Showing papers on "Directional solidification published in 2007"
TL;DR: In this paper, the formation of regular patterns is a common feature of many solidification processes involving cast materials, and regular patterns can be obtained in porous alumina by controlling the freezing of ceramic slurries followed by subsequent ice sublimation and sintering.
683 citations
TL;DR: In this paper, it was found that streaming of solute-enriched liquid at the tip of high-order branches promotes growth by local undercooling, which causes solute pile-up at the roots of the branches.
236 citations
TL;DR: In this article, the effect of TEMC on the cellular liquid-solid interface and the cellular morphology was evaluated at different scales and the result showed that the effect is different for different scales.
143 citations
TL;DR: In this paper, an extended lattice Boltzmann model is developed for simulating the convection-diffusion phenomena associated with solid-liquid phase transition processes, in which macroscopic hydrodynamic variables are obtained through the solution of an evolution equation of a single-particle density distribution function, whereas, the microscopic temperature field is obtained by solving auxiliary scalar transport equations.
Abstract: An extended lattice Boltzmann model is developed for simulating the convection–diffusion phenomena associated with solid–liquid phase transition processes. Macroscopic hydrodynamic variables are obtained through the solution of an evolution equation of a single-particle density distribution function, whereas, the macroscopic temperature field is obtained by solving auxiliary scalar transport equations. The novelty of the present methodology lies in the formulation of an enthalpy-based approach for phase-change modelling within a lattice-Boltzmann framework, in a thermodynamically consistent manner. Thermofluidic aspects of phase transition are handled by means of a modified enthalpy–porosity formulation, in conjunction with an appropriate enthalpy-updating closure scheme. Lattice-Boltzmann simulations of melting of pure gallium in a rectangular enclosure, Rayleigh–Benard convection in the presence of directional solidification in a top-cooled cavity, and crystal growth during solidification of an undercooled melt agree well with the numerical and experimental results available in the literature, and provide substantial evidence regarding the upscaled computational economy provided by the present methodology.
113 citations
TL;DR: In this paper, the interface morphologies of single crystal superalloy CMSX-2 were studied over a range of cooling rate with large variations in withdrawal speeds in directional solidification.
80 citations
TL;DR: In this article, models of the directional solidification processes for columnar-grain alloy GTD-444 are used to evaluate the various heat transfer steps and heat extraction modes within each process.
Abstract: Thermal profiles obtained during directional solidification of Ni-base superalloy castings by the Bridgman and liquid-metal-cooled (LMC) techniques have been used to develop and validate the finite-element models of these processes. Models of the directional solidification processes for the columnar-grain alloy GTD-444 are used to evaluate the various heat-transfer steps and heat extraction modes within each process. This analysis highlights the similarities and the fundamental differences in heat-transfer modes between the two solidification approaches. Ultimately, the unique limiting heat-transfer step of each process is identified. For the stepped plate configuration investigated, heat extraction is limited by radiation from the exterior mold surface in the Bridgman process. Heat transfer between the casting and the interior mold surface is the primary resistance in the LMC process, and heat extraction is enhanced by the lowering coolant temperatures in this process.
74 citations
TL;DR: In this article, a review of the technique as well as examples of images obtained during solidification of aluminum alloys are presented. But the results are expected to be important for validation of dendrite growth models.
Abstract: The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.
72 citations
TL;DR: A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN792+Hf.
Abstract: A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN792 + Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after γ/γ′ eutectic reaction. The (Ti + Ta + Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of γ/γ′ eutectic, the continual increase of (Ti + Ta + Hf)/Al ratio in the residual liquid eventually led to the completion of γ/γ′ eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M5B3 and M3B2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of γ/γ′ eutectic. Because of the limited solubility of Cr, Mo, and W in γ′ phase, these elements were enriched in the residual liquid during the solidification of γ/γ′ eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both γ and γ′ phases so that the possibility of boride formation in the residual liquid ahead of the γ/γ′ eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.
68 citations
TL;DR: In this paper, the incorporation of bubbles in Al 2 O 3 -YAG and AYZ eutectic crystals grown using the laser floating zone method was investigated and the effect of different growth experimental parameters was considered.
64 citations
TL;DR: In this article, a brief introduction to the phase-field model and its implementation is given, and its capabilities are illustrated by examples taken from the directional solidification of binary alloys, in particular, the morphological stability of hexagonal cellular arrays and of eutectic lamellar patterns.
64 citations
TL;DR: In this paper, the columnar-to-equiaxed transition of Al-Zn and Zn-Al alloys under different conditions of superheat and heat transfer efficiencies at the metal/mold interface is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradient, and recalescence determined from the temperature vs time curves.
Abstract: Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between –0.338 °C/mm and 0.167 °C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys.
