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The high thermal conductivity, lightweight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic and photonic packages and housings. It is particularly attractive for aerospace and space applications due to its lightweight. A main challenge for these applications is poor bondability. The native aluminum oxide (Al2O3) prevents aluminum from bonding by using popular die-attach materials such as solders. Zincating process is often needed to dissolve the Al2O3 layer and deposit a protection zinc layer which provides a basis for subsequent metallization or soldering process. However, the zincating and metallization processes could increase the processing cost and bring more reliability issues. In this research, a novel Ag foil bonding technique has been developed to bond Ag foils directly to Al substrates to produce Ag-cladded Al substrates. Two Ag-Al bonding processes are developed: solid-state and eutectic. Subsequently, Si chips are bonded to the Ag-cladded Al substrates using solid-state process at 300 °C without any additional die-attach material. For the Ag-Al bonding processes, no surface treatment is applied to Al substrates to remove the native Al2O3 layer. In the Ag-Al soli-state bonding process, Ag and Al atoms inter-diffused through the thin Al2O3 to react and form Ag2Al and Ag3Al compounds. In the Ag-Al eutectic bonding process, Ag2Al+(Al) eutectic structure forms at the Ag/Al interface without Ag3Al compound formation. The native Al2O3 layer, a potential fracture path, is broken into pieces during eutectic reaction and possibly dispersed into the eutectic structure. Shear test results of Si/Ag/Al joint samples far exceed the military criterion (MIL-STD-883H method 2019.8). The Si/Ag/Al structures break either along the Ag/Al interface or within the Si chip. With the advantages of high thermal conductivity, high reliability, lightweight, and process simplicity, the Ag-cladded Al structures should be highly valuable for applications in packages and housings where lightweight and high heat-conducting are necessary.

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.

Microstructure transition and selection of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics with minimal eutectic spacing of 50 nm are studied by laser zone melting under rapid solidification rate. When the eutectic spacings are in micronscale, the ‘Chinese script’ structure is the main microstructure. As increasing the solidification rate (≥ 50 µm/s), the eutectic colony structure is obviously formed. The irregular-regular structural transition and coexistence are produced around eutectic spacing of 100–200 nm. The mechanism of irregular/regular eutectic structure transition and coexistence is revealed by considering the relative kinetic undercooling of each phase. In addition, the strong formation tendency of amorphous/nano-eutectic structure is observed at the surface of as-solidified ceramic drops by rapid liquid-metal quenching.

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.

Dissimilar transient liquid phase joints were fabricated between Ni-based GTD-111 and Co-based FSX-414 superalloys using a 50 μm thick amorphous Ni Si B interlayer. The athermally solidified zone in the middle of joint contained the ternary eutectic of γ/Ni3B/Ni6Si2B and few amounts of Ni3Si. The amount of eutectic phase was decreased by increasing the bonding time. The complete isothermal solidification was obtained at the joining time of 100 min. The completion of isothermal solidification was controlled by several factors including the diffusion of Si and B from the bonding zone into the base metals, the diffusion of Ti, Al, Co and Cr from the base metals into the bonding zone and proportional re-distribution of them throughout the joint. The width of bonded zone (about 80 μm) was greater than the thickness of interlayer due to the coupled diffusion and dissolution phenomena. After the homogenization heat treatment, an increase of 34% in shear strength of the joint was observed due to the uniform distribution of strengthening precipitates across the bonding area.

A series of CoCrFeMnNiPdx (x = 0.2–2 at.%) eutectic high entropy alloys (EHEAs) were prepared and analyzed to show the alloying effect of Pd on the microstructure and phase constituents. It was found that the microstructure as the increase of x changes from the divorced eutectic to the coarse granular divorced eutectic around which are the fine lamellar dendrite eutectic and then to the seaweed dendrite eutectic. Hitherto, such seaweed eutectic dendrite was rarely reported in either high entropy alloys or conventional alloys. Specifically, a multimodal grain size distribution including several hundred microns for the eutectic-dendrite, several microns for the coarse granular eutectic, several hundred nanometers for the fine lamellar eutectics and the lath-shaped microstructures in the intermetallic phases, and even several nanometers for the nano-twins was observed. Since the CoCrFeNiMnPdx EHEAs obey all the design rules of HEAs with a single solid-solution phase but exhibits a two-phased microstructure, mere consideration of average physical properties of constituent elements was concluded to be not sufficient to the design of HEAs. The present work is not only enrichment of current EHEA systems but also of importance to the design of HEAs.

