Development of Amorphous Layered Al84Co8.5Ce7.5 Structures by
Laser Irradiation for Enhanced Corrosion Resistance
Introduction
Experimental
Sample Preparation
UV Laser
Amorphous metals have attractive properties, particularly in areas of corrosion resistance, mechanical
hardness, wear and fatigue. Advances in metallic glass chemistries with reduced critical cooling rates, Tc,
have furthered the development of bulk metallic glasses.[1] Glassy metals are formed by rapid
solidification of a liquid phase such that nucleation and growth of the preferential crystalline phases is
prevented, locking the super-cooled liquid into a metastable phase. Enhanced corrosion resistance from
an amorphous state stems from a lack of grain boundaries, secondary phases, compositional segregation,
and crystalline defects. Conventional melt spinning techniques with a maximum cooling rate of 106 K/s
create amorphous ribbons, however the application of ribbons is quite limited. Whereas irradiation of a
material with a short laser pulse, 3-100 ns, establishes rapid melting and solidification velocities at the
surface, 106-108 K/s and 10-1 -101 m/s respectively.[2]. This research was conducted using the Al-Co-Ce
alloy system, a system developed at UVa with an excellent glass forming ability.[6] Bulk polycrystalline
ingots were laser surface modified with an excimer laser. Resulting microstructures were correlated with
electrochemical analysis and devitrification behavior. This study represents initial research efforts to
correlate microstructure and global corrosion resistance as a function of pulsed ultraviolet laser
irradiation, specifically focused on the ability of laser surface modification as a means to duplicate
the global corrosion resistance of Al-Co-Ce melt spun ribbons.
Melt Depth
Melted Layer
Bulk Crystalline Target
Resolidification
Velocity
Target Velocity
Motivation and Background
The aerospace industry desires improvement upon pre-existing high-performance coatings designed
to maximize the lifetime of parts exposed to corrosive environments. Military aircraft are clad with a four
component system, a base metal, typically AA2024, is covered with Alclad, a chromate cladding, then a
chromate-containing primer coating, and finally an epoxy-based topcoat. Each component’s corrosion
resistance stems from different mechanisms. Alclad acts as a sacrificial anode, increasing resistance to
pitting and exfoliation while conventional chromate conversion coatings and chromate-containing primer
coatings protect AA2024 from its high susceptibility to stress corrosion cracking and poor resistance to
pitting and exfoliation while acting as anodic protection. [3] However, chromate claddings necessitate
replacement due to deleterious environmental hazards of hexavalent chromate, a known carcinogen.
Samples
were
composed
of
polyphase
Al84Co8.5Ce7.5 ingots arc-melted in an Ar
atomsphere using high purity powders. Care was
taken to prepare sample surfaces for irradiation and
characterization by polishing to a one micron
roughness using a Buehler Ecomet 4 polishing
wheel and Buehler diamond polish.
Typically
samples were 1 cm x 1 cm x 0.5 cm.
Laser Processing
Electrochemistry
A Lambda Physics KrF excimer laser (λ = 248 nm,
25 ns at FWHM, 25 Hz) operating at fluences
ranging from 0-5 J/cm2 irradiated a target surface
with corresponding velocity between 0-50 mm/s in a
controlled He atmosphere at a variety of backfill
pressures with a base pressure less than 50 mTorr.
A
programmable
Newport
ESP300
motion
controller/driver operated two ILS series high
precision motion control stages Samples were
irradiated from 1-2000 pulses per area (PPA).
Studies were performed to determine the effects of
fluence and PPA on melt depth, microstructure, and
crystallinity. Above a schematic illustrates the
principal experimental parameters and below the
experimental setup is shown.
Corrosion experiments were performed in a
standard three-electrode cell with deaerated 0.6 M
NaCl and a SCE reference electrode using a EG&G
273A potentiostat. Ni reference electrode leads
were bonded to the backs of samples with
conductive epoxy, while the surface was masked
with XP2000 StopOff to avoid preferential pitting of
voids and defects.
Open circuit scans were
followed by potentiodynamic scans to determine the
pitting potential, repassivation potential, open circuit
potential and behavior. Data was acquired for
native, melt spun, and laser surface modified
samples.
Below a schematic illustrates the
standard three-electrode cell used in the
electrochemical analysis.
The Al-Co-Ce alloy system has tremendous potential as a cladding material. The corrosion
resistance arises from its ability to actively inhibit corrosion, serve as an efficient corrosion barrier and act
as a sacrificial cathode while functioning in an amorphous state.[4] An absence of grain boundaries,
dislocations, secondary phase particles, and localized concentrations of alloying elements removes
preferential attack sites, resulting in a more protective oxide film.[7] Localized corrosion, or pitting, is
initiated at local flaws, heterogeneities within an oxide film, where damaging species such as chloride may
adsorb.
SEM
EDS
The production of metallic glasses requires the vitrification of a melt necessitating high cooling rates.
The glass forming ability increases as increasing constituents are added. Conventionally, amorphous
structures were produced by splat cooling techniques and more recently by melt spinning which cools the
molten liquid on a LN2 cooled copper wheel, producing an amorphous ribbon as seen on the left in the
figure below. The Al-Co-Ce system with enhanced glass solidification chemistries was developed at UVa
by Dr. Shiflet and his research group, as seen in the center of the figure below.[5] The figure also shows
this system with a reduced critical cooling rate can be amorphous over a wide range of alloy compositions.
High resolution secondary and backscattered
electron imaging were performed using a
JSM6700F. Samples were investigated in both
plane view and cross-sectional view. The fracture
procedure used to investigate melt depth and to
correlate melt depth and resultant microstructures to
the irradiation conditions is shown below.
Qualitative EDS spectra were obtained using a
JSM6700F in combination with a Spirit system by
Princeton Gamma-Tech. This technique was used
to compare the chemistries of the polyphase ingot
and laser surface modified specimens, confirming
the near 10 micron surface chemistry is similar in
laser processed specimens.
Both experimental and computational studies concerning the efficacy of the Al-Co-Ce system to
function as a tunable corrosion barrier determined the system posses the tunability to function as a barrier
coating with enhanced resistance to chloride-induced pitting as compared to pure Al or AA2024, where Ce
promoted amorphicity, lower pitting, open circuit, and repassivation potentials However, Co promoted
higher repassivation and open circuit potentials as seen below. The large range of open circuit potentials
this alloy system has are a benefit in sacrificial cathodic protection as the ability to select an appropriate
open circuit potential for a base metal is of great interest.
Open Circuit Potentials
Amorphous Al-Co-Ce Alloy Compostions
15
Technique
12
13
-0.5
-0.6
10
-0.2
9
-0.7
7
6
5
2
1
-0.3
Pure Aluminum
-0.8
-0.4
-0.9
-1.0
-T3
AA 2024
-0.5
-0.6
-1.1
0.09
0
3
4
5
6
7
8
9
10
Cobalt (at%)
Amorphous (Unlu et al.)
Amorphous (Inoue et al.)
Amorphous (Mansour et al.)
Crystalline (Inoue et al.)
Crystalline (Unlu et al.)
Amorphous + Crystalline (Unlu et al.)
11
12
13
14
15
16
-1.2
-1.3
Al-Co-Ce System
0.08
0.07
0.06
0.05
xCo
0.04
0.03
(at. fra
c.)
0.02
0.01
0.03
0.04
0.05
0.06
c.)
0.07
fra
0.08
(at.
0.09
x Ce
0.10
0.11
0.08
-0.7
-0.8
0.07
0.06
0.10
0.09
0.08
0.05
-T3
0.07
xCe (at.
0.06
frac.)
0.04
0.05
0.04
0.03
(at.
fr
2
o
1
xC
0
Laser Surface
Modified Region
ac
.)
8
3
Copper
Wheel
SEM, AES, EDS, XRD and
Electrochemistry
-T3
11
4
Amorphous
Ribbon
XRD
A Scintag LET 2400 X-ray diffractometer was used
to analyze and identify the crystalline and
amorphous states of polycrystalline ingots and melt
spun ribbons and to ascertain whether the
amorphous nature of laser processed samples.
