Woodruff School of Mechanical Engineering
Faculty Candidate Seminar
Microstructure engineering: toward a unified theory applied to nano-materials in extreme conditions
Dr. Aurelien Villani
Georgia Tech - Lorraine
Thursday, March 31, 2016 at 11:00:00 AM
MRDC Building, Room 4211
Many of tomorrow’s materials will be engineered at the nano scale: batteries with nano particles, ODS steels, ultra fine grains polycrystals, multilayers, etc. Most of the time, the key to their properties lies in the high density of interfaces 1. By understanding how these interfaces interact with point defects and dislocations, one can engineer a microstructure adapted to specific applications: creep resistance, radiation damage resistance, tolerance to damage, and to extreme conditions in general. Before being able to provide macroscopic models, applicable to industrial sized components, one has to build models allowing for deeper understanding of physical processes. At small scales, many things happens in the same place, at the same time: diffusion of vacancies and interstitials, glide and climb of dislocations, grain boundary migrations, defects interactions with interfaces, etc. These processes being tightly coupled, multi physics frameworks have to be developed. In this work, a novel continuum mechanics framework is proposed to describe the strain fields resulting from such a diffusion-driven process in a polycrystalline aggregate where grains and grain boundaries are explicitly considered. The choice of an anisotropic eigenstrain in the grain boundary region provides the driving force for the diffusive creep processes. The corresponding inelastic strain rate is shown to be related to the gradient of the vacancy flux. Dislocation driven deformation is then introduced as an additional deformation mechanism, through standard crystal plasticity constitutive equations. The fully coupled diffusion-mechanical model is implemented into the finite element method and then used to describe the biaxial creep behaviour of FCC polycrystalline aggregates. The corresponding results revealed for the first time that such a coupled diffusion-stress approach, involving the gradient of the vacancy flux, can accurately predict the well-known macroscopic strain rate dependency on stress and grain size in the diffusion creep regime. They also predict strongly heterogeneous viscoplastic strain fields, especially close to grain boundaries triple points. Finally, a smooth transition from Herring and Coble to dislocation creep behaviour is predicted and compared to experimental results.
Aurelien Villani is a post doctoral fellow at Georgia Tech - Lorraine and LEM3 laboratory since 2015, where he is working on FFT methods for field dislocation mechanics applied to interfaces. During his PhD, obtained at the Ecole des Mines de Paris, he specialized in stress diffusion coupled problems and phase field methods. Dr Villani also holds a Masters degree in Solid and Fluid Mechanics from Chalmers University, and a Diplome d’ingenieur from the Ecole des Mines de Douai.
Refreshments will be served.