Showing papers on "Directional solidification published in 2010"

Onset of sidebranching in directional solidification.

Abstract: We use a computationally efficient phase-field formulation [B. Echebarria et al., Phys. Rev. E 70, 061604 (2004)] to investigate the origin and dynamics of sidebranching in directional solidification for realistic parameters of a dilute alloy previously studied experimentally [M. Gorgelin and A. Pocheau, Phys. Rev. E 57, 3189 (1998)]. Sidebranching is found to result either from noise amplification or from deterministic oscillations that exist both in two dimensions and in a three-dimensional thin-sample geometry. The oscillatory branch of growth solutions bifurcates subcritically from the main steady-state branch of solutions and exists over a finite range of large array spacings. In contrast, noise-induced sidebranching is associated with a smooth transition where the sidebranching amplitude increases exponentially with spacing up to nonlinear saturation due to the overlap of diffusion fields from neighboring cells, as observed experimentally. In the latter case where sidebranching is noise-induced, we find that increasing the externally imposed thermal gradient reduces the onset velocity and wavelength of sidebranching, as also observed experimentally. We show that this counterintuitive effect is due to tip blunting with increasing thermal gradient that promotes noise amplification in the tip region.

Phase-field study of three-dimensional steady-state growth shapes in directional solidification

Abstract: We use a quantitative phase-field approach to study directional solidification in various three-dimensional geometries for realistic parameters of a transparent binary alloy. The geometries are designed to study the steady-state growth of spatially extended hexagonal arrays, linear arrays in thin samples, and axisymmetric shapes constrained in a tube. As a basis to address issues of dynamical pattern selection, the phase-field simulations are specifically geared to identify ranges of primary spacings for the formation of the classically observed "fingers" (deep cells) with blunt tips and "needles" with parabolic tips. Three distinct growth regimes are identified that include a low-velocity regime with only fingers forming, a second intermediate-velocity regime characterized by coexistence of fingers and needles that exist on separate branches of steady-state growth solutions for small and large spacings, respectively, and a third high-velocity regime where those two branches merge into a single one. Along the latter, the growth shape changes continuously from fingerlike to needlelike with increasing spacing. These regimes are strongly influenced by crystalline anisotropy with the third regime extending to lower velocity for larger anisotropy. Remarkably, however, steady-state shapes and tip undercoolings are only weakly dependent on the growth geometry. Those results are used to test existing theories of directional finger growth as well as to interpret the hysteretic nature of the cell-to-dendrite transition.

Influence of Particle Size on Ice Nucleation and Growth During the Ice‐Templating Process

Abstract: The solidification behavior of suspensions of alumina particles during directional solidification is investigated here by in situ observations using X-ray radiography and tomography. The objective of this study was to assess the influence of particle size on the solidification behavior of the suspensions during the early stages of solidification. Four powders with particle size in the range of 0.2–3.4 μm (median size) were investigated. Solidification is obtained by cooling at a constant rate, starting from room temperature. Attention is specifically paid to the nucleation and growth behavior of the ice crystals in these suspensions. We propose that the nucleation of ice crystals is controlled by the particle size, the surface of the particles acting as nucleation sites. Smaller particle size leads to a lower degree of supercooling because nucleation and growth can proceed at a higher temperature than with larger particles. The initial interface velocity is dependent on the degree of supercooling, and controls the extent of the initial structural gradient in the resulting porous materials.

Microstructure and mechanical properties of rapid directionally solidified Ni-base superalloy Rene′41 by laser melting deposition manufacturing

Abstract: Ni-base superalloy Rene′41 was produced by the laser melting deposition (LMD) manufacturing process. The microstructure was characterized by optical and scanning electron microscopy and X-ray diffraction. Results showed that ultra-fine rapid directionally solidified columnar grains with some fine MC carbide particles in the interdendritic zone were formed in the as-deposited samples along the deposited-direction due to the high thermal gradient and solidification cooling rate. The columnar grains were composed of well oriented cellular dendrites with a primary arm spacing of approximately 35 μm. The size of γ′ precipitates in the dendritic cores was larger than that in the interdendritic zones. Mechanical test data have shown that the LMD material had good high-temperature mechanical properties along longitudinal section.

The effect of the flake to fiber transition in silicon morphology on the tensile properties of Al–Si eutectic alloys

Abstract: The combined and separate effects of microstructural scale and silicon phase morphology on the mechanical properties of Al–Si eutectic alloys are investigated here. The Bridgman-type gradient-zone directional solidification method is employed to produce as-cast structures characteristic of the full range of practical (i.e. casting) growth velocities, and the corresponding mechanical properties are characterized by uniaxial tension testing. The results are analyzed in light of previously reported microstructural changes associated with the flake to fiber or “quench modification” transition [1] . Both tensile strength and elongation were found to increase with solidification rate. Application of the Nan–Clarke [2] micromechanical analysis to the Al–Si composite structure, incorporating the strengthening effects of reinforcement-induced dislocations, suggests that the decreasing microstructural scale alone is sufficient to account for the increase in tensile strength with solidification rate. However, the flake to fiber transition was found to have a particular relevance with regard to the fracture behavior of the alloy, increasing tensile elongation and decreasing the overall variability of tensile properties. A maximum in elongation was observed at approximately 600 μm/s, corresponding to the upper threshold of the flake to fiber transition associated with complete disappearance of the flake morphology and dominance of the fibrous structure. These results emphasize the importance of understanding and controlling the flake to fiber transition that occurs with increasing solidification rate in Al–Si eutectic alloys.

