Search

Aluminum-silicon alloys contain coarse, acicular Si plates that are detrimental to material properties. However, trace additions of Sr have been found to modify the eutectic Si to have a fine, fibrous structure, which improves elongation, strength, and thermal conductivity. In this study, in-situ neutron diffraction was conducted during the solidification of unmodified and Sr-modified binary Al-6 wt.% Si to obtain a novel view of the modification mechanism. Neutron diffraction intensity data was collected at temperatures ranging from 660 °C to 200 °C and was integrated to create fraction solid curves for the individual Al and Si phases in each alloy. Scanning electron microscopy revealed that the Sr-modified alloy microstructure contained many micro-sized Al-Si-Sr intermetallics scattered about the eutectic regions at the Al-Si interfaces, and a few Si flakes close to some β-(Al,Si,Fe) intermetallics, suggesting important interactions between both the Fe-bearing phases and Sr in the solidification of eutectic Si. Comparing the isolated phase evolution temperatures for each alloy revealed in-situ for the first time that Sr suppresses the nucleation temperatures of both the eutectic Al and eutectic Si phases by several degrees. Accordingly, the eutectic reaction was observed to evolve at a higher primary Al solid fraction with Sr modification.

Eutectic high-entropy alloys are potential replacements of structural alloys, due to their microstructure and properties. In this work, CoCrFeNiTax (x = 0.1, 0.2, 0.3, 0.395, 0.4 and 0.5, x value in molar ratio) eutectic high-entropy alloys were produced by arc melting technique. Two phases, FCC solid solution and Laves phase, are identified in the alloys. The alloys are transformed from hypoeutectic to hypereutectic solidification by increasing Ta content. CoCrFeNiTa0.395 eutectic high-entropy alloy exhibits an ultrahigh yield strength of 1.4 GPa, while CoCrFeNiTa0.3 hypoeutectic high-entropy alloy shows a compressive strength of 2.5 GPa and a considerable fracture strain of 44%. Such remarkable strengthening effects are shown to result from the ideal combination among the second-phase, solid solution, and boundary strengthening.

Ni-Sn anomalous eutectic is obtained by single track laser cladding with the scanning velocity from 1 mm/s to 10 mm/s using the Ni-32.5 wt.%Sn eutectic powders. The microstructure of the cladding layer and the grain orientations of anomalous eutectic were investigated. It is found that the microstructure is transformed from primary α-Ni dendrites and the interdendritic (α-Ni + Ni3Sn) eutectic at the bottom of the cladding layer to α-Ni and β-Ni3Sn anomalous eutectic at the top of the cladding layer, whether for single layer or multilayer laser cladding. The EBSD maps and pole figures indicate that the spatially structure of α-Ni phase is discontinuous and the Ni3Sn phase is continuous in anomalous eutectic. The transformation from epitaxial growth columnar at bottom of cladding layer to free nucleation equiaxed at the top occurs, i.e., the columnar to equiaxed transition (CET) at the top of cladding layer during laser cladding processing leads to the generation of anomalous eutectic.

In this paper, we report that a selective laser melted (SLM) AlSi10Mg alloy has unique hierarchical microstructures. There are micro-sized Al grains and transgranular ultrafine cellular structures around primary Al, and the cell boundary is made up of alternate eutectic Si and eutectic Al phases. The SLM AlSi10Mg alloy has both high tensile strength and high strain-hardening capability, which are superior to Al-Si alloys fabricated by conventional powder metallurgy and cast methods. Post-tensile high-resolution microscopy analyses provide further evidence supporting Orowan looping as a strengthening mechanism of the SLM Al-Si alloy.

The emergence of eutectic high-entropy alloys containing Laves phase provides a new and exciting research direction towards developing advanced structural alloys, in light of the inherent advantages of Laves phase at elevated temperatures. In this work, CoCrFeNiZrx alloys were prepared with varying zirconium contents by vacuum arc-melting method. Typical eutectic microstructure was identified in the as-cast alloy having x = 0.5. The alloys consist of face-centered cubic solid solution and C15 Laves phase in the form of lamellae. Crystallographic orientation relationship between these two phases was determined. With the increase of the volume fraction of hard C15 Laves phase, the resulting alloys showed an increase in strength, but became more brittle at room temperature; the fracture process changed from ductile inter-lamellar fracture to brittle trans‑lamellar fracture. With the increase of test temperature, however, the fracture mode converted to be ductile fracture. As such, the eutectic microstructure can accommodate considerable plastic strain and has potential for engineering applications involving elevated temperatures. Keywords: High-entropy alloy, Laves phase, Eutectic microstructure, Mechanical properties, Elevated temperature

A series of Al-40Zn-5Si alloys with different additions of Eu were prepared by permanent mold casting. The nucleation phenomenon incorporating the modification of silicon phase was investigated by using the synchrotron radiation imaging technology, scanning electron microscopy, electron probe microanalysis and high resolution high-angle annular dark-field scanning transmission electron microscopy. It was found that the primary phase was α-Al dendrites and the pre-eutectic Si particles were precipitated continuously at the front of the solid/liquid interface in Al-40Zn-5Si alloys. The nodular pre-eutectic Si particles were observed when the Eu addition was 0.3%. Meanwhile, a flake-to-fibrous transition of eutectic Si was also observed. The needle-like eutectic Si crystals were seen to shoot out from the vicinity of primary α-Al dendrites into melt with high velocities leading the irregular eutectic interface in the unmodified Al-40Zn-5Si alloy. However, the eutectic grains with smooth interfaces nucleated mainly near primary or secondary α-Al branches in 0.3% Eu-modified Al-40Zn-5Si alloy. The eutectic grains began to nucleate on the nodular pre-eutectic Si particles when the α-Al surface reached so close to them. The formation of nodular pre-eutectic Si particles was attributed to the adsorption of Eu atoms along the growth direction of Si and at the intersection of Si twins. This is fully consistent with the well-known poisoning of twin plane re-entrant edge (TPRE) and the impurity induced twinning (IIT) modification mechanisms proposed for interpreting the modification of eutectic Si.

Fine-grained aluminum alloys are very heat sensitive and overheating can reduce the strength of the material. Filler metals of Zn-5Al and Zn-5Al-3Cu alloys were used to join fine-grained 7034 aluminum alloy pieces by ultrasonic-assisted soldering at 420 °C. We observed excellent dissolution of the filler metal to the base metal. For the Zn-5Al solder, the Al content of the bond layer increased from 25 to 54 at.% when the ultrasound time increased from 5 to 60 s, resulting in a decrease of the volume fraction of the eutectic phase from 60% to 15%. The maximum tensile strength of the joints reached 208 ± 8 MPa and all fractures occurred in the brittle eutectic phase. For the Zn-5Al-3Cu solder, η-Zn and α-Al phases were identified in the bond layer. The maximum tensile strength of the joints was 249 ± 12 MPa, which was ~80% of the base metal after the same soldering thermal cycle. In this case, fracture occurred in the η-Zn phase. The higher strength is attributed to the absence of the eutectic phase. When the ultrasonic treatment time was increased to 60 s, the volume fraction of α-Al in the bond layer increased to 93%. The strength of the joints decreased as the brittle ZnAl eutectoid phase crystals appeared inside the α-Al phase in the bond layer. Keywords: Fine-grained Al alloy, Ultrasonic-assisted soldering, Zn-based solder, Eutectic phases, Mechanical properties

We report a novel microstructure evolution and corresponding mechanical behavior of bimodal eutectic (Ti 63.5 Fe 26.5 Co 10 ) 82 Nb 12.2 Al 5.8 alloys processed by semi-solid sintering (SSS) of the as-milled alloy powders with various glass contents resulted from various milling times. Results show that the as-milled alloy powders have more homogeneous element distribution and higher content of glassy phase with increased milling time. Correspondingly, although the SSSed bulk alloys possess the same constituted phases of the bcc β-Ti, bcc B2 Ti(Fe, Co) and fcc Ti 2 (Co, Fe), their eutectic structures containing bcc β-Ti and bcc B2 Ti(Fe, Co) evolve from irregular eutectic, to partial coarse eutectic, to fine cellular eutectic matrix, and finally to typical nano and ultrafine lamellar eutectic matrix. Interestingly, it is the first time to report that the lamellar eutectic matrix has an bimodal structure consisting of interleaving nano- and ultrafine-grained B2 Ti(Fe, Co) and bcc β-Ti lamellae. Corresponding to the evolution of eutectic structure, the SSSed bulk alloys exhibit a gradual increase in yield strength and plastic strain. Especially, the SSSed bimodal eutectic alloy has ultra-high yield strength of 2050 MPa and large plasticity of 19.7%, superior to those of equivalent counterparts. Theoretically, strengthening mechanism of the SSSed bimodal eutectic alloy can be mainly rationalized as ordering strengthening of B2 superstructured Ti(Fe, Co) and coherency strengthening between bcc β-Ti and bcc B2 Ti(Fe, Co) lamellae inside the lamellar eutectic matrix.

The results of investigation of the structure of a heat-resistant eutectic niobium–silicon alloy, which was prepared by directional solidification using a liquid-metal coolant, are reported. The typical areas of the macrostructure of an ingot undergone directional solidification are considered, and the composition and the volume fractions of the Nb–Si composite have been determined.

A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared. A unique eutectic-dendritic solidification pattern was observed in the as-cast alloy. Heretofore, this kind of pattern was rarely reported no matter in binary or multi-component alloys. Through a series of experimental characterization and theoretical analysis, it could be concluded that the sluggish diffusion effect of high-entropy alloys makes the interface deviate considerably from the local non-equilibrium condition and the alternate arrangement of CoCrFeNiPd-rich FCC and Tetragonal Mn7Pd9 lamellae results in weak interface energy anisotropy, thus forming the seaweed eutectic-dendrite. Application of the designing rules of high-entropy alloys to the current work indicates that mere consideration of the weighted average physical properties of constituent elements is not sufficient to the design of high-entropy alloys with a single solid-solution phase.

We report on a novel approach to design typical ultrafine lamellar eutectic structure in bimodal alloys fabricated by semi-solid sintering (SSS) of a eutectic mixture. In our work, ultrafine lamellar eutectic structure was implemented by controlling the phase composition of eutectic reaction, and consequently by regulating the structure of eutectic reaction-induced liquid phase through varying component number. Microstructure analysis indicate that although all SSSed alloys have the same three phase constitutions of bcc β-Ti, bcc Ti(Fe, Co), and fcc Ti 2 (Co, Fe), the morphology and distribution of the eutectic structure transforms from limited length and minor quantity, to partial fine alternating bcc β-Ti and bcc Ti(Fe, Co) lamellae, and further to typical complete ultrafine alternating continuous lamellae in the SSSed ternary Ti-Fe-Co, quaternary Ti-Fe-Co-Nb, and quinary Ti-Fe-Co-Nb-Al alloys. Interestingly, the SSSed Ti-Fe-Co-Nb-Al alloy presents a novel bimodal microstructure of coarse fcc Ti 2 (Co, Fe) surrounded by an ultrafine lamellar eutectic matrix containing ultrafine bcc β-Ti and bcc Ti(Fe, Co) lamellae. This bimodal microstructure exhibits ultra-high yield strength of 2050 MPa with plasticity in compression of 19.7%, which exceed published values of equivalent materials. Our results provide a novel pathway for fabricating new-structure metallic alloys for high-performance structural applications.

The present investigation is based on the results of a directionally solidified (DS) Sn–9 wt%Zn–2 wt%Cu alloy, including primary/secondary/tertiary dendrite arm spacings of the Sn-rich matrix, the morphologies of the eutectic mixture and the corresponding interphase spacing, the nature and proportion of the Cu–Zn intermetallic compound (IMC). The main purpose is to establish interrelations of these microstructure features with experimental solidification thermal parameters, such as cooling rates and growth rates (v), macrosegregation and hardness. Such interrelations are interesting for both industry and academy since they represent a tool permitting the preprogramming of final properties based on the design of the microstructure. In the case of Sn–Zn–Cu alloys, hardly anything is known about the combined effects of the length scale of the microstructure and fraction and distribution of the primary IMC on hardness. The alloy microstructure is composed of a β-Sn dendritic region, surrounded by a eutectic mixture of α-Zn and β-Sn phases and the γ-Cu5Zn8 IMC. The eutectic interphase spacing varies in the range 1.2–3.6 μm, with the α-Zn phase having a globular morphology for v > 0.5 mm/s and a needle-like morphology for v

The mechanism of refinement of eutectic Si with Sb additions in an Al–10 wt.% 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. Keywords: Al–Si alloys, High number-density nano-particle, Precursor-assisted nucleation, Atomic structure, Eutectic Si refinement

Ni-30wt%Sn alloy powder beds were remelted using a laser beam to examine regular eutectic and anomalous eutectic growth behavior. The remelted microstructure mainly consisted of primary α-Ni dendrites and refined regular lamellar eutectic in the interdendrite space. At the top of the molten pool, the competition between primary α-Ni dendrites and the regular lamellar eutectic can be explained by the maximum interface temperature criterion. Anomalous eutectic was observed at the bottom of the molten pool. The remelting of primary α-Ni dendritic arms formed in the first laser remelting scan had an important effect on the formation of the anomalous eutectic in the second laser remelting scan. This effect led to the formation of globular α-Ni particles with similar Euler's angles in the anomalous eutectic.

Sn-34 wt%Bi, Sn-34 wt%Bi-2wt%Ag and Sn-34 wt%Bi-0.7 wt%Cu alloys have been directionally solidified (DS) under a broad range of solidification cooling rates. Microstructures have been characterized with emphasis on both eutectic growth and precipitation of Bi within the β-Sn dendritic matrix. The eutectic growth, for all alloys examined, is shown to be associated with the coexistence of coarse and fine lamellar structures with different length-scale of lamellar spacing (λ). Experimental growth relations of λ vs. the cooling rate have been proposed. The length-scale of the lamellae in the fine eutectic ranges from 0.8 to 2.5 μm while in the coarse eutectic from 1.8 to 4.0 μm. Taking as reference the Sn-Bi alloy, both the spacing between Bi precipitates (λp) and the fine eutectic spacing (λfine) increase with Cu and Ag additions, whereas λcoarse remains roughly unaltered. Both ternary Sn-Bi-Ag and Sn-Bi-Cu alloys are shown to have worse distributions of both lamellae in the fine eutectic and of precipitates within Sn-rich dendrites, which resulted in decrease in hardness.

The microstructure and property of plasma-sprayed Al2O3–Y3Al5O12 (YAG) composite coatings were investigated. The sprayable feedstock was composed of Al2O3/Y2O3 granules obtained from the spray drying. As-sprayed Al2O3–YAG composite coating showed obvious amorphous characteristics. Post-heat treatment facilitated the complete crystallization of the Al2O3–YAG composite coating. The heat-treated Al2O3–YAG ceramic coating possessed the deposition microstructure comprising three-dimensional interpenetrating network of α-Al2O3/YAG eutectic phases, which is very similar to the directional solidification microstructure of Al2O3/YAG eutectic bulk ceramics. This eutectic coating microstructure does not correspond to a traditional eutectic microstructure. Al2O3–YAG ceramic coating with eutectic composition is obtained from the crystallization of an amorphous solid ceramic instead of the solidification of the liquid. Furthermore, the plasma-sprayed Al2O3–YAG eutectic ceramic coating with the heat treatment exhibits excellent microstructure and performance stabilities under high temperature.

Au-Ge-based solder alloys are promising alternatives to lead containing solders due to the fact that they offer a combination of interesting properties such as good thermal and electrical conductivity and high corrosion resistance in addition to a relatively low melting temperature (361 °C for eutectic Au-28Ge at.%). By adding a third element to the eutectic Au-28Ge alloy not only the Au content could be reduced but also the melting temperatures could be further decreased. In this study, in addition to the eutectic Au-28Ge (at.%) two ternary alloys were chosen from the Au-Ge-Sb and Au-Ge-Sn system, respectively. The soldering behavior of these alloys in combination with the frequently used metals Cu, Ni, and Ti was investigated. The interface reactions and microstructures of the joints were characterized in detail by SEM and EDX analysis. For the determination of the mechanical properties, shear tests were conducted. Mean shear strength values up to 104 MPa could be achieved.

The aim of the present study is to identify the ternary eutectic Mo-Si-B composition to produce directionally solidified materials, which are expected to have excellent high-temperature properties due to the well-defined microstructure. Different alloy compositions in the respective primary solidification areas of the phases were chosen to investigate the microstructural evolution. The results were compared to thermodynamic calculations of the liquidus projection and isopleth phase diagrams using the software FactSageTM. By carrying out these experiments the eutectic point was found to have a nominal composition of Mo-17.5Si-8B (at.%). In the next step, the eutectic alloy was directionally solidified by a zone melting (ZM) process. The evolution of a typical eutectic microstructure due to the growth of lamella-like structures is shown by microstructural investigations. Furthermore, we present a eutectic phase field model for the eutectic Mo-Si-B alloy. The equilibrium interface geometries and interface mobility were calculated using an isotropic model. The results are shown to be in an adequate conformity with the experimental observations.

