gurpreet kaur, p. jegadeesan, d. murali, b.k. panigrahi, m ...bemod12/slides/kaur_bemod12.pdfcluster...
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0.4 0.6 0.8 1.0 1.2 1.41E-27
1E-23
1E-19
1E-15
1E-11
KMC
Le Claire's model
Experiment
Y
Ti
FeD
iffu
sio
n C
oe
ffic
ien
t (m
2/s
)
1000/T (K-1
)
Gurpreet Kaur, P. Jegadeesan, D. Murali, B.K. Panigrahi, M.C. Valsakumar, C.S.SundarMaterials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, INDIA
gurpreet@igcar.gov.in
The strength of Nanostructured Ferritic Alloys depends on the distribution ofY-Ti-O nanoclusters in the matrix.
These nanoclusters are formed during the high temperature consolidation of the ballmilled (powdered) alloy.
Understanding the initial phase of formation of these nanoclusters is crucial to predicttheir overall behavior.
We have used first principles methods along with Lattice Kinetic Monte Carlo technique tostudy the kinetics of nanocluster formation in BCC Iron.
Introduction
Pairwise bondsFirst neighbour
(eV)Second neighbour
(eV)
Fe-Y 0.25 0.00
Fe-Ti -0.10 0.00
Fe-O 0.35 0.17
Fe- 0.25 0.00
Y-Y 0.19 0.01
Y-Ti 0.15 0.01
Y-O 0.45 -1.05
Y- -1.40 -0.25
Ti-Ti 0.23 0.13
Ti-O -0.25 -0.55
Ti- -0.25 0.16
- 0.15 -0.21
-O -1.55 -0.75
O-O 0.60 0.40
Pairwise bond energy parameters
Migration energy Em (eV)
Fe- 0.65
Y- 0.10
Ti- 0.40
O 0.55
Migration Energy Barriers
The bond energies are calculated
from Density Functional Theory
using VASP code with PAW (PBE)
psuedo potential with 128 atom
supercell.
The migration barriers for the solute
atoms in bcc iron were calculated
using the Nudged Elastic Band
(NEB) method
Diffusion coefficients of solute atoms
The diffusion coefficients of solute atoms in bcc Fe calculated using LKMC and compared
with the diffusion coefficients calculated from first principles using Le Claire’s nine
frequency model and also the available experimental values.
7.38906 20.08554 54.59815
2.71828
7.38906
Me
an
, S
igm
a
Time ( s)
Mean
Sigma
Size distribution of clusters
References:
G.R. Odette, M.J. Alinger and B.D. Wirth, Annu. Rev. Mater. Res. 38 (2008) 471.
D. Murali, B.K. Panigrahi, M.C. Valsakumar, Sharat Chandra, C.S. Sundar, Baldev Raj, J. Nucl. Mater. 403 (2010) 113-116
P. Jegadeesan, D. Murali, B.K. Panigrahi, M.C. Valsakumar, C.S. Sundar, International Journal of Nanoscience 10 (2010) 1-5 .
M. Ratti, D. Leuvrey, M.H. Mathon, Y.de, Carlan, J. Nucl. Mater. 386, 540-543 (2009).
V.A. Borodin and P.V. Vladimirov, J. Nucl. Mater. 386, 106 (2009).
1 10 1000
100
200
300
400
No
. o
f C
lus
ters
Time ( s)
3
10
100
Av
. C
lus
ter
Siz
e (
No
. o
f a
tom
s)
0 2 4 6 8 10 12 140
100
200
300
400
500
600
6.4 s
12.8 s
19.2 s
25.6 s
32.1 s
38.5 s
44.9 s
51.3 s
57.7 s
64.2 s
Nu
mb
er
of
clu
ters
R (in Angtrom)
3 4 5 6 7 8 9 10 11 12 13 140.0
0.2
0.4
0.6
0.8
1.0
Cu
mu
lati
ve
Dis
trib
uti
on
of
No
.of
clu
ste
rs
R (in Angtroms)
2.0x10-4
4.0x10-4
6.0x10-4
8.0x10-4
1.0x10-3
1.2x10-3
1.4x10-3
2.0x10-7
4.0x10-7
6.0x10-7
8.0x10-7
1.0x10-6
Do
Vacancy concentration
Ti
Fe
The LKMC Model
Two sublattices: 1st sublattice: regular bcc sites
with Fe, Y, Ti, vacancy ( )
2nd sublattice: octahedral interstitial sites
with Oxygen
One Monte Carlo sweep: all the Vacancy and O allowed to
jump once
A rigid bond model for total energy with interactions up to
second neighbor (fitted to first principle calculation)
Formation of Y-Ti-O clusters
Randomly distributed Y, Ti, O
and vacancy in Fe, evolved
by vacancy jumps and
interstitial O jumps, leading to
the formation of clusters
88 88 88 unit cells
(1.3 million sites) Y: 0.12%,
O: 0.2%, Vac: 1000 ppm
Temp. 1150 K
Evolution of clusters with time O to Y ratio in the clusters
Normal distribution is found to give the best fit for the
cluster size distribution (R >4 Å) out of Lognormal,
Normal and gamma distributions. [ignoring the initial
decaying portion(R < 4 Å) of size distribution].
51
tR
k ji
ijij knkE,
)()(
0 100 200 300 400 5000
25
50
75
100
125
150
175
200
Without titanium
Nu
mb
er
of
clu
ste
rs
Cluster size (No.of atoms)
0 100 200 300 400 5000
25
50
75
100
125
150
175
2004000
5000
With titanium (0.5at%)
Nu
mb
er
of
clu
ste
rs
Cluster size (No.of atoms)
Effect of Titanium
E-spE-init
E-final
Vacancy
Conclusions
Mechanism of growth is the coagulation of the clusters. The Cluster Size follow a normal distribution. Presence of titanium reduces the size of clusters .
Time exponent shows coagulation of clusters is the growth mechanism
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