Metastable phases in Zr-Excel alloy and their stability under heavy ion (Kr2+) irradiation
Introduction
Since Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) was proposed as a candidate material for pressure tubes for the CANDU Supercritical Water Reactor (CANDU-SWR), due to its high strength and good creep resistance, research work on the alloy which was originally developed by AECL in 1970s has been revived. Significant studies have been carried out on the thermodynamics [1], [2], [3], mechanical properties [4], irradiation induced damage [5], [6] and delayed hydride cracking (DHC) [7] of Zr-Excel alloy.
Like Zr-2.5 wt.% Nb alloy, of which the pressure tubes of current CANDU reactors are made, Zr-Excel alloy is a dual-phase material consisting of a majority of hexagonal close packed (hcp) α-Zr phase and a fraction of thermodynamically metastable body center cubic (bcc) β-Zr phase. In the Zr-2.5wt.%Nb pressure tube material, the metastable β-Zr is enriched with β stabilizing elements Fe and Nb [8], [9]. The concentration of Nb in β-Zr phase is about 20%, which corresponds to the composition at the monotectoid point of the equilibrium Zr–Nb binary phase diagram [10]. Decomposition of β-Zr to hexagonal ω phase with space group of P6/mmm or Nb enriched equilibrium cubic β-Nb phase could take place under irradiation or thermal treatment [8], [11], [12], [13], [14], [15], [16]. However the redistribution of minority alloying elements such as Fe and Nb between the α and β phases was only observed under irradiation. It was found that Fe is depleted in the β phase after both neutron irradiation [8], [14] and heavy ion (Ar+) irradiation [17]. An increased concentration of Fe was observed at the α-α boundary and in the α matrix [8], [14]. The concentration of Nb is increased in both the α matrix, caused by irradiation driven alloying element redistribution, and in the β-Zr matrix as a result of decomposition of β-Zr to ω phase [14], but is decreased in β-Nb if it is present before irradiation [8].
The volume fraction of β-Zr phase in Zr-Excel as-received pressure tube is reported as 13% [3]. The concentration of Nb in the β-Zr phase is just 4%, as evaluated from energy dispersive X-ray spectroscopy (EDS) [3], which is much less than the monotectoid point of the equilibrium Zr–Nb binary phase. However, the addition of Mo which is a strong β phase stabilizer and is enriched in the β-Zr phase contributes to the retention of the β-Zr phase in Zr-Excel. No ω phase particles were reported in the β-Zr phase [3], whereas, in our present study, cuboidal ω phase particles were observed in the β-Zr phase in the Zr-Excel as-received pressure tube. An early study on the stability of ω phase particles in a Zr–Nb alloy under HVEM demonstrated dissolution of coarse ω particles and re-precipitation of small fine ω particles at 350 °C [16]. However, an in-situ heavy ion irradiation at 400 °C did not show a noticeable change of the morphology of ω particles in Zr–20Nb alloy [18]. A recent ex-situ statistical study was carried out on a heat treated Zr-Excel alloy in which as-quenched ω particles were present in the β phase, showing that irradiation assists the precipitation and growth of ω phase in the β-Zr phase [6].
To date, three open questions remain for as-received Zr-Excel pressure tube alloy. 1) What are the lattice parameters of the β-Zr phase and the ω phase? 2) Are the minor phases in Zr-Excel alloy stable under irradiation? 3) How do the alloying elements redistribute between the β-Zr phase and the ω phase after irradiation? In this paper, a high resolution synchrotron X-ray diffraction experiment was carried out on the as received Zr-Excel pressure tube material to measure the lattice parameters of the two minor phases. In-situ heavy ion irradiation was carried out on a TEM disc cut from the material to directly observe the stability of the β-Zr and ω phases during irradiation. Before and after the irradiation, STEM-EDX elemental mapping was performed to characterize elemental distribution over the two metastable phases.
