Research Projects - Grain Boundaries

Grain boundaries are one of the most prominent defects in engineering materials separating different crystallites, which determine their strength, corrosion resistance and failure. Typically, these interfaces are regarded as quasi two-dimensional defects and controlling their properties remains one of the most challenging tasks in materials engineering. However, although more than 50 years ago the concept that grain boundaries can undergo phase transformations was established by thermodynamic concepts, they have not been considered, since they could not be observed. Through a combination of atomic resolution scanning transmission electron microscopy (STEM) and advanced atomistic modelling we establish pathways to directly observe and explore grain boundary transitions in metallic alloys. more

In this project, we investigate a high angle grain boundary in elemental copper on the atomic scale which shows an alternating pattern of two different grain boundary phases. This work provides unprecedented views into the intrinsic mechanisms of GB phase transitions in simple elemental metals and opens entirely novel possibilities to kinetically engineer interfacial properties.
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The mostly unknown influence of Ag as solute segregate at copper grain boundaries on mechanical properties is studied by aberration-corrected STEM from an atomistic structural point of view and by in-situ TEM nanocompression experiments to visualize dislocation-grain boundary interactions. more

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 scanning transmission electron microscopy (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 possible effect on liquid metal embrittlement are currently investigated by atomic scale microscopy. more

Grain boundaries are one of the most important constituents of a polycrystalline material and play a crucial role in dictating the properties of a bulk material in service or under processing conditions. Bulk properties of a material like fatigue strength, corrosion, liquid metal embrittlement, and others strongly depend on grain boundary properties such as cohesive strength, energy, mobility, etc. These boundary properties in turn are governed by the structure and chemistry of a grain boundary. Furthermore, it has recently been realized that grain boundaries themselves can be described as interface-stabilized phases. We are just at the advent to utilize the phase character of grain boundaries as a material design element. more

Grain boundaries (GB) are typically considered as 2-dimentional interfaces separating two differently oriented crystals inside a polycrystalline material. Understanding their structure and composition down to nano-scale regime is fundamental to explain their macroscopic properties. Discerning the contribution of GBs towards strength, corrosion resistance and high temperature properties is necessary to boost our efforts of making metals lighter, stronger and hence greener. Titanium (Ti) is one of the most attractive materials for aerospace and bio-medical industries where high strength to weight ratio and chemical inertness play a vital role. Ti also makes an interesting case owing to its allotropic transition from the hexagonal close packed (HCP) to the body-centred cubic (BCC) phase at 882 ºC. Despite of its widespread industrial use very little is known about its GB structure and their possible transitions. Hence, the aim of this project is to look deeply into the atomic structure of GBs in Ti and to resolve the impact of alloying additions on their structure through advanced transmission electron microscopy (TEM) techniques. A challenging, yet intriguing task is to obtain defined tilt GBs in Ti and we found that epitaxial thin films are excellent candidates for generating thin films containing pure tilt boundaries in them. more

Extensive research has been focusing on face-centered cubic (FCC) high entropy alloys (HEAs) to establish the underlying mechanisms for their outstanding mechanical properties, for instance, an impressive combination of strength and ductility at cryogenic temperatures. One possibility suggested is that these new types of alloys show stronger Hall-Petch strengthening, where the grain size has a stronger impact on their yield strength. The origin of this grain boundary strengthening in HEAs seems to be originating from the different atomic radii of the supersaturated solid solution inducing high lattice strains. Hence, resolving the impact of compositional complexity on the atomic structure of grain boundaries in HEAs is crucial to understand their role in the strengthening mechanisms. more

This project with the acronym GB-CORRELATE is supported by an Advanced Grant for Gerhard Dehm by the European Research Council (ERC) and started in August 2018.
The project GB-CORRELATE explores the presence and consequences of grain boundary phase transitions (often termed “complexions” in literature) in pure and alloyed Cu and Al. If grain size gets smaller and smaller - like in nanocrystalline materials - the grain boundary (GB) volume can exceed several 10% of the total material volume and become a powerful lever to manipulate and set properties. The atomic coordination and chemistry of such GBs may undergo phase transitions, abrupt changes in structure and/or chemistry, which will impact the material behavior - like strength, thermal stability, electrical resistance – even for conventional materials. However, this interplay between GB phases and material properties is poorly understood. Experimentally, GBs are difficult to study - it needs atomic resolution and sensitivity with respect to chemistry to uncover their structure and possible complexions. In addition, it is unknown under which conditions phase transformations of GBs occur. A fundamental understanding requires atomistic modelling connected with smart experiments.
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