Al-Co-Ce Alloy System
AA 2024
14
Erp (VSCE)
Alloy Melt
Repassivation Potentials
Aermet 100
16
OCP (VSCE)
Induction
Coil
Conventional
Melt Spinning
Cerium (at%)
Ejection
Pressure
AES
AES depth profiling was performed using a Perkin
Elmer PHI 560 ESCA/SAM system. Surface studies
of native and laser treated samples were completed
to investigate the nominal composition within 10 nm
of the surface, specifically concentrating on surface
oxides.
0.03
Reprinted From [5]
Sample
Fractured Surfaces
SEM
Jeffrey G. Hoekstra, Gary J. Shiflet, John R. Scully and James M.
M. FitzFitz-Gerald
University of Virginia Department of Materials Science & Engineering
Engineering
Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with .F = 1.0 J/cm 2
Homogenization
BEI Illustrating the
Effect of Multiple
Pulses Per Area (PPA)
on Microstructure
Ingot
Homogenization
SEI (top) and BEI
(bottom) Illustrating
the Effect of Fluence
on Microstructure
5 PPA
50 PPA
Key
Findings:
Native
specimens exhibit significant
carbonaceous and alumina
present on the surface.
Oxide thickness increased on
irradiated sample.
500 PPA
Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with 5 PPA
0.6
0.5
Atomic Fraction
SEM
Depth Profile of
Al84Ce7.5Co8.5 ingot
AES
Taken with 3kV Ar+ beam over 2mm x
2mm spot with 3kV e- beam with a
resolution of 3 eV/step, a data
collection rate of 200 msec/step, and 5
sweeps/measurement.
C
Al(0)
Ce
C
0.4
Depth Profile of Laser Surface Modified
(F= 1 J/cm2 and 500 PPA) sample
Al(0)
3+
Al
O
Co
0.5
3+
Al
Ce
0.3
0.2
O
Co
0.1
0.0
3+
Al
Al(0)
O
Ce
Co
C
0.6
0
5
10
15
20
Atomic Fraction
Surface Analysis
Al(0)
3+
Al
0.4
O
0.3
0.2
Ce
0.1
0.0
C
0
100
Sputtering time (s)
200
Co
300
400
500
600
Sputtering Time (s)
Bulk Analysis
F = 0.1 J/cm2
F = 0.25 J/cm2
F = 0.5 J/cm2
F = 0.75 J/cm2
EDS
XRD
F = 2 J/cm2 and 50 PPA
Al84Ce7.5Co8.5 ingot
Intensity
Cross Section View: Laser Surface Modified Al84Ce7.5Co8.5 Fracture Surface with 25 PPA
Increasing Melt Depth
SEI Illustrating the
Effect of Fluence on
Melt Depth
Energy (eV)
Ingot
SEI Illustrating
resolidification
dominated by
thermodynamics of
underlying bulk
Intensity (counts)
F = 2 J/cm2 and 50 PPA
F = 1 J/cm2
F = 2 J/cm2
Cross Section View: Laser Surface Modified Al84Ce7.5Co8.5 Fracture Surface with F = 2 J/ cm2 & 25 PPA
Key Findings: Increased PPA and fluence resulted in a significant degree of homogenization, while higher
fluences increased the cracking of Al-Ce rich phases. Increased fluences also resulted in larger melt
depths, which are limited by the reflectivity of 248nm photons by the metal alloy.
Electrochemistry
Al84Co8.5Ce7.5 Open Circuit Potentials
in 0.6 M Deaerated NaCl
Energy (eV)
F = 3 J/cm2
Al84Co8.5Ce7.5 Normalized Polarization Data
comparing Ingot, Melt Spun Ribbon, and
Irradiated Specimens in 0.6 M Deaerated NaCl
Conclusions and Future Work
From the nonequilibrium thermal nature of the process, SEM studies showed laser surface modification
created complicated microstructures exhibiting cellular resolidification in the nm regime on tensile
fracture surfaces and minimal suppression of voids and surface defects because of the low penetration
of UV photons in this metallic system. Future TEM studies will determine the degree of crystallinity
present in the laser surface modified layers and multi-step irradiation procedures.
