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

                                                                Introduction                                                                                                                                                                                                                                                                                            Experimental

  Amorphous metals have attractive properties, particularly in areas of corrosion resistance, mechanical                                                                                                                                                                                                                                         UV Laser
                                                                                                                                                                                                                                                                                                                                                                                                       Sample Preparation
  hardness, wear and fatigue. Advances in metallic glass chemistries with reduced critical cooling rates, Tc,
                                                                                                                                                                                                                                                                                                                                                                                         Samples      were   composed       of     polyphase
  have furthered the development of bulk metallic glasses.[1] Glassy metals are formed by rapid
                                                                                                                                                                                                                                                                                                                 Melt Depth              Melted Layer                                    Al84Co8.5Ce7.5 ingots arc-melted in an Ar
  solidification of a liquid phase such that nucleation and growth of the preferential crystalline phases is
                                                                                                                                                                                                                                                                                                                                                                                         atomsphere using high purity powders. Care was
  prevented, locking the super-cooled liquid into a metastable phase. Enhanced corrosion resistance from                                                                                                                                                                                                                                                          Resolidification       taken to prepare sample surfaces for irradiation and
  an amorphous state stems from a lack of grain boundaries, secondary phases, compositional segregation,                                                                                                                                                                                                                                                             Velocity
                                                                                                                                                                                                                                                                                                                                    Bulk Crystalline Target                              characterization by polishing to a one micron
  and crystalline defects. Conventional melt spinning techniques with a maximum cooling rate of 106 K/s
                                                                                                                                                                                                                                                                                                                                                                                         roughness using a Buehler Ecomet 4 polishing
  create amorphous ribbons, however the application of ribbons is quite limited. Whereas irradiation of a                                                                                                                                                                                                                                                              Target Velocity
                                                                                                                                                                                                                                                                                                                                                                                         wheel and Buehler diamond polish.           Typically
  material with a short laser pulse, 3-100 ns, establishes rapid melting and solidification velocities at the
                                                                                                                                                                                                                                                                                                                                                                                         samples were 1 cm x 1 cm x 0.5 cm.
  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                                                                                                                                                                                                                           Laser Processing                                                    Electrochemistry
  electrochemical analysis and devitrification behavior. This study represents initial research efforts to                                                                                                                                                                                                       A Lambda Physics KrF excimer laser ( = 248 nm,                         Corrosion experiments were performed in a
  correlate microstructure and global corrosion resistance as a function of pulsed ultraviolet laser                                                                                                                                                                                                             25 ns at FWHM, 25 Hz) operating at fluences                             standard three-electrode cell with deaerated 0.6 M
  irradiation, specifically focused on the ability of laser surface modification as a means to duplicate                                                                                                                                                                                                         ranging from 0-5 J/cm2 irradiated a target surface                      NaCl and a SCE reference electrode using a EG&G
  the global corrosion resistance of Al-Co-Ce melt spun ribbons.                                                                                                                                                                                                                                                 with corresponding velocity between 0-50 mm/s in a                      273A potentiostat. Ni reference electrode leads
                                                                                                                                                                                                                                                                                                                 controlled He atmosphere at a variety of backfill                       were bonded to the backs of samples with
                                                                                                                                                                                                                                                                                                                 pressures with a base pressure less than 50 mTorr.                      conductive epoxy, while the surface was masked
                                                                                                                                                                                                                                                                                                                 A programmable         Newport   ESP300     motion                      with XP2000 StopOff to avoid preferential pitting of
                                                Motivation and Background                                                                                                                                                                                                                                        controller/driver operated two ILS series high                          voids and defects.        Open circuit scans were
                                                                                                                                                                                                                                                                                                                 precision motion control stages Samples were                            followed by potentiodynamic scans to determine the
                                                                                                                                                                                                                                                                                                                 irradiated from 1-2000 pulses per area (PPA).                           pitting potential, repassivation potential, open circuit
                                                                                                                                                                                                                                                                                                                 Studies were performed to determine the effects of                      potential and behavior. Data was acquired for
        The aerospace industry desires improvement upon pre-existing high-performance coatings designed                                                                                                                                                                                                          fluence and PPA on melt depth, microstructure, and                      native, melt spun, and laser surface modified
  to maximize the lifetime of parts exposed to corrosive environments. Military aircraft are clad with a four                                                                                                                                                                                                    crystallinity. Above a schematic illustrates the                        samples.       Below a schematic illustrates the
  component system, a base metal, typically AA2024, is covered with Alclad, a chromate cladding, then a                                                                                                                                                                                                          principal experimental parameters and below the                         standard three-electrode cell used in the
  chromate-containing primer coating, and finally an epoxy-based topcoat. Each component’s corrosion                                                                                                                                                                                                             experimental setup is shown.                                            electrochemical analysis.
  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.                                                                                                                                                                                                                                                                                                                                       SEM                                                              EDS
        The production of metallic glasses requires the vitrification of a melt necessitating high cooling rates.                                                                                                                                                                                                High resolution secondary and backscattered                             Qualitative EDS spectra were obtained using a
  The glass forming ability increases as increasing constituents are added. Conventionally, amorphous                                                                                                                                                                                                            electron imaging were performed using a JSM6700F.                       JSM6700F in combination with a Spirit system by
  structures were produced by splat cooling techniques and more recently by melt spinning which cools the                                                                                                                                                                                                        Samples were investigated in both plane view and                        Princeton Gamma-Tech. This technique was used
  molten liquid on a LN2 cooled copper wheel, producing an amorphous ribbon as seen on the left in the                                                                                                                                                                                                           cross-sectional view. The fracture procedure used                       to compare the chemistries of the polyphase ingot
  figure below. The Al-Co-Ce system with enhanced glass solidification chemistries was developed at UVa                                                                                                                                                                                                          to investigate melt depth and to correlate melt depth                   and laser surface modified specimens, confirming
  by Dr. Shiflet and his research group, as seen in the center of the figure below.[5] The figure also shows                                                                                                                                                                                                     and resultant microstructures to the irradiation                        the near 10 micron surface chemistry is similar in
  this system with a reduced critical cooling rate can be amorphous over a wide range of alloy compositions.                                                                                                                                                                                                     conditions is shown below.                                              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                                                                                                                                                                                                                                  AES                                                              XRD
  coating with enhanced resistance to chloride-induced pitting as compared to pure Al or AA2024, where Ce
                                                                                                                                                                                                                                                                                                                 AES depth profiling was performed using a Perkin                        A Scintag LET 2400 X-ray diffractometer was used
  promoted amorphicity, lower pitting, open circuit, and repassivation potentials However, Co promoted
                                                                                                                                                                                                                                                                                                                 Elmer PHI 560 ESCA/SAM system. Surface studies                          to analyze and identify the crystalline and
  higher repassivation and open circuit potentials as seen below. The large range of open circuit potentials
                                                                                                                                                                                                                                                                                                                 of native and laser treated samples were completed                      amorphous states of polycrystalline ingots and melt
  this alloy system has are a benefit in sacrificial cathodic protection as the ability to select an appropriate
                                                                                                                                                                                                                                                                                                                 to investigate the nominal composition within 10 nm                     spun ribbons and to ascertain whether the
  open circuit potential for a base metal is of great interest.
                                                                                                                                                                                                                                                                                                                 of the surface, specifically concentrating on surface                   amorphous nature of laser processed samples.
                                            rol - l  i
                                           AhA e y p n
                                           m s o l C ss
                                           ouCA o t  o
                                            p - C om                                Open Circuit Potentials                                                                                       Repassivation Potentials                                                                                       oxides.
Ejection       Conventional                                                                                                                                  Aermet 100
                                                                                                                                                                                                                                                                                                                                                   SEM, AES, EDS, XRD and
                                       1                                                                                                                                                                                                                                                                                                                                                   Fractured Surfaces                  SEM
               Melt Spinning           4
                                       1                                                                                                                                      AA 2024                                                                   Al-Co-Ce Alloy System                                                                         Electrochemistry
                 Technique             2

