J. De Wilde

University of Leuven, Louvain, Flanders, Belgium

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Publications (10)5.97 Total impact

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    ABSTRACT: Combustion Synthesis experiments have been performed on the ISS (International Space Station) during the Belgian taxi-flight mission ODISSEA in November 2002, in the framework of the ESA-coordinated project COSMIC (Combustion Synthesis under Microgravity Conditions). The main objective of the experiments was to investigate the general physico-chemical mechanisms of combustion synthesis processes and the formation of products microstructure. Within the combustion zone, a number of gravity-dependent phenomena occur, while other phenomena are masked by gravity. Under certain conditions, gravity-dependent secondary processes may also occur in the heat-affected zone after combustion. To study the influence of gravity, a specially dedicated reactor ensemble was designed and used in the Microgravity Science Glovebox (MSG) onboard the ISS. In this work, the experiment design is first discussed in terms of the experimental functionality and reactor ensemble integration in the MSG. To investigate microstructure formation, a sample constituted by a cylindrical portion followed by a conical one, the latter being inserted inside a massive copper block, is used. The experiment focused on the synthesis of intermetallic matrix composites (IMCs) based on the Al-Ti-B system. Depending on the composition, different intermetallic compounds (TiAl and TiAl3) can be formed as matrix phase while TiB2 represents the reinforcing particulate phase. During the ISS mission, six samples with a relatively high green density of 65%TD have successfully been processed. The influence of the composition on the combustion process will be examined.
    Microgravity - Science and Technology 08/2007; 19(5):85-89. · 0.59 Impact Factor
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    ABSTRACT: The results obtained during a parabolic flight campaign sponsored by ESA on March 2002, and related to the combustion synthesis of TiB2 − xTiAl and TiB2 − xTiAl3 composites are reported. Besides classical SHS experiments performed using cylindrical pellets, combustion front quenching experiments using conical samples placed inside a copper block are also conducted. Similar experiments are carried out under terrestrial conditions, for the sake of comparison. As expected, under both low-gravity and terrestrial conditions, it is found that combustion temperature and front propagation velocity decrease as the system exothermicity is reduced, that is, when the aluminide/diboride molar ratio is augmented. However, it is observed that reaction front propagates relatively slower under low-gravity conditions. Consistently, the extinction of the combustion front occurs earlier when the reaction is performed under reduced gravity conditions. A theoretical analysis performed by means of appropriate dimensionless numbers is proposed to provide possible explanations of the main experimental evidences reported in the literature on this subject, including those shown in this work. Specifically, this analysis reveals that the relatively higher-propagation velocity observed under terrestrial conditions may be due to the correspondingly lower-sample porosity change, which in turn limits the thermal conductivity decreasing in the reaction zone. In addition, free convection phenomena taking place in the molten phase formed at the reaction zone are able to justify the difference in combustion wave velocity experimentally observed under terrestrial conditions between top and bottom ignition configurations. Moreover, it is found that finer microstructure typically observed under low-gravity is likely a consequence of the reduced coalescence phenomena taking place in such conditions. © 2006 American Institute of Chemical Engineers AIChE J, 2006
    AIChE Journal 10/2006; 52(11):3744 - 3761. · 2.58 Impact Factor
  • Jimmy De Wilde, Ludo Froyen
    Materials Science Forum - MATER SCI FORUM. 01/2006; 508:51-56.
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    ABSTRACT: Unidirectional solidification of ternary eutectic alloys with a composition close to the univariant eutectic reaction under certain conditions leads to two-phase planar growth, showing a lamellar morphology similar to what is known from binary (invariant) eutectic growth. In the latter case, the most applied analytical description is the Jackson–Hunt model. The present paper extends this model to steady-state two-phase planar and regular lamellar coupled growth in the case of the univariant eutectic reaction in ternary alloys as obtained during unidirectional solidification. It is shown that during steady state, a similar expression between the lamellar spacing λ and the growth velocity v , i.e., λ<sup>2</sup>v= constant, describes the spacing selection whenever minimum undercooling is assumed. The constant is only dependent on the material properties of the selected alloy. The theory is applied to growth along the univariant reaction L→α( Al )+θ- Al <sub>2</sub> Cu in Al–Cu–Ag alloys resulting in a good agreement between the calculations and the experimental results. However, some uncertainties concerning the variation of the material properties, in particular, the interfacial properties, remain when going from the binary system towards ternary univariant growth. So, finally a sensitivity analysis is made, being a good solution to show the effect of a change in alloy and material properties on the eutectic growth morphology, e.g., by a change in the amount of the alloying elements.
    Journal of Applied Physics 07/2005; · 2.21 Impact Factor
