1. 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

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

3. Part I. Methodology

4. Scope of Simulations
Multi-scale
simulation
Experiments

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

6. 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

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

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

9. 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

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

11. 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

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

13. 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).

14. 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!!!

15. Example of Output File

16. Part II. White Etching Area

17. 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

18. 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)

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

20. 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

21. Fe-C Phase Diagram

22. Proposed structure of WEA
Image from Timken

23. Proposed structure of WEA
Zone 0: original material
Zone 1: crack/shear band; strain rate /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.