Members of the Committee: Thesis Proposal Sina Moeini Ardakani Members of the Committee: Prof. M. J. Buehler Dr. S. R. Hyde Prof. J. Li Prof. R. J. –M. Pellenq Jul 9, 2015

Displacive VS. Diffusion Mechanical Twinning Dislocation glide Martensitic transformation Diffusion: Dislocation climb Precipitation Dissolution Civilian (Diffusion) Military (Displacive)

Part I. Methodology

Scope of Simulations Multi-scale simulation Experiments

Statistical Mechanics System consists of N atoms Phase-Space 6N dim. 3N Position 3N Momentum Poisson-Boltzmann weight distribution

Thermo. Quant. & Ergodicity Thermodynamic Quantities: Ergodicity: No need to integrate all over the phase space Follow the system trajectory over long periods of time Calculate the thermodynamics quantities by averaging out over time

MD Potential Energy : based on the given potentials from first principles EAM Potential:

MD Initial Configuration Update X Using Velocity & Timestep Assign Velocities Update Velocity using Newton’s 2nd Law Calculate Energy & Forces

Diffusive MD Capture diffusive timescale Passes through trillions of energy barriers No catalog event required Gaussian variation coupled with diffusion. Degrees of freedom per atomic site (5): Position (3) + Gaussian width (1) + occupation probability (1) Minimize free energy Evolution of occ. prob. due to diffusion master equation

Chemical integration step Diffusive MD Chemical integration step Minimize F w.r.t. and

Diffusive MD Advantage of a continuous site-probability representation at atomic resolution Probability Wave of Vacancy Random Walker Representing long-range mass transport by vacancy random walk with MD, hyper-MD or kinetic Monte Carlo could require trillions of vacancy hops – impractical and unnecessary

What is MAPP? MAPP = MIT Atomistic Parallel Package Open source MD/DMD software Written in C++, parallel using MPI Available online for public: http://github.com/sinamoeini/MAPP

Why write a new code? Easier to manipulate the source code. Designed and optimized for metallic potentials 38% faster than the most popular MD code (LAMMPS) (Benchmark performed on 100 processors). New parallelization schemes for faster data transfer between processors. Equipped with DMD. New minimization algorithm (L-BFGS) (not possible in LAMMPS).

Example of Input File Initial config. Force Field type Force Field file Record snapshot Minimization Print the thermo. quantity Set ensemble Boltz. const. Setup time step Strain the box RUN!!!

Example of Output File

Part II. White Etching Area

Achilles Hill of Wind Turbines Wind turbines bearings fail well before their life expectancy The macro cause of failure is usually material spalling or radial cracking of raceway Cracks are almost always accompanied by White Etching Areas (WEAs) WEA: Cause or symptom? Chicken or Egg issue But why it happens more often in wind turbines? Changes in speed and direction (not uniform) Engagement and disengagement with the gearbox

WEA Micro-Structural Features in WEA: Ferrite with supersaturated carbon Equiaxed, nano-grains sizes (10-100nm) Absence of Carbides Harder than surrounding matrix Possible causes of WEA in Wind Turbines: Hydrogen embrittlement Strain rate Plastic deformation in presence of cracks and voids Fig. 6. Scanning electron microscope image of resulting nanohardness indentations with corresponding measured hardness value. Five of the six indents shown; the sixth indent not shown measured 10.1 GPa. Greco, Sheng, Keller, Erdemir Wear 302 (2013)

Major Phases of Fe-C Mixture Austenite/Iron FCC (Υ): Cubic Cementite (Fe3C): Orthorhombic Ferrite/Iron BCC (α): Cubic Martensite/Iron BCT: Tetragonal

Major Microstructures of Fe-C Mixture Retained Austenite Martensite Pearlite: Fe3C+Fe(α), layered structure Bainite: Fe3C+Fe(α), needle shaped Fe(α) separated by elongated Fe3C Spherodite: Fe3C+Fe(α), spherical Fe3C particles inside Fe(α) matrix Pearlite Bainite

Fe-C Phase Diagram

Proposed structure of WEA Image from Timken

Proposed structure of WEA Zone 0: original material Zone 1: crack/shear band; strain rate 100-1000/s induces SB Zone 2: processed material, nano ferrite, can be considered amorphous like metallic glasses Zone 3: relaxed processed material. Finer grains than Zone 0 Zone properties: Very high/low aspect. Allows atom transfer across. Damps energy. It is shear soft.