Transcript EXAFS of Copper-Niobium Nanoalloy Thin Films

EXAFS of Copper-Niobium
Nanoalloy Thin Films
Soma Chattopadhyay1, S.D. Kelly2, T. Shibata1, A. Puthucode3, P.
Ayyub4, and R. Banerjee3
1CSRRI-IIT,
Advanced Photon Source, 2EXAFS Analysis,
3Department of Materials Science and Engineering, University of North Texas,
4Department of Condensed Matter Physics and Materials Science, Tata Institute
of Fundamental Research, Mumbai India.
UOP 5646-1
Outline
 Introduction
 Experimental
 Atom Probe Tomography
 X-ray Absorption Spectroscopy
 Molecular Dynamic Simulations
 Summary
Introduction
 Multi-component bulk metallic glasses
– high yield strength
– high elastic limit
– low modulus
– poor room temperature plasticity due to the localization of plastic
deformation into discrete shear bands.
 Addition of low concentrations of elements with a small positive
enthalpy of mixing improve properties
 Reasons not apparent
– compositional heterogeneity
– structural ordering at the 1–2 nm scale.
 Compositional heterogeneities in metallic glasses, typically arising
from phase separation in the under-cooled liquid prior to glass
formation
– Cu43Zr43Al7Ag7,
– Ni58.5Nb20.25Y21.25 ,
– Ti28Y28Al24Co20, and other systems
•J. C. Oh, T. Ohkubo, Y. C. Kim. E. Fluery, and, K. Hono, Scripta Mater., 53, 165 (2005).
•N. Mattern, U. Kühn, A. Gerbert, T. Gemming, M. Zmkerich, H. Wendrock et. al., Scripta Mater., 53, 271 (2005).
•B. J. Park, H. J. Chang, W. T. Kim, and, D. H. Kim, Appl. Phys. Lett., 85, 6353 (2004).
•A. R. Miedema, F. R. Deboer, and, R. Boom, CALPHAD, 1, 341 (1977).
Introduction
 Thermodynamics predict that glass (amorphous phase) forming
systems exhibit a large negative enthalpy (heat) of mixing, 𝛥Hmix<0
– Minority solute in a solvent
 Immiscible systems exhibit a large positive enthalpy of mixing (𝛥
Hmix>0), phase separating
 Completely immiscible binary metallic systems (such as Ag-Ni and
Cu-Nb) with large positive enthalpies of mixing and driven far from
equilibrium also exhibit glass formation.
– 𝛥Hmix of the liquid phase in these systems is not constant but a
function of temperature
– Stabilization of amorphous phases is caused by nanoscale phase
separation
•E. Ma, Prog. Mater. Sci., 50, 413 (2005).
•J. H. He, H. W. Sheng, P. J. Schilling, C. –L. Chien, and, E. Ma, Phys. Rev. Lett., 86(13), 2826 – 2829 (2001).
•J. H. He, H. W. Sheng, and, E. Ma, Appl. Phys. Lett., 78(10), 1343 – 1345 (2001).
Experimental
 CuNb thin films were deposited on silicon wafers by dc
magnetron co-sputtering using pure elemental Cu (99.99%)
and Nb (99.99%) targets.
 Films of composition, 55 atomic% Cu and 45 atomic%
niobium, were deposited on flat substrates (~2-3 m thick) for
EXAFS experiments and on flat-top silicon microtips for APT
studies.
50 nm
Atom Probe Tomography
 APT studies were carried out in a local
electrode atom probe (LEAP™)
microscope. Flat-top microtip samples
were sharpened to tip radii ~ 60 – 70 nm.
 All atom probe experiments were
carried out in the electric-field
evaporation mode at 70K, with the
evaporation rate maintained at ~ 0.2%
and the pulsing voltage at 30% of the
steady-state applied voltage.
 Nano scale clustering of Cu (red) and Nb
(blue) atoms is observed.
•M.K. Miller, Atom Probe Tomography: Analysis at the Atomic Level, Springer, 1st edition (2000).
Atomic Probe Tomography
 Compositional profiles for Cu
and Nb, averaged across a
cylinder of 3nm diameter reveal
compositional fluctuations
 Frequency distribution obtained
for 50-atom blocks as a function
of average composition.
 Bulk-normalized concentration
as a function of distance from
Cu center. CN=1 corresponds to
random solid solution. True for
distances larger than 1.5 nm.