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.

Glass-forming ability of Ge0.08Se0.92 (eutectic composition) and Ge0.20Se0.80 (Phillips-Thorpe threshold where the mean coordination number equals to 2.4) are compared employing very slow melts cooling. Using X-ray diffraction the presence of crystalline phase (Ge4Se9) was found in slowly cooled melt of Ge0.20Se0.80. It is suggested that in Se-GeSe2 system the maximum of glass-forming ability is shifted to the vicinity of the eutectic composition.

Effects of Re on the formation of surface eutectics have been investigated by using Ni-base single crystal superalloys with different Re additions. It was found that Re promotes the segregation of Al and Ta to the eutectic melt, leading to an increase of the surface and internal eutectics. In addition, the addition of Re also increased the freezing range, the local solidification time, and the permeability of the dendritic network within the mushy zone. These factors ultimately promoted the outflow of the interdendritic residual liquid with the action of solidification shrinkage, and led to the formation of more surface eutectics. In contrast, the addition of Re had no obvious influence on the surface eutectic microstructures.

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.

In order to confirm the eutectic composition, eutectic temperature and phases of Al-rich Al-RE (La, Ce, Pr, Nd) binary systems, the eutectic Al-RE alloys with the controversial compositions were investigated. The scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), differential scanning calorimetric (DSC) and cooling curve tests analysis indicated that a similar eutectic composition (∼12 wt.% RE) on the Al-rich region was observed in the Al-La, Al-Ce, Al-Pr and Al-Nd alloys, and the eutectic structure of the alloys was consisted of α-Al and α-Al11RE3 phases by combining with X-ray diffraction (XRD) tests. In addition, the eutectic temperatures determined by DSC and cooling curve tests were very close and around 645 °C.

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.

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.

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.

(Co 76 Sn 24 ) 100- x Nb x ( x = 0, 0.5, 0.8, 1.0) eutectic alloy melts were solidified at small undercooling for investigating the effect of Nb addition on microstructural development. With increasing Nb content, the solidification interface transits from eutectic seaweed ( x = 0) to eutectic dendrite ( x = 0.5) as a response to the change in the anisotropy of interfacial energy. Coupled eutectic growth can no longer be maintained within the main stems at x = 0.8 because the enrichment of Nb in the liquid ahead of the interface causes a significant difference in growth velocity between the α-Co and β-Co 3 Sn 2 phases and, as such, Co 3 Sn 2 doublons form. For x = 1.0, the difference in growth kinetics between the two eutectic phases is so large that divorced eutectic growth takes place on a large scale. Letting the undercooling prior to solidification increase, the eutectic interface morphology of (Co 76 Sn 24 ) 99.5 Nb 0.5 eutectic alloys returns from dendritic pattern back to factual seaweed pattern at more than 38 K undercooling and then transits to compact seaweed pattern at more than 181 K undercooling. The eutectic growth velocity slightly increases at low and intermediate undercooling but obviously decreases at large undercooling due with Nb addition.

Low-melting-point alloys have an extensive applications in the fields of materials processing, phase change energy storage, electronic and electrical automatic control, continuous casting simulation, welding, etc. Specifically, the eutectic compositions make up a large number of low-melting-point alloys that are exploited because of their desirable features like single melting peaks, excellent operational reliability, and casting fluidity. However, the fundamental physicochemical properties from the current available literature on low-melting-point multi-component eutectic alloys (LMP-MCEAs) are rather rare and lowly accurate, including the exact melting temperatures and compositions, constituent phases, microstructures and morphologies, melting enthalpies, specific heats, densities, and so on. This lack of information seriously limits the development and application of low-melting-point multi-component eutectic alloys. In this paper, the low-melting-point multi-component eutectic alloys composed of Bi, Cd, Sn, Pb, and In elements synthesized by high vacuum induction melting and fundamental data were investigated by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and density analysis instrument. Most of the LMP-MCEAs with complex eutectic morphology structures and XRD diffraction patterns could be explained with the fact that they were three-phase eutectic alloys with mixed growth way. Generally, LMP-MCEAs present an extremely low melting point between 48.3 and 124 °C and high density between 8 and 10 g/cm3.

