Experimental study of Fe-rich region in the Fe–Nb–Zr system at 1000 °C

Highlights

• The Fe-rich corner of the Fe–Nb–Zr phase diagram at 1000 °C is proposed.

• The synchrotron x-ray powder diffraction technique is able to differentiate the C36 and C14 hexagonal Laves phases.

• The MgNi₂ (C36) structure corresponding to Fe₂(Zr_1-xNb_x) Laves phase was not found at 1000 °C.

• The Fe₂₃Zr₆ is concluded to be a stable phase in the Fe–Nb–Zr system at 1000 °C.

Abstract

The Fe-rich corner of the Fe–Nb–Zr phase diagram was studied at 1000 °C by using synchrotron x-ray powder diffraction (SXPD) technique and quantitative electron probe microanalysis (EPMA). The MgCu₂ (C15) and MgZn₂ (C14) structures corresponding to Fe₂Zr and Fe₂Nb Laves phases were observed in the Fe₂Zr–Fe₂Nb pseudo-binary system. The MgNi₂ (C36) structure corresponding to Fe₂(Zr_1-xNb_x) Laves phase was not found in any of the compositions explored at this temperature. The existence of two three-phase fields (C15 + Fe₂₃Zr₆ + C14) and (C14 + Fe(α) + Fe₂₃Zr₆) and five two-phase fields (Fe(α) + C14), (Fe(α) + Fe₂₃Zr₆), (C15 + Fe₂₃Zr₆), (C15 + C14) and (Fe₂₃Zr₆ + C14) is proposed in the present work.

Introduction

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. Although Zr is a major component in this type of alloys, the knowledge of phase diagrams of its multi-component alloys is vitally important for nuclear and other technologies. Understanding the effect of alloying elements on the microstructure will allow tailoring the mechanical properties and the corrosion. Moreover, the experimental assessment of the complete phase diagrams is a prerequisite for modelling based on techniques such as the Calphad method, and feeds thermodynamic databases such as Zircobase [1].

To meet the requirement of higher burnup of nuclear fuels, a new generation of Nb-containing zirconium-based alloys has emerged. Such is the case of Zirlo™ (Zr-1.0Sn-1.0Nb-0.1Fe) [[2], [3], [4]]; M5^R (Zr-1.0Nb), E110 (Zr-1.0Nb-0.1Fe) and E635® (Zr-1.2Sn-1.0Nb-0.35Fe) [4,5], among others. One of the ternary systems of great technological interest that still presents challenges is Fe–Nb–Zr since alloying with Nb and Fe enhances the corrosion resistance in these new-generation zirconium alloys.

One of the first experimental studies of this ternary phase diagram was performed by Gruzdeva et al. [6] who focused on the Zr-rich corner of the Gibbs triangle at 700, 800, 900 and 1000 °C.

In this work, we will focus on the Fe-rich region of the Fe–Nb–Zr phase diagram with particular attention to the Laves phases. Studies on the Fe–Nb–Zr ternary system were carried out in the Fe₂Zr–Zr–Nb–Fe₂Nb area in Refs. [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. A very interesting critical review on Zr–Nb and Zr–Nb–Sn–Fe systems was published in 2018 by Harte et al. [19] in which the Fe₂Zr–Zr–Nb–Fe₂Nb area was analyzed in alloys with and without irradiation and under different heat treatments. After Korotkova [20] not many studies spanned the Fe₂Zr–Fe–Fe₂Nb field until recent years [[21], [22], [23]].

The most challenging issue in this composition region is related to the Laves phases Fe₂(Zr_1-xNb_x). Kanematsu [24] reported the crystal structure of Fe₂(Zr_1-xNb_x) alloys as a function of Nb content (x): MgCu₂(C15)-type for x ≤ 0.3, MgNi₂ (C36)-type for 0.35 < x < 0.5 and MgZn₂(C14)-type for x > 0.5. Kanematsu showed that for Fe₂(Zr_1-xNb_x) alloys, the MgNi₂(C36) phase has similar X-ray diffraction (XRD) patterns to the MgZn₂(C14) and concluded that it is hard to differentiate C36 from C14 in Fe–Nb–Zr alloys by only using the conventional XRD technique. Tang et al. [21], who studied the Fe₂Zr–Fe–Fe₂Nb area at 1200 °C, proposed tentative phase boundaries between Fe₂Zr(C15) and Fe₂Nb(C14) ternary Laves phases in the Fe₂Zr–Fe₂Nb pseudo-binary system. As the C14 phase could not be differentiated from C36, both were tentatively regarded as a single-phase region. Liang et al. [22] and Arreguez et al. [23] determined the phase boundaries between (C15), (C36) and (C14) in the Fe₂Zr–Fe₂Nb pseudo-binary system based on results from scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS) and electron microanalyses with wavelength dispersive spectrometry (SEM-WDS) respectively.

A second open question that arises in the Fe-rich region is related to additional traces of the Fe₂₃Zr₆ phase reported by some authors [21,22]. Tang et al. [21] did not include this phase in the equilibrium phase diagram, following Stein et al. [25], whereas on the contrary Liang et al. [22] did include it following Huang et al. [26]. Arreguez et al. [23] did not report the observation of the Fe₂₃Zr₆ phase.

The experimental knowledge on the Fe-rich region of the Fe–Nb–Zr phase diagram remains incomplete. No compelling evidence supports the discrimination between the C36 and C14 Laves phases. The present work explores the experimental phase diagram of the Fe-rich region in the Fe–Nb–Zr system at 1000 °C by using synchrotron x-ray diffraction (SXRD) technique, with the aim of increasing the sensitivity of the XRD technique to discern the equilibrium phases, combined with electron probe quantitative microanalysis (EPMA).