Eutectic Nucleation Research Articles

Effect of Vanadium Addition on Solidification Microstructure and Mechanical Properties of Al-4Ni Alloy.

The effects of vanadium addition on the solidification microstructure and mechanical properties of Al-4Ni alloy were investigated via thermodynamic computation, thermal analysis, microstructural observations, and mechanical properties testing. The results show that the nucleation temperature of primary α-Al increased with increased vanadium addition. A transition from columnar to equiaxed growth took place when adding vanadium to Al-4Ni alloys, and the average grain size of primary α-Al was reduced from 1105 μm to 252 μm. When the vanadium addition was 0.2 wt%, the eutectic nucleation temperature increased from 636.2 °C for the Al-4Ni alloy to 640.5 °C, and the eutectic solidification time decreased from 310 s to 282 s. The average diameter of the eutectic Al3Ni phases in the Al-4Ni-0.2V alloy reduced to 0.14 μm from 0.26 μm for the Al-4Ni alloy. As the vanadium additions exceeded 0.2 wt%, the eutectic nucleation temperature had no obvious change and the eutectic solidification time increased. The eutectic Al3Ni phases began to coarsen, and the number of lamellar eutectic boundaries increased. The mechanical properties of Al-4Ni alloys gradually increased with vanadium addition (0-0.4 wt%). The Al-4Ni-0.4V alloy obtained the maximum tensile strength and elongation values, which were 136.4 MPa and 23.5%, respectively. As the vanadium addition exceeded 0.4 wt%, the strength and elongation decreased, while the hardness continued to increase. Fracture in the Al-4Ni-0.4V alloy exhibited ductile fracture, while fracture in the Al-4Ni-0.6V alloy was composed of dimples, tear edges, and cleavage planes, demonstrating mixed ductile-brittle fracture. The cleavage planes were caused by the primary Al10V and coarse Al3Ni phases at the boundary of eutectic cells.

Solidification behavior and microstructure of Ag–Cu eutectic alloy at different sub-rapid cooling rates

The microstructures of rapidly solidified Ag-39.9 at. % Cu eutectic alloys have been widely studied, while the research between microstructures and sub-rapid cooling rates is still lacking. Ag-39.9 at. % Cu eutectic alloy was solidified in quartz crucibles under different conditions. The alloy that was melted and in-situ cooled in a quartz crucible to room temperature solidified into eutectic dendrites composed of regular lamellae. The alloy that was melted and injection cast into a quartz crucible solidified into equiaxed eutectic colonies with primary α-Ag phase embedded. If the quartz crucible was further immersed in a gallium-indium liquid, a new spherical eutectic colony structure was found, with anomalous eutectic or primary α-Ag phase at the central part and a “ring” of coarse lamellar eutectic structure separating the eutectic colony into two parts. It is demonstrated that the cooling rate influences not only the primary phase selection and eutectic lamellar spacing, but also the eutectic nucleation and growth mode.

Ultrasounds induced eutectic structure transition and associated mechanical property enhancement of FeCoCrNi2.1Al high entropy alloy

One dimensional (1D) and three dimensional (3D) ultrasound sources were applied to modulate the solidification process of FeCoCrNi2.1Al high entropy alloy whose liquidus temperature is up to 1614 K with the acoustic signal in liquid alloy in-situ measured. It was found that 1D ultrasound significantly refined the lamellar eutectic structure by decreasing both the eutectic cell width and the aligned α(B2)/γ(FCC) phase spacing. If ultrasound amplitude and dimension were increased, lamellar eutectic gradually transformed into anomalous eutectic without synergetic orientation. Meanwhile, a metastable σ phase enriched with Cr precipitated from γ phase in anomalous eutectic under 3D ultrasounds. Based on the combined analysis of eutectic structural evolution and sound field characteristics, the ultrasonic solidification mechanism was further clarified. The promoted dependent nucleation by stable cavitation and the fast growth rate caused by acoustic streaming were the main reasons for the significantly refined eutectic structure under 1D ultrasound. The induced independent eutectic nucleation and growth by transient cavitation, together with the enhanced solute and thermal diffusion by the intensive acoustic streaming under 3D ultrasounds were responsible for the formation of anomalous eutectic and the precipitation of metastable σ phase. The maximum ultimate strength and total elongation under ultrasound were simultaneously increased by 28% and 93%, showing comprehensive mechanical performance superior to those prepared by conventional approaches. The prominent strain hardening in γ phase and increase in α/γ interfaces induced by refining effect were the strengthening mechanisms, and the improvement of ductility was attributed to the intensified strain hardening in γ phase.

