WEB Atomic resolution study of impurity segregation to ∑5 (310) grain boundaries in ferritic iron
It is well known that grain boundaries (GBs) dominate the physical properties of polycrystalline materials . GB segregation of impurity elements can vary the structure, cohesive strength and therefore the mechanical properties. Therefore it is important to understand the atomic structure and segregation behavior of different elements of GBs to optimize the properties of polycrystalline materials. Iron and its alloys (steels) show high strength and ductility. However, segregation of impurities to GBs can reduce their cohesive strength and with this lead to embrittlement. The key to design strategies to prevent GB failure is to examin the atomistic structure of GBs to develop an holistic understanding of their properties. This is quite challenging as they depend on at least five geometrical parameters. Further, it is not well known how co-segregation of two or more elements influences the GB properties .
Here, we investigated the nanostructure and chemistry of ∑5  (310) tilt GBs in ferritic Fe-2wt%Al using aberration-corrected scanning transmission electron microscopy (STEM) as well as atom probe tomography (APT). The grain boundary was grown by a modified Bridgeman technique. In a first approach, electron backscatter electron diffraction (EBSD) measurements were applied to the samples to determine the microscopic structure of the GB.
Next, high resolution HAADF-STEM images provided information on the atomic arrangement of the previously characterized GB (Figure 1). It was found that the GB consists of kite-type structural units similar to predictions by first-principles calculations by Scheiber et al. . However, the the GB also contains defects such as facets or steps, leading to different local reconstructions, which can largely influence impurity segregation and GB migration. The decoration of impurity elements of the same GB was explored by APT experiments. A clear segregation of carbon and boron were observed, while surprisingly aluminum is depleted at the boundary (see Figure 2). The latter observation is in contradiction to first-prniciples calculations, where segregation of aluminum was predicted. The mechanisms leading to this unexpected segregation behavior and the connection to the atomic structure of the GB will be discussed in detail.
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|Presentation||Figure 1||HAADF-STEM image of a symmetric Σ5 (310)  tilt GB. The inset shows the fast Fourier transform (FFT) and confirms that both grains are misoriented by 37º around the common  tilt axis.||106 KB||Download|
|Presentation||Figure 2||a) Secondary electron micrograph of a sharp APT needle containing the GB. b) Atom distribution showing a segregation of carbon (C) and boron (B) to the boundary. c) The corresponding concentration profile extracted across the GB shows a peak concentration of of ~2.4% for C and ~1.1% for B, while the aluminium concentration even decreases at the GB.||124 KB||Download|