**Completed studies**

*Mean field modeling of dynamic recrystallization by homogenization of Full field models on 304L*

**Ludovic Maire, 2015-2018**

Mean field (MF) models of dynamic recrystallization (DRX) emerged in the last decades with the intention to implicitly describe the microstructure by considering grains sets as spherical classes. These models have the advantage to provide accurate results in terms of macroscopic results such as recrystallized fraction or grain size but also to provide additional information in terms of grain size distribution and dislocation density distribution [1,2,3,4].

In parallel, finer approaches called full field (FF) models have emerged in the last decades. These approaches consider a complete description of the microstructure topology at the polycrystal scale [5]. A review of the most significant numerical methods can be found in [6].

Several DRX models based on a full field approach can already be found in the literature [7,8,9]. Although literature already provides a large number of papers on full-field DRX models, major drawbacks are either they are developed in 2D and/or they only consider small deformations (< 20%).

In the present work, a 3D model based on the level-set method in a FE framework is employed to model the DRX phenomenon in austenitic stainless steel 304L at large deformations.

The level-set approach coupled to a remesher provides an accurate tracking of interfaces (i.e. grain boundaries) all along the recrystallization simulation while mean field laws are used for the nucleation and work hardening mechanisms.

The development realized are also dedicated to the elaboration of optimized improved mean field model concerning DRX thanks to the full field developments.

[1] Montheillet, F., Lurdos, O., and Damamme, G. (2009). Acta Materialia, 57(5):1602–1612.

[2] Bernard, P., Bag, S., Huang, K., and Logé, R. (2011). Science and Engineering: A, 528(24):7357–7367.

[3] Cram, D. G., Zurob, H. S., Brechet, Y. J. M., and Hutchinson, C. R. (2009). Acta Materialia, 57(17):5218–5228.

[4] Maire, L., Moussa, C., Bozzolo, N., Scholtes, B., Pino Muñoz, D., Bernacki, M. (2016). Journal of Materials Science, 51(24):10970-10981.

[5] Scholtes, B., Shakoor, M., Settefrati, A., Bouchard, P.-O., Bozzolo, N., and Bernacki, M. (2015). Computational Materials Science, 109:388–398.

[6] Hallberg, H. (2011). Metals, 1(1):16–48.

*Globularization phenomenon in α/β titanium alloys: experimental analysis and numerical modeling*

**Danai Polychronopoulou, 2014-2017**

Fragmentation of α lamellae and subsequent spheroidization of α laths in α/β titanium alloys occurring during and after deformation are well known phenomena. In this PhD works, the development of a new finite element methodology to model these mechanisms is considered. This new methodology is based on a level set framework to model the deformation and the *ad hoc* simultaneous and/or subsequent interfaces kinetics. Validation of these full-field numerical developments thanks to experimental results is also planned.

**MSc or MTech Internships**

**Finite element mesh constrained repartitioning using open-source libraries for HPC applications**

**Patrick Teyou, 2017**

Within this context of high performance computing in metallurgy, handled by the DIGIMU Chair, this internships (which can then open the road to a PhD position) is dedicated to improving our current mesh partitioning strategy implementation. Within the aforementioned context of a finite element library (FE) using unstructured meshes, partitioning algorithms are used in order to distribute the elements among the available CPUs and solve partial differential equations using the parallel computing. Sometimes the FE mesh must be adapted. This adaptation is achieved through the steps of remeshing. In the present approach, each processor remeshes its partition using a sequential algorithm without modified the borders of the partition. The computational domain is then ”re-partitioned” so that the old partition borders are strictly placed within the new partitions a asthe 2D example shown in the Figure (this can be seen as constrained repartitioning operations). New partitions are remeshed again, and repeating these operations multiple times, a mesh can be adapted in parallel.

The strategy adopted today has some limitations that we would like to remove. To this end, open-source partitioners (Metis, Scotch, …) will be tested and compared to the existing strategy on large meshes. The idea is to build a multi-partitioners interface that taking into account the specific weaknesses/strengths of each partitioner.

**2D full field grain growth modelling in ODS steels: Level-set versus Monte-Carlo**

**Flore Villaret, 2017**

Development of the next generation of nuclear reactor could be seen as a way to provide carbon-free and safer energy. These sodium-cooled fast reactors operate at much higher temperature than actual pressurized water reactor (up to ~700°C instead of ~300°C in the primary circuit) that is why new materials are needed to build these reactors. Fuel claddings are basics structural elements of reactors and need to coop with these high temperature and high neutron flux for long exposure times. Such conditions leads to special property demands like high temperature strength, thermal and irradiation creep strength or resistance to radiation embrittlement. Oxide Dispersion Strengthened (ODS) steels alloys are believed to complete these specifications and are regarded as prominent candidate materials for such applications.

ODS steels are ferritic matrix steels, strengthened by many Y-Ti-O oxide nanoparticles. During forming process, these materials are deformed and reheated several times in order to obtain the final shape. Every heating step has to be well controlled in order to obtain optimal microstructure and mechanical properties. Indeed during heating, different phenomena leading to microstructural changes can occur: recovery, recrystallization, and grain growth. Oxide nanoparticles slow down recrystallization and grain growth by grain boundary and dislocation pinning (known as Smith-Zener pinning). Numerical simulations will be used to check these mechanisms. Full field Monte-Carlo and level-set methods will be used in this perspective.

Main objective of this study will be to compare results from grain growth and recrystallization simulations on ODS steels with the two methods mentioned above, in order to get a better understanding of assumptions made in each model for these materials.

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