TL;DR: In this article, a laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated, and microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ.
Abstract: The laser welding of a directionally solidified INCONEL 738 superalloy (DS IN738) was investigated. Microstructural analysis of the fusion zone (FZ) and the heat-affected zone (HAZ) revealed that cracking occurred mainly in HAZ with only some cracks being extended into the FZ. However, the frequently observed centerline cracking in FZ of DS and single crystal (SX) alloys did not occur, which was attributed to the presence of negligible volume fraction of γ-γ′ eutectic in FZ. Constitutional liquation of secondary solidification constituents (MC carbides, M3B2 borides, M2SC sulphocarbides, and γ-γ′ eutectic) and γ′ precipitate particles was found to be the major cause of grain boundary liquation and the resultant intergranular microfissuring in the HAZ. The extent of HAZ microfissuring was, however, observed to be smaller in samples welded along the transverse direction (perpendicular to solidification direction) than when welding was done along the longitudinal direction (solidification direction). Nonetheless, more severe HAZ cracking occurred in samples of similarly welded conventionally cast (CC) alloy, and the present results indicate that the severity of HAZ liquation cracking in IN738 superalloy can be reduced by using a DS version of the alloy.
TL;DR: In this paper, a technical Al-7 wt pct Si-0.6 wt mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions.
Abstract: To obtain a quantitative understanding of the effect of fluid flow on the microstructure of cast alloys, a technical Al-7 wt pct Si-0.6 wt pct Mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions. The solidification was studied in an aerogel-based furnace, which established flat isotherms and allowed the direct optical observation of the solidification process. A coil system around the sample induces a homogeneous rotating magnetic field (RMF) and, hence, a well-defined flow field close to the growing solid-liquid interface. The application of RMFs during directional solidification results in pronounced segregation effects: a change to pure eutectic solidification at the axis of the sample at high magnetic field strengths is observed. The investigations show that with increasing magnetic induction and, therefore, fluid flow, the primary dendrite spacing decreases, whereas the secondary dendrite arm spacing increases. An apparent flow effect on the eutectic spacing is observed.
TL;DR: In this article, two Fe-based alloys with a small variation in the Ni content were fabricated on a martensitic stainless steel 1Cr13 substrate by laser surface cladding (LSC) using a CO2 laser and Ar shielding gas that was blown into a molten pool.
TL;DR: In this article, the successive relaxation of supercooling (SRS) method was used to solve the problem of polycrystalline silicon ingots directionally solidifying by the conventional traveling heater method.
TL;DR: Polycrystalline Ni50Mn30Ga20 was produced by directional solidification to achieve a coarse-grained, textured microstructure as discussed by the authors, and samples were cut along the principal crystal (1.0.0)-axes.
TL;DR: In this article, the microstructures and microsegregation of a binary Mg-4Al (wt.%) alloy were investigated under controlled solidification conditions, and the morphology of the solid liquid interface and the microstructure of both the mushy zone and the steady-state region were characterized at different levels of growth rates.
TL;DR: This study has experimentally determined the growth direction and the morphology of dendrites in a thin sample of succinonitrile alloy in configurations where the heat flow direction differs from the preferred crystalline orientations, and provided evidence of an internal symmetry.
Abstract: This study has experimentally determined the growth direction and the morphology of dendrites in a thin sample of succinonitrile alloy in configurations where the heat flow direction differs from the preferred crystalline orientations. A large data set has been obtained over a range of growth velocity, dendrite spacing, and misorientation angle between the two directions. Data analysis has provided evidence of an internal symmetry from which the expression of the orientational response of dendrites to the growth conditions has been identified. This has been complemented by the identification of a new dendrite scale, more relevant than the dendrite spacing to the present issue. Altogether, these results provide new insights on the growth direction and the morphology of dendrites that could be applied to more practical configurations.
TL;DR: In this article, the authors present new analytic results relating to the nonstationary Stefan-type problems for the unidirectional solidification of binary solutions or melts with a mushy layer.
25 Jan 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the microstructural evolution during solidification and subsequent solid-state transformation of AISI 304 stainless steel is studied by directional solidization and quenching methods, where the phase transformation sequence in the steel directionally solidified is that the precipitation of primary ferritic dendrites, ferrite-austenite eutectic reaction and the direct formation of austenite occur in sequence during the directional solidification of the austenitic stainless steel.
