Diffusive Growth Conditions Research Articles

Solidification furnace for in situ observation of bulk transparent systems and image analysis methods.

This paper aims to describe the experimental framework of the Directional Solidification Insert, installed onboard the International Space Station, dedicated to the in situ and real-time characterization of the dynamic selection of the solid-liquid interface morphology in bulk samples of transparent materials under diffusive growth conditions. The in situ observation of the solid-liquid interface is an invaluable tool for gaining knowledge on the time evolution of the interface pattern because the initial morphological instability evolves nonlinearly and undergoes a reorganization process. The result of each experiment, characterized by the sample concentration, a thermal gradient, and a pulling rate, is a large number of images. The interpretation of these images necessitates a robust identification of each cell/dendrite's position and size during the entire solidification. Several image analysis methods have been developed to reliably achieve this goal despite varying contrast and noise levels and are described in detail. Typical solidification experiments are presented, and the dynamics of the pattern formation are analyzed to illustrate the application of the image analysis methods.

Effect of sub-boundaries on primary spacing dynamics during 3D directional solidification conducted on DECLIC-DSI

To characterize the dynamical formation of three-dimensional (3D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile – 0.24 wt% camphor model alloy was carried out under low gravity in the DECLIC Directional Solidification Insert onboard the International Space Station. The continuous interface observation enables to construct space-time evolution maps of cell location and primary spacing. Both convergent and divergent sub-boundaries are identified and new insights on their effects on the spatiotemporal evolution of the pattern are thus evidenced. 3D phase-field simulations that reproduce the experimental sub-boundary configurations are performed to support the analyses. Even for the low angle sub-boundaries studied, the primary spacing increases or decreases in the vicinity of the boundary respectively for divergent and convergent sub-boundary. This effect may extend on a long distance within the different sub-grains and its magnitude depends on the average primary spacing and its positioning relative to the limits of the stability band. On the sample scale, the primary spacing profile is also influenced by the presence of sources and sinks at the crucible wall due to the pattern drift. Their type and distance from the sub-boundaries give rise to complex spatial distributions of primary spacing over the entire sample.

Investigation of Peritectic Solidification Morphologies by Using the Binary Organic Model System TRIS-NPG.

Under pure diffusive growth conditions, layered peritectic solidification is possible. In reality, the competitive growth of the primary α-phase and the peritectic β-phase revealed some complex peritectic solidification morphologies due to thermo-solutal convection. The binary organic components Tris-(hydroxylmenthyl) aminomethane-(Neopentylglycol) were used as a model system for metal-like solidification. The transparency of the high-temperature non-faceted phases allows for the studying of the dynamic of the solid/liquid interface that lead to peritectic solidification morphologies. Investigations were carried out by using the Bridgman technic for process conditions where one or both phases solidify in a non-planar manner. Different growth conditions were observed, leeding to competitive peritectic growth morphologies. Additionally, the competitive growth was solved numerically to interpret the observed transparent solidification patterns.

In Situ and Real Time Observation of Microstructure Formation during Directional Solidification of a 3D-alloy: Experiments in the DECLIC-DSI

To analyze the fundamental mechanisms active in the formation of three-dimensional (3D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of series of experiments on transparent model alloy was carried out in the DECLIC Directional Solidification Insert on-board the International Space Station, offering a very unique opportunity to in situ characterize the whole development of the microstructure in extended 3D patterns. Some of the most striking results are here presented. Microgravity environment provided the conditions to get benchmark data in diffusive transport conditions; the comparison with ground experiments will be performed to highlight the influence of convection in terms of primary spacing distributions. Oscillatory breathing modes observed for the very first time in bulk samples will also be described with the support of 3D-phase-field simulations.

