Effects of B on the Segregation Behavior of Mo at the Fe–Cr(111)/Cr2O3(0001) Interface: A First-Principles Study

Article Effects of B on the Segregation Behavior of Mo at the Fe–Cr(111)/Cr2O3(0001) Interface: A First-Principles Study Yanlu Zhang 1, Caili Zhang 1,*, Zhuxia Zhang 2, Nan Dong 1, Jian Wang 1, Ying Liu 1, Zhibo Lei 1 and Peide Han 1,* College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China; (Y.Z.); (N.D.); (J.W.); (Y.L.); (Z.L.) 2 College of Aeronautics and Astronautics, Taiyuan University of Technology, Taiyuan 030600, China; * Correspondence: (C.Z.); (P.H.) 1 Received: 12 March2020; Accepted: 24 April 2020; Published: 28 April 2020 Abstract: The addition of B can inhibit the precipitation of σ phases at the grain boundary to improve the hot workability and corrosion resistance for super austenitic stainless steel with high Cr and Mo content. This study focused on the interaction between B and Mo at the Fe– Cr(111)/Cr2O3(0001) interface and its effect on interfacial adhesion by employing the first-principles method, especially the effect of B on the segregation behavior of Mo. The most stable O-terminated Fe/Cr2O3 interface was chosen as the basic configuration. The segregation energy and the work of separation were calculated for the metal/chromia interface with Fe–Cr as the substrate. It has been demonstrated that B can promote the diffusion of Mo atoms into the oxide layer to increase the content of Mo in the passive film. In addition, the interfacial adhesion is higher at the most segregated sites. However, it is more difficult for two or more Mo atoms than a single Mo atom to diffuse into the oxide part with the effect of B, indicating that B can only improve the Mo content of the passive film to a small extent. The electronic properties were also further discussed to analyze the interactions and the binding characters between doped atoms and their surrounding atoms and to explain the underlying reasons for the variation of interfacial adhesion. Keywords: Mo content; interface; segregation; interfacial adhesion; passive film; first-principles 1. Introduction Super austenitic stainless steel, possessing the austenitic microstructure with a face-centered cubic crystal structure, has been extensively applied in harsh service conditions such as seawater, oilfields, cooling water systems, flue gases, chemical applications, and nuclear reactions [1–3] due to its excellent corrosion resistance. In general, a high content of Cr and Mo assures its unique corrosion resistance. Particularly, the addition of Mo can significantly heighten the resistance to pitting and crevice corrosion in a reductive and chloride environment. Nonetheless, higher Cr and Mo content is likely to induce precipitation of the brittle second phases such as sigma, chi, and laves phase [4–6], which deteriorates the hot workability and corrosion resistance. Fortunately, B atoms are easier to segregate at grain boundaries, suppressing the precipitation of the second phases. Bai et al. clarified the inhibition effect of B on the σ phase [7]. Besides, it has been demonstrated experimentally that B can elevate the content of Mo oxide in the passive film [8], which improves pitting corrosion resistance. The compactness and composition of the passivation film and its adhesion with the Metals 2020, 10, 577; doi:10.3390/met10050577 www.mdpi.com/journal/metals Metals 2020, 10, 577 2 of 13 substrate can affect the corrosion resistance of stainless steel to some extent. Thus, it is worthwhile to study the effects of the added microelement B on the metal/chromia interface. Currently, there have been many theoretical studies on metal/oxide interfaces [9–14]. The structure and properties of such heterogeneous interfaces can significantly affect the performance of materials, and they even play a decisive role in some special properties such as corrosion resistance and high-temperature oxidation resistance. Stainless steel is a typical example, with a dense protective passivation film formed on its surface. Dong et al. investigated the segregation behavior of different alloying additives of the Fe/Cr2O3 interface and their effects on the interfacial adhesive strength [15], which can provide theoretical guidance for the chemical composition design of austenitic heat-resistant stainless steels. However, the substrate was pure iron, the composition of which was inconsistent with the actual austenitic stainless steel enriched in Fe, Cr, and Ni. Kamiya et al. carried out ab initio calculations of electronic structures and carrier transport properties for ZnO– metal interfaces and revealed the nature of the interfaces [16]. First-principles calculations can describe the interactions among atoms in detail and obtain some information like the interfacial atomic coordination, bonding nature, and electronic structures, which compensates for such disadvantages in experiments. In our previous work [17], the effect of B on the segregation of Mo at grain boundaries was studied by first-principles calculations. The results showed that B can restrain the segregation of Mo at grain boundaries to reduce the precipitation of the σ phase. However, the effects of B on Mo at the metal/oxide interface have not been discussed yet. Moreover, the effects of B on the composition of the passive film and the interfacial adhesion play a significant role in corrosion resistance. In addition, it is hard to observe the effect of B on the diffusion of Mo by means of existing characterization methods. Therefore, in this work, first-principles calculations were adopted to study the segregation behavior and the interfacial adhesion of the Fe–Cr/Cr2O3 interface and to explore the reason for this from the perspective of charge interaction. 2. Computational Details The first-principles calculations based on density functional theory (DFT) [18–20] were implemented with the Vienna Ab initio Simulation Package (VASP) [21,22]. The Perdew–Burke– Ernzehof (PBE) [23] generalized gradient approximation (GGA) [24] was adopted to describe electron exchange and correlation. The interaction between valence electrons and ion cores was represented by projector-augmented wave pseudopotentials (PAW) [25,26]. The kinetic energy cutoff was 400 eV, and the Brillouin zone was sampled by a 5 × 3 × 1 Γ-centered k-points mesh following the Monkhorst– Pack method [27]. As for magnetism of the paramagnetic austenitic phase γ–Fe, there are different ways to deal with the directions of the magnetic moments, including disregard [28–30], setting spin according to the ferromagnetic phase [31], and adopting the disordered local moments model [32– 34]. Spin polarization was not adopted in this study. Unless otherwise stated, the convergence criteria of all optimizations were 10−4 eV and 0.01 eV/Å−1 for electronic and ionic relaxations, respectively. To verify the rationality of calculation parameters chosen in this paper and to build interface models for the next step, the lattice constants of face-centered-cubic (fcc) Fe a (...truncated)