[Paper Review] Atomic motifs govern the decoration of grain boundaries by interstitial solutes

Xuyang Zhou, Ali Ahmadian+5 more|arXiv (Cornell University)|Nov 15, 2022

Corrosion Behavior and Inhibition4 citations

TL;DR

This study reveals that atomic motifs—specific structural units within grain boundaries—govern the decoration of grain boundaries by interstitial solutes (B and C) in Fe-based alloys. Using multi-scale characterization from macroscale misorientation to electronic structure, the authors demonstrate that even identical grain boundary misorientations exhibit distinct chemical compositions and atomic arrangements when the boundary plane inclination changes, proving that atomic motifs are the decisive factor in controlling chemical segregation, enabling targeted design of grain boundaries to resist corrosion and embrittlement.

ABSTRACT

Grain boundaries, the two-dimensional (2D) defects between differently oriented crystals, control mechanical and transport properties of materials. Our fundamental understanding of grain boundaries is still incomplete even after nearly a century and a half of research since Sorby first imaged grains. Here, we present a systematic study, over 9 orders of magnitude of size scales, in which we analyze 2D defects between two neighboring crystals across five hierarchy levels and investigate their crystallographic, compositional, and electronic features. The levels are (a) the macroscale interface alignment and grain misorientation (held constant here); (b) the systematic mesoscopic change in the inclination of the grain boundary plane for the same orientation difference; (c) the facets, atomic motifs (structural units), and internal nanoscopic defects within the boundary plane; (d) the grain boundary chemistry; and (e) the electronic structure of the atomic motifs. As a model material, we use Fe alloyed with B and C, exploiting the strong interdependence of interface structure and chemistry in this system. This model system is the basis of the 1.9 billion tons of steel produced annually and has an eminent role as a catalyst. Surprisingly, we find that even a change in the inclination of the GB plane with identical misorientation impacts GB composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, which control the most important chemical properties of the grain boundaries. This finding 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.

Motivation & Objective

• To understand the structural and chemical factors governing interstitial solute decoration at grain boundaries in metals.
• To resolve the long-standing gap between grain boundary structure and chemical composition in materials science.
• To identify the hierarchical structural level that most strongly controls chemical segregation at grain boundaries.
• To enable the targeted engineering of grain boundary chemistry for improved material performance.

Proposed method

• Systematic analysis of grain boundaries across five hierarchical levels: macroscopic misorientation, mesoscopic plane inclination, atomic motifs and facets, chemical composition, and electronic structure.
• Use of Fe-B-C as a model system due to strong coupling between interface structure and chemistry.
• Employment of advanced characterization techniques including atom probe tomography and transmission electron microscopy to probe atomic-scale structure and composition.
• Computational modeling and electronic structure analysis to link atomic motifs to solute segregation energetics.
• Controlled variation of grain boundary plane inclination while holding misorientation constant to isolate structural effects.
• Correlation of atomic motifs with solute segregation behavior across multiple length scales.

Experimental results

Research questions

• RQ1 How does the inclination of the grain boundary plane affect solute segregation when misorientation is held constant?
• RQ2 Which hierarchical level of grain boundary structure most strongly governs the chemical composition of interstitial solutes?
• RQ3 What is the role of atomic motifs in mediating the segregation of B and C at grain boundaries in Fe?
• RQ4 How do electronic structure features of atomic motifs influence solute binding and decoration?
• RQ5 Can atomic motifs be used as design parameters to passivate grain boundaries against degradation?

Key findings

• Even with identical misorientation, changing the inclination of the grain boundary plane leads to significant differences in solute (B and C) segregation and atomic arrangement.
• Atomic motifs—localized structural units within the boundary—were identified as the dominant factor controlling chemical composition and solute decoration.
• The electronic structure of specific atomic motifs correlates strongly with interstitial solute binding energy and segregation tendency.
• Variations in boundary plane inclination alter the population and stability of atomic motifs, thereby modulating solute segregation.
• The findings close a critical gap in understanding the structure-composition relationship in grain boundaries, enabling predictive design of grain boundary chemistry.
• This work establishes atomic motifs as the key design parameter for engineering grain boundaries to resist corrosion and hydrogen embrittlement.