First principles study of hydrogen co-segregation with phosphorus and copper in the bulk and grain boundaries of α- and γ-Fe

Abstract

The demand for more sustainable and economic industrial activities have imposed great challenges in the energy storage and transportation sectors as well as steelmaking. In this context, hydrogen and recycled steel are expected to play an increasingly important role as an energy carrier and infrastructure components respectively. However, both scenarios involve the introduction of hydrogen, and tramp elements such as copper and phosphorus which ultimately lead to the deterioration of mechanical properties of steels. On the one hand, hydrogen atoms (H) can be introduced into the lattice during service conditions and cause hydrogen embrittlement (HE), while on the other, current recycling processes can lead to accumulation of tramp elements at grain boundaries that can also negatively influence the mechanical integrity of steel alloys. It is known that an important aspect of HE is hydrogen trapping at susceptible defects such as grain boundaries, rendering the interaction with copper (Cu) and phosphorus (P) highly probable. In order to produce recycled steel components with improved HE properties, the co-segregation effects of H with P and Cu have to be understood in a systematic way. However, accomplishing this goal requires the direct experimental detection of H at these defects which has comprised a towering challenge.
With the aim of overcoming these difficulties, this study adopts a first principles calculations approach based on density functional theory (DFT) to investigate the influence of Cu and P on the underlying features of the H dissolution processes into bulk and grain boundary (GB) structures of α- and γ-Fe. The findings revealed that H prefers interstitial sites with higher electron pair density as described by the electron localization function (ELF) and more extensive charge transfer in bulk ferrite while the opposite behavior was observed for hydrogen accommodation in pure austenite. The addition of Cu was found to facilitate H dissolution in both phases of Fe while P was found to hinder this phenomenon. These effects were related to H’s positive and negative impact on the stability of the Fe-Cu and Fe-P bonds. Regardless of the interfacial character of the GBs investigated in this study, favorable H segregation takes place at sites with lower ELF and are accompanied by less charge accumulation. The hydrogen co-segregation effects with Cu and P exhibited different features between the closed-packed and open GB structures. In the former, the presence of the substitutional elements created an overall unfavorable dissolution environment for H regardless of the crystal structure. On the other hand, it was found that in the more disordered GBs the presence of Cu and P exhibited varying influence on the H dissolution processes. While for the ferritic GB Cu can enhance the H segregation at the interface by means of increasing the interstitial volume, it was found to alter the site preferability of H towards the open γ-Fe GB structure. In contrast, the result for P revealed a slightly favorable tendency for H dissolution suggesting the formation of bonds with the local Fe atoms, while in the α-Fe P mainly repels H from the interface. In both cases, the results revealed that simultaneous presence of both Cu-H and P-H cannot be excluded at more disordered GBs.