Abstract: The microstructural evolution during solidification and subsequent solid-state transformation of AISI 304 stainless steel is studied by directional solidification and quenching methods in this paper The phase transformation sequence in the steel directionally solidified is that the precipitation of primary ferritic dendrites, ferrite-austenite eutectic reaction, and the direct formation of austenite occur in sequence during the directional solidification of the austenitic stainless steel At the eutectic reaction stage, plenty of columnar and cellular eutectic colonies composed of the coupled growth of lamellar ferrite and austenite take place During the course of subsequent solid-state transformation, austenite grows into ferrite gradually, resulting in the disappearing of eutectic colonies and thinning of primary ferritic dendrites, and dendrite cores of primary ferrite are retained as the final skeletal ferrite in the final microstructure (c) 2006 Elsevier BV All rights reserved
TL;DR: In this paper, the effect of rapid solidification on the microstructure characteristics, solidification behaviors and mechanical properties of the eutectics was analyzed under different growth conditions.
TL;DR: In this paper, an Al-3 wt.% Si alloy was solidified under steady-state conditions with a constant growth rate of 8.20 μm/s at different temperature gradients.
Abstract: Unidirectional solidification experiments have been carried out on an Al-3 wt.% Si alloy as a function of temperature gradient, G and growth rate, V. The samples were solidified under steady-state conditions with a constant growth rate of 8.20 μm/s at different temperature gradients (1.97-6.84 K/mm) and with a constant temperature gradient (6.84 K/mm) at different growth rates (8.20-492.76 μm/s). Microstructure parameters (primary dendrite arm spacing, λ1, secondary dendrite arm spacing, λ2, dendrite tip radius, R and mushy zone depth, d) were measured as a function of temperature gradient and growth rate. The experimental results have been compared with the current theoretical models and similar experimental works.
TL;DR: In this article, a new facility has been developed to investigate the directional solidification of transparent aqueous solutions forming mushy layers in a quasi-two-dimensional system, where experiments have been conducted on NaCl-H2O solutions by translating a Hele-Shaw cell at prescribed rates between fixed heat exchangers providing a temperature gradient of approximately 1°C mm−1.
Abstract: A new facility has been developed to investigate the directional solidification of transparent aqueous solutions forming mushy layers in a quasi-two-dimensional system. Experiments have been conducted on NaCl–H2O solutions by translating a Hele-Shaw cell at prescribed rates between fixed heat exchangers providing a temperature gradient of approximately 1°C mm−1. The mush–liquid interface remained planar at all freezing velocities larger than 8 μm s−1, while steepling occurred at lower velocities. No significant undercooling of the mush–liquid interface was detected at freezing velocities up to 12 μm s−1. Mathematical predictions of the steady-state temperature profile and mushy-layer thickness as functions of freezing rate are in excellent agreement with experimental measurements.
25 Jan 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effect of solidification rates on microstructure development was studied using an optical floating zone melting furnace within the solidification rate from 10 to 200mm/h.
Abstract: Unidirectional solidification experiments were performed on the (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems. The effect of solidification rates on microstructure development was studied using an optical floating zone melting furnace within the solidification rates from 10 to 200 mm/h. For the binary invariant eutectic alloy, a planar eutectic microstructure forms at 10 mm/h, while cellular eutectic microstructures develop at the higher solidification rates. The orientation relationship between Nb and Nb3Si formed upon planar eutectic solidification is determined as: {1 1 0}Nb//{1 1 0)Nb3Si, and 〈1 1 2〉Nb//〈0 0 1]Nb3Si. For the ternary univariant eutectic alloys, the formation of cellular eutectic microstructures is prevailing for all over the rates of solidification. The growth of Nb3Si is faster than that of (Nb). The size distribution of (Nb) particles spreads widely in an asymmetric manner. The cell size variation against solidification rates is in a reasonable agreement with the theoretical prediction.
TL;DR: In this paper, the columnar-to-equiaxed transition (CET) was investigated during the upward vertical directional solidification of hypoeutectic AlNi and Al-Sn alloys.
TL;DR: In this paper, the influence of a high magnetic field on the growth of MnBi, α-Al and Al3Ni dendrites in directionally solidified Bi-Mn, Al-Cu and Al-Ni alloys has been investigated.
TL;DR: In this paper, the authors investigated the relationship between solidification processing parameters (temperature gradient, G, growth rate, V, and cooling rate, T ˙ ) and cellular spacing, λ, in the Al-0.1 and Al-5.5 alloys grown at high growth rates.
TL;DR: In this paper, the effect of dendrite arm spacing on hot tearing susceptibility during directional solidification was explored by casting bi-crystal samples with the same grain boundary misorientation but different dendritic arm spacing.
Journal Article•
TL;DR: An overview of the development and current status of the directional solidification process assisted by liquid metal cooling (LMC) has been presented in this article, where the driving force of the rapid development of the LMC process has been analyzed by considering the demands of newer technologies that can provide higher thermal gradients for alleviated segregation in advanced alloy systems, and better production yield of the large directionally solidified superalloy components.
TL;DR: In this paper, the fracture toughness of directional solidified TiAl-Nb alloys was studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace.