Eutectic Front Research Articles

Effect of Solid/Liquid and Eutectic Front Velocities on Microstructure Evolution in Al-20%Cu Alloys

During the solidification process, microstructures are affected by the experimental conditions, the thermophysical characteristics of the alloy, and the type of grain-refining particles. Unidirectional solidification experiments were performed in a vertical Bridgman-type furnace to investigate the effect of the solidification front velocity on the solidified microstructure of a non-refined and refined Al-20%Cu alloy. The samples were solidified by rapidly increasing the sample velocity (v) range from 0.02 mm/s to 0.2 mm/s while maintaining an almost constant temperature gradient (~5 K/mm). As a result, despite changes in the solid/liquid front velocity along the sample, the microstructure of the non-refined alloys remained columnar. In the refined alloy, the columnar structure changed into an equiaxed structure at two different front velocities.

Structures in grain-refined directionally solidified hypoeutectic Al-Cu alloys: Benchmark experiments under microgravity on-board the International Space Station

Benchmark solidification experiments were successfully performed under microgravity conditions on-board the International Space Station (ISS) within the ESAprogramme CETSOL (Columnar-to-Equiaxed Transition in SOLidification Processing). Cylindrical samples of grain-refined Al-4wt.%Cu, Al-10wt.%Cu and Al-20wt.%Cu alloys were directionally solidified in a gradient furnace to investigate columnar and equiaxed dendritic growth structures as well as the columnar to equiaxed transition under diffusive conditions. The determination of temperature gradients; interface velocities; and cooling rates at liquidus, solidus, and eutectic front positions provides well-defined thermal experimental characterization. The evaluation of the flight samples demonstrates that no significant macrosegregation along the sample axis occurred and no radial effects were observed. Therefore, purely diffusive solidification behaviour without any residual melt convection can be assumed for these microgravity experiments. The analyses of the microstructure in longitudinal cross-sections show dendritic structures without any pore formation and the averaged eutectic fraction is largely constant along the sample. The samples of refined Al-4wt.%Cu alloy show a sharp CET from columnar dendrites to a fine equiaxed steady-state grain structure whereas in the samples of refined Al-10wt.%Cu and Al-20wt.%Cu alloy, only equiaxed dendritic grain growth is observed. A quantitative analysis of the equiaxed grain morphology shows, that the shapes of the equiaxed dendrites depend on the applied temperature gradient, but the grain sizes in radial and longitudinal directions are identical. Therefore, a fully equiaxed dendritic growth structure without dendrite elongation was obtained. Compared to experiments in microgravity with non-refined Al-Cu alloys the average equiaxed grain size is about three times smaller.

Influence of the Phase Fractions on the Formation of Eutectic Colonies: A Large‐Scale Phase‐Field Study

The growth of two‐phase eutectic colonies is a frequently observed phenomenon in the microstructure of directionally solidified ternary alloys. The formation of these macroscopic structures is driven by microscopic instabilities, caused by minor component impurities, diffusing into the liquid from the two solidifying phases. Due to an accumulation of this impurity component, a morphological instability at the eutectic front leads to the formation of eutectic colonies. In this work, the phase‐field method is used to investigate the influence of different melt composition and hence of the adjusting phase fractions on the growth of eutectic colonies. For this purpose, specially designed model systems, N‐xA‐yC, with defined phase fractions of the solids are generated on the basis of the high performance material NiAl‐34Cr. Based on these models, the evolution of eutectic colonies is investigated in two‐ and three‐dimensional (3D) large‐scale simulations with up to 3⋅109 cells. To perform these highly computationally intense large‐scale 3D simulations, the computational framework used is optimized in several layers. The results obtained show the influence of the melt composition on the formation and characteristics of the evolved eutectic colonies and provide new insights into the formation of these macroscopic structures.

Modeling Ablation Casting

Research published in open literature on ablation casting has been mainly experimental. Modeling of this innovative technology is scarce. This article describes an initial attempt in modeling ablation casting process. Material properties and boundary conditions were defined for calculating cooling curves and microstructural length scales in A356 alloy castings made using sand casting and ablation casting processes. Model predictions were compared with published data on cooling curves and solidification times in an A356 alloy casting. The validated model was then used to predict unique solidification features associated with the ablation casting process, including the eutectic solidification time, shrinkage porosity formation, eutectic front morphology, the advancing rates of the eutectic front and the associated eutectic silicon morphology. Simulation results indicate that the progressive advancing ablation cooling is capable of reducing secondary dendrite arm spacing, modifying the eutectic silicon phase owing to the water quench effect, and eliminating shrinkage porosity using much smaller risers than usual if the ablation is correctly driven towards the risers.

In-situ evidence for impurity-induced formation of eutectic colonies in an interdendritic liquid

Eutectic colony formation in near-eutectic alloys is often attributed to independent nucleation of eutectic grains on the surface of primary dendrites. In this paper, we report an in-situ study of eutectic solidification in hypoeutectic Zn–Al alloys using synchrotron X-radiography. While the α-Al/η-Zn eutectic is observed to grow from a single nucleus in the interdendritic liquid, a cellular instability at the eutectic front is observed for the first time. The cellular instability is induced by an impurity dissolved in the liquid, while a flow effect is negligible. This observation confirms an alternative mechanism of eutectic colony formation.

Effect of Zr on solidification segregation behavior of K417G alloy and its anomalous effect during rapid cooling process

Although Zr is classified as a beneficial element in superalloys, it noticeably increases the solidification segregation, worsens the as cast microstructure and lowers the melting point of the superalloys, but the detailed information about the behavior of Zr during the solidification remains unknown till now. In this paper, some evidences supporting the assumption that a film of Zr formed at the solid/liquid interface during solidification have been found, including the precipitation of ZrC, Zr2(SC) and Ni–Zr phases at the grain boundary or eutectic (γ+ γ′) front, and the concentration of Zr at the crack surface formed at the final stage of the K417G alloy solidification. The Zr-rich film at the solid/liquid interface markedly lowers the solidification rate of K417G alloy, and decreases the content of Al, Ti, Cr and Mo in the dendrite core. As a result, the precipitation of (γ+ γ′) eutectic is largely increased. Under water quenching conditions, the solidification of the final liquids contains two sequential eutectic reaction, including the (γ+ γ′) at the external part of the liquid pool and the (γ+ carbide) reaction at the central part of the liquid pool. Zr inhibits the second reaction and increases the crack formation tendency, implying that it would damage the weldability of the alloy.

Tailoring of Microstructures and Tensile Properties in the Solidification of Al-11Si(-xCu) Brazing Alloys

Ternary Al-11wt %Si-(xwt %)Cu alloys are highly recommended as commercial filler metals for aluminum brazing alloys. However, very little is known about the functional inter-relations controlling the solidified microstructures characterizing processes such as torch and furnace brazing. As such, we evaluated two commercial brazing alloys, which are the Al-11wt %Si-3.0wt %Cu and Al-11wt %Si-4.5wt %Cu alloys: Cu contents typically trend in between the suitable alloying spectrum. We analyzed the effects of solidification kinetics over features such as the dendrite arm spacing and the spacing between particles constituting the eutectic mixture. Also, tensile properties were determined as a function of the dendrite microstructure dimensions. The parameters concerned for translating the solidification kinetics were either the cooling rate, or growth velocity related to the displacement of the dendrite tip, or the eutectic front. The relevant scaling laws representing the growth of these brazing alloys are outlined. The experimental results demonstrated that a 50% increase in Cu alloying (from 3.0 to 4.5 wt %) could be operated in order to obtain significant variations in the dendritic length-scale of the microstructure across the produced parts. Overall, the microstructures were constituted by an α-Al dendritic matrix surrounded by a ternary eutectic consisting of α-Al + Al2Cu + Si. The scale measurements committed to the Al2Cu eutectic phase pointed out that the increase in Cu alloying has a critical role on refining the ternary eutectic.

Microstructural evolution and mechanical properties of Al-7Si-0.3Mg alloys with erbium additions

The mechanical properties of hypoeutectic Al-Si foundry alloys are generally controlled by their microstructures. The present study was performed to determine the influence of erbium (Er) in Al-7Si-0.3Mg alloys on the microstructures and mechanical properties. The evolution of microstructure was studied by thermal analysis and interrupted solidification technique. Optical microscopy, scanning electron microscope, and energy-dispersive X-ray were employed to characterize the alloy microstructures. The results demonstrated that the addition of Er significantly refined both the secondary dendrite arm spacing (SDAS) and the primary α-Al grain. Simultaneously, Er modified eutectic Si phase from coarse acicular plate-like structures into fine fibrous features. We observed that Er additions strongly suppress the nucleation of eutectic Si due to the preferential formation of ErP instead of AlP. Er additions change the eutectic growth mode from nucleation and growth on AlP particles to the propagation of a defined eutectic front from the mold walls. Moreover, Er addition that was cast for different holding times indicated that there was no fading effect on the SDAS, primary α-Al grain size, and eutectic Si morphology. Importantly, the tensile properties of the Er-modified alloys at as-cast and T6 conditions can be enhanced due to simultaneous refinement both the SDAS and the α-Al grains and modification of the eutectic Si.

Nucleation and Growth of Tin in Pb-Free Solder Joints

The solidification of Pb-free solder joints is overviewed with a focus on the formation of the βSn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, βSn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of ~4 K, even for small solder ball diameters down to <200 μm.

Eutectic Morphology of Al-7Si-0.3Mg Alloys with Scandium Additions

The mechanisms of Al-Si eutectic refinement due to scandium (Sc) additions have been studied in an Al-7Si-0.3Mg foundry alloy. The evolution of eutectic microstructure is studied by thermal analysis and interrupted solidification, and the distribution of Sc is studied by synchrotron micro-XRF mapping. Sc is shown to cause significant refinement of the eutectic silicon. The results show that Sc additions strongly suppress the nucleation of eutectic silicon due to the formation of ScP instead of AlP. Sc additions change the macroscopic eutectic growth mode to the propagation of a defined eutectic front from the mold walls opposite to the heat flux direction similar to past work with Na, Ca, and Y additions. It is found that Sc segregates to the eutectic aluminum and AlSi2Sc2 phases and not to eutectic silicon, suggesting that impurity-induced twinning does not operate. The results suggest that Sc refinement is mostly caused by the significantly reduced silicon nucleation frequency and the resulting increase in mean interface growth rate.

Effects of Strontium, Magnesium Addition, Temperature Gradient, and Growth Velocity on Al–Si Eutectic Growth in a Unidirectionally-solidified Al–13 wt% Si Alloy

Al–Si eutectic growth mechanism was investigated in a directionally-solidified Al–13 wt% Si alloy with different strontium (Sr) and magnesium (Mg) additions, growth velocities and temperature gradients. Macro- and micro-scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying Al–Si eutectic front and the final morphology of eutectic grains in the solidified Al–13 wt% Si alloys. By varying (tailoring) these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition (CET) of eutectic structure, which can be interpreted on the basis of Hunt's Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.

Thermal data processing to characterise quasi-unidirectional solidification of cast aluminium alloy AA 354

In this work the authors show how to build a semi-industrial scale macrothermal analysis experimental apparatus for low pressure aluminium casting AA 354 with quasi-unidirectional solidification. Several thermocouples were connected to a multichannel electronic device allowing a sampling rate up to 10 Hz; the thermocouples were installed in the mould at different locations to acquire the discontinuous cooling curves at those same locations.With this type of experiment and appropriate mathematical procedures it was possible to build a reasonable response surface T = f(x, t) and the respective derivatives: ∂T/∂t and ∂T/∂x. Exponential polynomials were applied for modelling the curves and linear interpolation to relate the several cooling curves. Mathematical tools applied to the modelled curves allowed the authors to identify different solidification events and correlate them with the specific thermal gradient, cooling conditions and solidification fronts phenomena such as columnar to equiaxed transition of α aluminium grains. Growth velocity of proeutectic aluminium α and eutectic fronts were also determined.

Solid–mush interface conditions for mushy layers

AbstractA subtle issue in the study of mushy zones which form during the solidification of binary alloys is that there are two distinct types of solid–mush interfaces which may occur. One of these is a eutectic front and the other is a front which separates the mushy layer and, assuming complete solute rejection, a layer of pure solid. For semi-infinite-domain configurations that admit similarity solutions, such as those at a uniform initial temperature and concentration with an imposed cold temperature at the lower boundary, only one of the two types appears, and the type of front is determined by the various parameters of the system. In a finite domain, it is possible for each type of front to appear at different times. Specifically, the advance of the eutectic front is restricted by the isotherm associated with the eutectic temperature, and the other front type will appear over a longer time scale. Leading up to the time when the front changes type, the concentration being frozen into the solid decreases. This process writes a history of the system into the solid.

In situ investigation of unidirectional solidification in Sn–0.7Cu and Sn–0.7Cu–0.06Ni

The dynamics of off-eutectic solidification are studied in two Pb-free solders using synchrotron X-ray radiography and post mortem scanning electron microscopy. The early stages of growth revealed the onset of coupled growth and phase competition. In Sn–0.7 wt.% Cu, the βSn–Cu 6Sn 5 eutectic then grew with Cu 6Sn 5 rods leading the βSn phase, with a range of rod spacings and local interface curvatures. However, these nonfaceted–faceted eutectic features were exhibited much more weakly than in the Al–Si and Fe–C eutectics, despite primary Cu 6Sn 5 crystals exhibiting marked faceting. In Sn–0.7 wt.% Cu–0.06 wt.% Ni, primary intermetallic laths grew ahead of a rod-like βSn–(Cu,Ni) 6Sn 5 univariant eutectic front. The univariant eutectic had a narrow freezing range and a similar spacing to the invariant eutectic at the same velocity. Composition measurements suggest that the univariant eutectic groove descends in the direction of the Sn + Cu 6Sn 5 binary eutectic point.

On the role of initial conditions in the selection of eutectic onset mechanisms in directional growth

The early-stage dynamics and onset mechanisms for eutectic solidification are investigated experimentally using slab-geometry slides of succinonitrile–(D)camphor (SCN–DC) transparent organic eutectic material. By specifically focusing separately on the pre-growth or holding period and the growth or pulling period, the critical roles of each in the establishment of initial conditions and the competition between eutectic initiation mechanisms, leading to the development of a steady-state eutectic front, are examined. It is found that a single-phase layer forms and increases in thickness monotonically with time during the holding period with a corresponding increase in the interface temperature. Because the thickness of this layer is observed to influence subsequent eutectic initiation mechanisms, it is concluded that the pre-existing structure, holding period duration, single-phase identity and thickness, and specimen slide geometry should all be reported as standard practice, along with the pulling velocity and thermal gradient, for a complete description of a gradient-zone directional solidification experiment.

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.

Effect of La and La+Sr on Structure and Properties of ZL101A

After heat treatment, the structures and properties of ZL101A alloy with different La and La+Sr contents were studied. The results show that a significant modifying effect can be achieved in ZL101A in metal mould casting. The Si phase in the alloy becomes finer and the average alloy grain size is reduced from 3 mm to 1 mm with La addition 0.6%, which is mainly attributed to the enrichment of La at the eutectic front. The tensile strength and elongation increased with the increasing of La addition. However, excessive amount of La will have negative effects on the tensile strength and elongation. Mixed 0.2% La and 0.015% Sr results in further refined average grain down to 0.5mm, exhibiting improved strength and elongation properties.

