Introduction

1. Development of Amorphous Layered Al84Co8.5Ce7.5 Structures by Laser Irradiation for Enhanced Corrosion Resistance Experimental Introduction Sample Preparation 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. 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. UV Laser Melted Layer Melt Depth Resolidification Velocity Bulk Crystalline Target Target Velocity Laser Processing 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. Electrochemistry 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. 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. 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. 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. 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. SEM 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. EDS 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 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. 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. Open Circuit Potentials Repassivation Potentials SEM, AES, EDS, XRD and Electrochemistry Ejection Pressure Conventional Melt Spinning Technique Fractured Surfaces SEM -T3 Alloy Melt Induction Coil -T3 Laser Surface Modified Region Amorphous Ribbon Copper Wheel -T3 Reprinted From [5] Sample

2. Surface Analysis Depth Profile of Al84Ce7.5Co8.5 ingot Depth Profile of Laser Surface Modified (F= 1 J/cm2 and 500 PPA) sample SEM Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with .F = 1.0 J/cm2 Homogenization BEI Illustrating the Effect of Multiple Pulses Per Area (PPA) on Microstructure Ingot 5 PPA 50 PPA 500 PPA Plane View: Laser Surface Modified Al84Ce7.5Co8.5 with 5 PPA Homogenization Bulk Analysis SEI (top) and BEI (bottom) Illustrating the Effect of Fluence on Microstructure 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 F = 2 J/cm2 and 50 PPA Intensity (counts) Intensity Increasing Melt Depth Cross Section View: Laser Surface Modified Al84Ce7.5Co8.5 Fracture Surface with 25 PPA SEI Illustrating the Effect of Fluence on Melt Depth Energy (eV) Energy (eV) SEI Illustrating resolidification dominated by thermodynamics of underlying bulk Ingot F = 1 J/cm2 F = 2 J/cm2 F = 3 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. Al84Co8.5Ce7.5 Normalized Polarization Data comparing Ingot, Melt Spun Ribbon, and Irradiated Specimens in 0.6 M Deaerated NaCl Al84Co8.5Ce7.5 Open Circuit Potentials in 0.6 M Deaerated NaCl 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-Gerald University of Virginia Department of Materials Science & Engineering 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. Key Findings: Native specimens exhibit significant carbonaceous and alumina present on the surface. Oxide thickness increased on irradiated sample. 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. Electrochemistry Irradiation Parameters for Electrochemistry: F = 2 J/cm2 and 50 PPA 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.