Showing papers on "Directional solidification published in 2008"

In-Situ and Real-Time Analysis of the Formation of Strains and Microstructure Defects during Solidification of Al-3.5 Wt Pct Ni Alloys

Abstract: Alloy solidification was investigated in situ and real time by using a unique experimental setup developed at the European Synchrotron Radiation Facility (ESRF) combining both synchrotron X-ray radiography and topography. Although synchrotron X-ray radiography enables the investigation of the solid-liquid interface of metallic alloys, white-beam synchrotron X-ray topography enables the investigation of the formation of strains and defects formation in the growing solid microstructure. In this article, we present results obtained during directional solidification experiments performed with Al-3.5 wt pct Ni samples. First, the initial state after thermal stabilization is characterized. Next, the interface morphological instability and the transition to the columnar growth regime are thoroughly investigated. Topography observation shows that several parts of each dendrite become disoriented while the microstructure is developing. Disorientations are quantified and the aluminum yield stress at the melting point is estimated from the bending of secondary arms. Last, coupled growth of eutectic and dendrites settles with the formation of the eutectic phase. The eutectic grains grow strained and the dendrites concomitantly undergo additional stress.

Growth directions of microstructures in directional solidification of crystalline materials.

Abstract: In directional solidification, as the solidification velocity increases, the growth direction of cells or dendrites rotates from the direction of the thermal gradient to that of a preferred cristalline orientation. Meanwhile, their morphology varies with important implications for microsegregation. Here, we experimentally document the growth directions of these microstructures in a succinonitrile alloy in the whole accessible range of directions, velocities, and spacings. For this, we use a thin sample made of a single crystal on which the direction of the thermal gradient can be changed. This allows a fine monitoring of the misorientation angle between thermal gradient and preferred crystalline orientation. Data analysis shows evidence of an internal symmetry which traces back to a scale invariance of growth directions with respect to a Peclet number. This enables the identification of the relationship between growth directions and relevant variables, in fair agreement with experiment. Noticeable variations of growth directions with misorientation angles are evidenced and linked to a single parameter.

Phase-field simulation of weld solidification microstructure in an Al-Cu alloy

Abstract: Since the mechanical properties and the integrity of the weld metal depend on the solidification behaviour and the resulting microstructural characteristics, understanding weld pool solidification is of importance to engineers and scientists. Thermal and fluid flow conditions affect the weld pool geometry and solidification parameters. During solidification of the weld pool, a columnar grain structure develops in the weld metal. Prediction of the formation of the microstructure during welding may be an important and supporting factor for technology optimization. Nowadays, increasing computing power allows direct simulations of the dendritic and cell morphology of columnar grains in the molten zone for specific temperature conditions. In this study, the solidification microstructures of the weld pool at different locations along the fusion boundary are simulated during gas tungsten arc welding of Al–3wt%Cu alloy using the phase-field model for the directional solidification of dilute binary alloys. A macroscopic heat transfer and fluid flow model was developed to assess the solidification parameters, notably the temperature gradient and solidification growth rate. The effect of the welding speed is investigated. Computer simulations of the solidification conditions and the formation of a cellular morphology during the directional solidification in gas tungsten arc welding are described. Moreover, the simulation results are compared with existing theoretical models and experimental findings.

Cellular/Dendritic Transition and Microstructure Evolution during Transient Directional Solidification of Pb-Sb Alloys

Abstract: Recent studies of lead-antimony alloys, used for the production of positive electrodes of lead-acid batteries, have assessed the influences of both the microstructural morphology and of solute redistribution on the surface corrosion resistance in sulfuric acid solution, and have shown that cellular structures and dendritic structures have different responses on the corrosion rate of such alloys. The present article focuses on the search of adequate solidification conditions (alloy composition, cooling rate, and solidification velocity), which determine the occurrence of a microstructural transition from the cellular to the dendritic regime during the transient unidirectional solidification of hypoeutectic Pb-Sb alloys and on the microstructural evolution after such transition. The experimental data refers to the solidification of four hypoeutectic Pb-Sb alloys (2.2, 2.5, 3, and 6.6 wt pct Sb) and of the eutectic composition. The experimental results include transient metal/mold heat-transfer coefficients, liquidus isotherm velocity, cooling rate, and cellular and dendritic spacings. It was found that the cooling rate dependence on cellular and primary dendritic spacings is characterized by an experimental law of the form $$ \lambda _{1} = A{\cdot}\ifmmode\expandafter\dot\else\expandafter\.\fi{T}^{{{\kern 1pt} {-0.55}}}, $$ which seems to be independent of composition where A = 60 represents the alloys undergoing a cellular growth and A = 115 can describe the dendritic growth. The sudden change on such multiplier has occurred for the Pb 2.2 wt pct Sb alloy, i.e., for the cellular/dendritic transition.

