Stable quasicrystalline phase in Al-Cu-Fe-Cr coating materials
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QUASICRYSTALS (QCs) exhibit rotational symmetries, which were previously believed to be forbidden in crystals. These extraordinary materials not only have unique structures, but also have unusual properties. Unlike conventional metallic materials, QCs exhibit high hardness and stiffness, but low electrical and thermal conductivity and exceptionally low fracture toughness. The QC materials are reported to exhibit a very low coefficient of friction, measured against diamond and steel counterfaces. This, together with their high hardness, suggests that QC materials may be valuable in tribological applications. The utilization of QC materials in bulk form is compromised by their low fracture toughness, which presents problems both in the manufacture of components and in service lifetimes. Materials with low fracture toughness may, however, be used in combination with more ductile materials. This is of particular interest for tribological applications, where the surface properties of QC materials (high hardness and low friction coefficient) may be combined with the bulk properties of conventional metals (high toughness and ductility) by means of a QC coating on a metallic substrate. Dubois and his co-workers were the first to investigate the tribological properties of coatings, starting with the AlCu-Fe system and then extending to the Al-Cu-Fe-Cr system.[1,2,3] Their attention was focused on one potential application, the preparation of nonstick thermal-sprayed coatings on cookware. The Al-rich corner of the phase diagram of the Al-Cu-Fe-Cr system was explored for promising QC coating materials.[4,5] A metastable icosahedral phase was X.Z. LI, formerly Researcher Associate, Department of Materials Science and Engineering, Northwestern University, is Research Staff Member, Center for Materials Research and Analysis, University of Nebraska–Lincoln, Lincoln, NE 68588. L.D. MARKS, Professor, is with the Department of Materials Science and Engineering, Northwestern University. J. MACIEJEWSKI and L. FEHRENBACHER, Scientists, are with Technology Assessment and Transfer, Inc., Annapolis, MD 21401. J. ZABINSKI and J. O’NEILL, Scientists, are with the AFRL/MLBT, Wright-Patterson AFB, OH 45433. Manuscript submitted August 28, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
found in rapidly quenched samples, which transforms into the approximants of the decagonal phase upon annealing. The decagonal phase was formed within a certain coolingrate limit. If the cooling rate was too high, the icosahedral phase formed, while slow solidification led to the formation of the crystalline approximants, including a microcrystalline (MC) structure with pseudo tenfold symmetry. Five polymorphous approximants have been observed, which belong to an orthorhombic and monoclinic system.[6] The MC structures are composed of different approximant components, thus leading to many kinds of MC structures. The coating materials produced by thermal spraying (or hot-projection) techniques are metastable quasicrystals and (or transformed into) stable
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