Experimental study of Fe-rich region in the Fe–Nb–Zr system at 1000 °C
Graphical abstract
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]]; M5R (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(Zr1-xNbx). Kanematsu [24] reported the crystal structure of Fe2(Zr1-xNbx) 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(Zr1-xNbx) 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).
Section snippets
Experimental
Six ternary alloys were strategically designed to analyze the area of interest. They were labeled R1 to R6 and their nominal composition is indicated in Fig. 1 using solid symbols, in a portion of the phase diagram corresponding to the Fe corner. The phase boundaries in Fig. 1 correspond to those proposed by Liang et al. at 700 °C [22] (black lines) and Arreguez et al. at 800 °C [23] (magenta lines).
Raw materials for preparing the alloys were zirconium (99.9 wt%, main impurities were 600 ppm Fe
Results
According to the metallographic observations shown in Fig. 2, sample R1 quenched from 1000 °C presents a two-phase field, with a dispersed phase in contact with bigger grains of the main phase. SXRD data for sample R1 HT at 1000 °C, shown in Fig. 3, reveals the coexistence of the cubic C15 phase along with a small fraction of Fe23Zr6. The lattice parameter reported in the literature [31] for the Fe23Zr6 phase was used as an internal standard to calibrate the zero-shift and the parameter for
Discussion
As mentioned in the Introduction, the C36 (MgNi2 structure type) and C14 (MgZn2 structure type) Laves phases could not be clearly distinguished before by using XRD techniques, as pointed out previously by Tang et al. [21], Liang et al. [22] and Arreguez et al. [23]. Therefore, the phase diagrams proposed by these authors in the Fe-rich region are only based on SEM-EDS and SEM-WDS measurements and previous works. In two critical assessments on the Laves phases by Stein et al. [35,36], they show
Conclusions
By using the results of characterization by SXRD and EPMA of samples heat treated for long annealing times, and based on the preliminary results published for the Fe–Zr, Fe–Nb and Fe–Nb–Zr systems, a phase diagram section at 1000 °C in the Fe rich corner of the Fe–Nb–Zr system has been constructed. The SXRD technique proved to be adequate to differentiate the C36 and C14 Laves phases, giving best results when measurement are done on finely powdered samples to avoid texture effects. The Fe23Zr6
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Argentine National Scientific and Technical Research Council (CONICET) through a PhD Scholarship, the National University of Tucumán Research Council (CIUNT) through Project PIUNT E649/1 and the Brazilian Synchrotron Light Laboratory (LNLS) through Research Proposal XPD 20180186.
The authors are thankful to Lucas Acosta (National University of Tucumán, UNT), Dr. Germán Bridoux (LAFISO-INFINOA, UNT-CONICET), Dr. Gladys Nieva and Dr. Julio Guimpel (Bariloche Atomic
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