Most of the metallic materials are composed of micrometer large crystalline grains. It is now possible to elaborate nanocrystalline (grain size less than 100nm) or ultrafine grained materials (size less than 1μm).
At this length scale, new physical properties emerge : for instance, metallic nanocrystals exhibit an increased yield stress up to 10 times larger than their microcrystalline counterparts.
- Micrographie en champ sombre extraite d’une séquence de déformation in situ dans l’aluminium nanocristallin. On remarque le mouvement rapide du joint de grain dans la partie inférieure de l’image. La ligne pointillée en b) indique la position du joint en a).
- © CEMES-CNRS
Thus, since few years, these materials stimulate both academic and industrial research works. The increased mechanical strength is partly explained by the confinement of the dislocations which are usually involved in the plastic deformation of crystals. The identification of elementary plastic deformation mechanisms in small scale materials has attracted a lot interest in the scientific community. These mechanisms are not very well documented, mainly because of the difficulty to assess the dislocation activity in very small grains. More, several different mechanisms can compete : atomic shuffling, long range diffusion mechanisms, partial dislocation nucleation...
Since several years, in situ Transmission Electron Microscopy (in situ TEM) has appeared as a powerful tool to probe the elementary plastic mechanisms at a pertinent length and time scales.
CEMES is recognized as a world leader in this field. We perform in situ mechanical tests in a large range of temperatures. We contribute to the improvement of dedicated TEM holders (straining, high temperature, nano indentation).Our research work concentrate on different micro- or nano-structured materials :
- Pure nanocrystalline metals (Cu, Al) obtained by
electrodeposition like freestanding thin films or films
inserted in Micro Electro Mechanical Systems
(MEMS) equipped with stress/strain gauges (coll.
UCL Louvain, Johns Hopkins U., Baltimore, Georgia Tech, LAAS).
- Bulk ultra fine grained materials (UFG) obtained by severe plastic deformation or by sintering of sub-micronic powder.
- Thin films obtained by epitaxy on different substrate.
- Al and Be monocrystalline sub-micron fibers (coll.
KIT Karlsruhe, U. Penn, Philadelphia)
Our efforts are focused on the understanding of :
- The plastic relaxation mechanisms caused by the
motion of grain boundaries (GB) under stress (so-
called shear-migration coupling) in nanocrystalline,
fine grained metals and bicrystals (coll. RWTH Aachen, ICMPE, Thiais)
- Dislocation mechanisms in ultrafine grained metals (anelastic effect, dislocation/grain boundary interactions,...).
- The role of dislocation sources in sub-micron fibers.
We give below two examples of currently investigated topics
Plasticity in sub-micron Al fibers
Sub-micronic Al fibers obtained from a selective etching of an Al/Al2Cu eutectic alloy have been strained both in an SEM equipped with a load cell in KIT, Karlsruhe and in a TEM. They exhibit a plastic behaviour from fragile to extremely ductile, largely dependent on the dislocation density rather than on the size itself (fig.below). We found that the yield stress is inversely proportionnal to the fiber size. This trend can be explain by the operation of spiral sources distributed randomly in the fiber.
a) Dislocation spiral sources (composed of a fixed and moving arm) were observed during plastic deformation of sub-micron Al fibers. The strength of these sources measured in situ fit with SEM mechanical tests and provide evidence of the size effect on the yield stress.
Grain boundary assisted plasticity
Plastic deformation is generally controlled by the motion of dislocations. Preventing this motion using grain boundaries (GB) as obstacles usually leads to structural hardening, but in nanocrystalline materials (involving a very high GB density), recent experiments have shown that GB migration can become an alternative plastic deformation mechanism. Unlike dislocation-based deformation, very little is known about the shear produced by the migration of a GB. We have investigated this process both experimentally and numerically in an Al bi-crystal. In-situ TEM experiments have evidenced that the GB migration occurred through the motion of steps (elementary or macro-steps), indirectly identified as disconnections. The shear produced by the GB migration exclusively depends on these moving disconnections. Atomistic models in which the minimum energy path of the shear-coupled GB migration was computed, confirmed this mechanism. Especially, it enabled the energetic characterization of the formation and motion of disconnections. Finally, high-resolution TEM experiments pointed to these GB disconnections.
This study (PhD work of Armin Rajabzadeh) has significantly improved the fundamental knowledge of GB migration mechanisms by underlying the role of disconnections in the GB migration and the direct relation between these disconnections and the shear produced by the migration.
GB Migration mechanism for a model Σ13(320)  tilt boundary. An Al Σ41(540) tilt boundary containing a
K. Hemker (J. Hopkins Univ., Baltimore), H. Mughrabi (Université d’Erlangen), T. Pardoen (UCL Louvain), G. Dirras (LPMTM, Villetaneuse), D. Molodov (Université de Aachen), O. Pierron (Georgia Tech.), D. Gianola (U. Penn), O. Kraft (KIT, Karslruhe), S. Lartigue (ICMPE, Thiais), L. Jalabert (LAAS).
Selected recent publications
- N. Combe, F. Mompiou, and M. Legros. "Disconnections kinks and competing modes in shear-coupled grain boundary migration." Phys. Rev. B, 93:024109, 2016.
- F. Mompiou and M. Legros. "Quantitative grain growth and rotation probed by in-situ tem straining and orientation mapping in small grained al thin films." Scripta Materialia, 99:5 – 8, 2015.
- F. Mompiou, M. Legros, C. Ensslen, and O. Kraft. "In situ tem study of twin boundary migration in sub-micron be fibers." Acta Materialia, 96:57 – 65, 2015
- L. Farbaniec, G. Dirras, A. Krawczynska, F. Mompiou, H. Couque, F. Naimi, F. Bernard, and D. Tingaud. Powder metallurgy processing and deformation characteristics of bulk multimodal nickel. Mater. Charact.,94:126–137, 2014.
- A. Rajabzadeh, F. Mompiou, S. Lartigue-Korinek, N. Combe, M. Legros, and D. A. Molodov. The role of disconnections in deformation-coupled grain boundary migration. Acta Materialia, 77:223–235, 2014.
- J.A. Sharon, Y. Zhang, F. Mompiou, M. Legros, and K.J. Hemker. Discerning size effect strengthening in ultrafine-grained mg thin films. Scripta Materialia, 75(0):10 – 13, 2014.
- F. Mompiou, M. Legros, A. Boe, M. Coulombier, JP Raskin, and T. Pardoen. Inter- and intragranular plasticity mechanisms in ultrafine-grained Al thin films : An in situ TEM study. Acta Mater., 61(1):205–216, 2013.
- A. Rajabzadeh, M. Legros, N. Combe, F. Mompiou, and D. A. Molodov. Evidence of grain boundary dislocation step motion associated to shear-coupled grain boundary migration. Phil. Mag., 93(10-12, SI):1299–1316, 2013.
- A. Rajabzadeh, F. Mompiou, M. Legros, and N. Combe. Elementary Mechanisms of Shear-Coupled Grain Boundary Migration. Phys. Rev. Lett.,110:265507, 2013.
- F Mompiou, D Caillard, M Legros, and H Mughrabi. In situ TEM observations of reverse dislocation motion upon unloading in tensile-deformed UFG aluminium. Acta Mater, 60(8):3402 – 3414, 2012.
- F Mompiou, M Legros, A Sedlmayr, DS Gianola, D Caillard, and O Kraft. Source-based strengthening of sub-micrometer Al fibers. Acta Materialia,60:977–983, 2012.