H. Jung

Fragmentation in an Al–7 wt-%Si alloy studied in real time by X-ray synchrotron techniques

Abstract One mechanism for the formation of equiaxed grains is the detachment of dendrite fragments which is believed to be at the origin of the central equiaxed core region in casting processes. Unfortunately, the dynamics of the fragmentation phenomena cannot be revealed by classical methods. Investigation of a unrefined Al–7 wt-%Si alloy using in situ and real time synchrotron X-radiography and X-ray topography at the European Synchrotron Radiation Facility, has allowed verification of the existence of dendrite fragmentation and of cascade fragmentation during directional solidification, and to study the evolution of the growth and sedimentation of the equiaxed grains formed from these fragments. An examination of the crystallographic misorientation of dendrites as fragmentation is ongoing. These results contribute to the understanding of the characteristics of the columnar to equiaxed transition and to knowledge of the origin of new equiaxed grains in unrefined alloys.

Modeling of heat and solute interactions upon grain structure solidification

Simulations of several laboratory experiments developed for the study of structure and segregation in casting are presented. Interaction between the development of dendritic grain structure and segregation due to the transport of heat and mass by diffusion and convection is modeled using a Cellular Automaton - Finite Element model. The model includes a detailed treatment of diffusion of species in both the solid and liquid phases as presented elsewhere in this volume [1]. Applications deal with prediction of columnar and equiaxed grain structures, as well as inter-dendritic and inter-granular segregations induced by diffusion and macrosegregation induced by thermosolutal buoyancy forces.

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.

CET by Fragmentation during the Solidification under Natural and Forced Convection of Non-Refined Al-Based Alloys

The columnar to equiaxed transition (CET) has been widely studied for many years [1] because this phenomenon is observed in metallurgical applications like castings. In non refined alloys, detachment of dendrite fragments is the most probable mechanism responsible for the formation of an equiaxed microstructure [1]. In this frame, melt convection influences the grain structure evolution by playing a role in the fragmentation phenomena [2].