nb alloys studied by microstructural observations and thermodynamic analysis

The as-solidified microstructures of Zr3Al-Nb alloys with varying Nb concentration, viz., Zr3Al, Zr3Al-3Nb, and Zr3Al-10Nb, have been investigated and the phases occurring in these alloys have been identified in terms of their crystal symmetry and their chemical compositions. The formation of various phases is influenced by solute partitioning. With increasing Nb concentration, the extent of solute partitioning increases, resulting in formation of new metastable phases. The phases that solidify first are rich in Al and lean in Nb, whereas the phases that solidify last are rich in Nb and lean in Al. The formation of these phases has been explained in terms of the driving force for nucleation. For this purpose, the free energy for nucleation has been computed for all pertinent binary intermetallics phases by using thermodynamic data available in the literature. Using solution thermodynamics, it has also been shown that the liquid phase in the Zr-Al-Nb system should exhibit a miscibility gap, and the boundaries of the miscibility gap have been estimated by invoking the regular solution model