Effect of microsegregation behaviors on solidification microstructure of IC10 superalloy fabricated by directed energy deposition

Due to the solidification process and thermal history, the microstructure and properties along the deposition direction are generally inhomogeneous in components fabricated by the directed energy deposition technique. By developing a microsegregation model, this study gives an innovative understanding of the elemental segregation behavior of an as-deposited IC10 superalloy along the different heights and its influence on the microstructure. Results indicate that the γ-γ ´ eutectic-free microstructure is only obtained at the initial deposition layer, and as the deposition height increases, eutectic phases gradually appear in interdendritic regions. By taking account of the nonequilibrium solidification and dendritic tip undercooling, the new microsegregation model can well explain the segregation behavior of the alloy at different deposition heights. Compared to the back diffusion coefficient, the partition coefficient plays a major role in microsegregation behavior under rapid solidification conditions, especially for the elements with a high diffusion coefficient in solid, such as Ti and Al. Moreover, microstructure transformation is dependent on varying solidification processes due to the change in microsegregation. Limited by the heat transfer efficiency as the increased deposition heights, the temperature gradient and solidification rate decrease, which exacerbates the enrichment of Al and Ti at interdendritic regions and promotes the eutectic phase nucleation. Restricted by the diffusion rate of alloying elements, the eutectic reaction rate slows down, γ and γ ´ lamellae thickness increase, and finally the petal-like eutectic phases form.

On the role of Sc in powders and spray deposits of hypoeutectic Al-Cu

ABSTRACT Scandium (Sc) is an expensive alloying element which has an important effect on the properties of aluminum alloys. It is of interest to determine for a variety of solidification conditions the effect of minor addition, in the hypoeutectic levels, of Sc on Al alloys. This study builds on past work on rapid and slow solidification of Al-4.5Cu-0.4Sc (wt%) by comparing previous microstructures to those obtained using spray forming of strip. The effect of Sc (0.4 wt%) on the scale of the microstructure of as-atomised powders and as-deposited strips (SD) is found to be negligible in the investigated alloys. The eutectic undercooling of the Al-4.5Cu-0.4Sc (wt%) alloy is the same for both powders and SD. This is attributed to eutectic nucleation and growth following the same path in both solidification processes, characterised by an extended solubility of Sc in the primary α-Al matrix, as evidenced by the resulting Sc-rich precipitates and inherent age hardening upon annealing of the as-solidified samples. This work contributes to building an understanding of rapidly cooled structures and their subsequent self-aging in advanced manufacturing processes.

A Critical Conception of Hot-Tearing Susceptibility: How Controlled Diffusion Solidification Enables Shape-Casting with Wrought Aluminum Alloys

Instrumented Constrained Rod Casting (CRC) method and the Controlled Diffusion Solidification (CDS) process were used to study the high susceptibility of the aluminum alloy AA 7068 to hot-tearing. A precise analysis of the measured hot-tearing curves is provided considering the casting design and different feeding mechanisms. Scanning Electron Microscopy was used to study the hot-tear surfaces. It was found that the solidification of the eutectic liquid at the film stage and the phenomenon of solid-feeding strongly affects the hot-tear formation. The early formation of an absorbed eutectic layer on the primary phase due to low rate of back-diffusion is proposed to explain the loss of ductility at the film stage. The layer can serve as an efficient substrate for the eutectic nucleation and growth, hence its formation can advance the eutectic solidification. The isolation of liquid eutectic by its partial solidification results in development of solid-feeding stresses breaking the solid eutectic and provide the required tearing initiators. By increasing the back-diffusion rate, the CDS process avoids formation of the adsorbed solid layer which postpones eutectic solidification and thereby mitigating the hot-tearing susceptibility.

Eutectic Nucleation in 7xxx Series Aluminum Alloys from a Non-classical Viewpoint

The early stages of eutectic solidification in a copper-containing 7xxx series aluminum alloy (AA 7068 or AMS 4331) were studied using the two-thermocouple computer-aided thermal analysis (CATA) technique. A feature was detected on the cooling rate curve at the equilibrium solidus temperature of the alloy which persists until the peak of the subsequent final eutectic solidification. Detailed analysis of the temperature difference between the wall and the center of the thermal analysis sample, together with examination of the eutectic solidified on the walls of porosities and a study of the eutectic nucleation on the basis of the non-classical theory of adsorption heterogeneous nucleation, indicated how the feature can be related to the faceting of the atomic structure of the solid/liquid (S/L) interface. The solidification of the remnant liquid after the faceting transition at the equilibrium solidus point depends on the interfacial undercooling and proceeds via either primary phase re-nucleation or secondary phase nucleation by adsorption. The eutectic solidification is affected by the presence of the primary phase which acts like an adsorbent.

