B. Billia

The Samson phase, β-Mg2Al3, revisited

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samsons model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.

In situ and real-time observation of the formation and dynamics of a cellular interface in a succinonitrile-0.5 wt% acetone alloy directionally solidified in a cylinder

In situ and real-time observation of the solid-liquid interface during upward Bridgman growth of succinonitrile-0.5 wt% acetone in a cylinder was carried out. This first article concentrates on the study of the emergence and building of nonplanar patterns. By observation under white light in the solidification direction, bright field images and video records of the interface microstructure were obtained, which allowed dynamical phenomena such as the ripening and arrangement of cellular arrays with time. All results show a significant influence of natural convection in the melt. In particular, a microstructure gradient was observed at the interface. From a phenomenological model based on a boundary layer approach, its origin can be attributed to a ramp in solute concentration along the growth front created by tangential fluid flow. Statistical analyses of the evolution of primary spacing and array disorder show that the stable arrangement can still be described as a honeycomb disturbed by Gaussian noise. Convection was also found to be responsible for primary spacings smaller than that given by theoretical diffusive models and experiments on thin samples.

Evolution of characteristic pattern parameters in directional solidification of thin samples of a dilute AlCu alloy

Abstract The evolution of solidification patterns (cells, dendrites) with pulling velocity is analyzed on X-ray topographs taken in situ and in real time during directional growth of thin samples of a dilute AlCu alloy. Systematic measurements of major characteristic parameters (primary spacing, tip radius) are also carried out. The results are compared with data on the same alloy system available from the literature, which permits us to bring out the influence of convection in the melt on these parameters and on the cell-dendrite transition.

Thermosolutal convection-induced morpologies of the solid-liquid interface during upward solidification of Pb-30wt%TI Alloys

Abstract Observations of the macroscopic shape of the solid-liquid interface and radial solute segregation have been carried out on quenched Pb-30 wt%TI alloys which were grown upwards, that is, for the case for which thermosolutal convection may take place. The solidification front exhibits a structure which is composed of solid plateaux separated by liquid channels and whose amplitude presents a maximum when plotted versus growth rate, which suggests a concomitant maximum in the level of thermosolutal convection. The convective flow adjacent to the solidification front has then been deduced.

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.

Interface dynamics and coupled growth in directional solidification in presence of bubbles

Abstract The formation and dynamics of gas bubbles in Bridgman growth of succinonitrile-acetone alloys is examined. The experimental results show for the first time the rich dynamics that are associated with the formation and propagation of bubbles during directional solidification of alloys. The strong coupling of bubbles with the solid-liquid interface is found to result in the growth of elongated bubbles, either attached to a flat solidification front or forming localized cellular as well as dendritic duplexes (bubbles wrapped by a solid envelope). The coupling of the bubble with the solidification front is shown to cause oscillations in the bubble, which are characterized by fast Fourier transforms. When several duplexes are formed, coupled growth and screening may occur. The basic factors that give rise to oscillations, namely competition between source and sink of acetone assisted by capillary convection at the bubble cap, are discussed qualitatively through the development of an internal oscillator model. Coherent sidebranching observed on dendritic duplexes is shown to be due to resonant modes between the bubble cap and the solid envelope.

Solutal convection during directional solidification of a binary alloy: influence of side walls

In this study the effect of rigid side walls on the onset of solutal convection in the melt is investigated for upward solidification of a binary alloy. The physics is described by the Navier–Stokes equations in the Boussinesq approximation and by the balance of solute, which is coupled with the appropriate boundary conditions. By using a Tau–Chebyshev spectral method the numerical resolution of the perturbed system has been carried out for two alloys: lead–tin, which is the reference in the literature, and lead–thallium, which is currently under study. For both cases the lateral confinement has a stabilizing effect for all the aspect ratios that have been considered.

Morphology of the solidification front during undirectional growth of Pb-TI alloys

Abstract Lead alloys containing 5 to 40 wt% thallium are grown upwards by the vertical Bridgman method. It is shown that matter transport in the melt occurs by simultaneous convection and diffusion for certain values of the growth parameters. The limit of morphological stability for planar growth is investigated. For Pb-30wt%Tl alloys the variation with the distance from the instability threshold is then determined for the cell tip radius of curvature and the cellular or dentritic periodicty.

Measurement of solute profiles by means of synchrotron X-ray radiography during directional solidification of Al - 4 wt% Cu alloys

In the present study, we report on an image analysis procedure, which enables to extract from synchrotron radiographs the long range solute profiles in the whole sample and in both phases (solid and liquid). This image analysis is based on the measurement of local density differences, and is applied to study the directional solidification of Al - 4wt% Cu alloy, from planar to onset of the initial instability. Dedicated experiments were carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In order to validate this analysis the value of a key solidification parameter, namely the partition coefficient, was experimentally determined during the planar solidification, and a very good agreement was found with value found usually in the literature. On a further step, the evolution of the microstructure and solute profile during the initial transient of solidification was analysed in detail.

Pattern selection during directional solidification

A model has been constructed for pattern selection during the directional solidification of binary alloys, in terms of characteristic shape parameters. A step by step procedure allows a direct comparison with experiments at each level of the analysis. The introduction of nondimensional periodicity and radius of curvature makes a unified approach possible, which depends only on a parameter ℒ and on the solute partition coefficient. It is found that the Pelce-Pumir relation for the tip supersaturation agrees well with available data and that marginal stability at the tip should be a limiting case of a more general capillarity condition whose formulation is still unknown. It is finally shown that the quantitative evaluation of the periodicity of the structure is very sensitive to the precision of the model, which is not the case for the tip radius of curvature and solute concentration.