Microstructural Development in Al-Ni Alloys Directionally Solidified under Unsteady-State Conditions

Abstract: Three Al-Ni hypoeutectic alloys were directionally solidified under upward unsteady-state heat-flow conditions. Primary (λ1) and secondary (λ2) dendrite arm spacings were measured along the castings for all alloys and correlated with transient solidification thermal variables. A combined theoretical and experimental approach was used to quantitatively determine such thermal variables, i.e., transient metal/mold heat-transfer coefficients, tip growth rates, thermal gradients, tip cooling rates, and local solidification time. The article also focuses on the dependence of dendrite arm spacings on the alloy solute content. Furthermore, the experimental data concerning the solidification of Al-1.0, 2.5, and 4.7 wt pct Ni alloys are compared with the main predictive dendritic models from the literature.

Microstructures, crystallography of interfaces, and creep behavior of melt-growth composites

Abstract: Oxide eutectic ceramics were prepared from Al2O3 and Ln2O3-based systems by unidirectional solidification from the melt. The microstructure consists of two single-crystal phases continuously entangled in a three-dimensional interpenetrating network without grain boundaries, pores or colonies. The outstanding stability of these microstructures gives rise to a high strength and creep resistance at high temperature. Preferred growth directions, orientation relationships between phases and single-crystal homogeneity of specimens were revealed. Creep behavior at high temperature has been studied, mechanisms of deformation by dislocation motion and twinning were revealed from Transmission Electron Microscopy (TEM) observations. Extension to ternary eutectics with a three-dimensional microstructure consisting in the addition of a toughening phase (ZrO2 )t o the previous binary eutectics has been investigated. By using this method, significant improvement of fracture toughness was obtained. © 2008 Elsevier Ltd. All rights reserved.

Thermoelectric properties of directionally solidified half-Heusler compound NbCoSn alloys

Abstract: Single- and multiphase samples of the n-type half-Heusler NbCoSn were prepared by directional solidification using the optical floating zone melting method, and the thermoelectric properties of these samples were evaluated. NbCoSn has an excellent thermoelectric power which exceeds −250μVK−1 at around 900K and a relatively high carrier concentration, 4.82×1026m−3. A metalliclike temperature dependence of the electrical resistivity indicates that NbCoSn is a degenerate semiconductor. NbCoSn also shows an excellent power factor, 2.5mWm−1K−2 at about 650K, even without any tuning of the electrical properties which are susceptible to coexisting metallic phases.

Silicon nitride coating and crucible—effects of using upgraded materials in the casting of multicrystalline silicon ingots

Abstract: A method for removal of metallic contaminants from commercial Si3N4-powders has been developed. The method is based on acid leaching and shows promising results. Analyses show that the content of both intermetallic compunds as Fe, Ti and Al as well as O are significantly reduced. Clean, synthetic fused silica crucibles have been used along with normal sintered ones based on natural quartz. The crucibles were coated with normal, commercial silicon nitride powder and with purified Si3N4 in a cleanest possible environment. The crucibles were then used as vessels for directional solidification of multicrystalline silicon in a pilot scale furnace. The average lifetime of minority charge carriers in the cast silicon was determined by quasi steady state photoconductance (QSSPC) from the bottom to the top of the ingots. These varied in a systematic way, so that the materials cast in the pure environment had significantly higher values than the materials cast with conventional coating- and crucible materials. The maximum values for the lifetimes in the individual ingots varied from 7 to 135 µs. Copyright © 2007 John Wiley & Sons, Ltd.

Unidirectional solidification of binary melts from a cooled boundary: analytical solutions of a nonlinear diffusion-limited problem.

Abstract: A model is presented that describes nonstationary solidification of binary melts or solutions from a cooled boundary maintained at a time-dependent temperature. Heat and mass transfer processes are described on the basis of the principles of a mushy layer, which divides pure solid material and a liquid phase. Nonlinear equations characterizing the dynamics of the phase transition boundaries are deduced. Approximate analytical solutions of the model under consideration are constructed. A method for controlling the external temperature at a cooled wall in order to obtain a required solidification velocity is discussed.

