Atomic Scale Analysis Reveals the Interplay between Grain Boundary Structure and Composition

Boron and carbon preferentially segregate at grain boundaries, i.e., two-dimensional defects between crystals with different orientations, to reduce the Gibbs energy of the system. The adsorption of these two elements can improve the cohesion of grain boundaries, making them important alloying elements for improving the mechanical properties of steels.<br/>However, even for the simplest Σ5-grain boundaries, the atomic structure and composition of the grain boundaries are poorly understood. Several factors contribute to this difficulty: 1) representing the structure of grain boundaries requires more than five degrees of kinematic freedom; 2) the interplay between grain boundary structure and composition remains elusive, especially when it comes to imaging and quantifying light interstitial solutes, e.g. boron and carbon.<br/>To address these challenges, we have designed special sample geometries that restrict the kinematic freedom of the grain boundary, allowing for a systematic investigation of the relationship between structure and composition. Additionally, we have developed custom software to quantify the solute composition and distribution along the grain boundary planes using atom probe tomography (APT). This information is then correlated with the atomic structure obtained by state-of-the-art differential phase contrast (DPC)–four-dimensional scanning transmission electron microscopy (4DSTEM), which allows direct imaging of both the light solute atoms and the heavier iron atoms at the grain boundaries.<br/>Our combined results demonstrate that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. It is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries.<br/>Furthermore, when alloyed with boron, a grain boundary with the same crystallographic configuration can exhibit a nanoscale morphology transition from wavy to serrated. In-situ transmission electron microscopy (TEM) heating experiments demonstrate that at elevated temperatures, wavy or serrated grain boundaries tend to flatten. Interestingly, the presence of boron increases the temperature at which flattening occurs by at least 200 K. We attribute this difference to local grain boundary phase transformations triggered by boron atoms, as well as the pinning effect of boron-enriched nano-particles. Density functional theory calculations support the underlying mechanism of boron-induced local grain boundary phase transformation.<br/>This work not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure.