Materials Science Forum Vols. 790-791

Abstract: In this study, a near peritectic transparent metal-like solidifying NPG-TRIS alloy was directionally solidified in a thin sample under process conditions which favor cellular growth. Dif-ferent to an identical horizontally processed sample, the vertically processed sample revealed a cel-lular array which was curved. This curving was attributed to a convection-induced higher alloy con-centration at the sample edges compared to the sample center. Surprisingly, it was found that the curved cellular array is inherently unstable as three times dendrite-like structures shot forwards. As origin of these rapid advancements two possibilities were discussed. Either a convection-induced variation of the solute boundary layer might be responsible. Or it can be explained by a more gener-ic approach which due to the curved form of the cellular array considers extended freedom for cel-lular branching in combination with a relative large deviation of the preferential crystal growth di-rection from the heat flow direction. For the discussed phenomena the presence of the peritectic phase within the intercellular spacing was found to be not of importance.

Abstract: Temperature Gradient Zone Melting (TGZM) occurs when a liquidsolid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. TGZM has a major influence on the preparation of the initial solid-liquid interface during the stabilization phase following the directional melting of an alloy and is at the origin of the diffusion of solute towards the top part of the mushy zone. TGZM is also causing the migration up the temperature gradient of dendrite secondary arms during directional solidification, which can have a significant impact on the micro-segregation pattern of the final microstructure. In this communication we report on a directional solidification experiment carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) on Al4.0 wt.% Cu alloy to study the dynamics induced by the TGZM phenomenon on an equiaxed grain that nucleated in front of a columnar structure. Based on in situ experimental observations obtained by synchrotron X-ray radiography, the dissolution of the bottom part of the equiaxed grain is characterized and measurements are compared with predictions of the TGZM theory in diffusive regime.

Abstract: Small angle grain boundaries have been grown in a small Bridgman furnace, using seeded growth method, at three different pulling rates i.e. 3 μm/s, 13 μm/s and 40 μm/s. In order to assess segregation mechanisms of impurities towards the central grain boundary, melt has been polluted by 50ppma of either copper or indium. Secondary ion mass spectrometry (SIMS) local analyses have been performed to investigate the impact of solid state diffusion and limited rejection of solute at the grain boundary for each growth rate. The results are discussed in connection with an atomistic model built on Vienna Ab-initio Simulation Package (VASP).

Abstract: Elementary Mn has a great importance as neutralizer of Fe intermetallics like β-Al₅FeSi, which have detrimental effect on mechanical characteristics of AlSi alloys. Presence of Mn in AlSi alloys causes the formation of other intermetallic phases. To understand the effect of solidification conditions and fluid flow on the microstructure of AlSi-based alloys and the addition of Mn leading to Mn-based intermetallics, Al-5 wt pct Si 0.2/0.4/1.0 wt pct Mn alloys have been directionally solidified under defined thermal (gradient 3 K/mm, solidification velocity 0.02-0.12 mm/s) and fluid flow (rotating magnetic field 6 mT) conditions. The primary Al-phase and Mn-based intermetallic phases were studied using 3D X-ray tomography. The spatial morphology of primary phase and intermetallics were characterized with respect to different fluid flow and solidification conditions. The tomography has showed the 3D complicated structure of Mn phases developed.

Abstract: Eutectic alloys from the ternary system Al-Cu-Ag are excellent model alloys for the investigation of coupled eutectic growth, not only because most materials properties are well known but also because the system offers a rich variety of crystal structures and crystal orientation relationships (ORs) being associated to distinct minima of the solid-solid interface energy. This paper describes three research topics specifically related to bulk lamellar Al-Al₂Cu eutectics, e.g. the maze-to-lamellar transition during early growth, the role of fault lines during lamellar spacing selection close to the pinch-off limit and the onset of eutectic cell formation above the constitutional supercooling limit. These topics are central to the microgravity experiments SETA presently being prepared for the MSL / SQF.

Abstract: A melt encasement (fluxing) method has been used to undercool Ag-Cu alloy at its eutectic composition. The recalescence of the undercooled alloy has been filmed at high frame rate. At low undercooling lamellar eutectic is observed to grow, giving way to a mixed anomalous-lamellar structure at higher undercooling. In situ observation of the spot brightness reveals, as expected, that the lamellar eutectic grows via a planar front mechanism, while the anomalous eutectic grows via a more complex process characterised by a double recalescence. The first recalescence is non-space-filling (dendritic) in character and is followed shortly afterwards by a second recalescence which appears to be of the planar front type. Moreover, the first recalescence event appears to be to a temperature in excess of the equilibrium eutectic temperature. This is strongly suggestive that the anomalous eutectic morphology arises due to the growth and subsequent partial remelting of a dendritic morphology, probably a two-phase (eutectic) dendrite, followed by planar front growth of a lamellar eutectic into the residual liquid.

Abstract: Microstructure evolution of a 3003 sheet cladded with 4004 brazing alloy is investigated during slow heating (1K/min) under secondary vacuum up to isothermal brazing temperature (590°C). Optical and scanning microscopies, EDS chemical analysis and EBSD orientation mapping are used. Experimental results are discussed in the light of thermodynamic calculations using Thermo-Calc. Comparisons show good agreement as long as Mg vaporization does not take place.

Abstract: The sessile drop technique was used to evaluate the contact angle between liquid eutectic Fe-11wt%V-2.3wt%C alloy and VC substrate. This alloy solidifies through a eutectic reaction that starts at 1320°C and leads to the formation of the faceted/non-faceted austenite-VC eutectic. The effect of sulfur on the contact angle and on morphology of the austenite-VC eutectic in both non-directionally and unidirectionally solidified specimens was investigated. It was shown that sulfur increases the contact angle and modifies the eutectic morphology, thus indicating a relationship between eutectic morphology and liquid eutectic/faceted phase VC interfacial energy.

Abstract: The modification level of Al-Si alloys is generally evaluated by the depression of the (Al)-Si eutectic temperature which can be recorded by thermal analysis. However, this method requires a reference temperature which should be the eutectic temperature evaluated on the relevant phase diagram. Various methods proposed to account for the effect of low level alloying elements on this reference temperature are reviewed and emphasis is put on the so-called Mondolfo's equation which is updated. Predictions are compared to experimental information from literature.

Abstract: In solidification processes the fluid flow occurs almost at every scale from the bulk, near the interfaces and deeply in the mushy zone. Numerical modeling is a valuable tool for understanding and master the solidification processes, however, macro-scale models are not always able to predict in detail the random behavior of the solidification process whereas models for micro scales are not capable to take into account a complex structure of flows which enter into the mushy zone. In the present paper the variety of the flows and imprints they left on solidification structure are discussed and illustrated with experimental data which naturally comprise every flow occurring in the process.