In situ study of quasicrystal growth by synchrotron X-ray imaging

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.