The Influence of Growth Rate on Porosity in Al-Pd-Mn Icosahedral Quasicrystals.

Ross, A. R.; Fisher, I. R.; Canfield, Paul C.; Lograsso, Thomas A.

doi:10.1557/proc-643-k1.5

Cited by 2 publications

(2 citation statements)

References 6 publications

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“…1), could explain the irregular distribution of these pores noticed by A. R. Ross et al [5]. In the same way, our observation agrees with some other results reported previously like, the absence of pore both close to the external surfaces and to grain boundaries [6], the pore morphology evolution from non-facetted to facetted (dodecahedral) [5] and the pore shrinking during post-growth grain annealing [11]. Accordingly our results confirm that pores have very likely a vacancy origin as stated by Beeli et al, but disagree with the pore formation mechanism they proposed.…”

Section: Discussionsupporting

confidence: 93%

In situobservation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

Gastaldi

Schenk

Reinhart

et al. 2006

Philosophical Magazine

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It is now generally admitted that pores are intriguing special features of quasicrystals. Therefore, we have performed an "in situ" and real time observation of the pore evolution during directional solidification and melting cycles of an icosahedral Al-Pd-Mn bi-grained sample, by synchrotron X-ray radiography. Rather surprisingly, no pore was observed to grow during the solidification stages. Nucleation and growth of pores were firstly seen during melting. These pores were subsequently shrinking either just being absorbed or during resumption of directional solidification. It is concluded that the vacancy origin of pores is certainly valid whereas the vacancy supersaturation, needed thereby to explain their growth, would be more probably related to the peculiar structure of quasicrystal than to the destruction of the thermal equilibrium. growth, it is difficult to decide between the two hypotheses which have been put forward to explain their origin, namely the thermal vacancy condensation [7] and the hierarchical structure [8]. Therefore, we have performed an "in situ" and real time observation of the pore evolution during directional solidification-melting cycles of an Al-Pd-Mn bi-grained sample, by synchrotron radiography, in order to grasp, more precisely, the origin of these intriguing species. II Experiments:The synchrotron experiments were carried out at the ID 19 beam line of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). One thin Al-Pd-Mn sample (40x6x0.7 mm 3 ) cut in an ingot prepared at the composition which gives quasicrystal grains soon after the beginning of the solidification process (Al 72.4 Pd 20.5 Mn 7.1 ), was melted and solidified two times inside a graphite crucible held vertically in a two heating-zone furnace. This furnace, described previously in detail [9], is permeable to X-rays and thereby allowed us to follow the course of the solidification/melting of the investigated sample, under a 20K/cm gradient, a pressure of about 3x10 -7 mbar and pulling velocities ranging from 0 to 3.6 µm/s. Radiographs were recorded all along the processes, at the X-ray energy of 24 keV, either by using a CCD camera developed at the ESRF [10]or exposing High Resolution (<1µm) photographic plates (Ilford L4). The camera was fitted with an optics leading to an effective pixel size of 7.46 µm and a large field of view (15x15 mm 2 ). According to the high coherence of the beam delivered by the ESRF synchrotron source, phase contrasts were recorded in addition to absorption contrasts on radiographs, which increased the sensitivity to grain inhomogeneities of the technique. III Results:The sample, as cut in the master ingot (section II), was completely melted before being submitted to two partial solidification/melting cycles. Rather surprisingly, no pore was detected during the solidification stages at the spatial resolution allowed by our technique (≅1µm). The appearance of micrometric pores was firstly observed during melting, at about 2 mm ahead of the regressive interface, whil...

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Section: Discussionsupporting

confidence: 93%

In situobservation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

Gastaldi

Schenk

Reinhart

et al. 2006

Philosophical Magazine

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“…These concentration values are in line with porosity levels of up to 4% found in Al-Pd-Mn quasicrystals. 34 The presence of different types of vacancies with different diffusion constants in Al-Pd-Mn quasicrystals should also be considered when interpreting anomalous diffusion data such as those in Ref. 3.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2003

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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

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The Influence of Growth Rate on Porosity in Al-Pd-Mn Icosahedral Quasicrystals.

Cited by 2 publications

References 6 publications

In situobservation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

In situobservation of pore evolution during melting and solidification of Al–Pd–Mn quasicrystals by synchrotron X-ray radiography

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

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