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

Abstract

The Zircobase thermodynamic database for zirconium alloys coupled with Thermo-Calc software 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 Zr(Nb,Fe)2 and cubic (Zr,Nb)4Fe2, allowed their crystal structures (P63/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 agreement between the thermodynamic predictions and the experimental data derived from calorimetric experiments and microstructural observations.

Contribution on the phase equilibria in Zr–Nb–Fe system

“…Though alloy 12 was slightly off-equilibrium, results from this alloy revealed that the tie-line of NbFe (m) with Zr(NbFe) 2 is impossible. This was consistent with our previous conclusion, in which the co-existence of bNb and NbFe (m) with (Zr 1Àx Nb x ) 2 Fe was observed [13], but is contradictory to that of Toffolon et al [12], who suggested equilibrium of NbFe (m) and bNb and Zr(NbFe) 2 . This conclusion was also substantiated by the phase identification in alloy 21, in which (Zr 1Àx Nb x )Fe 2 and bNb were detected.…”

“…They suggested that NbFe(m) equilibrated with Zr(NbFe) 2 . This [12] is different from our previous results, where we found that NbFe did not equilibrate with Zr(NbFe) 2 [13]. To obtain a more accurate thermodynamic description of the ZreNbeFe system, it is necessary to further experimentally investigate the phase equilibria in ZreNbeFe system.…”

“…were in good agreement with each other. The tie-triangles in the Zr-rich corners are [aZr þ bNb þ Zr(NbFe) 2 reported several isothermal sections of this system by thermodynamic modeling [12]. They suggested that NbFe(m) equilibrated with Zr(NbFe) 2 .…”

“…Recently, Toffolon et al solved the crystal structures of Zr(NbFe) 2 and (ZrNb) 2 Fe. Zr(NbFe) 2 adopted the P63/mmm space group, while (ZrNb) 2 Fe was proved to be of Fd3 m structure [12]. Another ternary compound, C36-type (Zr 1Àx Nb x )Fe 2 , exists in the Fe-rich corner.…”

Investigation of Particles and Gas Bubbles in Zr–0.8Sn–1Nb–0.3Fe Zr Alloys Irradiated by Krypton Ions

“…The element peaks of Fe, Nb, and Zn can be found in the particles. Due to the easy dissolution of Sn atoms in the Zr matrix, it is difficult to detect Sn in the particles [21,22]. Thus, these particles should be the alloy compounds of Zr, Nb, and Fe elements in these two kinds of modified N36 Zr alloys.…”