Thermal-field effects on interface dynamics and microstructure selection during alloy directional solidification

We carry out three-dimensional phase-field simulations to model unique experimental observations of cellular and dendritic solidification structures formed under diffusive growth conditions in the DSI (Directional Solidification Insert) of the DECLIC (DEvice for the study of Critical LIquids and Crystallization) aboard the International Space Station. We had previously shown experimentally that complex thermal conditions affect the stationary position of the solid-liquid interface, as well as its dynamics of relaxation towards this stationary position over a finite time after the onset of sample pulling. Here, we discuss the effects of thermal diffusion within the adiabatic zone of the directional solidification setup and of latent heat release at the solid-liquid interface by means of quantitative phase-field simulations. Simulations and experiments characterize the entire evolution of the primary spacing of cellular/dendritic array structures from the onset of morphological instability to the establishment of the final steady-state spacing, including the transient coarsening regime associated with a sharp increase of spacing. Accounting for these thermal effects leads to a major improvement in the agreement between simulations and microgravity measurements for both the time of occurrence of morphological instability after the start of the experiment and the subsequent spacing evolution, which are not accurately predicted using the standard frozen temperature approximation.

Convection Effects During Bulk Transparent Alloy Solidification in DECLIC-DSI and Phase-Field Simulations in Diffusive Conditions

To study the dynamical formation and evolution of cellular and dendritic arrays under diffusive growth conditions, three-dimensional (3D) directional solidification experiments were conducted in microgravity on a model transparent alloy onboard the International Space Station using the Directional Solidification Insert in the DEvice for the study of Critical LIquids and Crystallization. Selected experiments were repeated on Earth under gravity-driven fluid flow to evidence convection effects. Both radial and axial macrosegregation resulting from convection are observed in ground experiments, and primary spacings measured on Earth and microgravity experiments are noticeably different. The microgravity experiments provide unique benchmark data for numerical simulations of spatially extended pattern formation under diffusive growth conditions. The results of 3D phase-field simulations highlight the importance of accurately modeling thermal conditions that strongly influence the front recoil of the interface and the selection of the primary spacing. The modeling predictions are in good quantitative agreements with the microgravity experiments.

In situ observation of solidification patterns in diffusive conditions

We present a review of recent in situ experimentation studies of solidification front patterns and microstructures in alloys. Front-tracking diagnostics and real-time observation methods using high-resolution optical or X-ray imaging devices currently apply to model transparent systems as well as metallic alloys in thin and bulk samples. On a theoretical basis that spans the physics of nonequilibrium pattern formation and materials science, in combination with time-resolved numerical simulations, conclusive results of both fundamental- and applied-science interest have been obtained on major problems relative to multiscale microstructure selection, morphological transitions, and crystallographic effects during single- and multi-phase solidification. We will mainly focus on the dynamics of cellular, dendritic, and eutectic growth patterns in diffusive-growth conditions, that is, in the absence of convection in the liquid. This can be achieved in (semi-)thin samples, or, for bulk solidification, in the reduced-gravity environment of orbiting facilities. A selection of emerging work on, e.g., faceted growth and adaptive control of solidification patterns will furthermore be reported. We conclude by pointing out open questions and new perspectives for future research.

Dynamical microstructure formation in 3D directional solidification of transparent model alloys: in situ characterization in DECLIC Directional Solidification Insert under diffusion transport in microgravity

To clarify and characterize the fundamental physical mechanisms active in the dynamical formation of three-dimensional (3D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of series of experiments on transparent model alloy succinonitrile - 0.24 wt% camphor was carried out under low gravity in the DECLIC Directional Solidification Insert on-board the International Space Station. These experiments offered the very unique opportunity to in situ observe and characterize the whole development of the microstructure in extended 3D patterns. The experimental methods will be first briefly described, including in particular the observation modes and the image analysis procedures developed to quantitatively characterize the patterns. Microgravity environment provided the conditions to get quantitative benchmark data: homogeneous patterns corresponding to homogeneous values of control parameters along the whole interface were obtained. The sequence of microstructure formation will be presented as well as the evolution of the primary spacing which is one of the most important pattern characteristic. Time evolution of this primary spacing during the microstructure development will be analysed to identify the mechanisms of spacing selection and adjustment; the importance of the macroscopic interfacial curvature will be pointed out.