Higher fluences increased the homogenization of the microstructure, however lower fluences resulted
in smoother surfaces. High PPA also increased microstructural homogenization. Future samples will
undergo a series of homogenization and amorphization laser surface treatments to explore the possibility
of amorphous layer formation and evaluate the global corrosion resistance.
EDS confirms no dramatic shift in alloy composition within 10 microns of the near-surface composition,
however oxide formation is present as seen in the AES data. AES indicates oxide formation of both Al
and Ce in laser surface modified specimens. Oxide formation will be controlled by using a controlled Ar
atmosphere that will displace O2.
Conventional XRD does not enable detection of amorphous layer formation due to the penetration
depth of the X-rays into the bulk. Grazing angle XRD will be performed.
Electrochemical analysis indicates no advantageous increase in the pitting potential for irradiated
specimens as observed in melt spun samples, but a reduction in the open circuit potential was shown.
No significant increase in the overall corrosion rate or pitting potential was found and the production of
amorphous surface layers remains unseen.
Irradiation Parameters
for Electrochemistry:
F = 2 J/cm2 and 50 PPA
Behavior
Pure Aluminum
pitting
AA2024T3
pitting
Melt Spun Ribbon
passive
Laser Surface Modified metastable pitting
Bulk Ingot
pitting
Melt Spun Ribbon
Bulk Ingot
Laser Surface Modified
OCP (V)
-0.64
-0.85
-1.03
Epit (V)
-0.23
-0.85
-0.80
Erp (V)
-0.58
-0.80
-0.83
Acknowledgments and References
A Multi-University Research Initiative (Grant No. F49602-01-1-0352) entitled The Development of an Environmentally Compliant Multifunctional Coating for Aerospace Applications using Molecular and Nano-Engineered Methods under the direction of Dr. Paul C. Trulove at AFOSR
supported this study.
UVa SEAS Advanced Laser Processing Laboratory Group
UVa SEAS Center for Electrochemical Science and Engineering
M. Jakab, M. Goldman, N. Ünlü, M. Gao, and J. Poon
Key Findings: While bulk polyphase ingot samples, pure Aluminum, and AA2024T3 pitted at open circuit,
amorphous melt spun ribbons exhibited Epit = -0.23 V and laser surface modified samples exhibited
incidences of metastable pitting. The laser treated specimens exhibited decreased open circuit potentials.
Small improvement in pitting behavior were observed, however Epit = -0.75 V for the majority of samples.
[1] Wang, W.H., C. Dong, and C.H. Shek. Bulk Metallic Glasses. Materials Science and Engineering R 44 (2004) 45-89.
[2] Baeri, P. Pulsed Laser Quenching of Metastable Phases. Materials Science and Engineering, A178 (1994) 179-183.
[3] Jones, D.A. Principles and Prevention of Corrosion. (1996) 2nd ed. Prentice Hall, Upper Saddle River, NJ.
[4] Goldman, M.E., N. Ünlü, F.M. Preseul, G.J. Shiflet, and J.R. Scully. Amorphous Metallic Coatings with Tunable Corrosion Properties Based on Al-Co-Ce-(Mo) Alloy Compositions. NACE 2004. Paper 04276.
[5] Inoue, A., K. Othera, K. Kita, and T. Masumoto. New Amorphous Allots with Good Ductility in Al-Ce-M (M=Nb,Fe,Co,Ni, or Cu) Systems. Jpn. J. Appl. Phys. 2 Lett., Vol. 27, L1796-L1799 (1998). And unpublished Ünlü, N.
[6] Shiflet, G.J., J.R. Scully, and S.J. Poon. Amorphous Metallic Coatings with Tunable Corrosion Properties Based on Al-Co-Ce-(Mo) Alloy Compositions. Provisional Patent.
[7] Hashimoto K., Masumoto T. Corrosion Properties of Amorphous Allots. Glassy Metals: Magnetic, Chemical, and Structural Properties, CRC Press, 1983.