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One of the main challenges of quasicrystal science is to elucidate how the quasiperiodic order can extend so far, i.e. up to several centimetres according to the size of the single grains of various alloys routinely grown nowadays [M. Boudard, E. Bourgeat-Lami, M. de Boissieu, et al., Phil. Mag. Lett. 71 11 (1995); Y. Yokoyama, R. Note, A. Kimura, et al., Mater. Trans. JIM 38 943 (1997); A. Langsdorf, and W. Assmus, J. Cryst. Growth 192 152 (1998)]. Noticing that most of the present knowledge on the growth of quasicrystal grains has been collected after their cooling at room temperature, we have carried out the first in situ and real time observation of this peculiar process, which has clearly disclosed both the shape of the growing interface and its defective state. Therefore we have studied the solidification of an AlPdMn alloy giving quasicrystal grains by synchrotron X-ray imaging, combining thereby radiography and X-ray topography techniques. Radiography allowed us to clearly evidence a facetted growth proceeding by lateral motion of ledges at the solid-melt interface and controlled by the interface kinetics, rather than by local heat flow as was widely thought. Thus, a realistic estimate of the kinetic coefficient was deduced from the solid-melt interface undercooling, which indicates that quasicrystal growth is more comparable with the growth of both semiconductors and oxides than with that of pure metals. X-ray topographs, recorded simultaneously with radiographs, revealed that a lot of strain and defects are generated in the quasicrystal grains during their growth, which could be related to the growth process itself and very informative on the origin of the stability of the quasicrystal lattice. [ABSTRACT FROM AUTHOR]

In order to better understand the long-range propagation of quasicrystalline order, as well as quasicrystal stability, it is important to know if defects are generated in the quasicrystal grains during their growth. Previously, we studied the degree of perfection of about ten icosahedral quasicrystal grains of various alloys (Al–Pd–Mn, Al–Cu–Fe, Zn–Mg–Y), as grown or annealed, and we disclosed that some of them were much more perfect than the others. In this work we have concentrated on another slice of such a grain of the Al–Pd–Mn alloy. Similarly, we have performed an extensive synchrotron X-ray topographic investigation of strain and defects in this grain, combined with phase-contrast radiography and high resolution X-ray diffraction examinations. Very few two-lobe contrasts associated with pores and no loop-shaped contrasts were observed on the X-ray topographs, but straight line segments and band contrasts have been identified. Line segments could be considered as the result of the climbing of polygonal dislocation loops, as observed by TEM by Caillard and coworkers. This would indicate that most strains and defects observed in quasicrystal grains, at room temperature, are the result of stresses (external and internal) acting after growth. [ABSTRACT FROM AUTHOR]

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 first seen during melting. These pores were subsequently shrinking either just before 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. [ABSTRACT FROM AUTHOR]

The structure of real icosahedral quasicrystals of the Al-Pd-Mn system can be described in terms of a hierarchical self-similar packing of overlapping atomic clusters. An inflation scale factor tau 3 preserves long-range order but generates a hierarchy of holes and a fractal structure. Such holes have actually been observed and they might be of basic importance in the growth mechanism and stability of quasicrystals. [ABSTRACT FROM AUTHOR]

Presents a reply to the commentaries on hierarchical porosity in real quasicrystals. Use of radiography in determining the quasicrystals; Identification of fractal properties with structural holes; Determination of the diffraction pattern of quasiperiodic structure.

Abstract: The porosity was studied using coherent synchrotron X-ray radiography, firstly in several bulky icosahedral Al–Pd–Mn quasicrystals, grown in different laboratories by various techniques (Bridgman, Czochralski, Flux) and then in quasicrystal grains of many other alloys and structures (Part II). In this part I, we report a quantitative analysis which has been carried out in order to find some correlation of this porosity with some growth or after-growth cooling parameters (growth parameters). The size distribution of pores was observed to change drastically among samples grown with different growth parameters as well as in those extracted from the same ingot and no mode values were evidenced when we plotted the distribution of pores in all investigated as-grown samples. These changes point out the influence of the solidification process on the occurrence of porosity, especially the influence of the vacancy migration. These results are used to discuss the two main current hypotheses on the origin of porosity. [Copyright &y& Elsevier]