Surface structure of Al-Pd-Mn quasicrystals: Existence of supersaturated bulk vacancy concentrations

Abstract

We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two-and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys. We identified the presence and chemical nature of bulk vacancies in Al-Pd-Mn quasicrystals by measuring the structure and composition of two-and fivefold cleavage surfaces of different preannealed quasicrystals subjected to postcleavage heat treatments using scanning electron microscopy and Auger electron spectroscopy. A strong dependence of the surface structure from the preannealing is observed and explained by varying concentrations of bulk vacancies. The analysis of the data shows that as-grown Al-Pd-Mn quasicrystals contain Al, Pd, and Mn vacancies in supersaturated but near stoichiometric concentrations, while long-term preannealed samples contain a much lower vacancy concentration, with a tendency to have excess Al vacancies. We found that Al and Mn vacancies are more mobile at lower temperatures than Pd vacancies, and that the diffusion of Mn vacancies is directly coupled to the mobility of Al vacancies. The results demonstrate that the evolution of the surface structure is primarily affected by bulk vacancies migrating toward the surface during heat treatments and provide a new methodology to characterize the vacancies in intermetallic alloys. Keywords Ames Laboratory Disciplines Physical Chemistry Comments This article is from

Terrace selection during equilibration at an icosahedral quasicrystal surface

“…10,34,35 Indeed, there is evidence that this type of diffusion eventually produces a vacancy-depleted near-surface region that is a few microns deep. 34 In that case, one would expect the voidrich terraces to occur early in the life of the sample and to disappear over time.…”

“…For example, the root-mean-square displacement of a bulk Mn atom ͑the slowest diffuser in this alloy͒ is 25 and 50 m, at temperatures of 900 and 950 K, over 3 h. 33 The fact that this displacement is very large and differs only by a factor of 2 over the temperature range 900-950 K supports the hypothesis that mass transport is extensive already at or below 900 K and does not change dramatically between 900 and 950 K. It has been suggested that surface modification due to migration of bulk defect vacancies also begins in a significantly lower temperature range. 10 Hence, it seems unlikely that new diffusion processes begin around 900 K.…”

“…There is evidence that migration of bulk vacancies to the surface contributes to this roughness. 10 At higher temperatures, the surface usually changes to a flat quasicrystalline phase. 24 An intermediate phase that is both quasicrystalline and rough may form around 700 K. 8 Flat crystalline phases can also form above 700 K. [25][26][27][28][29] The factors that select between crystalline and quasicrystalline surface phases above 700 K are poorly understood.…”

“…First, a rich diversity of structures can form, depending upon the history of sample treatment and the length scale of the structural examination. This array of surface morphologies includes rough surfaces, 4-9 faceted voids, 10 and smooth ͑to within a few tenths of an angstrom͒ terraces. 6,8,9,[11][12][13][14][15][16] The latter type of structure-flat terraces-is the topic of this paper.…”

Nanodomains due to Phason Defects at a Quasicrystal Surface

“…Defects in crystalline solids-deviations from an idealized structure-have always been recognized as extremely important, since they can control fundamental properties such as mechanical strength, conductivity, color and corrosion. They affect surface properties as well, such as corrosion, nucleation, or phase selection [1,2]. The relationship between surface and bulk defects has emerged recently as a topic of great importance for understanding the unusual strength of nanostructures.…”