Contribution of Thermodynamic Calculations to Metallurgical Studies of Multi-Component Zirconium Based Alloys

The Zircobase thermodynamic database for zirconium alloys coupled with Thermo-Calc soft- ware represents a powerful tool for prediction of thermodynamic and metallurgical data such as activities, formation enthalpies, phase transformation temperatures, solubility limits, existence temperature range, and chemical compositions of second phase precipitates. This database was built up with binary and ternary descriptions assessed according to the CALPHAD methodology. It is sometimes necessary to take into account new systems, but also new versions of binary descriptions as recently experienced. For example, the two binary systems Zr-Fe and Zr-Sn had to be updated in order to fit new experimental results believed to be more accurate than previously available.This paper aims at showing the improvements of the database taking into account new descriptions of binary systems, as also ternary description such as Zr-Fe-Cr, Zr-Fe-Ni, and Zr-Nb-Fe. For this last ternary system, new experimental data were necessary. New experimental study of the two ternary phases, hexagonal ZrNb,Fe 2 and cubic Zr,Nb 4 Fe 2 , allowed their crystal structures P6 3 /mmc and Fd3 ¯ m, respectively to be checked. This was useful to build up a sublattice model giving account of the existence of a composition range for these two intermetallic phases. Moreover, several specific ZrNbFe alloys were fabricated and annealed for times ranging from 1000 h to 10,000 h at 550, 700, 800, and 900°C to determine the equilibrium binary and ternary phase domains as a function of the temperature. All these data were used to obtain an improved thermodynamic modelling of this system. Finally, we illustrate some thermodynamic predictions of the different phases evolutions as a function of the temperature on multi-alloyed industrial type alloys. These examples show quite good agree- ment between the thermodynamic predictions and the experimental data derived from calorimetric experi- ments and microstructural observations.