Christian Brandl

Plasticity in Cu(111)/Cu46Zr54 glass nanolaminates under uniaxial compression

We perform large-scale molecular dynamics simulations to investigate plasticity in Cu/Cu46Zr54 glass nanolaminates under uniaxial compression. Partial and full dislocations are observed in the Cu layers, and screw dislocations are seen near the amorphous-crystalline interfaces (ACIs). Nucleation of shear bands in a glass layer is directly induced by the dislocations in the neighboring crystalline Cu layer through ACIs, and they grow from the ACIs into the glass layer and absorb ambient shear transformation zones. Plasticity in the glass layers is realized via pronounced, stable shear banding.

Slip transfer through a general high angle grain boundary in nanocrystalline aluminum

The atomistic details of a slip transfer through a general high angle grain boundary in three dimensional nanocrystalline Al are reported and discussed in terms of possible implications for mesoscopic simulation models.

Strain rates in molecular dynamics simulations of nanocrystalline metals

To study deformation mechanisms using molecular dynamics, very high strain rates have to be applied. The effect of lowering the strain rate by two orders of magnitude on the deformation characteristics of nanocrystalline Al was investigated. For the highest strain rate, the onset of dislocation propagation is delayed, resulting in a stress overshoot. With decreasing strain rate the grain-averaged resolved shear stress reduces and cross-slip occurs more frequently. However, even at the lowest applied strain rate the grain boundary network can not accommodate all arriving dislocations, illustrating the challenge to determine the rate-limiting deformation mechanisms for experimental conditions.

Dislocation mediated plasticity in nanocrystalline Al: the strongest size

Molecular dynamics simulations of nanocrystalline FCC metals revealed the strain rate dependence to be reflected in the critical resolved shear stresses for dislocation propagation. The strain rate and temperature dependence study is complemented by a stress and grain size dependence supporting consistently the importance of thermally activated dislocation mediated plasticity in the inverse Hall–Petch regime. A model is presented demonstrating that the strongest size can as well be explained on the sole basis of dislocation mechanisms.

Atomistic simulation of a dislocation shear loop interacting with grain boundaries in nanocrystalline aluminium

An atomistic simulation methodology is presented in which well-defined dislocation loops can be introduced on arbitrary slip-systems of nanocrystalline (nc) grains. This approach allows one to study loop expansion and deposition of dislocation segments into the surrounding grain boundaries (GBs) at finite temperature. Such a dislocation loop creation method is intended to aid in the systematic study of the dislocation/GB interaction within a fully three-dimensional GB network geometry, and will also facilitate the atomistic study of the pile-up phenomenon as a function of GB misorientation.

In situ observation of deformation processes in nanocrystalline face-centered cubic metals

Summary The atomistic mechanisms active during plastic deformation of nanocrystalline metals are still a subject of controversy. The recently developed approach of combining automated crystal orientation mapping (ACOM) and in situ straining inside a transmission electron microscope was applied to study the deformation of nanocrystalline PdxAu1− x thin films. This combination enables direct imaging of simultaneously occurring plastic deformation processes in one experiment, such as grain boundary motion, twin activity and grain rotation. Large-angle grain rotations with ≈39° and ≈60° occur and can be related to twin formation, twin migration and twin–twin interaction as a result of partial dislocation activity. Furthermore, plastic deformation in nanocrystalline thin films was found to be partially reversible upon rupture of the film. In conclusion, conventional deformation mechanisms are still active in nanocrystalline metals but with different weighting as compared with conventional materials with coarser grains.

Grain Boundary Motion under Dynamic Loading: Mechanism and Large-Scale Molecular Dynamics Simulations

Grain boundaries (GBs) are not static structures during shock loading, despite the short timescales. We present a mechanistic explanation for why non-coherent Σ 3 GBs are particularly mobile, due to their consisting of coherent twin boundaries every third (111) glide plane, separated by incoherent twin boundary segments with three Shockley partial dislocations that can readily glide into either grain. Asymmetric GBs with such structures can thus move in response to the elastic driving force provided by uniaxial compression. We present large-scale molecular dynamic simulations that illustrate this mechanism, which explains the Σ 3 GB faceting recently observed in shock-recovered copper multi-crystals.

Self-Healing and Shape Memory Effects in Gold Microparticles through the Defects-Mediated Diffusion

Some metal alloys subjected to irreversible plastic deformation can repair the inflicted damage and/or recover their original shape upon heating. The conventional shape memory effect in metallic alloys relies on collective, or “military” phase transformations. This work demonstrates a new and fundamentally different type of self‐healing and shape memory in single crystalline faceted nano and microparticles of pure gold, which are plastically deformed with an atomic force microscope tip. It is shown that annealing of the deformed particles at elevated temperatures leads to nearly full recovery of their initial asymmetric polyhedral shape, which does not correspond to global energy minimum shape. The atomistic molecular dynamic simulations demonstrate that the shape recovery of the particles is controlled by the self‐diffusion of gold atoms along the terrace ledges formed during the particles indentation. This ledge‐guided diffusion leads to shape recovery by the irreversible diffusion process. A semiquantitative model of healing developed in this work demonstrates a good agreement with the experimental data.

The role of the structure of grain boundary interfaces during shock loading

In order to understand the role of interface structure during shock loading, and specifically the role of interfaces in damage evolution due to shock, four copper bi-crystal grain boundaries (GBs) were studied under shock loading and incipient spall conditions. These boundaries, two [100]/[111] boundaries and two [100]/[100] boundaries, were characterized prior to deformation using optical microscopy (OM), electron back scattered diffraction (EBSD), and transmission electron microscopy (TEM) to determine axis/angle pair relationships and interface plane. Samples containing these boundaries were then subjected to incipient spall at 2.1 GPa and shock loading at 10 GPa, respectively, in an 80 mm gas gun. Samples were soft recovered and characterized post-mortem via EBSD and TEM. Preliminary results indicate that typical GBs readily form damage during shock loading but that special boundaries, such as twin boundaries, are resistant to failure. Differences in slip and defect transmissibility across these types...

Athermal critical stresses for dislocation propagation in nanocrystalline aluminium

Molecular dynamics simulations investigating the mechanical properties of nanocrystalline Al have revealed the importance of both dislocation nucleation at grain boundaries and subsequent propagation as possible rate-limiting microscopic plastic process. Here, we present an approach to determine the athermal stress–strain behaviour of selected dislocations as they propagate within the nanocrystalline environment. In the first case study, the critical resolved shear stress to depin resulting into a planar propagation is found to be 735 MPa. In a second case study, a pinned dislocation is found to cross-slip at a critical resolved shear stress of 372 MPa to circumvent a near unfavoured grain boundary region.