Simultaneous Refinement of Primary Si and Modification of Eutectic Si in A390 Alloy Assisting by Sr-Modifier and Serpentine Pouring Channel Process.

In this study, A390 alloy was prepared using the combined process of a water-cooled copper serpentine pouring channel (SPC) and strontium (Sr) modifier, in order to simultaneously refine primary silicon (Si) and modify eutectic silicon (Si). The nucleation and growth mechanisms of the Si phase were discussed by morphology analysis and non-isothermal analytical kinetics. The results indicate that the size of primary Si is refined to 25.2–28.5 µm and the morphology of eutectic Si is modified from acicular into fibrous. The serpentine pouring channel process stimulates primary Si nucleation due to chilling effect and has no influence on eutectic Si nucleation. Impacts of Sr-modifier on primary and eutectic Si are similar, including three aspects: (1) poisoning of the nucleation site; (2) decreasing the interface energy between Si phase and liquid; (3) raising the activation energy for diffusion across solid-liquid interface. The content of Sr determines which one of the three aspects mentioned above is the dominant factor to promote or restrain the nucleation and growth of the primary and eutectic Si in hypereutectic Al-Si alloy.

Co-growing mechanism of γ/γ′ eutectic on MC-type carbide in Ni-based single crystal superalloys

To clarify and comprehend the co-growing mechanism of γ/γ′ eutectic on MC-type carbide in a single crystal CM247LC superalloy, the interface and the elements' segregation between MC-type carbide and γ/γ′ eutectic was investigated in this paper. The results show that a semi-coherent boundary was observed having an orientation relationship of {1¯1¯1}MC‖{1¯1¯1}γ/γ′ with <011>MC || <011>γ/γ′. This interface structure resulted in a high interface energy, and a reduced preference of γ/γ′ eutectic's nucleation on MC-type carbide. However, this unfavorable interfacial feature was offset by the elements' segregation profile during the MC-type carbide's solidification, which is characterized by Al, Ni, Co and Cr enrichment and a co-growing mode.

The early nucleation of a eutectic alloy during high-energy milling

The nucleation of a eutectic alloy –at an early stage– and the factors that determine whether rod or hexagonal growth occurs were evaluated as a function of the process control agent (PCA)—or the lack of it. The study is subdivided into three separated experiments; in all cases, the samples (elemental powders of Pb and Sn) were processed by high-energy milling under composition, pressure and temperature (c-P-T) vial conditions. In the first case, it was detected an excessive mechanical kneading when the powders were processed without PCA and a monolith –as long as 5 mm in length– was obtained. However, this route should not be underestimated because the presence of α−, β− and eutectic-phases –at the surface of the monolith– were traced as nanoparticles (<100 nm). In the second and third cases, to prevent the excessive mechanical kneading and interparticle bond of the powders during milling, two different mechanochemical routes with PCA were considered: (i) tributyl phosphate (TBP) and (ii) ethanol (ETOH). Both of them were proposed to modify the viscoelastic behaviour of the powders. Nanorods resulted when TBP was used. Although structural disorder –at nanometric scale– was not traced, other effects associated with TBP like organic debris and the presence of by-products were detected. Based on the results with the ETOH option, it was shown that the precursors store energy as structural defects during milling. This suggests that the early eutectic nucleation (hexagonal crystal structure) occurred due to structural defects, crystals interfaces and phase boundaries. These acted as preferential sites for nucleation. Moreover, a typical eutectic phase distribution was detected. Finally, it was demonstrated that even after prolonged milling time and nonetheless the kind of PCA –or the lack of it, only the eutectic formation was scarcely traced; that is to say, a partial eutectic formation took place.

Influence of Sr addition on microstructure of the hypereutectic Zn\u2013Al\u2013Cu alloy

The purpose of the presented work is to answer the questions: how does the addition of strontium to the Zn–8Al–1Cu alloy crystallisation kinetics influence? What will the significance of the low temperature of heat treatment, characteristic for eutectic Zn–Al alloys, have for the formation of intermetallic phases? To describe the phenomena that occur in the metal during solidification under various conditions caused by the variable cooling rate, and as a result of the application of modifiers that provide the basis for heterogeneous nucleation, it was decided to use thermal-derivative analysis method. The mentioned method allows to accurately describe and interpret the kinetics of crystallisation of the tested materials. The effectiveness of the modification was assessed based on a degree of fragmentation of the structural components and changes in the properties of the material being modified, as well as by analysis of the cooling curves of the tested alloys. The thermal events occurring during solidification process, i.e. liquidus (TL) and solidus (TSol) temperatures and the beginning of phases and eutectic nucleation, were determined with the thermal-derivative analysis and calculation of total heat of crystallisation of the analysed material.

The formation of bulk β-Al3Ni phase in eutectic Al-5.69wt%Ni alloy solidified under high pressure

The microstructure and formation thermodynamic of bulk β-Al3Ni phase in eutectic Al-5.69wt%Ni alloy synthesized under different pressures were systematically investigated. The results demonstrate that α, bulk β-Al3Ni, and eutectic phase exist simultaneously after solidified under 1 GPa and 2 GPa. A mathematical model was proposed to describe the interfacial solidification velocity of dilute solution alloy under high pressure, with combination of the growth model of BCT and TMK, it reveals that the β-Al3Ni phase takes precedence over the eutectic phase nucleation when the solidification pressure is greater than 0.66 GPa.

Coupling eutectic nucleation mechanism investigated by phase field crystal model

Eutectic solidification has been investigated for decades; however, our understanding of the mechanism of eutectic nucleation remains very limited. In this work, we investigated the regular eutectic solidification process by an atomistic simulation method, and a new eutectic nucleation scenario is proposed. We found that, near the eutectic composition, the nucleation of a metastable triangle phase is more energy favorable for diffusion-controlled binary eutectic systems, and the solidification often passes through a transient state. With the proceeding of solidification, alternated A-rich and B-rich domains will be generated at the interface between the transient phase and liquid phase by pseudospinodal decomposition. When the compositions of alternated domains are close to the respective equilibrium values, eutectic phases will be formed inside the domains simultaneously, which further develops into coupling eutectic colonies. Furthermore, the construction of solute interdiffusion field and the formation of interface curvature are explained self-consistently with our eutectic nucleation mechanism.

Effects of cooling rate and magnetic field on solidification characteristics of Au80Sn20 eutectic solder

The fragility of Au80Sn20 eutectic solder is the major bottleneck of its application, which origins from the coarse primary ζ′-phase and the intrinsic brittleness of irregularly eutectic structure. To tackle this problem, various solidification processes have been adopted in this work, and the influence of those processes on phase composition and eutectic microstructure is studied. The results show that simply increasing the cooling rate does not prevent the formation of detrimental ζ′-Au5Sn phases, although the resulting eutectic morphology changes from dendritic to isometric. Applying magnetic field into rapid solidification can effectively remove the primary ζ′-Au5Sn phases, which is attributed to the fission of primary crystal nucleus during the fast eutectic nucleation course. The alloys through magnetic-field-induced rapid cooling exhibit a superior room-temperature compressive property, and the fracture stress and strain reach up to 1456.7 MPa and 64% respectively. Our finding on the effect of cooling rate and magnetic field on solidification not only sheds light on future research direction but also provides practical guidance to the engineers in solder industry.

Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel

Rapid solidification of D2 tool steel is investigated experimentally using electromagnetic levitation (EML) under terrestrial and reduced gravity conditions and impulse atomization (IA), a drop tube type of apparatus. IA produces powders 300 to 1400 μm in size. This allows the investigation of a large range of cooling rates (~100 to 10,000 K/s) with a single experiment. On the other hand, EML allows direct measurements of the thermal history, including primary and eutectic nucleation undercoolings, for samples ~6 to 7 mm in diameter. The final microstructures at room temperature consist of retained supersaturated austenite surrounded by eutectic of austenite and M7C3 carbides. Rapid solidification effectively suppresses the formation of ferrite in IA, while a small amount of ferrite is detected in EML samples. High primary phase undercoolings and high cooling rates tend to refine the microstructure, which results in a better dispersion of the eutectic carbides. Evaluation of the cell spacing in EML and IA samples shows that the scale of the final microstructure is mainly governed by coarsening. Electron backscattered diffraction (EBSD) analysis of IA samples reveals that IA powders are polycrystalline, regardless of the solidification conditions. EBSD on EML samples reveals strong differences between the microstructure of droplets solidified on the ground and in microgravity conditions. While the former ones are polycrystalline with many different grains, the EML sample solidified in microgravity shows a strong texture with few much larger grains having twinning relationships. This indicates that fluid flow has a strong influence on grain refinement in this system.

