Tiny Defects in Metals Mapped with Atomic Precision Reveal Hidden Strength

Triple junctions, critical components of grain boundary networks, profoundly influence material properties yet remain poorly understood as defective structures. Tobias Brink, Saba Saood, and Peter Schweizer, alongside colleagues from the Max Planck Institute for Sustainable Materials, have now characterised a triple junction within an aluminium thin film using a combined approach of scanning transmission electron microscopy and atomistic simulation. Their research reveals that these junctions exhibit dislocation-like behaviour, establishing a direct link between junction crystallography and microscopic grain boundary degrees of freedom. Significantly, the team developed a method for calculating the Burgers vector of triple junctions and demonstrated that their line energy follows the same principles as bulk dislocations, offering new insights into grain boundary evolution and network behaviour.

Atomic structure and dislocation character of aluminium triple junctions

Researchers have revealed the intricate defect structure of triple junctions in aluminium thin films, demonstrating a novel technique to connect their dislocation character to the microscopic degrees of freedom of the adjoining grain boundaries. These triple junctions, where three grain boundaries converge, profoundly influence the behaviour and properties of metallic materials, yet a comprehensive understanding of their atomic-scale structure has remained elusive.
This work combines high-resolution scanning transmission electron microscopy with advanced atomistic simulations to investigate a specific triple junction within an aluminium film exhibiting a strong {111} texture. By employing sampling methods, the research team successfully recreated the experimentally observed junction structure within a computational model, enabling detailed analysis of its fundamental properties.

A key achievement of this study is the development of a method for calculating the Burgers vector of the triple junction, a measure of the lattice distortion caused by the defect. This calculation establishes a direct link between the junction’s dislocation character and the specific crystallographic orientations of the surrounding grain boundaries.

Furthermore, the model allows for the precise determination of the junction line energy, revealing that it adheres to the same physical laws governing bulk dislocations. Investigations revealed a surprising finding: the experimentally observed triple junction does not represent the lowest possible energy configuration.

The research demonstrates that a range of possible triple junction structures exist for the observed grain boundaries, each differing in the magnitude of its Burgers vector. The fact that the actual junction observed is not the most energetically favourable suggests that the transformation kinetics of the junction line are too slow to be driven by the minimal energy difference.

This implies that factors beyond simple energy minimisation govern the evolution of grain boundary networks, opening new avenues for materials design and engineering. The findings have implications for controlling the mechanical properties of metals and developing strategies for manipulating grain boundary behaviour in thin films.

Atomic structure determination and computational modelling of an aluminium triple junction

Scanning transmission electron microscopy and atomistic simulations underpin the investigation of a triple junction within an Al thin film exhibiting a {111} texture. High-resolution STEM imaging initially captured the atomic structure of the junction, providing experimental data for comparison with computational models.

Subsequent to imaging, sampling methods were employed to construct an identical triple junction structure within a computer simulation environment. This modelling approach facilitated detailed analysis inaccessible through experiment alone. A novel technique was developed to calculate the Burgers vector associated with the triple junction.

This calculation directly links the junction’s dislocation character to the microscopic degrees of freedom present in the adjoining grain boundaries. The line energy of the modelled junction was then determined using an embedded atom method potential, mirroring the behaviour of a bulk dislocation. This allowed for quantitative comparison between the simulated and theoretical expectations for defect energetics.

The research further explored the range of possible triple junction configurations for the observed grain boundaries, varying the magnitude of their Burgers vector. Analysis revealed that the experimentally observed junction does not represent the lowest energy configuration, suggesting kinetic limitations prevent the system from readily adopting the most stable state.

The slow kinetics are likely due to the relatively small energy contribution of the triple junction itself, hindering spontaneous transformation of the junction line. This work establishes a methodology for connecting junction structure to its dislocation character and provides insights into the factors governing grain boundary network evolution.

Triple junction structure, dislocation character and grain boundary stability in aluminium thin films

Researchers utilized scanning transmission electron microscopy and atomistic simulations to investigate a triple junction within an Al thin film exhibiting a {111} texture. Through sampling methods, a model replicating the experimental junction structure was constructed, enabling the calculation of the junction’s Burgers vector and linking its dislocation character to the microscopic degrees of freedom of the adjoining grain boundaries.

The model’s junction line energy adhered to the same principles governing bulk dislocations. Simulations were performed using an embedded atom method potential for Al, selected for its accuracy in modeling grain boundary phases. A search for matching grain boundary phases to the experimental observations was conducted using GRIP, varying super-cell sizes from 1×1 to 5×5 in the grain boundary plane.

Molecular dynamics simulations at 300 K, with a 2 fs time integration step, were employed to assess the stability of the grain boundary structures. STEM image simulations were completed with a 300 keV electron probe, a spherical aberration of 10μm, and an annular detector range from 90 to 150 mrad, averaging 50 to 100 consecutive frames for image acquisition.

The study discovered a range of potential triple junctions for the observed grain boundaries, differing in the magnitude of their Burgers vector. The experimentally observed junction did not possess the lowest possible Burgers vector or energy, suggesting that kinetic limitations likely impede transformation to a lower energy state.

Random insertion of 8 to 21 atoms into a donut-shaped sample around the junction center was used to sample the triple junction structure, with at least 10 statistically independent realizations performed for each configuration. Minimization involved rigid body cooling from 50 K to 0.1 K over 50ps, followed by standard minimization with non-rigid bodies.

Molecular dynamics simulations at 300 K for 1ns confirmed the stability of selected triple junctions. The excess free energy of grain boundaries was defined as γA = [U]N−T[S]N−σ33[V ]N−A X i=1,2 tiσ3i−μ[N]N, where γ represents the grain boundary energy and [Z]N is defined as Z − N Nbulk Zbulk. This framework allows for the expression of defect thermodynamics in the isobaric grand canonical ensemble, with the resulting nonzero Φ equal to [Φ].

Burgers vector quantification links triple junction structure and energy in aluminium thin films

Triple junctions within grain boundary networks possess dislocation character, evidenced by a measurable Burgers vector. Investigations combining scanning transmission electron microscopy and atomistic simulations of an aluminium thin film reveal a direct connection between the Burgers vector of a triple junction and the microscopic structure of the adjoining grain boundaries.

A novel technique was developed to calculate this Burgers vector, demonstrating its relationship to the degrees of freedom within the grain boundaries themselves. The calculated line energy of the triple junction follows the same logarithmic law as that of bulk dislocations, indicating a divergence for infinitely-sized systems and highlighting the importance of considering system size when reporting line energies.

Numerous possible configurations exist for a given set of grain boundaries forming a triple junction, limited by the periodicity of coincident site lattice structures. The experimentally observed junction did not exhibit the lowest possible Burgers vector or line energy, suggesting that kinetic limitations prevent the system from readily reaching a minimum energy state.

Transformation of the junction requires coordinated movement of the grain boundaries and rearrangement of the core region, a process likely too slow to occur within the timeframe of the experiment. Furthermore, the interdependence of triple junction energies within a grain boundary network, coupled with elastic interactions, implies that junction types are influenced by sample history and network interactions, making accurate prediction challenging.