Phase transformations in multicomponent melts

Abstract: Foreword. Preface. List of Contributors. Part One: Thermodynamics. 1. Phase Formation in Multicomponent Monotectic Al-based Alloys (Joachim GrAbner, Djordje MirkoviA , and Rainer Schmid-Fetzer). 1.1 Introduction. 1.2 Experimental Methods. 1.3 Systematic Classification of Ternary Monotectic Phase Diagrams. 1.4 Selected Ternary Monotectic Alloy Systems. 1.5 Quaternary Monotectic Al-Bi-Cu-Sn System. 1.6 Conclusion. 2. Liquid-Liquid Interfacial Tension in Multicomponent Immiscible Al-based Alloys (Walter Hoyer and Ivan G. Kaban). 2.1 Introduction. 2.2 Measurement Technique. 2.3 Experimental Details. 2.4 Results. 2.5 Discussion. 2.6 Summary. 3. Monotectic Growth Morphologies and Their Transitions (Lorenz Ratke, Anja MA ller, Martin Seifert, and Galina Kasperovich). 3.1 Introduction. 3.2 Experimental Procedures. 3.3 Experimental Results. 3.4 Discussion. 3.5 Outlook. 4. Thermal Expansion and Surface Tension of Multicomponent Alloys (JA rgen Brillo and Ivan Egry). 4.1 Introduction. 4.2 General. 4.3 Results. 4.4 Conclusion and Summary. 5. Measurement of the Solid-Liquid Interface Energy in Ternary Alloy Systems (Annemarie Bulla, Emir Subasic, Ralf Berger, Andreas BA hrig-Polaczek, and Andreas Ludwig). 5.1 Introduction. 5.2 Experimental Procedure. 5.3 Evaluation of the Local Curvature of the Grain Boundary Grooves. 5.4 Results and Discussion. 5.5 Sumamry and Conclusions. 6. Phase Equilibria of Nanoscale Metals and Alloys (Gerhard Wilde, Peter Bunzel, Harald RAsner, and JArg WeissmA ller). 6.1 Introduction. 6.2 Phase Stability and Phase Transformations in Nanoscale Systems. 6.3 Summary. Part Two: Microscopic And Macroscopic Dynamics. 7. Melt Structure and Atomic Diffusion in Multicomponent Metallic Melts (Dirk Holland-Moritz, Oliver Heinen, Suresh Mavila Chathoth, Anja Ines Pommrich, Sebastian StA ber, Thomas Voigtmann, and Andreas Meyer). 7.1 Introduction. 7.2 Experimental Details. 7.3 Results and Discussion. 7.4 Conclusions. 8. Diffusion in Multicomponent Metallic Melts Near the Melting Temperature (Axel Griesche, Michael-Peter Macht, and GA nter Frohberg). 8.1 Introduction. 8.2 Experimental Diffusion Techniques. 8.3 Influence of Thermodynamic Forces on Diffusion. 9. Phase Behavior and Microscopic Transport Processes in Binary Metallic Alloys: Computer Simulation Studies (Subir K. Das, Ali Kerrache, JA rgen Horbach, and Kurt Binder). 9.1 Introduction. 9.2 Transport Coefficients. 9.3 A Symmetric LJ Mixture with a Liquid-Liquid Demixing Transition. 9.4 Structure, Transport, and Crystallization in Al-Ni Alloys. 9.5 Summary. 10. Molecular Dynamics Modeling of Atomic Processes During Crystal Growth in Metallic Alloy Melts (Helmar Teichler and Mohamed Guerdane). 10.1 Introduction. 10.2 Entropy and Free Enthalpy of Zr-rich Ni x Zr 1-x Melts from MD Simulations and Their Application to the Thermodynamics of Crystallization. 10.3 Bridging the Gap between Phase Field Modeling and Molecular Dynamics Simulations. Dynamics of the Planar [Ni x Zr 1-x ]liquidA ~ Zr crystal Crystallization Front. 10.4 Entropy and Free Enthalpy in Ternary A1 y Ni 0.4-y Zr 0.6 Alloys Melts. 10.5 Concluding Remarks. 11. Computational Optimization of Multicomponent Bernala s Liquids (Helmut Hermann, Antje Elsner, and Valentin Kakotin). 11.1 Introduction. 11.2 Methods. 11.3 Results and Discussion. 11.4 Conclusion. 12. Solidification Experiments in Single-Component and Binary Colloidal Melts (Thomas Palberg, Nina Lorenz, Hans Joachim SchApe, Patrick Wette, Ina Klassen, Dirk Holland-Moritz, and Dieter M. Herlach). 12.1 Introduction. 12.2 Experimental Procedure. 12.3 Results. 12.4 Conclusions. Part Three: Nd-Fe based Alloys. 13. Phase-Field Simulations of Nd-Fe-B: Nucleation and Growth Kinetics During Peritectic Growth (Ricardo Siquieri and Heike Emmerich). 13.1 Introduction. 13.2 Phase-Field Model with Hydrodynamic Convection. 13.3 Investigating Heterogeneous Nucleation in Peritectic Materials via the Phase-Field Method. 13.4 Conclusion. 14. Investigations of Phase Selection in Undercooled Melts of Nd-Fe-B Alloys Using Synchrotron Radiation (Thomas Volkmann, JArn Strohmenger, and Dieter M. Herlach). 