Initial transient behavior in directional solidification of a bulk transparent model alloy in a cylinder

To characterize the dynamical formation of three-dimensional (3-D) arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of a series of experiments on a transparent succinonitrile–0.24wt.% camphor model alloy was carried out under low gravity in the Device for the Study of Critical Liquids and Crystallization (DECLIC) Directional Solidification Insert on board the International Space Station (ISS). The present paper focuses on the study of the transient solid–liquid interface recoil. Numerical thermal modeling led us to identify two thermal contributions to the interface recoil that increase with the pulling rate and add to the classical recoil associated with the solute boundary layer formation. As a consequence of those additional contributions, the characteristic front recoil is characterized by a fast initial transient followed by stabilization to a plateau whose location depends on pulling rate. The analysis of comparative experiments carried out on the ground shows the absence of stabilization of the interface position, attributed to longitudinal macrosegregation of the solute induced by convection. This behavior is surprisingly also observed in space experiments for low pulling rates. An order of magnitude analysis of the mode of solute transport reveals that for these conditions, the effective level of reduced gravity on board the ISS is not sufficiently low to suppress convection so that the interface recoils with longitudinal macrosegregation in a similar way as in ground experiments.

Two-phase separated growth and peritectic reaction during directional solidification of Cu–Ge peritectic alloys

Abstract

Phase-field modeling of the columnar-to-equiaxed transition in neopentylglycol-camphor alloy solidification

We have performed phase field simulations in order to explore the effect of equiaxed grain nucleation undercooling on the columnar to equiaxed transition "CET" in the NPG-DC alloy system. Our phase field model is based on the multiphase-field method. The simulation parameters are adapted to a microgravity experiment performed under purely diffusive growth conditions. The experimental investigation is made during the sounding rocket campaign TEXUS-47.

Eutectic Growth in Three Dimensions

Critical experimental studies have been carried out to examine the stability of eutectic morphology in three-dimensional (3-D) samples under diffusive growth conditions. By directionally solidifying capillary samples of the well-characterized Al-Cu eutectic alloy, it is shown that the observed minimum spacing agrees with the value predicted by the Jackson and Hunt (JH) model, but the range of stable spacing is reduced significantly in three dimensions. The ratio of the maximum to minimum eutectic spacing in three dimensions is found to be only 1.2 compared to the predicted value of 2.0 in two dimensions. The narrow range of stable spacing is shown to be due to the instabilities in the third dimension that forms when the local spacing becomes larger than some critical spacing value, which corresponds to the maximum stable spacing. A new mechanism of lamellar creation in the third dimension is observed in which lamella with a local spacing larger than the critical value becomes unstable and forms a sidewise perturbation that becomes enlarged at the leading front and then propagates parallel to the lamella to create a new lamella. Alternately, an array of sidewise perturbations form, which then coalesce at their leading fronts and then become detached from the parent lamella to form a new lamella.

Spatio-temporal microstructure evolution in directional solidification processes

Experimental studies in Al–4.0 wt% Cu alloys were conducted in samples of different sizes in which the effect of convection increases with increasing sample diameter. Detailed measurements of dendrite tip radii and the primary arm spacing have been carried out under both diffusive and convective growth conditions. The experimental results on tip radius and primary spacing show a good agreement with the microsolvability theory and Hunt–Lu model, respectively, for diffusive growth processes. In larger samples where convection effects are present, an inhomogeneous microstructure develops. A localized growth model is proposed to explain the spatio-temporal microstructure formation under convective growth conditions. The key parameter in this model is the effective diffusion coefficient which increases with the sample diameter and plays the decisive role in selecting the length scales of the solidification microstructures under convective growth conditions.

