Analysis of Gas-Phase Clusters Made from Laser-Vaporized Icosahedral Al−Pd−Mn
An icosahedral Al-Pd-Mn quasicrystal sample is laser vaporized to form metal clusters by gas aggregation. The clusters are subsequently laser ionized and mass analyzed in a time-of-flight mass spectrometer. The mass spectra show cluster compositions which are qualitatively similar to that of the sample. This is consistent with a kinetically controlled cluster growth process. Cluster thermodynamic stability is probed by multiphoton ionization/fragmentation, which induces primarily Al and Mn loss. The resulting spectra are composed of a series of Pd-rich Al-Pd clusters. The average cluster composition is 60 (′1)% Pd. This composition is close to a known eutectic in the Al-Pd system. When manganese is seen on these clusters, it is always in units of Mn 3 . These results are discussed in terms of relative binding strengths in the Al-Pd-Mn alloy system.
Phase Formation and Stability
Locally similar icosahedral structural ordering between parent phase and nucleating phase is believed to be responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. Microscopic mechanisms for the formation and stabilization of quasicrystals are discussed, with emphasis on the role of clusters. The different types of experimentally observed phase transitions will be discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline, and quasicrystalline to crystalline as a function of temperature, pressure, irradiation, and high-energy ball milling, respectively.
Mechanical properties of single-quasicrystalline AlCuCoSi
The mechanical properties of single-quasicrystals of decagonal AlCoCuSi have been studied for the first time by applying the Vickers indentation method. The hardness has been determined as H≃9.6 MPa. Estimates for the modulus of elasticity and the fracture toughness are given. The quasicrystals are highly strained internally. Scratching experiments shown slight anisotropies in the abrasive friction coefficient. The dominant abrasive mechanism is microplowing