Intermediate Annealing Behavior And Grain Growth Of Al-Cu-Fe Quasicrystalline Coatings
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INTERMEDIATE ANNEALING BEHAVIOR AND GRAIN GROWTH OF Al-Cu-Fe QUASICRYSTALLINE COATINGS M. J. Daniels1, D. King2, J. S. Zabinski3, J. C. Bilello1 1 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 ABSTRACT A powder composite AlCuFe target was used to produce 10 micron films of a quasicrystalline precursor phase by RF sputtering onto polycrystalline alumina substrates. X-ray studies have determined this precursor to be a nano-quasicrystalline phase, despite previous reports of an amorphous structure. Anneals for various times at temperatures between 450°C and 550°C showed the evolution and grain growth of the quasicrystalline approximant structure as a function of both variables. Synchrotron diffraction studies showed a bimodal grain size distribution of the quasicrystalline approximant structure for anneals at 450°C for 1 hour, below a previously reported transition temperature of 475°C. Composition analysis of annealed films by XPS depth profiling showed the surface composition to be nearly stoichiometric Al2O3 and extensive oxygen penetration to 160 nm. X-ray diffraction studies have been correlated with cross section microscopy to characterize the development of quasicrystalline films from their precursors. Anneals for longer times and at higher temperatures showed no evidence of a bimodal distribution or remaining nanoquasicrystalline phases. INTRODUCTION Quasicrystalline structures are aperiodic, yet exhibit short-range order and form polycrystalline diffraction patterns. Stable quasicrystals were first observed in the AlCuFe system by Tsai, et al. [1], although they also exist in many aluminum-transition metal systems. They have been determined to have low coefficients of friction and large measured hardnesses [2,3]. For this reason, they are under investigation as wear resistant coatings for a variety of tribological applications. In addition to the icosohedral quasicrystal present above 700°C at a composition of approximately Al63Cu25Fe12, a rhombohedral approximant phase (r-phase, a=32.16 Å, α=36°) forms at the same composition at lower temperatures [4]. Approximant phases are observed in other quasicrystalline systems and are very similar in structure and local order to their quasicrystalline counterparts, but are instead periodic on large (several nanometer) length scales. As a result, the diffraction patterns of quasicrystals and approximant phases are often nearly indistinguishable [5]. Due to the similarities in structure to the quasicrystalline phase, approximant phases are believed to have similarly useful properties. The phase diagram for the icosohedral quasicrystal has a single-phase region of only 3 atomic percent at 800°C, while there is indication that the single-phase region for the rhombohedral structure at lower temperatures may be twice as large [6,7]. The large
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