Segregation effects of interstitial and substitutional elements at grain boundaries in ferritic iron and their effect on liquid metal embrittlement
The segregation of impurity elements to grain boundaries largely affects interfacial properties and is a key parameter in understanding grain boundary (GB) embrittlement. Furthermore, segregation mechanisms strongly depend on the underlying atomic structure of GBs and the type of alloying element. Here, we utilize aberration-corrected scanning transmission electron microscopy (STEM) in combination with atom probe tomography (APT) and first-principles density functional theory (DFT) calculations to explore the atomistic and thermodynamic origins of co-segregation of interstitial boron and carbon as well as substitutional aluminum in bcc-Fe. The impact on zinc segregation and its possible effect on liquid metal embrittlement are currently investigated by atomic scale microscopy.
In this study, ferritic bulk Fe-2wt%Al bicrystals containing a ∑5  (310) tilt grain boundary (GB) were obtained by a modified Bridgman method. Aluminum (Al) was added to stabilize the body-centered cubic (bcc) structure throughout the solidification process. This bicrystal geometry presents an ideal template system to study co-segregation effects of interstitial trace impurities relevant in Fe-based alloys, such as carbon (C) and boron (B), with substitutional alloying elements, here Al. A detailed atomistic understanding of the correlation between GB structure, the interaction of solutes and its impact on grain boundary properties in bcc-Fe is missing.
In a first approach, SEM-EBSD experiments were applied to determine the macroscopic structure of the GB, confirming the presence of a planar ∑5  (310) tilt GB. Site-specific focused ion beam (FIB) sample preparation was used to extract samples for atomic scale characterization. The atomic resolution HAADF-STEM image shown in Fig. 1a) provides information on the evolution of structural units and local atomic arrangements at the GB. The GB core consists mainly of repeating kite-type motifs that are in some locations disrupted by GB defects. Correlative STEM - APT experiments reveal strong segregation of C and B, while surprisingly Al is even depleted at the boundary (Fig. 1b)). The atomic scale structural in compositional information from the GB was used as input for density functional theory based calculations to further elucidate the interaction of solutes at the GB. It was found that B and C both exhibit a stronger segregation tendency than Al, while it was also observed that B and Al repel each other. This could explain the observed decrease in Al concentration at the GB1. This study provides new insights into the complex interplay of multiple solutes at GBs and paves the way to understand their impact on interfacial properties in complex alloy systems.
In the next step, we wanted to use our model Fe-bicrystal to study atomistic behavior of zinc (Zn) segregation in a compositionally complex GB. Zinc is a technologically very important element in steel production, since it is used as a coating layer to protect the underlying material from corrosion. However, Zn can also lead to liquid metal embrittlement (LME) along the GBs ultimately leading to catastrophic material failure. In a first step to better understand the atomistic origins of this complex process, we studied the effects of local GB structure and impurity segregation of B and C on the impact of Zn segregation. For this, we have emulated a hot-dipping process by immersing the Fe-bicrystal into a liquid Zn bath followed by subsequent high temperature heat treatment. Near atomic scale energy dispersive X-ray spectroscopy (EDS) measurements in the STEM (STEM-EDS) reveal that Zn forms regularly spaced nanometer sized patches along the GB instead of homogeneously decorating it (Fig. 2 a) ). The 3D arrangement of Zn obtained from APT indicates that Zn is arranged in the form of segregation lines. The atomic structure of the GB shows a high density of interfacial defects and incorporates (210) type facets in asymmetric regions of the boundary (Fig. 2b). The intrinsic interplay of the GB structure and solute segregation leading to this anomalous segregation pattern and its effect on the GB properties are currently under investigation.