X-Ray Video Microscopy Studies of Irregular Eutectic Solidification Microstructures in Al–Si–Cu Alloys

In-situ studies of Al-Si eutectic growth has been carried out for the first time by X-ray video microscopy during directional solidification of Al-Si-Cu alloys with and without Sr-addtions. The unmodified eutectics showed distinctive non-isothermal growth dynamics, where Si-crystals attained needle-like tip morphologies and progressed under significantly higher undercooling than Al, leading to formation of an irregular eutectic with Si as the leading phase and subsequent nucleation of Al on the Si-surfaces. In the Sr-modified alloys, the eutectic reaction was found to be strongly suppressed, occurring with low nucleation frequencies at undercoolings in the range 10-18 K. In the Cu-enriched melt, the eutectic front was found to attain meso-scale interface perturbations, sometimes evolving into equiaxed cellular rosettes in order to accommodate to the long-range redistribution of Cu from the composite eutectic interface. The eutectic front also attained shortrange microscale interface perturbations consistent with characteristics of a fibrous Si growth. However, further improvements in spatial resolution are required in order to study the microscale structure formation in greater detail. Evidence was found in support of Si-nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form on the Si-particles. To what extent nucleation mechanisms in the Cu-enriched systems are transferable to binary or commercial Al-Si alloys remains uncertain. © 2010 ISIJ.

Microstructural selection in conventionally and rapidly solidified Sm–Co–T alloys

Abstract Microstructural selection maps under conventional solidification conditions have been determined in the vicinity of the binary eutectic point for Sm–Co–T alloys, with T = Nb, Mo, Ag, Zr, Ti, Hf and V up to 3 at.%. All additions except V were observed to disrupt the eutectic solidification front and lead to primary Co or Sm 2 Co 17 formation from the melt. An alloy with a composition of (Sm 0.09 Co 0.91 ) 97 V 3 was observed to have a eutectic structure. The alloying additions of Nb, Mo, Ag and Zr resulted in an expansion of primary Co formation, ultimately leading to deleterious effects on the magnetic properties. The addition of Ti and Hf extended the region over which primary Sm 2 Co 17 formed, potentially allowing the production of permanent magnet materials with higher Co content. Subsequent analysis of rapidly solidified Sm–Co–V alloys resulted in composition-magnetic property maps. Excellent magnetic properties were observed in the as-solidified state for (Sm 0.09 Co 0.91 ) 97 V 3 . The coercivity was determined to be 5 kOe and the energy product was 4.3 MGOe, which decreased as both the Sm/Co ratio and V content were changed. The melt-spun microstructure consisted of primary Sm 2 Co 17 and interdendritic Co more indicative of peritectic solidification, and the morphology of the phases changed as the Sm/Co ratio changed. The different conventionally and rapidly solidified microstructures suggest alteration of the equilibrium phase relationships due to high undercoolings.

Fabrication of Lotus-Type Porous Al-Si Alloys Using the Continuous Casting Technique

Lotus-type porous Al-Si (4, 8, 12, 14, and 18 wt pct) alloys were fabricated using the continuous casting technique under a hydrogen gas pressure of 0.1 MPa at various transference velocities, and the effects of the silicon content level and transference velocity on the pore morphology and porosity were investigated. Both the porosity and the average pore diameter increase as the silicon content level increases and decrease as the transference velocity increases. In particular, the velocity dependence is obviously exhibited at a silicon content level higher than 12 wt pct. The pore shape is changed from irregular in the higher-dendrite fraction to nearly circular in the lower-dendrite fraction. The porosity and the pore morphology are influenced by the silicon content level and transference velocity. In the model, these results can be understood with the explanation that the pores, which contribute to the increase in porosity, are generated at the eutectic fronts. This indicated that the porosity and the pore size in lotus-type porous Al-Si alloys can be well controlled by varying the silicon content level and the transference velocity.