Influence of forced/natural convection on segregation during the directional solidification of Al-based binary alloys.

Abstract: We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.

Control of Si Crystal Growth during Solidification of Si-Al Melt

Abstract: The growth of Si crystals from a Si-55.3 at%Al melts was investigated with the aim of developing a new Si refining process for SOG-Si production. Si crystals were grown by directional solidification of the Si-Al alloy. The temperature at the bottom of the sample was controlled from 1273 K (the alloy’s liquidus temperature) to 1173 K. Bulk Si crystals were successfully obtained in this study, and the crystal growth was found to be controlled by the diffusion of Si in the melt. By controlling the crystal growth conditions, the Al content of the Si crystals could be decreased to the level of the solid solubility of Al in Si; furthermore, other impurity elements could be efficiently removed by this refining method. [doi:10.2320/matertrans.M2010001]

Influence of directional solidification variables on the microstructure and crystal orientation of AM3 under high thermal gradient

Abstract: Solid–liquid interface morphologies of a nickel-base single crystal superalloy AM3 were investigated under high thermal gradient. The critical velocities of planar–cellular and cellular–dendritic transition were greatly increased by high thermal gradients. A high thermal gradient was of great benefit to dendrite refinement. Experimental results showed that the primary and secondary dendrite arm spacings decreased with increasing cooling rate. As expected, the segregation of elements was suppressed and the size of the gamma prime (γ′) phase decreased significantly with increasing withdrawal rates. The shape of γ′ in interdendritic region kept cuboidal at higher withdrawal rate. It was found that the withdrawal rates had little influence on the crystallographic orientation in high thermal gradient directional solidification.

Process-induced microstructural characteristics of laser consolidated IN-738 superalloy

Abstract: Laser consolidation (LC) is a laser cladding based material additive process that could fabricate a “net-shape” functional metallic part through a “layer-upon-layer” deposition directly from a CAD model. The LC process can produce nickel-base IN-738 superalloy with metallurgical soundness. Nevertheless, due to process-induced rapid directional solidification, the LC IN-738 alloy demonstrates certain unique microstructural characteristics, such as, the presence of non-uniform coarse columnar grains (a majority of the grains is in the range of about 56–158 μm in diameter; but their lengths could vary from several hundred μm to the height of the wall being built up) in combination with exceptionally fine directionally solidified dendrites inside (the secondary dendritic arm spacing is about 1.7 μm). Moreover, the “as-consolidated” LC IN-738 alloy, in nature, is a supersaturated γ solid solution, and any precipitation of γ′ particles from the γ matrix is effectively suppressed. Post heat treatment, thus, is essential to achieve the required operational microstructure. On the other hand, the nature and role of conventional “solution plus aging” treatment to the LC IN-738 alloy seem to be different as compared to “as-cast” or wrought IN-738 alloy. In this paper, process-induced microstructural characteristics of the LC IN-738 alloy and its development brought by post heat treatment were fully investigated. The implication on high-temperature mechanical performance of the LC alloy was discussed as well.

Measurement of Solute Profiles by Means of Synchrotron X-Ray Radiography during Directional Solidification of Al-4 wt% Cu Alloys

Abstract: 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.

Grain growth and loss of texture during annealing of alloys, and the translation of Earth’s inner core

Abstract: [1] The Earth's inner core is seismically anisotropic, with the direction parallel to the rotation axis both fast and more attenuating. There is also increasing evidence that the inner core is asymmetric, with the western hemisphere exhibiting slower direction-averaged P wave velocity, less overall attenuation, and greater elastic anisotropy. It was recently suggested that the hemispherical variations might result from convective translation, whereby enhanced solidification in the western hemisphere leads to a net eastward translation of inner core material, with melting occurring in the east. Annealing accompanies this eastward movement. This study examines experimentally a previously unobserved sequence of grain growth and loss of texture during the annealing of a directionally solidified alloy. The growth of newly nucleated grains results because the original grains that resulted from directional solidification have a high energy associated with intragranular interphase boundaries, and because the minor element has a very low solubility in the primary phase so that a more traditional sequence of coarsening is not possible. This supplies a physical mechanism for the loss of texture that is suggested seismically.

High-quality multi-crystalline silicon growth for solar cells by grain-controlled directional solidification

Abstract: We report simple ideas for grain control by using active cooling and crucible insulation for high-quality multi-crystalline silicon (mc-Si) growth for solar cells. The method employed an active cooling spot to induce initial dendrite growth, and the solidification front was controlled to be slightly convex through crucible insulation. It was found that the percentage of grains having twins was significantly increased by the present approach. The dislocation density for those grains was also significantly lower. More importantly, the successful improvement showed that the grain size and the minority carrier lifetime increased along the growth direction. And the laser beam induced current (LBIC) measurement also showed much higher quantum efficiency for the twin area. Copyright © 2010 John Wiley & Sons, Ltd.