X-ray Absorption Spectroscopy
 Cu and Nb fluorescence measurements made at MR-CAT 10-ID
at the Advanced Photon Source
 Structure about Cu and Nb is to be compared to bulk metal
structures.
 Average structure about all Cu and Nb atoms within the
sample
 Detection of like (Cu-Cu) and unlike (Cu-Nb) neighbors
 EXAFS results similar to an average atomic radial distribution
about Cu and Nb in the sample
 Unlike pairs of Cu—Nb must be present in both the Cu edge
and Nb edge spectra. The distance between the pairs and s2
values must be the same.
Nb
Cu
Nb
Copper, FCC Structure
Cu, FCC Structure, a = 3.61 Å
4 atoms per unit cell
1
1
1
1
1
1
1
1
1
1
1
1
Atom type
Number
Distance
Cu1
12
2.56 Å
Cu2
6
3.62 Å
EXAFS Models for Cu Foil
1
Re FT[(k)·k2] Å-3 |FT((k)·k2)| Å-3
4
2
1
1
1
1
Cu Foil
Model
1
1
1
·
·
1
1
1
1
0
4
2
Neighbor
CN
R
s2 (x10-3)
-2
CuCu1
12
2.55
9±1
-4
CuCu2
6
3.71
12 ± 2
CuCu3
24
4.43
13 ± 1
CuCu4
12
5.11
11 ± 1
0
·
·
0
2
4
R (Å)
6
8
 Radial distances are determined through a = -0.3 ± 0.2%
 Strong EXAFS signals present in foil spectra at 5 Å and
beyond.
Niobium, BCC Structure
Nb, BCC Structure, a = 3.30 Å
2 atoms per unit cell
2
1
1
1
1
2
2
2
2
1
1
1
1
2
Atom type
Number
Distance (Å)
Nb1
8
2.86
Nb2
6
3.30
Nb3
12
4.67
EXAFS Models for Nb Foil
2
-3
Re FT[(k)·k2] Å-3 |FT((k)·k )| Å
2.0
1.5
1.0
·
·
0.5
0.0
2
Neighbor
CN
R
s2 (x10-3)
1
NbNb1
8
2.86
6±1
NbNb2
6
3.30
7±1
-1
NbNb3
12
4.67
11 ± 2
-2
NbNb4
24
5.47
13 ± 3
0
·
·
0
2
4
R (Å)
6
8
 Radial distances are determined through a = -0.2 ± 0.2%
 Strong EXAFS signals present in foil spectra at 5 Å and
beyond.
Compare Cu in CuNb thin film to Cu foil
XANES
 Cu XANES spectrum has similar
adsorption edge energy supporting
reduced Cu in CuNb thin film.
 Cu Fourier transform spectrum
from CuNb thin film has much less
order structure about Cu as
compared to Cu foil.
 The 1st nearest neighbor distance is
significantly contracted (arrow)
 Lack of signal in Fourier transform
at R distances greater than 3 Å
FT
Compare Nb in CuNb thin film to Nb foil
XANES
 Nb XANES spectra is
similar to Nb foil
 Nb Fourier transform
shows very little signal.
The strongest signal is at
much shorter distance as
compared to Nb foil.
FT
Comparison of Cu and Nb spectra from
thin film
 If the distribution of atom types and
distances are the same for both Cu
and Nb their EXAFS signals will be
very similar. This is not the case.
 Cu EXAFS signal is much stronger
than the Nb EXAFS signal.
 Very little structure in Fourier
transform beyond 4 Å indicating the
lack of ordered structure at longer
distances.
Cu and Nb Models of CuNb thin film
Nb Edge Spectrum
Cu Edge Spectrum
1.0
0.5
Re FT[(k)·k2] Å-3
Re FT[(k)·k2] Å-3
0.2
0.0
-0.2
Nb-O
Nb-Cu1
Nb-Nb1
Nb-Cu2
Nb-Nb2
·
·
-0.4
-0.6
0
1
2
3
4
R (Å)
5
6
7
8
0.0
Cu-Cu1
·
-0.5
·
Cu-Nb1
-1.0
Cu-Nb2
-1.5
0
1
2
3
4
5
6
7
R (Å)
 Simultaneous refinement of both the Cu and Nb edges.
 Constrain Nb-Cu signals to have the same distance and s2 for
both data sets.
