Methods

We combine Transmission and Scanning Electron Microscopy (TEM, in-situ, high resolution and automatic crystalline orientation mapping), in-situ Scanning and Atomic Force Microscopy (STM/AFM), and atomic modeling to address both the elementary mechanisms at play and their statistical impact on many grains.

Materials

Considering the complexity of the mechanisms, aluminium is chosen as a model material to perform both experiments and simulations. We will use both ultrafine grain (UFG) Al and Al bi-crystals through international collaborations with the groups of Prof. Molodov, RWTH, Aachen, Germany (for bicrystals) and Prof. Pippan Erich Schmid Inst., Leoben, Austria (for bulk UFG metals). We also work with O. Pierron in Georgia Tech (for nc-Au thin films), T. Pardoen in U. Louvain (for nc-Al films) and K.J. Hemker in Johns Hopkins University (for nc-Al and nc-Cu films). We also have some recent agreement with J. Rajagopalan, at Arizona State University to share nc-Al and nc-Au thin films.

In-situ experiments

In situ TEM and in situ UHV STM/AFM allows the dynamic observation of changes happening under controlled stress application in crystalline materials. Their combination allows a 3D reconstruction of the shear associated with GB migrations.

For this we will use the TEM facilities at CEMES (Jeol2010, CM20FEG and Hitachi3300 microscopes) in combination with customed holders (high temperature straining or "push-to-pull" device with nanoindentation holder), and CCD camera (MEGAVIEW III and One View, Gatan) For STM/AFM experiments, we use the capabilities of the unique Nanoplast experimental device, available at Institut Pprime.

High resolution TEM

High resolution TEM is probably the only experimental method able to describe accurately the structure of grain boundaries and their defects (disconnections). Because we use aberration-corrected TEMs, we also avoid the interfacial image shifts. This is of importance, especially to compare real GB structures with snapshots obtained by MD simulations. Image simulations and interpretation in HR mode are carried out at ICMPE.

Modeling

Nudge Elastic Band method, using an EAM potential, is extremely time consuming, but is a method truly adapted to explore the mechanisms involved in GBs. It is implemented in the molecular dynamics package LAMMPS and run either at CEMES on small clusters or at CALMIP (regional scientific computing facility).