Archive 29 May 2019

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