This study is focused on the effect of boron addition, in the range of 0.011 wt% to 0.048 wt%, on the solidification behavior and microstructure of U720Li alloy. It has been found that boron increased the eutectic (γ + γ′) precipitation significantly, while reduces the formation of η phase and Zr-rich particles precipitated in front of the eutectic (γ + γ′) obviously. Boron greatly retards the γ matrix solidification which is verified by the fact that it markedly reduces the ’segregated area’ around the final liquid pool or eutectic (γ + γ′). Based on this phenomenon, boron is assumed to form a layer at the solid/liquid interface and inhibits the attachment of atoms to attach on or jumping off the solid front, and hence lowers the γ matrix solidification rate. Boron increases the eutectic (γ + γ′) precipitation mainly by retarding the γ matrix solidification, which resulting in deeper undercooling and more eutectic forming elements such as Ti left in the residual liquids. Subsequently, boron reduces the η-Ni 3 Ti and Zr-rich phase formation because the increasing eutectic (γ + γ′) precipitation favors the consumption of Ti and dissolving Zr in γ′ phase.

Sb is an effective additive to modify eutectic Si of Al-Si alloys and the modification mechanism has not yet been studied very clearly. In this paper, computer aided cooling curve thermal analysis and microstructure assessment were carried out to investigate the modification effects of Sb on Al7SiMg alloy. The results show that the Sb addition amount of 0.1~0.25% has improvement effect on eutectic Si morphology, which can be further confirmed from the thermal analysis results. The addition of Sb decreases the eutectic growth temperature by 5 °C to 7.8 °C, indicating an undercooling effect resulted from Sb modification. Compared to that of short rods, a better eutectic Si morphology of nearly granular particles can be obtained in a faster cooling rate. The length of silicon has been gradually shortened as holding time increased to 2.5 h from 0.5 h, which means a long period modification effect of Sb in Al-Si alloys. Owing to undercooling resulted from Sb addition in Al-Si alloys, the refinement structure of eutectic silicon can be achieved.

The aim of the present work was to investigate various means of obtaining a fine eutectic silicon structure in A356.2 alloy and therefore improve the alloy mechanical properties. The effects of solidification rate, Sr modification and melt thermal treatment (MTT) on the silicon particle characteristics of A356.2 (Al–7 %Si–0.4 %Mg) alloy were studied. The particle characteristics measured were the average particle area, average particle length, average particle roundness and average particle aspect ratio, using image analysis and optical microscopy. The results showed that Sr modification-, superheat- and Sr modification-MTT (SrMTT)-processed castings provide fine eutectic Si particles, the SrMTT process giving the best modification results. Both size and morphology of the eutectic silicon particles are affected by the modification process used. The Sr-grain-refined, Sr-melt superheat and SrMTT castings show well-modified fibrous Si particles, whereas the MTT casting exhibits Si particles that, although refined to a certain extent, still retain their acicular morphology. Solidification rate affects the eutectic Si particle size in that a higher solidification rate produces finer Si particles. However, within the range of solidification rates provided by the end-chill mold used in this work, the solidification rate does not affect the morphology of the Si particles.

Aluminium alloys possess excellent mechanical, physically chemical and technological properties which have found an extensive application in the automotive industry. Alloy AlSi11 belongs to a group of eutectic alloys characterized by high fluidity, low pouring temperature and narrow solidification interval which comprehend to the uniformly distributed eutectic microstructure indicating superior mechanical and technological properties. Study of AlSi11 alloy solidification sequence has been accompanied by calculation of equilibrium phase diagram, and simultaneous thermal analysis, all in correlation with microstructure development. Thermodynamic calculation revealed solidification sequence with corresponding temperatures in equilibrium state. Differential scanning calorimetry enables determination of exact temperatures of phases’ transformations correlated at different cooling rates. Dilatometer thermal analysis defined dimensional changes of AlSi11 alloy. Microstructural examination revealed development of following constituents: primary aluminium (αAl), high-temperature intermetallic phases on the iron base (Al5FeSi and/or Al15(Fe,Mn)3Si2), binary eutectic (αAl + βSi) and secondary eutectic (Mg2Si and Al5Mg8FeSi6) as a last solidifying phases. Synergy of performed phase diagram calculation, thermodynamic and microstructural investigation enables determination of thermodynamic stability of AlSi11 alloy exposed to different cooling conditions, with respect to phase’s evolution.

The influence of treating the melts by electromagnetic acoustic fields on the structure and properties of Al–12% Si and Al–20% Si binary alloys is investigated. In the course of experiments, the frequency of the electromagnetic field induced in the loop antenna varies as 500, 1000, and 2000 kHz. The melts are treated after their degassing and refining. It is established that this treatment method of the melts leads to a reduction of the total preparation time of alloy by 12% on average. The short-term treatment of the melts by electromagnetic acoustic fields promotes the refinement of the main phase components of alloys and an increase in their mechanical properties. When treating the Al–12% Si eutectic alloy with a frequency of 500 kHz, α-Al dendrites are refined from 30 to 22 μm and eutectic Si crystals are refined from 13 to 10 μm. When treating the Al–20%Si eutectic alloy with a frequency of 1000 kHz, eutectic Si crystals diminished from 8 to 5 μm and these of primary Si diminished from 90 to 62 μm. The ultimate tensile strength of the Al–12%Si eutectic alloy increases 13% under the mentioned treatment modes, while the relative elongation increases 17%; as for the Al–20% Si eutectic alloy, the same characteristics increases 9 and 65%, respectively. Based on these investigations, it is concluded that the selection of the treatment parameters of the melts of the Al–Si system by electromagnetic acoustic fields should be determined by the silicon content in the alloy. It is necessary to treat the melt by waves with a higher oscillation frequency with an increase in the silicon concentration. This treatment method makes it possible to form the modified fine-crystalline structure of alloy and, consequently, improves their mechanical properties. It can be successfully used when fabricating fine-crystalline foundry alloys and in the production of alloys of the Al–Si system. To determine the optimal treatment parameters depending on the structure of the initial charge and alloy nature, additional investigations are required.

A set of aluminum–nickel alloys has been studied. The elemental composition of the samples has been determined by atomic emission and atomic absorption spectrometry. X-ray diffraction analysis has revealed that the alloying of the metals leads to the formation of Al3Ni and Al3Ni2 intermetallic compounds, while a portion of Al remains in a metallic phase. The local chemical composition and surface morphology of the original alloys and the alloys activated with the liquid Ga–In eutectic have been studied by scanning electron microscopy and X-ray microanalysis. It has been shown that the original alloys are characterized by a pronounced morphological heterogeneity of interfacial regions in the near-surface layers. It has been found that the studied Al–Ni alloys are activated by the liquid Ga–In eutectic; however, one of the alloy components—the Al3Ni intermetallic compound—does not undergo significant morphological and chemical changes in contact with the liquid eutectic.