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    ABSTRACT: The research described in this report will increase the understanding of fundamental aspects of self-propagating combustion synthesis processes and synthesis of materials that cannot be formed under normal gravity conditions. Self-propagating combustion synthesis processes involve several stages that are significantly affected by gravity: melting of reactants and products, spreading of the melt, droplet coalescence, diffusion and convection, buoyancy of solid particles and densification of liquid products. High temperatures in the combustion wave generate liquids that are subject to gravity- driven flow. In this context, the removal of such gravitational effects is likely to provide increased control of the reaction front, with a consequent improvement in the microstructure of the synthesised product. In addition, microgravity experiments will lead to advances in understanding fundamental aspects of combustion and structure formation during self-propagating combustion synthesis. Since self-propagating combustion synthesis technology has the potential to prepare advanced materials and net-shape articles with tailored physical and mechanical properties in one step, and has extremely low external energy requirements, it is well-suited for use on space platforms.
    05/2005; 1281:86-93.
  • J. De Wilde, L. Froyen
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    ABSTRACT: The solutal configuration, both in the solid as in the liquid, during coupled two-phase planar growth as it can be obtained during unidirectional solidification of ternary Al-Cu-Ag and Al-Cu-Si alloys with a composition close to the univariant eutectic reaction L → α (Al) + θ−Al2Cu has been investigated during preparatory ground experiments. During solidification, both Al and Cu can be redistributed in the liquid by cross-diffusion parallel to the planar solid/liquid interface. However the third element, i.e. Ag and Si, must be segregated over a much longer distance. In the case of Ag, a diffusive stable solute boundary layer has been observed. Si however is found to be solutal unstable and gravity driven solutal convection breaks up the boundary layer. In both cases, however, the interface is prone to a Mullins-Sekerka instability giving rise to two-phase cellular growth at a critical value of the growth velocity. In the solid, the α(Al) composition, determined by the solidus line, changes with changing growth velocity.
    Microgravity - Science and Technology 01/2005; 16(1):40-44. · 0.59 Impact Factor
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    ABSTRACT: During casting, unconstrained solidification occurs most frequently. The objectives of the present study were to describe the different stages of a ternary eutectic solidification path, its unconstrained solidification and to assess gravity dependent phenomena as a preparation for a MAXUS sounding rocket microgravity experiment. Two alloys within the Al–Cu–Ag system were selected. They form a different primary phase α(Al) or θ-Al2Cu that however share the further solidification path. Nucleation of primary non-faceted α(Al) was enhanced by the addition of TiB2. The same addition did not influence the nucleation of faceted θ-Al2Cu. Gravity driven flotation of primary α(Al) was observed. Nucleation of the univariant eutectic α(Al)+θ-Al2Cu is triggered by θ-Al2Cu, whereas growth occurs with a lamellar, rod-like or irregular morphology. Growth of the invariant eutectic α(Al)+θ-Al2Cu+ξ-Ag2Al occurs either with an irregular, semi-regular brick type or regular double-rod type morphology, however, always with a strong association between θ-Al2Cu and ξ-Ag2Al.
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing - MATER SCI ENG A-STRUCT MATER. 01/2005; 413:514-520.
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    ABSTRACT: Multiphase solidification in multicomponent alloys is pertinent to many commercial materials and industrial processes, while also raising challenging questions from a fundamental point of view. Within the past few years, research activities dedicated to multiphase solidification of ternary and multicomponent alloys experienced considerable amplification. This paper gives an overview of our present understanding in this field and the experimental techniques and theoretical methods research relies on. We start with an introduction to thermodynamic databases and computations and emphasize the importance of thermophysical property data. Then, we address pattern formation during coupled growth in ternary alloys and cover microstructure evolution during successive steps of phase formation in solidifying multicomponent alloys. Subsequently, we review advances made in phase field modeling of multiphase solidification in binary and multicomponent alloys, including various approaches to crystal nucleation and growth. Concluding, we address open questions and outline future prospects on the basis of a close interaction among scientists investigating the thermodynamic, thermophysical and microstructural properties of these alloys.
    Materials Science and Engineering: R: Reports. 01/2004;
  • Jimmy De Wilde, Ludo Froyen, Stephan Rex
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    ABSTRACT: Coupled two-phase planar univariant eutectic growth occurs in Al–Cu–Ag alloys for different compositions and over a range of solidification velocities along the univariant eutectic reaction L → α(Al) + θ(Al2Cu). The classical relationship λ2v=constant is obeyed. Two limits of stability are present: morphological stability and stability of the coupling.
    Scripta Materialia. 01/2004; 51(6):533-538.
  • Ludo Froyen, Jimmy De Wilde
    Materials Science Forum - MATER SCI FORUM. 01/2003;