See how grain boundaries transform in a metal


Fig. 1: Atomic resolution scanning transmission electron microscopy (STEM) image of a phase transformation at grain boundaries in elemental copper. The corresponding grain boundary phases are predicted by the grain boundary structure search. The dynamics of phase transformation at grain boundaries as observed experimentally is modeled by molecular dynamics simulations. Credit: Christian Liebscher, Max-Planck-Institut für Eisenforschung GmbH

Grain boundaries are one of the most important defects in engineering materials separating different crystallites, which determine their strength, corrosion resistance and fracture. Typically, these interfaces are considered to be near two-dimensional defects, and controlling their properties remains one of the most difficult tasks in materials engineering. However, over 50 years ago, the concept that grain boundaries can undergo phase transformations was established by thermodynamic concepts, but they were not taken into account because they could not be observed. . Dr. Christian Liebscher, head of the Advanced Transmission Electron Microscopy group and members of his team at the Max-Planck-Institut für Eisenforschung (MPIE), have now found a way to directly observe grain boundary transitions experimentally. With colleagues from Lawrence Livermore National Laboratory (LLNL) in the United States who modeled grain boundary transformations, the researchers published their recent findings in Nature.

Their results are surprising: “The search for congruent transformations has all aspects of finding a needle in a 6 + C dimensional haystack,” explains John W. Cahn, materials scientist and thermodynamics expert. The team even found two of these “needles”. The key was to use the MPIE’s atomic resolution microscopes to directly visualize the transformation interfaces.

“We did not expect to see phase transformations at grain boundaries, but our results clearly show that two grain boundary patterns coexist with different atomic arrangements. However, the orientation of the grain boundary plane, the disorientation crystallites and chemical composition do not change. With these observations, we need to rethink the behavior of interfaces while exposing a material to temperature and / or stress, ”says Liebscher.

He and his colleagues analyzed thin films of pure copper, particularly by atomic-resolution transmission electron microscopy. In this way, they unlocked the phases at the grain boundaries and proved their coexistence with atomic precision. The phases can be atomistically described as patterns with a structure in the form of pearls and dominoes (see Fig. 1). Dr Timofey Frolov and Dr Robert Rudd of the Lawrence Livermore National Laboratory modeled the phases of grain boundaries. They used a new grain boundary structure search algorithm, capable of finding structures observed experimentally. In addition, their finite temperature molecular dynamics simulations explore transformation kinetics. The predicted structures not only closely resemble experimental observations, but demonstrate that phases at grain boundaries can transform into each other by changing temperature or stress. In addition, the simulations indicate that the phase junction at the grain boundaries, a new line fault that was not taken into account before, controls speed.

“We found through modeling that the speed of transformation largely depends on phase junction migration. It only takes a few tens of nanoseconds in the event of a short fault to complete the transformation of the domino structure into a pearl. Whereas no transformation is observed when the length of the defect exceeds a few nanometers and takes place below 500 K ”, explains Dr Thorsten Meiners, first author of the publication and former doctoral student at MPIE. In addition, the grain boundary phases are characterized by different properties, which determine how the interface phases move, how they absorb the elements of impurities or how they deform mechanically.

“Therefore, understanding how grain boundaries transform offers a new insight into as yet unexplained material phenomena, such as abnormal grain growth, and opens up new avenues for viewing interface transitions as an element of material design,” says Prof. Gerhard Dehm, director of the MPIE. The different states of grain boundaries or interfaces can have a strong impact on the corrosion behavior of materials, their behavior under catalytic conditions or even play an important role in the failure of microelectronic devices. Scientists aim to extend current observations to experiments performed at different temperatures, under stress and in the presence of impurities. The objective is to establish a complete understanding of these phase transformations, thus making it possible to design the properties of materials based on holistic grain boundary engineering.

Find order and structure in atomic chaos where materials meet

More information:
Thorsten Meiners et al. Observations of phase transformations at grain boundaries in an elementary metal, Nature (2020). DOI: 10.1038 / s41586-020-2082-6

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