The equilibrated grain boundary groove shapes of solid CuAl2 in equilibrium with (Al + Cu + Si) eutectic liquid were observed from a quenched sample by using a radial heat flow apparatus. The Gibbs-Thomson coefficient, (solid + liquid) interfacial energy and grain boundary energy of the solid CuAl2 were determined from these observed shapes. The thermal conductivity of the eutectic solid and the thermal conductivity ratio of eutectic liquid to the eutectic solid in the (Al + 26.82 wt.% Cu + 5.27 wt.% Si) eutectic alloy at its eutectic melting temperature were also measured with a radial heat flow apparatus and a Bridgman-type growth apparatus, respectively. The three phases of (Al + Cu + Si) alloy have detected as Al solution, Si and theta (CuAl2) phases with EDX composition analysis and the microstructure of these phases were photographed by SEM. (C) 2016 Elsevier Ltd. All rights reserved.

Additions of niobium (Nb) and molybdenum (Mo) were made to an Alloy 690 base alloy in order to investigate the formation of a eutectic constituent at the end of solidification and to evaluate the effect of the eutectic liquid on backfilling (or healing) of solidification cracks. Solidification cracking was induced using the cast pin tear test (CPTT) and regions of backfilling were located and characterized via optical and electron microscopy. Computational predictions of fraction eutectic and composition of the eutectic constituent were compared to experimental findings and were found to correlate well in both cases. The extent of crack backfilling increased significantly with increasing Nb content, but the addition of Mo did not seem to influence the amount of eutectic constituent or the degree of backfilling. SEM/EDS analysis confirmed that the eutectic composition is constant and that increasing Nb above 4 wt% has little effect on expanding the solidification temperature range, but has a beneficial effect on mitigating solidification cracking by a crack healing effect.

The influence of small amount of Ca addition on the microstructures and mechanical properties of three near-eutectic Mg-x Li (x = 6.8, 7.5, 8.4 wt. %) binary alloys under rapid solidification condition by using Copper-mould Suction Casting technique were investigated. The resultant microstructures were very distinct from those by conventional gravity casting technique in the morphology, grain size and the volume fraction of α-Mg phase. Mechanical properties of the Ca-modified Mg-x Li binary alloys were significantly enhanced. More interestingly, a eutectic growth mode transition from their regular divorced eutectic growth to the regular coupled eutectic structure was induced in Mg-8.4Li alloy via Ca addition. The eutectic transition mechanisms in three Mg-x Li alloys by Ca addition were discussed in terms of the freezing rate, Mg2Ca particle segregation, the relative volume fraction of α-Mg phase.

Rare earth metals can create a fine eutectic Si structure in cast Al-Si10.6-Cu2.5 (ADC12) alloys produced through heated mold continuous casting. Fine and spherical eutectic Si phases are created in the ADC12 alloys through the addition of Sr0.04, and fine lamellar eutectic Si phases are created through Sb and Bi addition. Crystal orientation on the face perpendicular to the casting direction is formed by [110]; however, this uniform formation is collapsed in the ADC12 alloy with an increasing amount of Sr addition, such as Sr > 0.04%. The shape of the eutectic Si is statically analyzed, and the effects of the eutectic Si characteristics on the mechanical properties are examined experimentally. On the one hand, the mechanical properties of the ADC12-Sr alloy increase with increasing Sr content because of the fine eutectic Si, the randomly orientated crystal formation, and so on. On the other hand, the material ductility increases in the ADC12 alloy with increasing addition of Sb and Bi elements. A high fracture strain of approximately 14% is obtained for the ADC12-Bi1.5 alloy.

Rapid solidification of Co-Si alloys was investigated by using vacuum suction casting in this study. Different microstructures and intermediate phases were obtained. Eutectic εCo phase and eutectoid εCo + αCo2Si structures were obtained in the first eutectic Co76.9Si23.1 alloy. The microstructures of hypereutectic Co70Si30 alloys were composed of primary αCo2Si phase and interdendritic lamellar eutectoid εCo + αCo2Si. While for hypoeutectic Co63Si37 alloy at the second eutectic point, CoSi dendrites were the primary phase, and αCo2Si+CoSi eutectoid structures can be seen at the interdendritic region. Especially, a metastable CoSi2 phase was found in Co63Si37 alloy. This indicates that eutectoid decomposition βCo2Si→ CoSi+αCo2Si is restrained in eutectic Co60.3Si39.7 alloy. For rapid solidified Co55Si45 and Co52Si48 alloys, αCo2Si+CoSi eutectoid structures were not observed, while metastable CoSi2 were obtained. The higher hardness achieved in Co-Si alloys at the second eutectic point, for the reason of the higher volume fractions of compound phases.

The effect that the Al–Ni eutectic composition has on the microstructure and tensile properties of Al–Zr–V alloys is discussed. It is found that the change in the alloy's solidification behavior brought about by the presence of the eutectic composition mitigates segregation of the zirconium and vanadium atoms, and by doing so it enhances the precipitation hardening characteristics of the alloy and improves its yield strength.

Based on surveying the existing binary phase diagrams with eutectic points, a strategy of designing eutectic high entropy alloys (EHEAs) with desired high strength and ductility is proposed. Based on the computer-aided thermodynamic calculations, the pseudo eutectic binary alloy system of CoCrFeNiNbx (x = 0.1, 0.25, 0.5 and 0.8) was designed. The experimental results show that the eutectic alloys are composed of a ductile face centered cubic (FCC) phase and a hard Laves phase with fine laminar structures. The designed alloys show excellent integrated mechanical properties of ductility and strength. For the CoCrFeNiNb0.5 alloy, the compressive fracture strength and strain can reach above 2300 MPa and 23.6%, respectively.