To fabricate a higher melting point joint by bonding at a low temperature, bonding characteristics of Au/Cu joints bonded at various bonding conditions using Sn-57Bi-1Ag solder were investigated. They were compared with those of the Cu/Cu joint. In the Au/Cu joint bonded at 170 oC for 1 min, a large scallop-shaped AuSn4 layer formed in the joint interface of Au and solder. A Au-diffused layer formed in the tip of the AuSn4 layer. The Sn-Cu-Au layer including approximately 20 mol% Au formed in the joint interface of Cu and solder. With increasing the bonding time, both the eutectic microstructure with β-Sn and Bi and the Au layer disappeared. As the result, the solder area changed from the eutectic microstructure to Bi phases including AuSn4 and Ag3Sn phases in the bonding time over 30 min. In contrast, the eutectic microstructure formed in the Cu/Cu joint bonded at 170 oC regardless of the bonding time. The bond strength of the Au/Cu joint was approximately one-third of that of the Cu/Cu joint. In both joints, the bond strength reduced with increasing the bonding time due to the fracture mode change.

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.

Upward directional solidification experiments were carried out on various Pb-Sn alloys using water cooled copper chills. Different combinations of growth rate V and composition Co were used to investigate their effect on microstructure evolution during solidification. The effect of solidification velocity and composition on the structure formation is analyzed. It is observed that the hypoeutectic Pb-Sn alloys exhibit columnar dendritic structure with α phase rich region. In case of hypereutectic alloys the columnar dendritic structure is with β phase rich region. In both the cases dendritic structure was found to surround by the eutectic structure, a typical result of non equilibrium cooling. The eutectic composition exhibits complete eutectic structure aligned in the direction of solidification.

A novel melt falling-drop quenching method with rapid cooling rate is carried out to prepare the Al 2 O 3 /Er 3 Al 5 O 12 nanoeutectic ceramic based on laser floating zone melting. The regular eutectic structure with minimum interphase spacing of 30 nm is obtained, which is much smaller than previous reports. The continuous transition of irregular eutectic to regular eutectic and to amorphous structure is found under rapid solidification, and the instability mechanism of eutectic growth is discussed. On the basis, the corresponding eutectic growth rate during rapid solidification, which is calculated by Jackson-Hunt model and Trivedi-Magnin-Kurz model, is between 8.65 μm/s ~ 5.32 × 10 4 μm/s.

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.

Transient liquid phase (TLP) bonding of IN738LC superalloy was carried out using a rapidly solidified MBF-15 Ni-based foil. The effects of bonding temperature (1130–1170 °C) and time (5–120 min) as well as foil thickness (35–140 µm) were studied on the microstructure of joint region and its mechanical properties. The solidification sequence in the joint region was found to be (i) formation of γ solid solution in the isothermally solidified zone, followed by (ii) ternary eutectic of γ + Ni3B + CrB, and finally (iii) binary eutectic of γ + Ni3Si in the athermally solidified zone. Fine Ni3Si particles were also formed via a solid state transformation within the γ matrix in the vicinity of eutectic products. A deviation of isothermal solidification kinetics from the standard parabolic TLP model was observed by increasing the bonding temperature to 1170 °C, which resulted in the formation of eutectic constituents at the joint centerline. The analysis of mechanical and fractographic test results revealed that the samples with complete isothermal solidification exhibit the highest shear strength, whereas the hard eutectic constituents act as preferential failure sites and lead to a significant reduction in the joint shear strength in samples with incomplete isothermal solidification.

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.

A Ge thin film deposited by chemical vapor deposition (CVD) was used to obtain a uniform bonding between Au and Ge films for applications of wafer level packages (WLPs). This Ge CVD thin film showed selective growth on Au and Cu metals when the substrate has both metal and oxide. A one-step and two-step Ge deposition followed by eutectic bonding method was employed to bond the wafers. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy equipped with an energy dispersive spectroscopy (FESEM-EDS), atomic force microscopy, high resolution Field emission transmission electron microscopy, IR inspection tool and secondary ion mass spectroscopy (SIMS). According to the IR inspection results, the two-step Ge deposited sample showed more uniform film compared to one-step deposition after eutectic bonding. Moreover, an improved bonding quality was obtained from the two-step process. Based on FESEM observations, a uniform and crater-free interface was detected between the bonded 4-inch wafers, in which the presence of Ge beside Au and Si was confirmed by EDS. SIMS profiles proved the formation of a thin Au-Ge interlayer at the bonded interface, which enhanced the bonding conditions.

