Phd Defence:” Investigation of grain size and shape effects on crystal plasticity by dislocation dynamics simulations”

Phd candidate: Maoyuan JIANG

Directeur de thèse : Benoit DEVINCRE

Co-directeur de thèse : Ghiath MONNET

Dislocation Dynamics (DD) simulations are used to investigate the Hall-Petch (HP) effect and back stresses induced by grain boundaries (GB) in polycrystalline materials.

The HP effect is successfully reproduced with DD simulations in simple periodic polycrystalline aggregates composed of 1 or 4 grains. In addition, the influence of grain shape was explored by simulating grains with different aspect ratios. A generalized HP law is proposed to quantify the influence of the grain morphology by defining an effective grain size.  The average value of the HP constant K calculated with different crystal orientations at low strain is close to the experimental values. 

The dislocations stored during deformation are mainly located at GB and can be dealt with as a surface distribution of Geometrically Necessary Dislocations (GNDs). We used DD simulations to compute the back stresses induced by finite dislocation walls of different height, width, density and character. In all cases, back stresses are found proportional to the surface density and their spatial variations can be captured using a set of simple empirical equations. The back stress calculation inside grains is achieved by adding the contributions of GNDs accumulated at each GB facet.

These back stresses are found to increase linearly with plastic strain and are independent of the grain size. The observed size effect in DD simulations is attributed to the threshold of plastic deformation, controlled by two competing mechanisms: the activation of dislocation sources and forest strengthening. Due to strain localization in coarse-grained materials, the pile-up model is used to predict the critical stress. By superposing such property to the analysis we made from DD simulations in the case of homogeneous deformation, the HP effect is justified for a wide range of grain sizes.

Tuesday 04/05/2019, 13h30
Amphi 3, Bâtiment Eiffel, CentraleSupélec, 8-10 rue Joliot-Curie, 91190 Gif-sur-Yvette

“HDR” Defence: Mathieu FEVRE

Mathieu Fèvre will defend his HDR thesis entitled  “Modelling and characterization of alloys and alloy nanoparticles” in order to obtain the diploma of  “Habilitation à Diriger des Recherches”, Tuesday 29th January2019 at 2 p.m., in the BLANDIN amphitheatre of the Laboratoire de Physique des Solides on the site of the Faculté des Sciences d’Orsay (Building 510).
The jury will be composed by  Pascal ANDREAZZA (ICMN), Alexis DESCHAMPS (SIMAP), Alphonse FINEL (ONERA), Philippe GOUDEAU (INSTITUT P’), Christine MOTTET (CINAM) and Sylvain RAVY (LPS).

PhD Defence : Ouafi Mouhoub

Mr. Ouafi Mouhoub will defend his PhD thesis entitled “Structural and electronic properties of graphene, hexagonal boron nitride and their heterostructures” on Wednesday, December 19 at 2 pm. His defense will take place at the University Paris Diderot, Condorcet Building, 10 rue Alice Domon et Léonie Duquet, 75013 Paris – Salle Luc Valentin (454A).