Experimental identification of the degenerated equilibrium and thermodynamic modeling in the Al–Nb system

In order to ascertain the reaction type for the degenerated equilibrium among liquid, (Al) and NbAl 3, differential scanning calorimetry (DSC) measurements with heating and cooling rates of 2, 5 and 10 K/min are performed for the annealed pure Al and Al 0.97Nb 0.03 (atomic fraction) samples. The degenerated equilibrium is identified to be a peritectic one by extrapolating the heating rates to 0 K/min. A set of self-consistent thermodynamic parameters for the Al–Nb system is then obtained by means of the calculation of phase diagram (CALPHAD) approach applied to the experimental phase diagram and thermodynamic data from the literature and the present work. The calculated phase diagram and thermodynamic properties agree well with the corresponding experimental ones. The unusual feature associated with the previous modeling ( i.e., the appearance of ordered Bcc_B2 phase at low temperatures) is avoided in the present modeling.