PSI - Issue 13

Eunan J. McEniry et al. / Procedia Structural Integrity 13 (2018) 1099–1104 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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cars, requiring in turn the development of lighter components, while retaining overall mechanical strength. These requirements have led to the development and application of steel grades with ultimate tensile strengths (UTS) at or above the GPa level. Such materials, however, are generally deemed to be particularly susceptible to HE, thus hindering their application in real-world applications. As a result, a firm understanding of embrittlement effects in such materials has become a key issue. Due to the relatively low solubility of hydrogen in most bulk metallic systems of interest, it is apparent that hydrogen-induced damage is typically initiated at microstructural defects in the material, where hydrogen is expected to accumulate. As a result, an understanding of the energetics, kinetics and mechanical influence of hydrogen in the vicinity of grain boundaries, dislocations and other structural defects is essential in understanding the HE phenomenon. 1.1. Atomistic modelling and tight-binding An extremely valuable tool in investigating the behaviour of hydrogen at such defects is atomistic simulation. Simulations in the framework of ab initio density functional theory (DFT) have been performed in order to investigate a number of features of hydrogen behaviour (Hickel et al. 2014), with examples such as the interactions between hydrogen and vacancies in austenite (Nazarov et al. 2010), between hydrogen and grain boundaries in α - and γ -Fe (Du et al. 2011) , and on the effect of H on the stability of phase boundaries between α -Fe and cementite (McEniry et al. 2018). The major drawback with the DFT approach is its computational complexity, which limits the size of the effective simulation cell to a few hundreds of atoms at best. As a result, one is typically limited to studying model systems in terms of chemical composition, as well as somewhat simplified models of microstructural features. In order to partially overcome this limitation, simplified electronic structure methods within the framework of tight binding (TB) theory (Slater and Koster 1954, Sutton et al. 1988) have been developed. Such methods enable a fully quantum-mechanical description of atomic interactions, while reducing the computational complexity by around two orders of magnitude with respect to the DFT approach. Developments in the methodology in recent years have been predominantly aimed at improving the description of transition metals and alloys thereof, with particular emphasis on describing magnetic properties of such materials (Paxton and Finnis 2008, Madsen et al. 2011, McEniry et al. 2011). More recently, some of the present authors have developed an extended environmental-dependent tight-binding approach (McEniry et al. 2013, McEniry et al. 2017), which allows for a more robust description of atomic behaviour in the vicinity of extended defects or surfaces, which is essential in understanding hydrogen-related phenomena. . An important consideration in such simulations is competition between hydrogen and additional light elements which may also segregate to structural defects. It is indeed somewhat illogical to consider the segregation of hydrogen, which is typically present in ppm quantities in iron-based materials, while ignoring carbon, whose atomic concentration is typically of the order of 1%. Moreover, one should also consider the other first-row elements of boron, oxygen and nitrogen, whose segregation may play an important role in the mechanical response of our chosen microstructural features. As a result, our aim is to develop atomistic approaches for the study of light-element segregation in Fe-based materials. The development of appropriate atomistic models for such multicomponent systems is a technically challenging procedure. The major advantage of the environmental-dependent tight-binding approach is the fact that the energetics of multicomponent systems can be determined in terms of sets of three-body interaction parameters. As a result, the determination of the tight-binding model for a chemically complex system can be achieved in a straightforward manner. The focus here has been on developing a model for the Fe-C-H ternary system, with the intent of applying it to understanding segregation and possible co- segregation effects of C and H to a selection of grain boundaries in α -Fe. In order to assess the carbon segregation to these boundaries, we have considered a wide range of possible interstitial sites in the vicinity of the grain boundaries. Particularly interesting sites are those which offer higher local Voronoi volumes than in the bulk of the material, and those whose coordination number differs from the 4-fold or 6 fold coordination of a tetrahedral or octahedral hole. In each case, we have calculated the segregation energy = −( + − − ) , 2. Segregation at grain boundaries in Fe