Study Of Al Surface Segregation In Icosahedral A1₆₂Cu_25.5Fe_12.5

Abstract

Surface segregation of aluminium was observed during oxidation experiments of icosahedral A162Cu25.5 Fel12.5, performed in-situ and at different temperatures in the ultra-high vacuum chamber of a scanning Auger electron spectrometer. Two regimes, below and above 770K, were observed in relation with severe segregation of Al atoms at the surface for T > 770K. We postulate that this temperature dependent segregation rate is representative of the aluminium transport towards the surface of the quasicrystal. By analogy with classical diffusion experiments, we can thus determine reasonable estimates of the activation energy for Al self-diffusion in this quasicrystal. The results are consistent with the existence of phason flips below 770K and thermal vacancies above this temperature.

Discussion on the surface science of quasicrystals

“…observed, and a similar interpretation was given, in a study of Al migration from the bulk to the surface of i-Al-Cu-Fe[34]. In the latter work, migration of Al to the surface was triggered by exposure to oxygen, which caused selective oxidation of Al at the surface.…”

“…The effect of phonons and phasons at surfaces of quasicrystals has received some attention [34][35][36][37][38], but relative to atomic or electronic structure the level of scrutiny has been small. There are at least two surface phenomena that could be affected by these types of dynamics: Mass transport between the surface and the bulk; and energy dissipation at surfaces.…”

Effect of temperature on the initial stages of oxidation of γ-Al4Cu9(110)

“…Although it may seem surprising to observe aluminium segregation at such a low temperature, it can be related to the low value of the activation energy for aluminium diffusion in ␥-Al 4 Cu 9 estimated to be 0.65 ± 0.12 eV in our previous study [13]. This value is much smaller than those usually ascribed to the vacancy diffusion mechanism of aluminium in crystalline Al-based intermetallics (∼1.6 eV) [18] or in Cu (1.81 eV) [19] and has been explained by considering that the activation energy of diffusion is composed of a vacancy formation and a vacancy migration contributions to enthalpy, which have close values (0.68 and 0.62 respectively in pure Al [20]). As the ␥-Al 4 Cu 9 phase has a structure based on a 3 × 3 × 3 CsCl superstructure containing two vacancies [17,21] only the vacancy migration enthalpy must be considered in the case of ␥-Al 4 Cu 9 , thus leading to a very low value of the activation energy for Al segregation in this material.…”

Real time study of the growth of i-AlCuFe in very thin films obtained by simultaneous deposition of the components

“…When we try to describe these kinetics by a parabolic law (R ¼ ðDtÞ 1=2 ) in the first stage of the reaction, Q D is calculated as equal to 154 kJ/mol, indicating a diffusioncontrolled transformation. Lacking any comparable study on the quasicrystallization of AlCuFe amorphous alloys, it is reasonable to compare the value of this activation energy to the value of Q D ¼ 142 kJ/mol in the case of the Al auto-diffusion [6] or for Al diffusion in AlCuFe (Q D ¼ 168 kJ/mol), calculated indirectly in a previous study in the first stage of i-AlCuFe oxidation [7]. After this peritectoid transformation period, the second part of the kinetic growth (Fig.…”