In this study, the mechanism controlling the formation of a chemical bonding between a ceramic melt and a solid body prior to the melt solidification was investigated using the thermal spraying process as an example. The formation of a chemical bonding between TiO2, LCO, LZO, YSZ splats and ceramic substrates of the same material was investigated focusing on the effect of the deposition temperature. The bonding state was examined by studying both fractured cross-section samples and samples prepared utilizing the focused ion beam (FIB) technique. The microstructure at the interface of typical FIB-prepared cross section samples was examined by high-resolution transmission electron microscopy. The results show that an effective bonding at the splat interface can be formed only when the deposition temperature is higher than a critical bonding temperature. The critical bonding temperature linearly increases with the melting point of the ceramic material. The interface temperature, directly influencing the bonding formation, was calculated utilizing a one-dimensional heat transfer model. The maximum interface temperature corresponding to the critical bonding temperature was found to be close to the glass transition temperature of the splat material. Thus, the concept of an intrinsic bonding temperature and a sufficient condition for the formation of an efficient bonding are proposed. Furthermore, a model for the bonding mechanism at the melt/solid interface is established to explain the bonding formation and the microstructure at the interface.

The interaction between Eu and P in high purity Al-7Si alloys was investigated. Interrupted solidification experiments were undertaken to study macrostructure evaluation and thermal analyses were performed to obtain characteristic temperatures of the eutectic reaction. Without the presence of Eu, increasing P concentrations results in a significant increase in the number of eutectic grains due to the formation of AlP particles, which can act as nucleation sites for eutectic Si. With the presence of Eu, a decrease in nucleation frequency and thereby an increase in eutectic grain size was observed in alloys containing 5 and 30 ppm P due to the interaction between Eu and P, which can be further supported by the results of μ-XRD patterns and SADP of TEM. It was found that most P-rich particles were covered by Eu-rich intermetallics, which are indexed to be Al2Si2Eu phase, indicating that the formation of the pre-eutectic Al2Si2Eu phase may lead to a depletion of pre-existing AlP particles or residual P in the melt. This investigation highlights the effect of the interaction between Eu and P on the nucleation of eutectic Si and eutectic grains.

The present study investigated the eutectic bonding on the long-term performance of flip-chip LEDs under varying aging stresses. A direct eutectic bond was used to adhesive the power LED chip to the Al2O3 ceramic substrate. The flip-chip structure was capable to perform both high thermal conductance and good light extraction efficiency of the LED device. The varying sputtered copper thickness on the ceramic substrate was investigated to reduce junction temperature. The experimental results showed that flip-chip LEDs using eutectic bonding exhibited superior die shear strength and also performed good long-term reliability after applying 1000 h aging test under various ambient temperature and relative humidity (RH). Thermal resistance decreases as thickness of copper was decreased from 60 μm to 15 μm. The lumen maintenances were above 98.5% and 104% for the eutectic flip-chip LEDs under temperature of 298 K and 358 K, respectively. When the RH was increased from 50% to 90%, lumen maintenance was higher than 100% for all eutectic flip-chip LEDs. However, the luminous decay was at 31.6% for sliver pate LED sample at 90% RH after aging tests. The experimental results are applicable for optimizing the direct eutectic bonding process and for designing highly reliable LED devices.

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 directionally solidified Al2O3/MgAl2O4/ZrO2 ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al2O3/MgAl2O4/ZrO2 eutectic ceramic rods with diameters of 10 mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al2O3, MgAl2O4 and ZrO2 phases. In the eutectic microstructure, the MgAl2O4 and Al2O3 phases form the matrix, the ZrO2 phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO2 phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12 GPa and 6.1 MPa·m 1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO2 phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.

We present a thorough characterization of the thermal properties and thermal reliability of the eutectic mixture of decanoic acid with tetradecanoic acid, as a phase change material (PCM) of potential interest for passive temperature control in buildings. From the temperature-composition binary phase diagram we found that the eutectic composition is 0.82 ± 0.02 mole fraction (78 ± 2 mass%) decanoic acid. We thoroughly characterized the thermal properties of the eutectic mixture. The eutectic composition has a high latent heat of fusion Δ fus H = 153 ± 15 J g − 1 and a melting temperature T onset = 20.5 ± 1.5 °C. The heat capacity measured as a function of temperature for the solid and liquid phases just below and above the melting point is 1.9 and 2.1 ± 0.2 J K − 1 g − 1 , respectively. The average value of the thermal conductivity of the solid phase measured between −33 and 9 °C is κ s = 0.20 ± 0.02 W m − 1 K − 1 and for the liquid phase, the thermal conductivity is κ l = 0.23 ± 0.03 W m − 1 K − 1 for 28 and 38 °C. The mixture has a good long-term thermal stability as indicated by negligible changes in Δ fus H and T onset after 3000 melt–freeze cycles. The parameters determined in this work allow more accurate modeling and optimization of the behavior of the eutectic mixture in preparation for implementation as a thermal energy storage PCM.

