Development of Amorphous Layered Al84Co8.5Ce7.5 Structures by Laser

Development of Amorphous Layered Al84Co8.5Ce7.5 Structures by Laser

Development of Amorphous Layered Al84Co8.5Ce7.5 Structures by
Laser Irradiation for Enhanced Corrosion Resistance
Experimental

Introduction

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, T c,
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 10 6 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

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 ( = 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.

AES

XRD

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.

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.

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 components 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.

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.
Amorphous Al-Co-Ce Alloy Compostions

Ejection
Pressure

Conventional

Open Circuit Potentials
Aermet 100

16

Melt Spinning

-0.5

12

Cerium (at%)

-0.2

-0.6

10
9

-0.7

8
7

-0.3

Pure Aluminum

-0.8

6

-0.4
-0.9

4
3

Copper
Wheel

-T3

11

5

Amorphous
Ribbon

Al-Co-Ce Alloy System

AA 2024

14

Technique
Alloy Melt

SEM, AES, EDS, XRD and
Electrochemistry

15
13

Induction
Coil

Repassivation Potentials

-T3

2
1

AA 2024

-0.5

-1.0

-0.6

-1.1

0.09

0
0

1

2

3

4

5

6

7

8

9

Cobalt (at%)

10

11

12

13

14

15

16

-1.2

Al-Co-Ce System

-1.3
0.08

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.)

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11

Laser Surface
Modified Region

0.08

-0.7

0.07
0.06

-0.8
0.10

0.09

0.08

0.05

-T3

0.07

0.06

0.04
0.05

0.04

0.03

0.03

Reprinted From [5]

Sample

Fractured Surfaces

SEM

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