ARTEX — In situ observation of directional solidification of binary aluminium alloys on TEXUS

A binary Al-6wt. % Si alloy was directionally solidified during the TEXUS 39 mission to compare diffusive solidification conditions with convective ones. In addition, the operativeness of a new furnace technology ARTEX using fragile aerogels as a crucible material in microgravity was demonstrated. The TEXUS 39 sample is evaluated with regard to the processing parameters and microstructural features. A reduction of the secondary dendrite arm spacing and spacing of the interdendritic eutectic and an increase of the primary dendrite spacing is observed under microgravity compared with experiments under 1g conditions. To obtain a fundamental understanding of the influence of convections on the microstructure evolution the ARTEX facility is now equipped with a rotating magnetic field device being able to generate a controlled fluid flow in the melt during solidification. The forthcoming flight experiment ofARTEX+ on TEXUS 41 in November 2004 should lead to a direct comparison of pure diffusive growth conditions (TEXUS 39) and controlled convective conditions (TEXUS 41) using the same alloy and also to compare the results with extensive lab research.

Phase-field modeling of microstructural pattern formation during directional solidification of peritectic alloys without morphological instability.

During the directional solidification of peritectic alloys, two stable solid phases (parent and peritectic) grow competitively into a metastable liquid phase of larger impurity content than either solid phase. When the parent or both solid phases are morphologically unstable, i.e., for a small temperature gradient/growth rate ratio (G/v(p)), one solid phase usually outgrows and covers the other phase, leading to a cellular-dendritic array structure closely analogous to the one formed during monophase solidification of a dilute binary alloy. In contrast, when G/v(p) is large enough for both phases to be morphologically stable, the formation of the microstructure becomes controlled by a subtle interplay between the nucleation and growth of the two solid phases. The structures that have been observed in this regime (in small samples where convection effects are suppressed) include alternate layers (bands) of the parent and peritectic phases perpendicular to the growth direction, which are formed by alternate nucleation and lateral spreading of one phase onto the other as proposed in a recent model [R. Trivedi, Metall. Mater. Trans. A 26, 1 (1995)], as well as partially filled bands (islands), where the peritectic phase does not fully cover the parent phase which grows continuously. We develop a phase-field model of peritectic solidification that incorporates nucleation processes in order to explore the formation of these structures. Simulations of this model shed light on the morphology transition from islands to bands, the dynamics of spreading of the peritectic phase on the parent phase following nucleation, which turns out to be characterized by a remarkably constant acceleration, and the types of growth morphology that one might expect to observe in large samples under purely diffusive growth conditions.

Directional solidification microstructures in diffusive and convective regimes

Most theoretical models on the development of solidification microstructures are based on diffusive growth conditions, whereas convection effects are generally present in most directional solidification experiments. This paper examines the effect of convection on the development of microstructures in directionally solidified Al–Cu alloys. A numerical model of convection is first discussed to characterize fluid flow as a function of sample diameter for the thermal profile of the directional solidification used in this study, and it was established that sample diameters of 1 mm or smaller is required for diffusive growth. Based on this result, a new experimental design is developed for directionally solidifying several thin samples of different diameters in the same experimental run to systematically study the effect of sample diameter on microstructure development. Sample diameters, ranging from 0.2 to 5.0 mm, were used to access both the diffusive and convective regimes. Experiments have been performed over a range of velocities and temperature gradients to characterize the effect of convection on planar, cellular and dendritic microstructures. In very thin samples of ⩽1.0 mm diameter, diffusive growth was obtained. The cell/dendrite tip composition and primary spacing were found to increase, and the tip temperature was found to decrease, as the convection effects were reduced in thinner samples. The planar to cellular transition was suppressed in the presence of fluid flow. Also, convection was found to slightly increase the critical velocity for cellular to dendritic transition.

Mechanisms of Layer Structure Formation in Peritectic Alloys

ABSTRACTThe mechanisms of layer structure formation in the two phase region of peritectic systems are discussed. Under diffusive growth conditions, a banded structure is predicted within a narrow composition range in the hypoperitectic composition. Experimental studies show the formation of an oscillatory structure in the hyperperitectic region. It is shown that this structure is induced by convection in the melt, and it is a novel structure in which a large single tree-like structure of primary phase is surrounded by the peritectic phase. Basic ideas of convection that lead to this tree-like morphology are described.