On the temperature dependence of the hardness of quasicrystals

Results of mechanical micro- and nanotesting of single-quasicrystalline phases in the Al–Co–Ni-, Al–Pd–Mn- and RE–Mg–Zn-systems (RE=Y, Ho or Dy) are reported. Particular emphasis was devoted to the icosahedral phase in the Y–Mg–Zn-system, since — owing to its low melting point — high homologous temperatures Th can be attained almost without heating. At room temperature indentation of quasicrystals with icosahedral structure results in extended formation of type-III cracks, i.e. shear cracks with different height levels of the crack banks. This indentation-induced surface fragmentation is considerably less pronounced with decagonal phases, indicating that the quasicrystalline structure has a strong influence on brittleness. In contrast to the fracture behaviour the Meyer hardness value HM does not seem to follow a sequence HM (three-dimensional quasicrystal)》HM (two-dimensional quasicrystal)》HM (crystalline matter) but is mainly determined by the sequence of the solid→liquid transformation (or decomposition) temperature Tm, i.e. HM(Al73Ni14Co13)>HM(Al70Pd21Mn9)>HM(Y10Mg30Zn60). The hardness seems to be primarily governed by the complexity of the structure and by type and strength of bonding. At room temperature, all quasicrystals exhibit a strong indentation size effect with the hardness increasing with decreasing load. This effect becomes inverted at higher temperatures. The hardness variation with temperature depends on load: negligible variation for very small loads (nanotesting), very strong variation for moderate loads, and less drastic variation for higher loads. This observation is interpreted in the frame of two models: indentation-induced fragmentation, and dislocation involved deformation.