Section snippets
Experiments
The material used in this study is an as-received Zr-Excel alloy from a pressure tube provided by Atomic Energy Canada Limited (AECL) Chalk River Laboratory (now Canadian Nuclear Laboratory) with specified chemical composition given in Table 1. The as-received tube was fabricated by hot extruding the hollow billet at 850 °C at a ratio of 10:1 followed by 25% cold drawing. The tube was then annealed at 750 °C for 30 min and stress relieved at 400 °C for 24 h. The microstructure of as-received
Lattice parameters
The lattice parameters of the major phase (hcp α-Zr) and minor metastable phases (bcc β-Zr and hexagonal structural ω-Zr) in Zr-Excel as-received pressure tube material are investigated with synchrotron X-ray diffraction. The synchrotron X-ray diffraction pattern which contains peaks from all the three phases is shown in Fig. 2. All the peaks are identified and shown in the figure except two peaks locating at 2θ = 15.073° and 32.174° which correspond to interplanar spacings of 2.748 Å and
Effect of alloying element on lattice parameters
Since the as-received tube was annealed and stress relieved in the final processing stage, deviation of the lattice parameter induced by residual stress could be ignored, except for those associated with cool down from the annealing temperature. These thermal stresses will induce tension on the c-axis and contraction on the a-axis, with expected elastic strains of 0.05–0.1% and 0.01–0.1% respectively depending on texture [26], [27]. The observed change of the lattice parameter compared to pure
Conclusion
In this study, synchrotron X-ray diffraction was used to identify the phases in as-received Zr-Excel pressure tube material and to determine their lattice parameters. Subsequent in situ heavy ion irradiation at 200 and 450 °C was performed to investigate the stability of the metastable phases. Post irradiation Chemi-STEM EDS characterization was employed to analyze irradiation induced elemental redistribution. The conclusions drawn are as follows:
- (1)
Compared with pure Zr, there is 0.3% shrinkage
Acknowledgement
This work is supported by the NSERC/NRCan Gen-IV project, and the NSERC/UNENE/Nu-Tech Precision Metals Industrial Research Chair Program at Queen's University. The in-situ ion irradiation and synchrotron X-ray diffraction were accomplished at Argonne National Laboratory, a US Department Office of Science Laboratory under Contract No. DE-AC02-06CH11357 managed by University of Chicago. The authors are grateful for Peter Boldo's assistance with in-situ ion irradiation and Dr. Levente Balogh's
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A study on the evolution of ω-phase in Zr-20Nb alloy under the influence of electron irradiation
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2021, Acta MaterialiaCitation Excerpt :Almost all of them have an α-Zr matrix, but if they are quenched from the high temperature β region during the manufacturing process, they will contain the high temperature β-Zr phase (or its decomposition products) in the final microstructure. Zr-Nb alloys can therefore be divided into two general categories: those recrystallised below the monotectoid temperature which only contain the equilibrium β-Nb phase in the form of small second phase precipitates (SPPs) such as E110, ZIRLO, M5 [4–7]; and the β-quenched and annealed alloys that contain some residual β-Zr phase such as the Zr-2.5Nb and Zr-Excel alloys used in the CANDU reactors [8–10]. The morphology of the β-Zr phase also depends on the manufacturing route.
In-situ study of heavy ion irradiation induced lattice defects and phase instability in β-Zr of a Zr–Nb alloy
2019, Journal of Nuclear MaterialsCitation Excerpt :However, it should be noted that concentration of irradiation induced point defects is much lower under heavy ion irradiation than under electron irradiation because of the thermal spike associated with the displacement cascade under heavy ion irradiation, which enhances the recombination of the Frenkel pairs. On the other side, the displacement cascades cause ballistic mixing, which may cause the dissolution of newly formed ω phase [14]. It is striking that tetragonal zirconia has been extensively observed after irradiation outside the in-situ observed area, i.e., outside the electron illumination.
Retention of metastable β phase in a two-phase quaternary zirconium alloy
2018, Materials and DesignThe characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical review
2018, Journal of Nuclear MaterialsCitation Excerpt :%) there is debatably no detectable Nb in the alpha phase (see discussion by Griffiths in Ref. [14]). Although values for the α-phase Nb content of 0.23 wt% have been reported for Excel alloy [15], these values are not supported by the spectra shown in the paper and the accuracy of the quantitative analysis at such low values is therefore questionable. Nb content in the α-Zr phase of Excel has only been convincingly shown in alloys that are water quenched from between 690 and 876 °C, resulting in the retention of Nb in the α-Zr phase to between 0.37 and 0.59 wt% [16].
Stacking faults observed in {101−2} extension twins in a compressed high Sn content Zr alloy
2017, Scripta Materialia