Effect of Nb Precipitates on Cu-Nb Multilayer Composites
B. C. O'Meara, H.M. Zbib, I.N. Mastorakos
School of Mechanical and Materials Engineering
Washington State University
Introduction
Effect of Varying Layer Thickness
Effect of Varying Precipitate Radius
Dislocations cause thin film materials to deform and fail
under loading. Finding ways of preventing dislocations
from spreading would increase the strength of a
material. This project focuses on Cu-Nb multilayer
composites and the effect of Nb precipitate in the Cu
layer on the material's strength.
• Research has shown that for layer thicknesses greater
than 5nm, strength tends to increase with decreasing
thickness
6.5 nm Layer Thickness
Representative Precipitate Radii
12
2. Dislocations begin
to nucleate at the
precipitate
10
• For precipitate radius 1.0nm, strength increases with
decreasing layer thickness. This is the expected result.
Precipitate Radius 1.0 nm
12
Dislocations
8
3. Material fails
Direction of slip
8
St ress (GPa)
Extra Net
Plane
Stress (GPa)
10
6
4
1. Structure free of
dislocations
No precipitate
R=5
R=10
R=20
0
-2
0.00000
0 .0 0 0 0 0
0 .0 5 0 0 0
0 .1 0 0 0 0
0 .1 5 0 0 0
0.05000
0.10000
0.15000
0.2 0000
0.2 5000
Strain
• However, for precipitate radius 2.0 nm, the strength is
about the same for both 6.5nm and 8.5nm. This is
attributed to the thinning of the Cu layer by the presence
of the precipitate that brings the total layer thickness
close to critical optimum thickness.
-2
0 .2 0 0 0 0
0 .2 5 0 0 0
Strain
Defects in the crystal structure of a material
Lattice structure effects direction and plane of
dislocation propagation.
T = 6.5nm
T=8.5 nm
0
Dislocation
Core

4
2
2

6
• The stress-strain curves indicate that the size of the precipitate
determines whether the material strengthens or weakens
Precipitate Radius 2.0 nm
12
10
Cu-Nb Multilayer Composites
Interface
St ress (GPa)
8
Effect of Precipitate Radius on Maximum Stresses
6
4
2
T=6.5 nm
T=8.5 nm
12
0
Cu
-2
0.00000

Cu and Nb have different lattice structures

Interface prevents propagation of dislocations

More obstacles to dislocations = stronger material?
8
6
•Embed a Nb precipitate inside the Cu layer
•Layer thickness: 6.5 nm, 8.5 nm
•Apply uniaxial tensile load
0.15000
0.2 0000
0.2 5000
Precipitate Radius 3.0 nm
4
ultimate stress
(second peak on curve)
yield (where curve
becomes non-linear)
2
12
10
8
0
0
0.5
1
1.5
2
2 .5
3
3.5
Radius (nm)
Method
•Molecular dynamics (MD) simulations
0.10000
• For precipitate radius 3.0 nm, the strength is again
about the same for both 6.5 nm and 8.5 nm layer
thickness.
St ress (GPa)
Interface
Interface
0.05000
Strain
Stress (GPa)
Nb
10
6
4
2
T=6.5nm
T=8.5nm
0
• The ultimate stress reaches a maximum with a precipitate radius
between 1.0nm and 1.5nm.
• The yield stress tends to decrease with increasing precipitate
radius. This could be because the precipitate interface favors
dislocation nucleation
-2
0.00000
0.05000
0.10000
0.15000
0.2 0000
0.2 5000
Strain
• By changing the size of the precipitate, a thicker
material can be made as strong as a thinner material.
Adding precipitates to Cu-Nb multilayer composites will
help to improve the performance of these materials.
This work was supported by the National Science
Foundation’s REU program