Segregation effects of interstitial and substitutional elements at grain boundaries in ferritic iron and their effect on liquid metal embrittlement

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 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 effect on liquid metal embrittlement are currently investigated by atomic scale microscopy.

In this study, bulk ferrite Fe-2wt%Al bicrystals containing a ∑5 [001] (310) tilt grain boundary (GB) were grown by means of a modified Bridgman method. Aluminum (Al) was added to stabilize the body-centered cubic (bcc) structure throughout the solidification process. Mintz et al. [1] showed that Al segregation to GB would lead to an increase of the strength with a decrease of ductility of iron (Fe). This was also proven by DFT calculation [2]. However, Geng et al. concluded from first principle studies based on the full-potential linearized  augmented plane-wave method that Al does not have any effect on the GB cohesion strength [3]. On the other hand, trace impurities such as carbon (C) and in particular boron (B) are expected to enhance GB cohesion and suppress the segregation of impurity elements such as phosphorous (P), which would embrittle the interface [2]. However, a detailed atomistic understanding of the structure of GBs and the correlated segregation of intestinal impurities in bcc-Fe is missing. In a first approach, SEM-EBSD experiments were applied to determine the macroscopic structure of the GB as shown in Figure 1a, confirming that for tens of micrometers planar ∑5 [001] (310) tilt GB was obtained. Site-specific focused ion beam (FIB) sample preparation was then applied to extract samples for atomic scale characterization. The atomic resolution HAADF-STEM image shown in Fig. 1b provides extensive information on the structural units and atomic arrangements at the GB. The GB core consists of kite-type structural units similar to predictions by first-principles DFT calculations (Figure 1b). In other locations, the perfect GB structure is disrupted by GB defects, which influence impurity segregation. APT experiments reveal segregation of C and B, while surprisingly Al is even depleted at the boundary (Figure 1c). Furthermore, C segregation seems to be inhomogeneous following a particular segregation pattern, whereas B decorates the GB uniformly. In Figure 2 it can be seen that the GB contains defects which result in faceting of the interface. In future steps, we will explore the effects of local GB structure and impurity segregation of B and C on the impact of zinc (Zn) and manganese (Mn) adsorption and ultimately its impact on GB embrittlement.

Figure 1: Complete structural and compositional investigation of the symmetric Σ5 (310) [001] Fe tilt GB starting from the EBSD (a), HAADF-STEM imaging in combination with DFT (b) and APT (c) measurements.
Figure 2: Structural reconstruction by nanofacetting to compensate for grain boundary plane inclination.
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