Yuanzineng kexue jishu (Dec 2024)

Stability Study of bcc-Fe/FeCr2O4 Interface Based on First Principles

  • YANG Yu1, 2, ZHANG Yan’ge2, LI Xiangyan2, XU Yichun2, , , WU Xuebang2, LIU Changsong2

DOI
https://doi.org/10.7538/yzk.2024.youxian.0202
Journal volume & issue
Vol. 58, no. 12
pp. 2570 – 2580

Abstract

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Ferritic/martensitic (F/M) steels are regarded as the ideal candidates for structural materials in advanced nuclear reactors because of their high thermal conductivity, low thermal expansion and high resistance to irradiation. However, in contact with oxygenated coolants (air, water and liquid metals), they undergo oxidative corrosion and multiple layers of oxides are formed on their surface. In addition, F/M steels must be subjected to high doses of neutron irradiation in the reactors in service. The irradiation not only leads to increased temperature and thermal stress, but also induces defects, which increases the diffusion rate of Fe atoms outward and O atoms inward, further accelerates the growth rate of the oxide layer. The growing oxides not only reduce the thermal conductivity of the structural material, but also detach from its substrate under stress, leading to a thin substrate and a reduction in the mechanical properties of the material. Therefore, the effect of irradiation on the stability of the oxide layer must be clarified for the safe operation of the reactor. In this paper, the interface stability of a bcc-Fe and FeCr2O4 with or without a vacancy was investigated using a first principles calculation method. Firstly, the Fe(001) surface and FeCr2O4(001) surface with Fe- and CrO-termination (FeT and CrOT) were constructed and optimized by their surface energies and inter layer distances. The surface energy of FeCr2O4(001) with CrOT is lower than that with FeT in oxygen-rich and iron-rich environments, which is thermodynamically more stable. Based on the optimized Fe(001) with three symmetry sites (bridge, hollow and top) and FeCr2O4(001) surfaces with FeT and CrOT, constructed six different configurations of Fe(001)/FeCr2O4(001) interfaces and evaluated the interface stability by interface energy and adhesion work. The results show that the CrOT-hollow model with the lowest interface energy and the largest adhesion work is the most stable model. Based on the most stable model, the most stable position of a vacancy was investigated by calculating the vacancy formation energy at the interface and nearby. The results show that Fe vacancies are more likely to be formed and distributed in the first layer on the Fe side of the bcc-Fe/FeCr2O4 interface, which is consistent with the experimentally observed vacancies in the internal oxidation region. By comparing the adhesion work of the interface with and without the most stable vacancy, it shows that the minimum adhesion work of the interface with vacancies decreases and the fracture surface changes from the bcc-Fe/FeCr2O4 interface to the second layer on the FeCr2O4 side. This suggests that irradiation-induced defects not only make the oxide layer more susceptible to detachment, but also change the fracture behavior of the oxide layer.

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