Extensive effort was expanded to elucidate the growth and morphology of the stable eutectic grains during early solidification of continuous cooled Fe–C–Si alloys. To this purpose, quenching experiments at successive stages during solidification have been carried out on five cast irons with various magnesium and titanium levels designed to produce graphite morphologies ranging from lamellar to mixed compacted–spheroidal. The graphite shape factors were measured on the metallographic samples, and their evolution as a function of the chemical composition and the solid fraction was analyzed. Extensive scanning electron microscopy was carried on to evaluate the change in graphite shape during early solidification, to establish the fraction of solid at which the transition from spheroidal-to-compacted-to-lamellar graphite occurs, and to outline the early morphology of the eutectic grains. It was confirmed that solidification of Mg containing irons started with the development of spheroidal graphite even at Mg levels as low as 0.013 mass%. Then, as solidification proceeds, when some spheroids developed one or more tails (tadpole graphite), the spheroidal-to-compacted graphite transition occurs. The new findings were then integrated in previous knowledge to produce an understanding of the eutectic solidification of these materials. It was concluded that in hypoeutectic lamellar graphite iron austenite/graphite eutectic grains can nucleate at the austenite/liquid interface or in the bulk of the liquid, depending on the sulfur content and on the cooling rate. When graphite nucleation occurs on the primary austenite, several eutectic grains can nucleate and grow on the same dendrite. The primary austenite continues growing as eutectic austenite and therefore the two have the same crystallographic orientation. Thus, a final austenite grain may include several eutectic grains. In eutectic irons the eutectic grains nucleate and grow mostly in the liquid. The eutectic austenite has different crystallographic orientation than the primary one. The solidification of the austenite/spheroidal graphite eutectic is divorced, with graphite spheroids growing on primary austenite dendrites. The eutectic austenite grows on the primary austenite and has the same crystallographic orientation. The result is large austenite (primary and eutectic) dendrites that incorporate numerous graphite spheroids. A eutectic grain cannot be defined.

Results of electron back-scattered diffraction and transmission electron microscopy studies reveal that the orientation of aluminium in the eutectic phase is different from the surrounding aluminium dendrites and that twin spacing of eutectic silicon increased significantly due to the interaction between strontium and antimony. In addition, the formation of ternary Mg2Sb2Sr intermetallic compound that precedes the eutectic reaction was proposed to be responsible for interaction.

Liquid ternary Fe–6.5%Mo–18.5%Si eutectic and Fe–4%Mo–19%Si peritectic alloys were highly undercooled and rapidly solidified under containerless and microgravity condition inside the drop tube. In this situation, the cooling rate and undercooling of the alloy droplets increased obviously when their diameters decreased, and the alloy droplets exhibited maximum undercoolings up to 184 K and 468 K respectively. For the Fe–6.5%Mo–18.5%Si eutectic alloy, the ternary eutectic is composed of Fe3Si, Fe2Si and Fe5MoSi4 intermetallic phases. With the decrease of the droplet diameter, the volume fraction of the ternary (Fe3Si + Fe2Si + Fe5MoSi4) eutectic increases and the morphology of the ternary eutectic vary from lamellar structure to granular structure. In the Fe–4%Mo–19%Si peritectic alloy, the primary phase is Fe2Si intermetallic phase, and the peritectic structure consists of Fe3Si and FeSi intermetallic phases. With the increase of the undercooling, the primary phase vanishes, the peritectic transformation nearly accomplishes, and the growth of (Fe3Si + FeSi) peritectic phases transform from faceted blocks into non-faceted granules.

With modern microtechnology, there is an aggressive miniaturization of smart devices, despite an increasing level of integration and overall complexity. It is therefore becoming increasingly important to be achieve reliable, compact packaging. For implantable medical devices (IMDs), the package must additionally provide a high quality hermetic environment to protect the device from the human body. For chip-scale devices, AuSn eutectic bonding offers the possibility of forming compact seals that achieve ultra-low permeability. A key feature is this can be achieved at process temperatures of below 350315°C, therefore allowing for the integration of sensors and microsystems with CMOS electronics within a single package. Issues however such as solder wetting, void formation and controlling composition make formation of high-quality repeatable seals highly challenging. Towards this aim, this paper presents our experimental work characterizing the eutectic stack deposition. We detail our designmethods and process flow, share our experiences in controlling electrochemical deposition of AuSn alloy and finally discuss usability of sequential electroplating process for the formation of hermetic eutectic bonds.

High temperature tensile properties and microstructure morphologies of directionally solidified NiAl-Cr(Mo) alloy under different compositions and withdrawal rates were investigated. Tensile strength of 513.8 MPa was obtained in the directionally solidified NiAl-36Cr-6Mo hypereutectic alloy at 1000 °C, which is the highest value reported in the NiAl-Cr(Mo) alloy until now. In addition, perfect cellular eutectic alloy solidified at fast withdrawal rate possessed higher tensile strength and elongation than planar eutectic alloy solidified at low withdrawal rate when they had the same composition. This is because bonding strength between eutectic cells in the perfect cellular eutectic alloy was high, which resulted in high crack propagation resistance and harmonious deformations in the intracellular and intercellular regions. The results above may change the locked-in view that only regular eutectic microstructure morphology grown with planar solid/liquid interface could be used in the eutectic in-situ composites. Perfect cellular microstructure morphology grown in a far fast rate could be also used, which would result in high efficiency in the industrial production. Refined microstructure, larger volume fraction of strengthening phase and dispersion strengthening of second phase particles were contribute to the improvement of high temperature tensile strength of the directionally solidified NiAl-Cr(Mo) alloy. However, the bonding strength of interface was obviously different in the alloys with different microstructure morphologies, which had significant effect on the tensile strength. When bonding strength of phase interface was low, the other strengthening factors could not be able to play their due roles, thus leading to relatively low tensile strength.

