Abstract: The physical sciences research activities implemented by ESA in the framework of the European Life and Physical sciences in Space (ELIPS) programme build up on more than two decades of scientific progress and experience with developing and operating instruments on various space carriers. Research projects are mostly defined and developed in the context of International Topical Teams supported by ESA in coordination with other international partner agencies. Formal project proposals are eventually submitted to regular Announcements of Opportunity, reviewed by international independent peers and their technical feasibility is assessed by technical experts of the facilities considered for the implementation of the space experiments. For most projects, experiments and numerical modelling go hand in hand. As concerning Materials Sciences and the realisation of solidification experiments several platforms and facilities are utilised. This spans from centrifuges to - with increasing experiment duration - the Drop Tower in Bremen, the A-300 aircraft flying parabolic trajectories, sounding rockets and eventually the International Space Station (ISS). Several solidification instruments from typical Bridgman type furnaces to electromagnetic levitation and heating of spherical samples were developed, some of which are already operational in orbit. A dedicated X-ray set-up has also recently been operated on a sounding rocket to enable in-situ observations of the solidification of a metallic alloy. The paper will provide an overview of solidification projects, their objectives and corresponding instruments available for experimentation. It will lay out the objectives and strategy of ESA’s research programme, including the consolidating the international cooperation that prevails in Europe and in the ISS project.
Abstract: This paper gives an overview of the experiments on-board the International Space Station (ISS) performed so far by the CETSOL team. Al-7 wt% Si alloys with and without grain refiners were solidified in microgravity. Detailed grain structure analysis showed columnar growth in case of non-refined alloy, but the existence of a columnar to equiaxed transition (CET) in refined alloy. One main result is a sharp CET when increasing the solidification velocity and a progressive CET for lowering the temperature gradient. Applying a front tracking model this behavior was confirmed numerically for sharp CET. Using a CAFE model both segregation and grain structures were numerically modeled and show a fair agreement with the experimental findings.
Abstract: Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ (Ni31Si12) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β1-Ni3Si and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-Ni3Si would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase Ni25Si9 and β1-Ni3Si is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β2/β3) being suppressed from the melt.
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: At present, our understanding of the interaction between melt flow and solidification patterns is still incomplete. In columnar dendritic growth buoyancy driven flow may alter the dendrite tip and spacing selection and consequently the microsegregation of alloying elements. With the aim of supporting directional solidification experiments under hyper-gravity using a large diameter centrifuge (LDC), phase field simulations of β (Ti) dendrite growth have been performed under various gravity conditions for the binary alloy Ti-45at.%Al. The results show that Al segregation at the growth front causes convection rolls around the dendrite tips. The direction of the gravity vector is an essential parameter. When g is opposite to the direction of dendrite growth, increasing gravity leads to a marked decrease of the primary dendrite spacing and to a decrease of the mushy zone length. When g is aligned parallel to the direction of dendrite growth, the primary dendrite spacing and mushy zone length are almost unchanged, however the secondary dendrite arms grow more prominently as the magnitude of g increases.
Abstract: This paper provides an analysis of the formation of intermetallic phases in AlSi7Fe1 alloy in samples processed onboard the ISS. Based on axial 2D cross-sections obtained from regions of pure diffusive growth and also solidified with forced melt flow, the sizes and distribution of intermetallic β-Al5FeSi phases were determined for different solidification velocities. In diffusive case the phases are larger and more homogeneously distributed than in case of induced melt flow. Additionally, especially for lower solidification velocity, the enrichment of Si and Fe in the centre part of the sample results in a few but rather large β-Al5FeSi particles.
Abstract: Recently several experiments on directional solidification of Al-6.5wt.Si-0.93wt.%Fe (AlSi7Fe1) alloy were performed under terrestrial conditions and onboard the International Space Station (ISS) in the Materials Science Lab (MSL) with use of electromagnetic stirring and without it. Analysis of the samples showed that stirring with a rotating magnetic field lead to the accumulation of iron-rich intermetallics in the center of the sample and influenced the primary dendrite spacing while the secondary dendrite arm spacing were not affected. In the present paper the accumulation of the intermetallics b-Al5SiFe in the center of the samples due to RMF stirring is demonstrated numerically and the evolution of primary and secondary dendrite arm spacing is discussed.
Abstract: During solidification of metallic alloys, thermosolutal natural convection plays a significant role in grain nucleation, subsequent growth and morphology, as well as the formation of casting defects. In this work, an Al-5wt%Ti-1wt%B inoculated Al-20wt%Cu alloy was solidified, near-isothermally, using a Bridgman-type gradient furnace, while being monitored in real-time via in-situ X-radiography as part of a parabolic flight microgravity campaign. Each parabola consisted of a transition through 24 seconds of hypergravity (1.8 g), followed by 22 seconds of microgravity, and a then a further 24 seconds of hypergravity. Solidification was controlled such that nucleation occurred coincident with the onset of microgravity. This allowed for the effects of microgravity on equiaxed nucleation and initial growth, followed by continuing solidification in hypergravity, to be observed, as well as the effect on the semi-coherent grain structure when transitioning between the two.
Abstract: The study proves that by introducing the iron powder to low-sulphur cast iron still before the inoculation carried out with a conventional graphitising inoculant, the mechanical properties similar to those obtained during the inoculation treatment carried out on cast iron with the recommended high sulphur content are achieved. The said operation increases the number of crystallisation nuclei for of the primary austenite dendrites. In this case, the iron particles act as substrates for the nucleation of primary austenite due to a similar crystallographic behaviour of the regular face centered cubic lattice The more numerous are the dendrites of primary austenite, the less free space is available in the interdendritic spaces for the formation of graphite eutectic grains, which makes the mechanical properties higher.