Depth Profile of Structure, Strain, and Composition in an Annealed Al-Cu-Fe-Cr Quasicrystalline Film
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DEPTH PROFILE OF STRUCTURE, STRAIN, AND COMPOSITION IN AN ANNEALED Al-Cu-Fe-Cr QUASICRYSTALLINE FILM M.J. Daniels1, D. King2, J.S. Zabinski3, Z.U. Rek4, J.C. Bilello1 Center for Nanomaterials Science, Department of Materials Science and Engineering, University of Michigan, 2300 Hayward St., Ann Arbor, MI 48109-2136 2 Technology Assessment and Transfer, Annapolis, MD, 21108 3 Air Force Research Laboratory, Wright-Patterson AFB, OH, 45433 4 Stanford Synchrotron Radiation Laboratory, Stanford University, Palo Alto, CA 94309
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ABSTRACT Quasicrystalline films were formed by RF sputtering from a powder composite target onto Inconel substrates, which produces a polymorphic nanoquasicrystalline grain structure, ~2.5 - 10 nm. Subsequent annealing at 500°C for 4 hours, at base pressures of below 5*10-5 Torr, and with Ar flow to 5 - 10 mT, fully develops the quasicrystalline structure with decagonal phase predominating, except near the termination surface. Analysis by XPS indicated extensive oxygen incorporation and limited aluminum enrichment at the termination surface. These results are correlated with structure and strain analysis via synchrotron grazing incidence x-ray scattering (GIXS). By varying the incident angle, hence the x-ray penetration depth, the evolution of an amorphous and crystalline crystalline secondary phases at the surface of the film has been detected. Residual strain analysis shows that this second phase induces a compressive residual strain of ≈ 0.10% as measured from the displacement of the major quasicrystalline peaks in the surface layers of the film. INTRODUCTION Quasicrystalline structures exhibit traditionally forbidden crystallographic symmetries and are found in a variety of Al-transition metal systems. Bulk quasicrystals are inherently brittle due to impeded dislocation flow, but they have also shown the potential to form low friction coatings.1 Typically, coatings are deposited via a PVD process, then annealed at around 500°C to develop the quasicrystalline structure. Oxidation of the surface is usually observed, and other groups have studied the oxide under different annealing atmospheres, temperatures and pressures.2-5 The present study correlates the resultant structure, composition and strain of an oxidized quasicrystalline film as a function of depth into the film using synchrotron diffraction methods and XPS. EXPERIMENT A more complete description of the experimental details are given elsewhere.6 A powder pressed composite target of composition Al84Cu3.5Fe6Cr6.5 was used to form ≈ 10 µm coatings by RF sputtering. In this case, an argon carrier gas pressure of 5 mT and sample bias of 156 V produced both a nanoquasicrystalline and nanoquasicrystalline approximants structure in the asdeposited state. Composition analysis by XPS showed this sample to be Al74Cu4Fe8Cr10O4. Annealing was performed for 4 hours at 500°C at a base pressure of 5*10-5 Torr under argon L12.4.1
flowing at 5 - 10 mT to promote the quasicrystalline structure via a grain growth process. The resulting film was charac
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