A new mathematical model was developed to predict the average eutectic spacing under high pressure solidification. The model can be used in the binary alloy system which contains solid solution and intermetallic compound. Following this model, the change of eutectic spacing under high pressure can be predicted if the volumetric conditions of the phases are known. Application of the model to the binary Mg-20.3wt.%Al solidified under different pressure was conducted. It was found that the results predicted by this model show an excellent agreement with the eutectic spacing of Mg-20.3wt.%Al alloy solidified under high pressure. And during experiment, an ultimate pressure was found, the microstructure tends to grow as divorced eutectic when the solidified pressure is higher than this ultimate pressure, the formula is not applicable.

The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that constitutional supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

A kind of Ni–Cr–Fe–B–Si system amorphous alloy was used as interlayer in transient liquid-phase bonding (TLP bonding) of polycrystalline superalloy K465. The bonding behavior, microstructure feature and the tensile properties of the joints were investigated. There are B-rich phase and Si-rich phase formed in the center of the seam after bonding at 1210 °C for 30 min. The isothermal solidification is complete after bonding at 1210 °C for 4 h. The relationship of the average width of the remnant eutectic zone and bonding time at 1210 °C is nonlinear. The tensile strength of the bonded joint at room temperature and 900 °C is comparable to that of K465 alloy.

This study shows that a eutectic Al–12Si alloy with controllable ultrafine microstructure and excellent mechanical properties can be achieved by using selective laser melting and subsequent solution heat treatment. This provides a novel and promising approach to the refinement of eutectic Al–Si alloys. Unlike Al–12Si alloys fabricated and refined by traditional methods, the as-fabricated Al–12Si in this study contains nano-sized spherical Si particles surrounding a supersaturated Al matrix. During solution heat treatment, precipitation and coalescence of the Si particles occur, which decreases the Si concentration in the matrix and sub-micron to micron-sized spherical particles embedded in an Al matrix form. The as-fabricated Al–12Si exhibits significantly better tensile properties than the traditionally produced counterparts; while the solution treated Al–12Si has an extremely high ductility of approximately 25%. Importantly, the mechanical properties of the Al–12Si can be tailored through controlling the precipitation and coalescence of the Si particles by varying the solution heat treatment time. A detailed transmission electron microscopy study was conducted to investigate this Al–12Si alloy with ultrafine eutectic microstructure. The excellent tensile properties have been attributed to the refined eutectic microstructure containing spherical Si particles. The formation of this unique microstructure is due to the super heating and an extremely high cooling rate during selective laser melting and the subsequent solution heat treatment, which enables Si to grow along its most stable plane {1 1 1}Si.

Anomalous eutectics are argued to form due to the remelting of the primary solid during solidification of deeply undercooled eutectic alloy melts. As an indicator of the tendency for anomalous eutectic formation, the remelted fraction of the primary eutectic was analyzed systematically based on the eutectic dendrite growth theory. For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes, the primary eutectic is highly supersaturated with solute and more prone to remelting. When the eutectic composition is set to different values, (e.g. the eutectic point is closer to one phase of the eutectic), the two eutectic phases under rapid growth change their compositions simultaneously, but their remelted fractions during temperature recalescence do not vary significantly. Three representative binary eutectic alloys Ag–39.9 at.%Cu, Ni–19.6 at.%P and Pd–16.0 at.%P – their eutectic products are solid solution–solid solution, solid solution–stoichiometric intermetallic compound and stoichiometric intermetallic compound–stoichiometric intermetallic compound, respectively, were solidified at large undercooling. Anomalous eutectics were observed in the first two eutectic alloys whose eutectic structure consists of at least one solid solution phase, whereby the stoichiometric intermetallic compound phase remained highly oriented whereas the solid solution phase had a near random distribution of orientations. For the Pd–16.0 at.%P alloy, however, the primary eutectic retained its original morphology as the final solidification structure regardless of the degree of undercooling. All these experimental results validate the argument that anomalous eutectics result from the remelting of the primary solid.

(Fe, Co, Ni) rich dendrites nucleate primarily in CoCrFeMoNi and CoCrCu0.1FeMoNi alloys, followed by peritetic and eutectic reactions. The quasi-peritectic reaction occurs between the primary Mo-rich dendrites and liquids in the CoCrCu0.3FeMoNi melts, and transfers to a eutectic coupled-growth at the edge of the quasi-peritectic structure. Subsequently, eutectic reaction happens in the remnant liquids. Liquid-phase separations have occurred in CoCrCuxFeMoNi alloys when x≥0.5. Meanwhile, some nanoscale precipitates are obtained in the Cu-rich region. Two crystal structures, FCC and BCC, are identified in CoCrCuxFeMoNi high entropy alloys. Amazingly, a pretty high plastic strain (51.6%) is achieved in CoCrCu0.1FeMoNi alloy when the compressive strength reaches to 3012 Mpa. With the increase of Cu content, atomic size difference (ΔR) and electro-negativity difference (ΔX) decrease while valence electron concentration (VEC), mixing enthalpy (ΔH) and mixing entropy (ΔS) increase. Consequently, the valence electron concentration (VEC) values range for the formation of mixture of FCC and BCC structures can be enlarged to 6.87–8.35 based on the study of this paper. It is the positive enthalpies of mixing that causes the liquid-phase separation in CoCrCuxFeMoNi high entropy alloys.

The structural, morphological, and chemical characteristics of aluminum-copper alloys before and after contact with the liquid Ga-In eutectic have been studied. X-ray diffraction (XRD) analysis has revealed that alloying results in the formation of Al9Cu11.5 and Al2Cu intermetallic compounds and CuAl0.005, CuAl0.14, and AlCu0.004 substitutional solid solutions. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have shown that, for the CuAl0.005 and CuAl0.14 substitutional solid solutions of aluminum in copper and the individual Al9Cu11.5 intermetallic compound in contact with the liquid Ga-In eutectic, surface spreading of the eutectic without any significant morphological and chemical changes in the studied systems is observed. At the same time, the alloy containing the Al2Cu intermetallic compound and the phase of the AlCu0.004 substitutional solid solution of copper in aluminum interacts with liquid Ga-In eutectic owing to the activation of aluminum as a solid solution component. This activated Al-Cu alloy reacts with liquid tert-butyl chloride in the presence of ethylene to form catalyst Al-Cu-Cl complexes in situ. It has been speculated that these complexes are mostly formed owing to the dissolution of the phase of the AlCu0.004 substitutional solid solution of copper in aluminum in a medium of liquid tert-butyl chloride.

