Elsevier

Calphad

Volume 72, March 2021, 102240
Calphad

Experimental investigation of phase equilibria in the Fe-Hf-Zr system at 1173 K and 1373 K

https://doi.org/10.1016/j.calphad.2020.102240Get rights and content

Highlights

  • Isothermal sections at 1173 K and 1373 K of the Fe-Hf-Zr system were determined.

  • The Fe23Zr6 phase was confirmed to exist without the oxygen contamination.

  • The dissolving of Hf increases the thermodynamic stability of the Fe(Hf,Zr)3 phase.

  • The liquid phase shows a wide homogeneity range at 1373 K.

  • No ternary compound was detected on these isothermal sections.

Abstract

A series of Fe-Hf-Zr alloys were designed to determine the phase equilibria for the entire composition range by annealing at 1173 K and 1373 K for 440–720 h and 12–360 h respectively, followed by X-ray diffraction (XRD) measurements and electron probe microanalysis (EPMA). At the Fe-rich corner, it was argued that the Fe23Zr6 phase is a stable phase without oxygen contamination, of which the formation mechanism was discussed. The Fe2(Hf,Zr)_C15 phase forms a continuous solid solution. The two-phase region of β(Hf,Zr) + Fe(Hf,Zr)3 was detected at 1173 K. The Fe(Hf,Zr)3 phase was observed in this ternary system at 1173 K, which is attributed to the increasing thermodynamic stability by dissolving Hf. The solubility of Fe in α(Hf,Zr) was determined to be extremely low. The liquid phase appears at 1373 K in a large composition range. No ternary compound was identified at both temperatures in this system. Based on the experimental results and reasonable extrapolations, the isothermal sections of 1173 K and 1373 K were constructed, which respectively consist of seven and five three-phase regions.

Introduction

The metal hafnium is considered as an ideal neutron absorbing material due to the combination of relatively large thermal neutron absorption cross section, satisfactory corrosion resistance, high radiation resistance and good mechanical machining performance. The hafnium materials are extensively used in boiling water reactors (BWR), pressurized heavy-water reactor (PHWR), power reactors and research reactors [[1], [2], [3], [4]]. Recently the hafnium hydride [5,6] and hafnium boride [7,8] have also been investigated for the control rod in the sodium-cooled fast reactor to prolong its service lifetime, superior to the widely used B4C control rod. In typical reactors, the ferritic-martensitic stainless steel (SS) is considered as a reference cladding material for the hafnium/hafnium hydride control rod [9,10], surrounded by the zircaloy guide tube [11,12]. In the event of a nuclear severe accident, the temperature inside the reactor core may increase rapidly due to the loss of coolant, leading to the degradation of control rod segments (Hf/SS/Zircaloy). To investigate the interaction among the hafnium/hafnium hydride control rod, the stainless steel cladding and the zircaloy guide tube, it is beneficial to better understand the degradation process of control rod segments. Thus, the investigated system is simplified to be the Fe-Hf-Zr system. Besides, the Fe-Hf-Zr system also attracts some interest as a subsystem of the in-vessel corium in the nuclear reactor during a severe accident. The thermodynamic investigation of the in-vessel corium system plays a crucial role in the configuration of the corium pool, affecting the effectiveness of the in-vessel retention (IVR) strategy [[13], [14], [15]].

As discussed above, the phase equilibria of the Fe-Hf-Zr system should be experimentally investigated. To our knowledge, only the isothermal section of the ternary Fe-Hf-Zr system at 1173 K [[16], [17], [18]] and the isopleth of Fe2Zr–Fe2Hf [19] have been experimentally studied. However, the experimental data in the Hf-rich corner is far from complete, Moreover, the annealing time of these investigations maybe not enough to achieve phase equilibrium, especially for the Fe23Zr6 phase as a peritectic product. For further thermodynamic assessment of the Fe-Hf-Zr system, more experimental data should be measured at different temperatures. Thus, the present work will utilize the equilibrated alloy method to construct the isothermal sections at 1173 K and 1373 K over the entire composition range, which is fundamental and essential to the future modelling work.

Section snippets

Literature review

In order to construct ternary isothermal sections, the previous investigations related to the binary sub-systems and the ternary system are summarized as follows.

Experimental details

The phase relations of the Fe-Hf-Zr system were investigated using the equilibrated alloy method. Pure elements of iron (99.99 wt%), hafnium (99.99 wt%) and zirconium (99.95 wt%) were used as raw materials. Table 2 shows the main impurities in these raw materials. Each alloy button with 20 g in total was prepared by a non-consumable arc melting furnace (WK-II, Opto-electronics Co.Ltd., Beijing, China) under a high-purity argon atmosphere with titanium as oxygen getter material placed in the arc

Phase equilibria of the Fe-Hf-Zr system at 1173 K

The phase equilibria of the Fe-Hf-Zr system at 1173 K were experimentally determined covering almost the whole composition range. Table 3 presents the analyzed compositions of alloys, the annealing time, the identified phases in each alloy and the corresponding phase compositions at 1173 K. The chemical compositions, BSE images and the XRD results were combined to identify the phase relations.

The alloy A1 was employed to study the stability of Fe23Zr6 in the Fe-rich corner at 1173 K. As

Conclusions

Based on the EPMA measurements and the XRD analyses, the isothermal sections of the Fe-Hf-Zr system at 1173 K and 1373 K covering the entire composition range have been constructed by equilibrated alloys, and the main conclusions are obtained as follows:

  • (1)

    Due to the mutual substitution of Hf and Zr atoms, Fe2(Hf,Zr)_C15 phase forms a continuous solid solution from the Fe–Zr side to the Fe–Hf side at both 1173 K and 1373 K;

  • (2)

    The three-phase region of α(Fe) + Fe23Zr6 + Fe2(Hf,Zr)_C15 was observed at

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.

Acknowledgement

The authors are grateful to the financial support from the National Natural Science Foundation of China (Grant number: 51971127) aad the National Key R&D Program of China (Grant number: 2017YFB0701904). The study was also supported a project funded by the Science and Technology on Reactor System Design Technology Laboratory (Nuclear Power Institute of China).

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