Crystalline defects and deformation mechanisms
Staff: M. Benoit, D. Caillard, A. Couret, A. Coujou, N. Combe, J. Douin, M. Legros, C. Levade, F. Mompiou, J. Morillo, F. Pettinari-Sturmel, H. Tang
Displacement field of a screw dislocations dipole in hcp titanium (numerical simulation with periodical conditions).
At the nanoscopic scale, the plastic deformation of crystalline materials proceeds by nucleation and motion of defects trough the periodic stacking of atoms. The mobility these defects - mainly dislocations and interfaces - is accordingly the factor which determines the mechanical properties as a function of stress and temperature. This shows the importance of understanding the structure of defects, and their mechanisms of motion.
Bright field micrography showing a cross section of a 200nm thick Al polycrystalline thin film deposited on Si. Note the presence of a single dislocation in the grain in the middle of the picture.
The core structure and the mobility of dislocations and interfaces are studied by simulations at the atomic scale (ab-initio and empirical), and by transmission electron microscopy and/or in situ straining experiments in the electron microscope. Their individual and collective behaviour are then modelled at various scales, in order to determine the exact origin of the mechanical properties.
Micrographie en champ clair montrant une coupe transverse d’un film d’aluminium polycristallin de 200nm d’épaisseur, épitaxié sur du silicium. Notez la présence d’une unique dislocation (grain au centre).
The materials studied are metals and alloys with various structures, semiconductors, and quasicrystals. Model materials are privileged to study elementary mechanisms.Special attention is given to the plastic deformation of nanostructured materials and thin layers, where the mean-free path of defects is very small. This results in fairly high mechanical properties. Lastly, dislocations which strongly disturb the normal behaviour of semiconducting devices must be eliminated, which requires a good knowledge of their physical properties.