Sn–Cu eutectic modified by minor In, Cr and Ni additions are one of the major alternatives to lead-free solders. The results show that nanostructured solders produced by rapid solidification are dependent on melting properties and the melting temperature. It is found that the In, Cr or Ni addition has the effect of suppressing the formation of eutectic rapidly solidified Sn–0.7Cu alloy. The results indicated that the melting temperatures (Tm) of Sn–0.7Cu are modified to lower temperature by In, Cr and Ni additions. The formation of new intermetallic compounds such as In3Sn, Cu6Sn5, Cu10Sn3, and NiSn are more uniformly distributed inside Sn-rich phase effectively enhancing the hardness and creep resistance of the eutectic Sn–0.7Cu solder joint at room temperature. The results of these tests are consistent with positive effect of the In, Cr and Ni in enhancing the performance of the eutectic Sn–0.7Cu solder as a practical to lead-free solder.

Ni 40 Ti 40 Nb 20 is reported in the literature as a eutectic composition in the quasi-binary NiTi–Nb system. Eutectic and near-eutectic alloys find applications in various fields, including hydrogen permeation, energy damping, and liquid phase bonding of NiTi structures. In this study, we examine a cast Ni–Ti–Nb alloy with the above eutectic composition. Prealloyed, near-stoichiometric NiTi powders and pure Nb powders are blended and heated above the eutectic temperature to create a melt with the eutectic composition. After solidification, the ingot shows some elemental segregation at its top and bottom, but a large middle region exists with a homogeneous eutectic structure. The eutectic has average composition of Ti–40.1Ni–19.6Nb at% and consists of Nb-rich lamellae (Nb–19Ti–10Ni) dispersed within a NiTi-rich matrix (Ti–41Ni–15Nb). Under monotonic compressive deformation, the eutectic alloy yields at 630 MPa, and then shows a linear hardening region, where both plastic and superplastic deformation are active, until a stress of 1080 MPa is reached at an applied strain of 14.7%. On unloading, some superelastic strain is recovered. Upon unloading during compressive load–unload cycling, the eutectic alloy exhibits approximately twice as much superelastic recovery as elastic strain recovery, regardless of the maximum cycle strain.

In this work, Au–Sn eutectic bonding and Au–Sn thermo-compression bonding are studied for applications in hermetic packaging at wafer level. Eutectic bonding experiments were performed under vacuum or pure nitrogen at temperatures between 310 and 400 °C while thermo-compression bonding experiments were performed under vacuum at 270 °C. Nickel layers 350 nm thick are used as wetting layers on both cap and device sides. During these experiments, the interaction of electrodeposited Au–Sn alloy with nickel layers is studied either in solid or liquid state. The diffusion paths at 270 °C as well as during isothermal solidification of the joint at 310, 350 and 400 °C are determined in relation with the thicknesses of the reaction layers formed at the interfaces and the microstructure of the joint. The effective interdiffusion coefficient in the ζ-Au5Sn phase is deduced at 270 and 310 °C. Moreover, the melting and solidification process of electrodeposited Au/Sn layers with an overall eutectic composition were studied by Differential Scanning Calorimetry in a specific configuration that does not involve the interaction of Au–Sn alloy with the nickel layer. Bonded wafers with good mechanical properties were characterized by cross-section Scanning Electron Microscopy using Energy Dispersive X-ray mode. The mechanical strength of the seal was checked by blade plate insertion technique.

Effects of adding bismuth content on structure, thermo-mechanical and wetting behavior of SnAg 3.5 eutectic alloy have been investigated. Matrix structure of SnAg 3.5 eutectic alloy, such as crystallinity, crystal size and lattice parameters, changed after adding bismuth content which effect on all measured properties. Melting temperature of SnAg 3.5 eutectic alloy decreased after adding bismuth content. Elastic modulus and contact angle of SnAg 3.5 eutectic alloy varied after adding bismuth content. The Sn 66.5 Ag 3.5 Bi 30 alloy has the best solder properties for electronic applications such as lower melting temperature, contact angle and elastic modulus.

Irradiation of the molten eutectic Bi43Sn57 alloy with ultrasonic energy produced different bismuthtin phases that were not reported in the equilibrium binary phase diagram. The X-ray powder diffraction data, refined using the Rietveld method, allowed the identification of these metastable phases and their crystal structure. The Bi2Sn and β′-Sn phases, with distorted β-Sn type structure, were obtained in the sample which was rapidly quenched after ultrasonication. On the other hand, the equiatomic phase BiSn was obtained in the quenched sample when the ultrasonication was continued during the rapid cooling below the eutectic point. The ultrasonicated samples were characterized by differential scanning calorimetry. The results suggested that the metastable phases, which were kinetically stable at RT, decomposed into equilibrium components during the heating treatment at 320 °C, and that this conversion is an irreversible process. Observations by scanning electron microscopy and energy dispersive X-ray analysis complete the characterization of samples.

We present the thermophysical properties and eutectic growth of undercooled liquid Zr91.2Si8.8 alloy at electrostatic levitation state. The obtained maximum undercooling is 371 K, which reaches up to 0.2TE. The density of liquid alloy decreases linearly with increasing temperature. The ratio of specific heat to emissivity is measured and the specific heat is derived accordingly. The solidification microstructure is composed of αZr and Zr3Si phases and displays a transition from lamellar eutectic to anomalous eutectic with the enhancement of undercooling. The growth velocity of lamellar eutectic is measured to be only 1 mm/s, whereas it increased to 90 mm/s for anomalous eutectic.