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Grain (Interfaces) Boundaries: Behavior and Segregation – GiBBS

GiBBS

Grain (Interfaces) Boundaries: Behavior and Segregation

Understanding the Anisotropy and Environment-Dependence of GB at the atomic scale

The majority of solid materials are polycrystals, i.e. a collection of single crystals joined by a 3D network of internal interfaces called grain boundaries (GBs). These extended defects have a structure and composition different from the ones of the adjacent grains. This impacts not only the mechanical, thermal and transport properties, but also the integrity and lifetime of these materials.<br /> While modelling is widely developed for metallic alloys, the case of non-metal/metal segregation still has to be unravelled. Recent ab initio studies have underlined the importance of chemical effects (charge transfer) on embrittlement mechanisms. But a large gap exists between these 0K studies of GBs that necessarily assume both the structure and chemical configurations, and the available experimental data on interfaces that are limited by the high-tech tools necessary to access information at the monolayer scale and include temperature and compositional effects.<br />The GiBBS project aims at filling this specific gap, by gathering together experimentalists and simulators from the metallurgical and thermodynamical communities (IMN, CINaM, ARMINES-SMS) who will work on non-metallic solute segregation at GBs by integrating new concepts such as complexions and GB energy anisotropy which are necessary to explain phenomena as core grain structural transitions and embrittlement. By coupling atomic-scale computer simulations and experiments on surfaces and GBs of bi-crystals and textured polycrystals of the nickel-sulfur system, the GiBBS consortium aims at bringing an original and critical contribution to the understanding of the behavior of interfaces in polycrystalline materials while accounting for their diversity.

-Computational atomic-scale approaches couoling first principles, force-field methods and development of knwoledge-based models
- Surface and bi-crystal characterization
- Segregation quantification by WDS and XPS techniques, set up of XPS spectra (simulated and experimental)
- Synthesis of textured thin films

- Segregation quantities strongly differ with the GB type in excellent agreement with experimental observations.

- Enligthning limitations of current force-field methods to describe metal/non-metal interactions.

- Synthesis of a limited number of grain boundaries in <111> Ni textured films on saphir r(1-102) substrate

- Quantification of S surface segregation by different techniques (XPS and WDS) that reveal in excellent agreement

- Excellent agreement between simulated and experimental XPD spectra of Ni(100) surface

- Classification of GB type as a function of segregation quantities and embritlling potencies

- Studying the influence of the GB core

- Loading bi-crystals and textured films

Two-dimensional versus Three-dimensional Constraints in Hetero-Epitaxy/Orientation Relationships
P. Wynblatt, D. Chatain
Journal of Materials Science 52(16), 9630–9639 (2017)
DOI: 10.1007/s10853-017-1145-z

The vast majority of solid materials are polycrystals, i.e. a collection of single crystals joined by a 3D network of internal interfaces called grain boundaries (GBs). These extended defects have a structure and composition different from the ones of the adjacent grains. This impacts not only the mechanical, thermal and transport properties, but also the integrity and lifetime of these materials.
While studied for now a century if we refer to the very pioneered works (1912), these GBs still raise many topical issues on their behavior as a function of their misorientation and composition. These questionings all share a common point: it is not possible to use an "average" GB to investigate and understand a polycrystal properties since GBs have anisotropic properties depending on the crystallographic orientation of their two abutting grains. This is clearly illustrated in the GB distribution within a pure polycrystal which has been recently demonstrated to strongly depend on the GB energy anisotropy.
Real materials contain impurities - especially nonmetallic - which accumulate (segregate) at certain GBs, either as low-dimensional compounds or solid solutions, and strengthen but more often weaken the metallic polycrystal. Besides, as a function of their concentration in the bulk, these species can infer discontinuous changes and structural phase transitions in the segregation profiles. In order to include both the chemistry and structural specificities of an interface state into a unique framework, a new terminology referred to as complexions has been introduced and made compatible with the classical rules of interface thermodynamics [Tan06, Fro13, Fro13b].
As surface science reaches maturity, interface science still needs large input to reach a better understanding of interfaces at all scales. While modelling is widely developed for metallic alloys, in particular by separating the driving forces of impurity segregation, the case of non-metal/metal segregation still has to be unravelled. Recent ab initio studies have underlined the importance of chemical effects (charge transfer) on embrittlement mechanisms. But a large gap exists between these 0K studies of GBs that necessarily assume both the structure and chemical configurations, and the available experimental data on interfaces that are limited by the high-tech tools necessary to access information at the monolayer scale and include temperature and compositional effects.
The GiBBS project aims at filling this specific gap, by gathering together experimentalists and simulators from the metallurgical and thermodynamical communities (IMN, CINaM, ARMINES-SMS) who will work on non-metallic solute adsorption at GBs of a metal. They will integrate new concepts such as complexions and GB energy anisotropy which are necessary to explain phenomena as core grain structural transitions, abnormal grain growth or embrittlement. By coupling atomic-scale computer simulations and experiments on surfaces and GBs of bi-crystals and textured polycrystals of the nickel-sulfur system, the GiBBS consortium aims at bringing an original and critical contribution to the understanding of the behavior of interfaces in polycrystalline materials while accounting for their diversity, their misorientation-induced energy anisotropies and their property-dependent thickness. In this strictly fundamental-oriented study, the Ni-S system is chosen as a model system for metal/non-metal (M/NM) interactions; it should be noted that polycrystalline Ni is widely studied and largely employed in metal industry as a superalloy that may contain not less than 20 elements among which sulfur is a well-known embritlling impurity.

Project coordination

Isabelle Braems (Institut des Matériaux Jean Rouxel)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ARMINES-SMS ARMINES Centre SMS de l'Ecole des Mines de Saint-Etienne
IMN Institut des Matériaux Jean Rouxel
CINAM Centre National de la Recherche Scientifique délégation Provence et Corse - Centre Interdisciplinaire de Nanoscience de Marseille

Help of the ANR 414,321 euros
Beginning and duration of the scientific project: November 2015 - 48 Months

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