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.

T0 curves in the phase diagrams have been proven useful to understand thermodynamically glass formation in metallic alloys, emphasizing the importance of the metastable solid solutions crystallized partitionlessly. Here we focus on four typical binary eutectic alloys with distinct glass-forming abilities and interatomic interactions, Ag60Cu40, Sb17.5Pb82.5, Au81.4Si18.6, and Ni24Zr76. The thermodynamics involved in the liquid–solid solution transition at T0 temperatures for the alloys of eutectic compositions are quantified, and the validity of the thermodynamic properties is evaluated. The comparison of the melting entropies for the equilibrium and partitionless solidifications reveals a basic relation. Based on the thermodynamics of the equilibrium phases and the solid solutions, an understanding of the glass formation of metallic alloys is proposed.

In order to improve the hardness and toughness simultaneously, B 4 C–HfB 2 –SiC ternary composites were synthesized by arc-melting using B 4 C, HfB 2 and β-SiC powders as raw materials. The ternary eutectic composition of 45B 4 C–15HfB 2 –40SiC (mol%) was first identified with a lamellar eutectic texture. HfB 2 about 500 nm in thickness and SiC about 700 nm in thickness were uniformly dispersed in B 4 C matrix of the eutectic composite, much smaller than raw powders. The Vickers hardness and fracture toughness of B 4 C–HfB 2 –SiC ternary eutectic were 35.7 GPa and 6.5 MPa m 1/2 at the eutectic composition, respectively.

We report a eutectic-type crystallization phenomenon during the fracture of a Zr-based bulk metallic glass. The eutectic crystalline colonies are found to have an average visible diameter of 390±190 nm. This process gives a direct evidence for the deformation-induced crystallization caused by temperature rise, which can reach as high as 1000 K. Meanwhile, the occurrence of eutectic crystallization is found to be closely correlated with the plasticity of bulk metallic glass.

Directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 (EAG) eutectic in situ composites are prepared by laser floating zone melting (LFZM) to investigate the solid–liquid interface characteristic and growth rate range under non-equilibrium solidification conditions. The solid–liquid ( S–L ) interface is in situ obtained by rapidly quenching, and its microstructure morphology and primary phase based on different eutectic compositions are analyzed. In stable growth zone, the composite presents typically “Chinese script” (CS) irregular eutectic structure consisting of interpenetrated Al 2 O 3 and EAG phases, but in quenched region the regular eutectic lamellae and CS structure are coexisted. Primary Al 2 O 3 phase in hypoeutectic composition is found both in S–L interface front and quenched region. Different from the stable growth zone, in quenched region as the solidification rate increases, the eutectic lamellae spacing does not show obvious decrease. The minimum eutectic lamellae spacing is refined to about 200 nm when the solidification rate is increased up to 100 μm/s. On the basis, according to the Jackson-Hunt (J-H) model, the maximum solidification rate in quenched region is calculated to be smaller than 1.26 × 10 3 μm/s, and the undercooling degree is 4.15 K.

The electro-migration test of Sn-15Bi solder joint was conducted at 150 °C with a current density of 160 A/cm 2 . The near eutectic phase layer thickening as the extending of electro-migration time was observed on the anode side interface of Sn-15Bi solder joint. The Bi fraction of cathode side reduced as the increase of electro-migration time. The Bi atom of near eutectic phase layer was from solid solution Bi atom in β-Sn phase. The diffusion of Bi atom was up-hill diffusion under the effect of electron flow.

Vertically-aligned nanostructures are of interest for novel system-on-a-chip applications including sensing and energy storage. Templated synthesis of such structures has been previously demonstrated via anodized aluminum oxide (AAO). However, the performance of AAO-on-Si devices has been limited by current methods of obtaining well-adhered, thick AAO templates on Si substrates. Thicker AAO templates allow for longer nanostructures of active material, which is expected to increase performance of devices reliant on the surface area or volume of the active material. In this study, