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Correlating the State and Properties of Grain Boundaries

Periodic Reporting for period 3 - GB-CORRELATE (Correlating the State and Properties of Grain Boundaries)

Reporting period: 2021-08-01 to 2023-01-31

The project GB-CORRELATE deals with one class of imperfections in materials - grain boundaries - which are of outmost importance in nanocrystalline materials. Crystalline materials are composed of a multitude of grains (each grain is a single crystal) separated by grain boundaries. If grain size gets smaller and smaller - like in nanocrystalline materials - the grain boundary volume can exceed several 10% of the total material volume and become most important for the resulting properties. The atomic coordination and chemistry of such grain boundaries may undergo phase transitions, abrupt changes in structure and chemistry, which again will impact the material behavior - like strength, thermal stability, electrical resistance – even for conventional materials. However, this interplay between grain boundary phases and material properties is not yet understood. Experimentally, grain boundaries are difficult to study - it needs atomic resolution and sensitivity with respect to chemistry. In addition, it is unknown under which conditions phase transformations of grain boundaries occur. A fundamental understanding requires atomistic modelling connected with smart experiments.
In the first part of the project we started to prepare grain boundaries which we can study further by experiments and simulations. Our manuscript "Synthesis and mechanical testing of grain boundaries at the micro and sub-micro scale" by Nataliya V. Malyar, Hauke Springer, Jürgen Wichert, Gerhard Dehm and Christoph Kirchlechner, Materials Testing 61 (1), 5-18 (2019) summarizes on sample preparation routes for grain boundary studies. First atomistic insights in a chemically driven change in grain boundary phase (i.e. atomic structure and chemical composition changes) was reported in our manuscript "Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper " by Nicolas J. Peter, G. Dehm et al, Physical Review Letters 121, 255502 (2018). This grain boundary phase transformation seems to impact also the mechanical properties and is currently under further investigation.

If material scientists understand fundamentally grain boundaries and their possible phase transitions a new door into materials design is opened. Materials with extreme thermal stability, high strength and high fracture resistance can be designed, but also materials that act as sensors and fail for example quickly under certain environmental conditions reporting critical conditions (like acidic or certain gas pollution). These materials may enter into applications for transportation, energy harvesting, medical applications and/or electronic material systems.
The project had a smooth start in August 2018 with hiring of a first PhD student (Thorsten M.; finished Nov. 2019) and fabrication of samples with dedicated grain boundary structures. For measurements of the electrical conductivity of samples with and without grain boundaries a high resolution electrical resistance instrument has been installed and a postdoc (Dr. Hanna B.) has started to measure the influence of different grain boundary types on the electrical properties of metals as a function of tilt angle and chemical composition. First interesting results on grain boundary structures, chemistry and phase transformations have been accomplished on pure and segregated grain boundaries in Cu. For pure Cu, a (Sigma 19b) tilt grain boundary was found to possess two different grain boundary motifs presenting two different grain boundary states. A novel line defect separating both phases has been discovered and atomistic simulations by our external collaborator Tim F. (USA) indicate that the mobility of this line defect decides on the grain boundary phase transformation. Based on these results and chemical aspects new project leaders have been implemented (Dr. Jazmin D., Dr. Tobias B.) and several new PhD students (Lena F., Saba A., Nicolas P.) complement the team.
Our finding that grain boundaries can undergo phase transformations (abrupt changes in atomic structure) in pure metals was unexpected and beyond the state of the art. These observations have been accepted for publication in Nature in February 2020. A new discovery was that although Cu exists only in a face centered cubic structure grain boundaries can switch between different grain boundary states. In pure Cu especially shear stresses are required to induce phase transformations. The newly discovered line defect separating both phases may become important for thermal stability of grain boundary phases and as such for material properties. The velocity of its motion seems to trigger or block the phase transition from one state to the other. Based on this breakthrough we hope to unravel further grain boundary transitions and to link them to grain boundary mediated properties, like electrical resistance, grain boundary mobility and mechanical properties.
Schematic of a nanocrystalline material. Black lines are grain boundaries.
Atomic resolved electron microscopy image of a grain boundary in Cu with two different phases.