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Deformation of materials: many secrets still retained

Developing new materials and understanding how they deform is the main challenge of engineers in order to follow and predict the fast evolutions of our society. For instance, in a framework of energetic cost reductions, increasing the mechanical performances of materials is synonymous of constant improvements of experimental techniques leading to new insights on their fundamental deformation mechanisms.

In mechanics of materials, stress is a measure of the effect of loads on an object; more specifically, it expresses the internal forces that neighboring particles of a material exert on each other. When an object becomes stressed, it may change shape i.e. deform or break, depending on the magnitude of stress on the object and the material the object is made from. The measure of this deformation is called strain.

 Most of used materials are generally polycrystalline. They consist of millions of single crystals called grains. Grains have different orientations of the atom lattice and they are separated from the neighboring grains by interfaces called Grain Boundaries (GB). It is well established that the irreversible (or plastic) deformation of a sample originates mainly from the nucleation and the propagation of more than hundreds of billions per cm³ of micrometric (even nanometric) linear defects of the regular crystal lattice called dislocations. Dislocations move through the grain and interact with each other or with GB. GB may act in several ways: sinks, traps and sources of dislocations.

Nowadays, one knows almost how one dislocation interacts with one model GB, but understanding the response of several real GB (contained in a real bulk polycrystalline sample) after receiving numerous dislocations is still a major scientific challenge. The mystery becomes inextricable when one considers that there are more than hundreds of billions of dislocations per cm³ of sample interacting together and with billions of GB… Even more inextricable if one wants to take into account the influence of the distribution of GB, other types of interfaces, grain shape, grain orientation and defects in the bulk sample i.e. its microstructure. Due to this inherent complexity, we have to link two extreme scales: sample (or macro-) scale and dislocation (or micro-) scale. It is obvious that these two worlds interact each other but their connections remain extremely difficult to understand because of the need of extrapolations.

Here, the role of micromechanical modeling brings new insight. Such computational schemes need constitutive equations that have to be “fed” with experimental criteria and parameters, capturing the important operative mechanisms. However, such valuable experiments are still marginal and all suffer numerous constraints making interpretations not reliable statistically.

How will the history of a material influence its mechanical performances? This key question is at the heart of my research work. Despite intense research on this field, no direct correlation and no constitutive physical models were made to date. Therefore, I want to face this challenge i.e. linking the macroscopic behavior of a sample with the microscopic operating mechanisms. Only then, I will be able to understand the deformation of any materials at every scale where it takes place.

I am interested in several other fundamental research topics as well. In order to open new research tracks in Materials Sicence, I work gladly on the theories of diffraction and electron microscopy. Indeed, I am strongly convinced that an in-depth use of such techniques requires a complete understanding of the physical phenomena at their origin.