Amorphous-to-Quasicrystalline Transformation in the Solid State

Abstract

Al 84 Mni6 multilayer films have been amorphized by room-temperature ion-beam irradiation. The amorphous phase was transformed into the quasicrystalline state through two routes: thermal and ion-beam-assisted thermal. The intensity of the quasicrystalline electron diffraction increases continuously with annealing between 270 and 350 °C. Ion irradiation of the amorphous phase produces a more complete set of icosahedral diffraction lines than thermal annealing.PACS numbers: 64.70. Kb, 61.16.Di, 61.50.Cj, 61.80.Jh The quasicrystalline state of matter has received a large amount of study in the last year. The quasicrystalline state is characterized by long-range icosahedral order. This state was first reported by Shechtman et al. l and has since been the topic of many experimental studies. 2 " 7 In all these instances the quasicrystalline state was prepared by rapid quenching from the liquid state with use of melt spinning 1 "" 7 or a scanning electron beam. 7 In this Letter we report the first observation of the formation of the quasicrystalline state by a solid-state polymorphic transformation from the amorphous phase by either thermal annealing or ion-beam irradiation. Although both processing techniques are capable of producing this transformation, only the ionirradiation-induced transformation generates a nearly complete set of icosahedral powder diffraction lines. These transformations allowed us to probe the thermodynamic hierarchy of amorphous, quasicrystalline, and crystalline free energies. At 270 °C we confirm the expected thermodynamic hierarchy that the free energy of the amorphous phase is greater than the free energy of the quasicrystalline state, which in turn is greater than the free energy of the equilibrium crystalline phase. Our results for the amorphous-toquasicrystalline transformation are consistent with a first-or second-order transformation. We propose a structural model for the amorphous-to-quasicrystalline transformation which can account for the kinetics in both thermal annealing and ion-beam mixing.Multilayer Al-Mn films were deposited on NaCl substrates in a turbomolecular-assisted ion-pumped vacuum system by electron-beam evaporation. Rutherford-backscattering spectrometry determined that the films were 500 A thick and had a composition of Al 84 Mn 16 . The films were floated off of the substrates onto transmission-electron-microscopy (TEM) grids and were amorphized by a room-temperature ion-beam irradiation of 8xl0 15 Xe ++ /cm 2 . The amorphous phase was then transformed to the quasicrystalline state by irradiation (4x 10 15 /cm 2 Xe + + ) at 150 °C. Irradiation experiments performed on coevaporated amorphous Al 80 Cr 2 o thin films have also produced similar results. 8 For all of the ion irradiations, the ion energy was 600 keV, the beam current was 0.025 juA/cm 2 , and the vacuum in the implant chamber was less than 8xl0~7 Torr. Ion ranges for 600-keV Xe are approximately 4 times the Al-Mn film thickness, thus making the incorporation of Xe unlikely. Thermal anneals were performe...

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