14.1 Introduction. 14.2 Description of the Investigations. 14.3 Experimental Results and Discussion. 14.4 Analysis Within Models of Nucleation and Dendrite Growth. 14.5 Summary and Conclusion. 15. Effect of Varying Melt Convection on Microstructure Evolution of Nd-Fe-B and Ti-Al Periteric Alloys (Regina Hermann, Gunter Gerbeth, Kaushik Biswas, Octavian Filip, Victor Shatrov, and Janis Priede. 15.1 Introduction. 15.2 Methods Developed. 15.3 Sample Preparation. 15.4 Results and Discussion. 15.5 Conclusion. 16. Nanosized Magnetization Density Profiles in Hard Magnetic Nd-Fe-Co-Al Glasses (Olivier Perroud, Albrecht Wiedenmann, Mihai Stoica, and JA rgen Eckert). 16.1 Introduction. 16.2 SANS with polarized neutrons in unsaturated magnetic systems. 16.3 Experimental Procedure. 16.4 Results and Discussion. 16.5 Conclusion. 17. Microstructure and Magnetic Properties of Rapidly Quenched (Nd 100-x Ga x ) 80 Fe 20 (x=0, 5, 10, and 15 at%) alloys (Mihai Stoica, Golden Kumar, Mahesh Emmi, Olivier Perroud, Albrecht Wiedenmann, Annett Gebert, Shanker Ram, Ludwig Schultz, and JA rgen Eckert). 17.1 Introduction. 17.2 Sample Preparation and Experimental Investigations. 17.3 Binary Nd 80 Fe 20 Rapidly Quenched Alloys. 17.4 Ternary (Nd 100-x Ga x ) 80 Fe 20 (x=5, 10, and 15) Rapidly Quenched Alloys. 17.5 Conclusions. 18. Solidification of Binary Alloys with Compositional Stressesa A Phase-Field Approach (Bo Liu and Klaus Kassner). 18.1 Introduction. 18.2 Equations of Motion. 18.3 Neutral Curves. 18.4 Phase-Field Model. 18.5 Simulation Results. 18.6 Conclusions. 19. Elastic Effects on Phase Transitions in Multi-component Alloys (Efim A. Brener, Clemens Gugenberger, Heiner MA ller-Krumbhaar, Denis Pilipenko, Robert Spatschek, and Dmitrii E. Temkin). 19.1 Melting of Alloys in Eutectic System. 19.2 Combined Motion of Melting and Solidification Fronts. 19.3 Continuum Theory of Fast Crack Propagation. 19.4 Summary. 20. Modeling of Nonisothermal Multi-component, Multi-phase Systems with Convection (Harald Garcke and Robert Haas). 20.1 Introduction. 20.2 Phase-field Models for Multicomponent, Multiphase Systems. 20.3 Multiphase Ginzburg-Landau Energies. 20.4 Convective Phase-Field Models. 20.5 Mathematical Analysis. 21. Phase-field Modeling of Solidification in Multi-component and Multi-phase Alloys (Denis Danilov and Britta Nestler). 21.1 Introduction. 21.2 Phase-field Model for Multicomponent and Multiphase Systems. 21.3 Modeling of Dendritic Growth. 21.4 Solute Trapping During Rapid Solidification. 21.5 Comparison of Molecular Dynamics and Phase-field Simulations. 21.6 Modeling of Eutectic Growth. 22. Dendrite Growth and Grain Refinement in Undercooled Melts (Peter K. Galenko and Dieter M. Herlach). 22.1 Introduction. 22.2 Solidification of Pure (One-Component) System. 22.3 Solidification of Binary Alloys with Constitutional Effects. 22.4 Solidification of a Ternary Alloy. 22.5 Summary and Conclusions. 23. Dendritic Solidification in the Diffuse Regime and Under the Influence of Buoyancy-Driven Melt Convection (Markus Apel and Ingo Steinbach). 23.1 Introduction. 23.2 The Multiphase-field Model. 23.3 Rapid Solidification in Ni 98 Al 1 Zr 1 . 23.4 Directional Solidification with Buoyancy-driven Interdendritic Flow. 23.5 Sumamry and Conclusion. 24. Stationary and Instationary Morphology Formation During Directional Solidification of Ternary Eutectic Alloys (Bernd BAttger, Victor T. Witusiewicz. Murkus Apel, Anne Drevermann, Ulrike Hecht, and Stephan Rex). 24-1 Introduction. 24.2 Investigations on the Eutectic System Ag-Cu-Zn. 24.3 Investigation on the Ternary Alloy System In-Bi-Sn. 24.4 Transient Growth. 24.5 Summary and Conclusion. 25. Dendritic Microstructure, Decomposition, and Metastable Phase Formation in the Bulk Metallic Glass Matrix Composite Zr 56 Ti 14 Nb 5 Cu 7 Ni 6 Be 12 (Susanne Schneider, Alberto Bracchi, Yue-Lin Huang, Michael Seibt, and Poppannan Thiyagarajan). 25.1 Introduction. 25.2 Experimental Procedures. 25.3 Results and Discussion. 25.4 Summary. Index.

