Papers by Author: Ludo Froyen

Abstract: A special type of divorced eutectic growth mode (symbiotic growth) in a ternary Al-Mn-Si alloy, triggered by addition of titanium boride (TiB2) has been studied under both ground and microgravity conditions. During directional solidification, α (AlMnSi) particles nucleate ahead of the planar solidification front and are pushed and later engulfed by the interface forming a banded particle layer structure. The presence of fine titanium boride particles (clusters) in front of the growing α (AlMnSi) particles makes the interaction between the intermetallic α (AlMnSi) particles and solidification front much more complex than most proposed models for particle/interface interactions. Microgravity experiments can eliminate the gravity related effects and thus provide an opportunity to better understand the formation mechanism of symbiotic growth. In this study, hypoeutectic Al-1Mn-3Si alloys with addition of 0.33 wt% TiB2 were directionally solidified in ESA Solidification and Quenching Furnace (SQF) on board of the International Space Station (ISS). The ground experiment was conducted in a replica of this furnace prior to the microgravity experiments. Non-destructive X-ray tomography and its 3D reconstruction software was used to characterize the particles and their distribution. Comparison between ground and microgravity experiment results is presented. The particle pushing and engulfment of symbiotic growth is discussed based on a particle pushing and engulfment model.

Abstract: In this work the transition from columnar to equiaxed growth is studied during the solidification of the univariant eutectic L=> α(Al) + θ-Al2Cu in the ternary Al-Cu-Ag system. The columnar to equiaxed transition (CET) in a ternary system behaves very similar to the CET observed in binary systems [1]. It is observed that the CET occurs at lower temperature gradients when the amount of the third alloying element, in this case Ag, is increased. Another important solidification parameter is the cooling rate of the furnace. When the cooling rate is increased, the CET will occur at lower positions in the sample where the temperature gradient is lower.

Abstract: The authors are members of the integrated project Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS), funded within the European Framework (FP6). One of the aims of IMPRESS is to develop new alloys and processes for the casting of TiAl-based turbine blades for the next generation of aero and industrial gas turbine engines. Within IMPRESS, two related issues have been identified during the primary solidification stage, namely, segregation and the columnar-to-equiaxed transition (CET). The authors have set out to isolate the effects of thermo-solutal convection, by designing a microgravity experiment to be performed on a European Space Agency platform. This experiment will investigate the CET formation during solidification. It is planned to use a sounding rocket providing a microgravity time of approximately twelve minutes. The results of this microgravity solidification experiment will be used as unique benchmark data for development and validation of new computational models of TiAl solidification. This in turn will produce accurate models and ultimately new robust industrial processes by project partners in the aerospace industry. The evolution of the design of the microgravity experiment is discussed and the results of preliminary ground reference experiments are presented. Future plans and objectives for the project are also highlighted.

Abstract: Spark-Plasma Sintering (SPS) involves rapid heating of powder by electric current with simultaneous application of external pressure. Numerous experimental investigations point to the ability of SPS to render highly-dense powder products with the potential of grain size retention. The latter ability is of significance for the consolidation of nano-powder materials where the grain growth is one of the major problems. A model for spark-plasma sintering taking into consideration various mechanisms of material transport is developed. The results of modeling agree satisfactorily with the experimental data in terms of SPS shrinkage kinetics.

Abstract: Melt-spun flakes and air atomised powder of a multi-component Al-Si-Fe-X alloy were consolidated by field assisted (FAST) or spark plasma sintering (SPS) in vacuum using steel dies and punches. Experiments were carried out at 350, 400, 450 and 500°C under applied loads ranging from 81 to 283 MPa. The resulting compacts were microstructurally and mechanically characterized. Ultimate strength values up to 1000 MPa and plastic strains up to 20% were observed during compression tests. The effect of the powder shape on the sintering behaviour is compared. The effect of process parameters such as temperature and applied load on the densification and mechanical properties is discussed. It was found that compacts sintered from melt-spun flakes resulted in a higher strength and ductility than compacts produced from air-atomized powder, sintered under identical conditions.

Abstract: Within the frame of the ESA research program SETA, “Solidification along a Eutectic Path in Ternary Alloys”, experiments have been performed focussing on several distinct subtopics. One of these subtopics is to study coupled growth along the univariant eutectic reaction: L → α + β. In this paper, the influence of the growth velocity v on the morphology of the solid/liquid interface is evaluated in a ternary Al-Cu-Si alloy with a composition close to the univariant eutectic groove L → α(Al) + θ-Αl2Cu. Different structural regions can be identified in terms of the stability of the solid-liquid interface (morphological stability) and the stability of the coupling (competitive growth) during unidirectional solidification as function of the solidification parameters. It is found that two-phase planar growth with a lamellar arrangement can be obtained at a sufficiently low growth rate v. The measured interlamellar spacing follows the Jackson and Hunt relationship λ2v = constant. At a higher growth velocity first a destabilisation of the solid/liquid interface is observed and finally competitive growth is observed revealing primary θ-Al2Cu growing ahead of the eutectic interface. It is assumed that the cellular break-up is a two-step process related to the crystallography of the system. Fitting the different morphologies into one microstructure map, an extension of the coupled zone concept as has been proposed for binary alloys is necessary.