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.

Conclusions and Future Work

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

[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.

Jeffrey G. Hoekstra, Gary J. Shiflet, John R. Scully and James M. Fitz-GeraldJeffrey G. Hoekstra, Gary J. Shiflet, John R. Scully and James M. Fitz-Gerald

University of Virginia Department of Materials Science & EngineeringUniversity of Virginia Department of Materials Science & Engineering

Surface Analysis

Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with .F = 1.0 J/cm2

Ingot 5 PPA 50 PPA 500 PPA

Homogenization

Cross Section View: Laser Surface Modified Al84Ce7.5Co8.5 Fracture Surface with F = 2 J/ cm2 & 25 PPA

BEI Illustrating the Effect of Multiple

Pulses Per Area (PPA) on Microstructure

SEI Illustrating the Effect of Fluence on

Melt Depth

SEI Illustrating resolidification dominated by

thermodynamics of underlying bulk

SEI (top) and BEI (bottom) Illustrating the Effect of Fluence

on Microstructure

Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with 5 PPA

F = 0.1 J/cm2

Homogenization

F = 0.25 J/cm2 F = 0.5 J/cm2 F = 0.75 J/cm2

Cross Section View: Laser Surface Modified Al84Ce7.5Co8.5 Fracture Surface with 25 PPA

Ingot F = 1 J/cm2 F = 2 J/cm2 F = 3 J/cm2

Increasing Melt Depth

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.

SEMSEM

Key Findings: Native specimens exhibit significant carbonaceous and alumina present on the surface. Oxide thickness increased on irradiated sample.

0 5 10 15 200.0

0.1

0.2

0.3

0.4

0.5

0.6

Co

Ce

O

Al(0)

Al3+

C

C Al3+

Al(0) O Ce Co

Ato

mic

Fra

ctio

n

Sputtering time (s)

Depth Profile of Al84Ce7.5Co8.5 ingot

0 100 200 300 400 500 6000.0

0.1

0.2

0.3

0.4

0.5

0.6

CoC

Ce

O

Al(0)Al

3+

Al3+

Al(0) O Ce Co C

Ato

mic

Fra

ctio

n

Sputtering Time (s)

Depth Profile of Laser Surface Modified (F= 1 J/cm2 and 500 PPA) sample

ElectrochemistryElectrochemistry

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.

Al84Co8.5Ce7.5 Open Circuit Potentials in 0.6 M Deaerated NaCl

BehaviorPure Aluminum pittingAA2024T3 pittingMelt Spun Ribbon passiveLaser Surface Modified metastable pittingBulk Ingot pitting

OCP (V) Epit (V) Erp (V)Melt Spun Ribbon -0.64 -0.23 -0.58Bulk Ingot -0.85 -0.85 -0.80Laser Surface Modified -1.03 -0.80 -0.83

Irradiation Parameters for Electrochemistry:

F = 2 J/cm2 and 50 PPA

Bulk Analysis

EDSEDS

F = 2 J/cm2 and 50 PPA

Inte

nsity

(co

unts

)

Energy (eV)

Inte

nsity

Energy (eV)

Al84Ce7.5Co8.5 ingot

XRDXRD

F = 2 J/cm2 and 50 PPA

AESAES

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.

Al84Co8.5Ce7.5 Normalized Polarization Data comparing Ingot, Melt Spun Ribbon, and

Irradiated Specimens in 0.6 M Deaerated NaCl