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Thesis Proposal

Sina Moeini Ardakani

Members of the Committee:Prof. M. J. Buehler

Dr. S. R. HydeProf. J. Li

Prof. R. J. –M. Pellenq

Jul 9, 2015

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Displacive VS. Diffusion

Displacive:• Mechanical Twinning• Dislocation glide• Martensitic transformationDiffusion:• Dislocation climb• Precipitation• Dissolution

Military (Displacive)Civilian (Diffusion)

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Part I. Methodology

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Scope of Simulations

Multi-scale simulation

Experiments

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Statistical Mechanics

• System consists of N atoms

• Phase-Space 6N dim. 3N Position 3N Momentum

• Poisson-Boltzmann weight distribution

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Thermo. Quant. & 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

• Thermodynamic Quantities:

• Ergodicity:

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MD

Potential Energy : based on the given potentials from first principles

EAM Potential:

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MD

Initial Configuration

Assign Velocities

Calculate Energy& Forces

Update Velocityusing Newton’s 2nd

Law

Update X Using Velocity & Timestep

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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

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Diffusive MD

Chemical integration stepMinimize F w.r.t.

and

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Diffusive MD

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

Advantage of a continuous site-probability representation at atomic resolution

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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

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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).

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Example of Input File

Initial config.Force Field typeForce Field file

Record snapshot

Minimization

Print the thermo.quantity

Set ensembleBoltz. const.Setup time step

Strain the boxRUN!!!

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Example of Output File

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Part II. White Etching Area

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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

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WEA

Micro-Structural Features in WEA:• Ferrite with supersaturated carbon• Equiaxed, nano-grains sizes (10-

100nm)• Absence of Carbides• Harder than surrounding matrix

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)

Possible causes of WEA in Wind Turbines:• Hydrogen embrittlement • Strain rate• Plastic deformation in presence of cracks

and voids

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Major Phases of Fe-C Mixture

Cementite (Fe3C):Orthorhombic

Austenite/Iron FCC (Υ):Cubic

Ferrite/Iron BCC (α):Cubic

Martensite/Iron BCT:Tetragonal

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Major Microstructures of Fe-C Mixture

• Retained Austenite• Martensite• Pearlite: Fe3C+Fe( ), layered α

structure• Bainite: Fe3C+Fe( ), needle α

shaped Fe( ) separated by αelongated Fe3C

• Spherodite: Fe3C+Fe( ), spherical αFe3C particles inside Fe( ) matrixα

Pearlite

Bainite

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Fe-C Phase Diagram

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Proposed structure of WEA

Image from Timken

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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:1. Very high/low aspect.2. Allows atom transfer across. 3. Damps energy.4. It is shear soft.