                                       0                                            -0.6                                                                                                                  -0.2
Induction    Alloy Melt                    9
                                           8                                        -0.7
   Coil                                    7
                                                                                                                                                                                                                                                                                             Pure Aluminum

                                                                       OCP (VSCE)

                                           6                                         -0.8

                                                                                                                                                                                                           -0.4                                                                                                                Laser Surface
                                                                                                                                                                                           E rp (V SCE)

                                                                                     -0.9                                                                                                                                                                                                                  -T3
                                           3                                                                                                                                                                -0.5                                                                                 AA 2024                      Modified Region
    Copper            Amorphous            1                                          -1.1                                                                                                                  -0.6

    Wheel              Ribbon              0                                                                                                                                                                                                                                                     0.09
                                           0 1234567890 1 3 1 6
                                                      11 14 1
                                                       12 15                          -1.2                                                                                          0.03                                                                                                     0.08

                                                        C( )
                                                         l                                           Al-Co-Ce System                                                         0.05

                                                                                      -1.3                                                                               0.06      c.)                                                                                                0.06
                                                                                                                                                                                 ra                          -0.8

                                                                                             0.08                                                                     0.07 . f

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                                                oua r a                                                      xC                     0.03                     0.10                                                                               0.06
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                                                t not .
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                                                                                                                                                                                                                                                                            Reprinted From [5]                                                                Sample

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