8
Modeling Results for CuNb thin film
Neighbors
Number
Distance (Å)
s2 (x 10-3 Å2)
Nb  O
0.3
2.09 ± 0.02
2±1
Nb  Cu1 (bcc)
2.4 ± 1.0
2.71 ± 0.02
14 ± 6
Nb  Nb1 (bcc)
3.1 ± 1.0
2.81 ± 0.05
21 ± 8
Nb  Cu2 (bcc)
4.8 ± 2.4
3.27 ± 0.03
24 ± 10
Nb  Nb3 (bcc)
3.8 ± 2.8
4.81 ± 0.05
30 ± 12
Neighbors
Number
Distance (Å)
s2 (x 10-3 Å2)
Cu-Cu1 (fcc)
2.6 ± 0.6
2.43 ± 0.02
8±1
Cu-Nb1 (bcc)
2.8 ± 1.1
2.71 ± 0.02
14 ± 6
Cu-Nb2 (bcc)
3.9 ± 1.8
3.27 ± 0.03
24 ± 10
 Simultaneous refinement of both the Cu and Nb edges.
 Constrain Nb-Cu signals to have the same distance and s2 for both data sets.
 Equal numbers of like and unlike atoms in the first shell consistent with
random distribution.
 Second coordination shell contains only unlike atom pairs.
 Small but longer 3rd shell for Nb is similar to bcc structure.
Molecular Dynamics Simulations
 Ab initio molecular dynamics (AIMD)
simulations using the Vienna Ab-inito
Simulation Package (VASP) within the
framework of density functional theory
(DFT) and classical embedded atom
method (EAM) simulations of
amorphous structures of the Cu45at%Nb films.
 Large-scale (1.15 Million atom) MD
using EAM potentials coupled with 3D
atom probe clustering behavior at large
length scales.
Molecular Dynamics Simulations
 Compositional profiles for Cu and Nb, averaged across
simulation super-cell and running along the central axis are
shown.
 Three independent profiles were calculated along lines parallel
to X, Y, and Z axes, and their average is plotted here.
 They reveal characteristic compositional fluctuations with an
average wavelength of ~3 nm.
Common Neighbor Analysis
icosahedral
fcc
bcc
 Common Neighbor Analysis (CNA) used to determine number of
pairs of different types, indicative of different local environment.
Each pair type is an average over 100 picoseconds.
 CNA labels each nearest neighbor atom pairs using a 3 index
notation jkl.
 Predominance of icosahedral and distorted icosahedral pairs
indicates amorphous structure.
AIMD and EXAFS Bond lengths and CN
Radial Distances
Neighbor EXAFS, R (Å)
Cu-Cu
2.43  0.02
Cu-Nb
2.71  0.02
Nb-Nb
2.81  0.05
AIMD, R (Å)
2.53
2.77
2.84
Number of Neighbors
Neighbor
Cu-Cu
Cu-Nb
Cu_sum
Nb-Cu
Nb-Nb
Nb_sum
EXAFS, CN
2.6  0.6
6.7  1.8
9.3
7.2  2.4
3.1  1.0
10.3
AIMD, CN
5.6
6.1
11.7
7.8
6.4
14.2
 Comparison of bond lengths and coordination numbers from
EXAFS and ab initio MD (AIMD) simulations.
 Neighboring distances are consistent within 0.1 Å.
 Coordination numbers are lower for EXAFS compared to AIMD
for like pairs but very similar for unlike pairs.
 Consistency between two techniques gives added validity to
measurements and calculations.
Conclusions
 Using EXAFS, 3D atom probe, and atomistic simulations we
have established that (a) there is a competition between
clustering and local chemical ordering tendencies, which may
lead to frustration, and (b) that the Nb-rich clusters exhibit
medium range topological order leading to a sub-unit cell
crystallinity.
 Nanoscale structural ordering and compositional
heterogeneities appear to thwart the crystallization kinetics
and promote glass formation.
 Nanoscale compositional clusters and/or nanocrystals within
an amorphous matrix deflect, blunt, or branch shear bands
and thereby encourage homogeneous plastic deformation,
and consequently enhance the room temperature plasticity of
BMGs.
UOP 5646-23
EXAFS Model of Cu and Nb Spectra
0.2
0.8
0.6
0.4
0.2
-3
Real part of FT (Å )
0.8
-3
0.1
0.0
0.2
0.0
Real FT (Å )
Nb data
Model
-3
Mag FT (Å )
Cu data
Model
-3
Mag FT (Å )
1.0
0.4
0.0
-0.4
-0.8
0
2
4
R (Å)
6
8
0.1
0.0
-0.1
-0.2
0
2
4
R (Å)
6
8