Mechanical Behavior of Quasicrystals
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of about 1-1.5 MPa m1/2, quasicrystals are very brittle. Young's modulus measured by mechanical spectroscopy on Al7o.5Pd2iMn85 single quasicrystals is 194 GPa. The evaluation of Vickers indentations in the same material yielded lower values dependent on the applied load—for example, 99 GPa at 100 g. This, as well as the large scatter in the hardness data,24'5 may be explained by microstructural changes. Evidence exists that the limited room-temperature ductility is related to a microcrack mechanism. Cross-sectional electron microscopy of the area under Vickers indentations revealed a high density of shear cracks.4'6 Scratching experiments using a diamond tool showed ploughing-type friction with low abrasive friction coefficients of
typically 0.05 (needle radius: 0.8 mm) in the single-quasicrystalline decagonal phase,1 for Al-Cu-Fe-Cr icosahedralpolyquasicrystalline alloys and thickfilm coatings.78
Fracture Interesting in regard to fracture mechanics but also significant to quasicrystal structure are the results of scanning tunneling microscopy (STM). Figure 1 shows at two different magnifications a surface produced by cleavage in ultrahigh vacuum of a Czochralski-grown Al705Pd2iMn8.5 single quasicrystal perpendicular to a twofold axis.' The microstructure predominantly consists of clusters (a) based on aggregates of an elementary cluster (b) about 1 nm in diameter. In diffraction experiments, this cluster was identified as the basic structural element of the Al-Pd-Mn icosahedral-quasicrystal lattice.1"11 It closely relates to Mackay's icosahedron, a threeshell arrangement of 54 atoms.12 This cluster and also its aggregates have the effect of hard obstacles. The cleavage crack circumvents them, exposing them to view. This interpretation is corroborated by molecular-dynamics simulations of crack propagation in a model quasicrystal.13 Scanning tunneling microscopy shows a smooth distribution for the electron density of states rather than the atomic-scale corrugation expected if "dangling" bonds were to exist at the cluster surface. This supports recent ideas1415 concerning the structure of quasicrystals and their particular elec-
Low-Temperature Mechanical-Property Data Quasicrystalline alloys are relatively hard materials. For example the Vickers hardness of single-quasicrystalline decagonal Al62Coi5Cu20Si3 measured at room temperature is 9.5 GPa1 and that of icosahedral Al72Pd20Mn8 is 7.8 GPa.2 For comparison, hardness values for steels range from 1.8 to 7.7 GPa. On the other hand, with a fracture toughness Klc MRS BULLETIN/NOVEMBER 1997
Figure 1. Scanning-tunneiing-microscope image at (a) low magnification and (b) high magnification of a fracture surface obtained by cleavage along a twofold plane of an Al-Pd-Mn single quasicrystal. Arrows indicate fivefold, twofold, and threefold directions of the quasicrystal lattice.
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Mechanical Behavior of Quasicrystals
tronic properties where the Mackay-type cluster is considered a particularly rigid unit forming a deep potential well to which the electrons are strong
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