Experimental Phase Diagram of the Fe-rich region in the Fe-Zr, Fe-Nb-Zr and Fe-Sn-Zr systems

PEDRAZZINI PABLO; ARIAS DELIA; AURELIO GABRIELA; GONZALEZ RUBÉN; TOLOSA MARTÍN RODRIGO; CASTELLANO GUSTAVO; NIEVA NICOLÁS

Singapur

Congreso; XLVIII CALPHAD, International Conference on Computer Coupling of Phase Diagrams and Thermochemistry; 2019

CALPHAD Committee

Zirconium-based alloys are widely used as fuel cladding in nuclear pressurized water reactors for their excellent mechanical properties, irradiation stability and resistance to corrosion. Alloyed with Fe, Nb and Sn, Zr is the main element in the Zirlo-type alloys, currently used as structural elements and as containers of burnable elements in nuclear reactors. Although Zr is a major component in this type of alloys, it is most important to know the phase diagrams of their components as well as possible. Further knowledge on the effect of the alloying elements will allow advancing in the understanding of the microstructure of these alloys, on which mechanical properties and corrosion strongly depend. Besides, the experimental data of the phase diagrams is a prerequisite before modelling by techniques such as the Calphad method and inclusion of the binary and ternary systems in thermodynamic databases such as the Zircobase.The information on the binary and ternary phase diagrams of the main components of the Zirlo type alloys (Fe-Sn-Zr, Fe-Nb-Zr) is still incomplete and, according to many authors, some uncertainties persist. A systematic study should include the study of all binary, ternary and even quaternary metallurgical systems at different temperatures.The binary phase diagram of the Fe-Zr system has been studied for some time now by several authors. However, in the Fe-rich region the existence of the Fe23Zr6 compound, which was first described in 1962, remains controversial [1-3]. To clarify the origin of this phase, the present work deals with the manufacture and prolonged heat treatments at different temperatures of alloys located in the Fe-rich region of the Fe-Nb-Zr, Fe-Sn-Zr and Fe-Zr phase diagrams. The experiments have been performed with raw materials whose degree of purity was varied. The present phases were identified by using X-ray diffraction (XRD), semi-quantitative microanalysis by using scanning electron microscopy analysis with energy dispersive spectrometry (SEM-EDS) and quantitative microanalysis by using electron microprobe with wavelength dispersive spectrometry (SEM-WDS).Particularly, in the Fe-rich corner of the Fe-Nb-Zr ternary system the experimental information is scarce. Recently, Liang et al. [3] and Arreguez et al. [4] have studied this region without differentiating the lattice parameters of the (Zr1-XNbX)Fe2, Zr(NbFe)2 and (ZrNb)2Fe phases. In the present work SEM-WDS, XRD, synchrotron radiation light source and neutron diffraction techniques were used to find the boundaries of the phases (Zr1-XNbX)Fe2, Zr(NbFe)2 and (ZrNb)2Fe with their lattice parameters. Finally, by using the results of characterization of heat-treated alloys for long annealing times at 800°-900°-1000°-1100°-1200°-1300°C, it is suggested that the presence of the Fe23Zr6 compound is an equilibrium phase of the Fe-Zr binary system, as shown in Fig. 1. Also, by using the results of characterization of heat-treated alloys for long annealing times at different high temperatures and the preliminary results published in previous works [2-4], the phase diagrams sections at 800°-900°-1000°-1200°C and 800°-900°-1100°-1200°C in the Fe rich corner of the Fe?Nb?Zr and Fe-Sn-Zr systems respectively has been constructed.