Gilles Demange

Anticiper l’évolution de la microstructure d’un matériau en condition d’usage est d’une importance cruciale pour l’industrie. Cette maîtrise du vieillissement nécessite une compréhension claire des mécanismes sous-jacents, qui agissent sur une large gamme d’échelles spatiales et temporelles. Dans cette optique, ce travail de thèse a choisi d’appliquer la méthode de champ de phase qui, en raison du saut d’échelle qu’elle réalise naturellement, est un outil intensivement employé dans le domaine des matériaux, pour prédire l’évolution en temps long de la microstructure. L’enjeu de l’étude a été d’étendre cette méthode à un système porté loin de l’équilibre thermodynamique, en particulier en présence d’irradiation. Nous avons ainsi adopté le formalisme du mélange ionique, introduit par Gras-Marti pour décrire le mélange balistique au sein d’une cascade de déplacements. Par l’utilisation conjointe d’un schéma numérique et d’une approche analytique, il nous a été possible d’établir le diagramme de phase générique d’un matériau irradié. Nous avons ensuite étudié le vieillissement de l’alliage binaire test AgCu sous irradiation, par l’utilisation conjointe de la méthode du champ de phase et d’approches atomistiques, dans une démarche multi-échelles. En fixant les paramètres de contrôle que sont le flux d’irradiation et la température, il nous a ainsi été possible de prédire la taille,la concentration ainsi que la distribution spatiale des nodules de cuivre produits sous irradiation dans cet alliage. La connaissance de ces informations a permis de simuler un diagramme de diffraction en incidence rasante, directement comparable aux diagrammes expérimentaux.

The present work is devoted to the phenomenon of induced side branching stemming from the disruption of free dendrite growth. We postulate that the secondary branching instability can be triggered by the departure of the morphology of the dendrite from its steady state shape. Thence, the instability results from the thermodynamic trade-off between non monotonic variations of interface temperature, surface energy, kinetic anisotropy and interface velocity within the Gibbs–Thomson equation. For the purposes of illustration, the toy model of capillary anisotropy modulation is prospected both analytically and numerically by means of phase-field simulations. It is evidenced that side branching can befall both smooth and faceted dendrites, at a normal angle from the front tip which is specific to the nature of the capillary anisotropy shift applied. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’.

In the present work, atomistic modeling based on the quasiparticle approach (QA) was performed to establish general trends in the segregation of solutes with different atomic size at symmetric ⟨100⟩ tilt grain boundaries (GBs) in α-Fe. Three types of solute atoms X₁, X₂ and X₃ were considered, with atomic radii smaller (X₁), similar (X₂) and larger (X₃) than iron atoms, respectively, corresponding to phosphorus (P), antimony (Sb) and tin (Sn). With this, we were able to evidence that segregation is dominated by atomic size and local hydrostatic stress. For low angle GBs, where the elastic field is produced by dislocation walls, X₁ atoms segregate preferentially at the limit between compressed and dilated areas. Contrariwise, the positions of X₂ atoms at GBs reflect the presence of tensile and compressive areal regions, corresponding to extremum values of the σ_XX and σ_YY components of the strain tensor. Regarding high angle GBs Σ5 (310) (θ = 36.95°) and Σ29 (730), it was found that all three types of solute atoms form Fe₉X clusters within B structural units (SUs), albeit being deformed in the case of larger atoms (X₂ and X₃). In the specific case of Σ29 (730) where the GB structure can be described by a sequence of |BC.BC| SUs, it was also envisioned that the C SU can absorb up to four X₁ atoms vs. one X₂ or X₃ atom only. Moreover, a depleted zone was observed in the vicinity of high angle GBs for X₂ or X₃ atoms. The significance of this research is the development of a QA methodology capable of ascertaining the atomic position of solute atoms for a wide range of GBs, as a mean to highlight the impact of the solute atoms’ size on their locations at and near GBs.

This article is devoted to the study of the tip shape of dendritic crystals grown from a supercooled liquid. The recently developed theory (Alexandrov & Galenko 2020 Phil. Trans. R. Soc. A 378 , 20190243. ( doi:10.1098/rsta.2019.0243 )), which defines the shape function of dendrites, was tested against computational simulations and experimental data. For a detailed comparison, we performed calculations using two computational methods (phase-field and enthalpy-based methods), and also made a comparison with experimental data from various research groups. As a result, it is shown that the recently found shape function describes the tip region of dendritic crystals (at the crystal vertex and some distance from it) well. This article is part of the theme issue ‘Transport phenomena in complex systems (part 1)’.

Motivated by an important application of dendritic crystals in the form of an elliptical paraboloid, which widely spread in nature (ice crystals), we develop here the selection theory of their stable growth mode. This theory enables us to separately define the tip velocity of dendrites and their tip diameter as functions of the melt undercooling. This, in turn, makes it possible to judge the microstructure of the material obtained as a result of the crystallization process. So, in the first instance, the steady-state analytical solution that describes the growth of such dendrites in undercooled one-component liquids is found. Then a system of equations consisting of the selection criterion and the undercooling balance that describes a stable growth mode of elliptical dendrites is formulated and analyzed. Three parametric solutions of this system are deduced in an explicit form. Our calculations based on these solutions demonstrate that the theoretical predictions are in good agreement with experimental data for ice dendrites growing at small undercoolings in pure water.

We present a multi-scale phase field modeling of stationary microstructures produced under 1 MeV krypton ion irradiation in a phase separating concentrated solid solution of silver and copper. We show that the mixture reaches ultimately a stationary micro-structural state made of phase domains with composition and size distribution mapped to the values of the incident flux of particles and of the temperature, variables that help defining a non equilibrium phase-diagram for the irradiated alloy. The modeling predicts the formation of diverse microstructures likely connected to spinodal hardening, thus opening the perspective of the on-purpose tuning of mechanically resistant microstructures and the preparation of metastable alloys with mechanical properties improved by comparison to counterparts obtained via classical thermo-mechanical treatments.