Directional solidification of Al2O3-Al2TiO5 system

Abstract: Directional solidification of Al2O3–Al2TiO5 eutectic system was investigated to design in situ composites that could exhibit optimized properties of strength and toughness Directional solidification of an alumina rich melt was expected to produce primary Al2O3 dendrites, these would act as load bearing component of the structure, separated by a Al2O3–Al2TiO5 eutectic matrix and Additional aim was to utilize the differential expansion coefficient between the two phases to produce microcracks and thus weak interfaces for load deflection Three different melt compositions were investigated; two off-eutectic compositions, at 11, 26 mol% TiO2, and one eutectic at 439 mol% TiO2 The crystallized phases were different than as one would expect from the published phase diagram Off-eutectics were composed of Al2O3 dendrites separated by a Al6Ti2O13 matrix Eutectic structure was composed of Al2TiO5 lamellae in an aluminum titanate matrix This matrix consisted of a superstructure made of 5 cells of Al6Ti2O13 for one cell of Al2TiO5 along [0 0 1] direction Al6TiO13 phase decomposed by a eutectoid reaction into Al2O3 and Al2TiO5 Eutectoid reaction occurred during post heat treatment at 1400 °C and decomposition of Al6Ti2O13 was completed after 10 h of annealing at 1500 °C Based on these observations, a modification of the alumina rich part of Al2O3–TiO2 phase diagram was proposed A second intermediate compound, Al6Ti2O13 is introduced as a high temperature phase Two invariant points, a peritectic (L + Al2O3 → Al6Ti2O13) and a eutectic (L → Al6Ti2O13 + Al2TiO5) are also added between Al6Ti2O13 and Al2TiO5

Microtomographic characterization of columnar Al–Cu dendrites for fluid flow and flow stress determination

Abstract: During the twin roll casting of Al alloys, the interdendritic liquid may flow as the two solidification fronts are compressed together between the rolls. This can lead to defects such as centerline segregation. To understand the flow properties of the interdendritic liquid, samples of Al–12 wt.% Cu were solidified directionally in a Bridgman furnace and quenched to capture the growing columnar dendritic structures. The quenched samples were scanned using a laboratory X-ray microtomography (XMT) unit to obtain the 3D structure with a voxel resolution of 7.2 μm. Image analysis was used to separate the Al dendrite from the interdendritic Al–Al2Cu eutectic. Flow between the dendrites was simulated by solving the Stokes equation to calculate the permeability tensor as a function of the fraction solid. The results were compared to prior experimental measurements and calculations using synchrotron tomography observations of equiaxed structures. Elasto–plastic finite element (FE) simulations were performed on the dendritic structures to determine flow stress behavior as a function of fraction solid. It was found that the standard approximations for the reduction in flow stress in the semi-solid have a variation in excess of 100% from that calculated using the true structure. Therefore, it is critical to simulate the actual dendrite for effective flow stress determination.

Surface Segregation during Directional Solidification of Ni-Base Superalloys

Abstract: Some aspects pertaining to the increased microsegregation at the external casting surface during directional solidification of a typical Ni-base superalloy, CMSX 10N, are presented. Increased eutectic coverage was observed at the external surface along the solidification length. This eutectic appears as a thin segregated layer proud of the secondary dendrite arms preventing them from impinging onto the mold wall. The extent of surface eutectic coverage was represented as a fractional measure of the ingot perimeter. Possible mechanisms focusing on the following: (1) interaction between mold and metal, (2) inclination of primary dendrite, and (3) contraction of the dendrite network have been investigated in relation to the observed phenomenon. We deduce that the most likely explanation is associated with the contraction of the dendritic network, which qualitatively accounts both for the observed morphology and the increased eutectic fraction at the external surface of the casting.