Effect of Sr on the Microstructure of Electromagnetically Stirred Semi-solid Hypoeutectic Al–Si Alloys

The present study investigated the effects of Sr addition on the microstructure of semi-solid Al A356 alloy during electromagnetic stirring. Different amounts of Sr (0–350 ppm) were added to the alloy, and the effect of Sr on eutectic nucleation and growth temperature of the eutectic silicon phase was determined. The results indicate that the addition of the Sr up to 90 ppm increased the solidification range (T L–T S) of the alloy and the further addition of Sr decreased the value of T L–T S. Image analysis revealed that increasing of Sr up to 90 ppm increased the globularity (roundness) of primary aluminum particles. However, the further addition of the Sr reduced the roundness of particles. Also, it was found that the addition of Sr in all amounts increased the area percentage of primary aluminum particles compared to the non-modified alloy. The mechanisms for changes in T L–T S, roundness and area percentage of primary aluminum phase are also discussed.

Enhanced nucleation and refinement of eutectic Si by high number-density nano-particles in Al–10Si–0.5Sb alloys

The mechanism of refinement of eutectic Si with Sb additions in an Al–10wt.% Si alloy is studied. The increased holding time designates a transition of the eutectic nucleation temperature from a decreased value to an increased one, accompanied by a morphological transformation of eutectic Si from lamellar to a shorter, rod-like shape. By ab initio molecular dynamics simulation, the chemical short-range order of −0.002 demonstrates weak affinity in the Al–Sb pair, which does not favor the existence of AlSb clusters in the melts. Sb–Sb segregation and the Sb–Si repulsive force are revealed, which promote the formation of Al-enriched precursors and the accumulation of Si atoms adjacent to precursors, inducing precursor nucleation. Continuous attachment of precursors and individual Si atoms during Si growth results in the high number-density nano-particles and suppresses the formation of twin grains within eutectic Si.

Modification of Eutectic Si in Hypoeutectic Al-Si Alloys with Erbium Addition

Effect of erbium (Er) on the eutectic Si morphologies in hypoeutectic Al-Si based alloys was investigated using thermal analysis and microstructure examination. The microstructural observations show that the addition of Er causes significant modification of the eutectic silicon morphology from a coarse plate-like to a fine fibrous one. Furthermore, the results of thermal analysis reveal that the addition of Er decreased the temperatures of eutectic nucleation and growth, and increased the eutectic undercooling. The eutectic undercooling caused by the presence of Er plays an important role in the modification of eutectic silicon.

Quantification of Primary Dendritic and Secondary Eutectic Nucleation Undercoolings in Rapidly Solidified Hypo-Eutectic Al-Cu Droplets

This paper reports on the quantification of primary dendritic and secondary eutectic nucleation undercoolings during rapid solidification of impulse atomized hypo-eutectic Al-Cu droplets. The procedure consists in determining the eutectic fraction of each investigated droplet from the fraction of intermetallic Al2Cu obtained by Rietveld refinement analysis of neutrons scattering data. The corresponding eutectic nucleation undercooling is then deduced from the metastable phase diagram of the alloy. The primary dendritic nucleation undercooling is subsequently determined using semi-empirical coarsening models of secondary dendrite arms. The two nucleation undercoolings are finally used as input variables to run a microsegregation model for binary alloys. The fractions of eutectic computed by the microsegregation model compare very favorably with the experimental results.

Role of cerium, lanthanum, and strontium additions in an Al – Si – Mg (A356) alloy

Abstract The effects of individual and combined additions of cerium (Ce), lanthanum (La), and strontium (Sr) on the eutectic modification and solidification characteristics of an Al – Si – Mg (A356) aluminum alloy were investigated using optical microscopy and thermal analysis techniques. Addition of Ce, La, and Sr resulted in different depression levels of the eutectic nucleation temperature and eutectic growth undercooling, generating modified eutectic structures exhibiting different levels of modification. Microstructural results showed that the best modification levels using individual additions were achieved by Sr which produced a fine fibrous eutectic structure, followed by La, which produced a refined lamellar structure, with Ce providing the lowest level of modification. On the other hand, a combined addition of Ce and Sr provided the highest modification level, with the production of a very fine fibrous eutectic silicon structure. In general, the addition of Sr helped to further increase the refinement obtained in the alloys containing La or Ce + La additions. In the latter alloy, the main intermetallic phases observed were La(Al,Si)2 and Al20(La,Ce)Ti2Si. The improved modification levels were found to be proportional to the depression in the eutectic nucleation temperature and the eutectic growth undercooling. A high cooling rate also improved the modification level by at least one level.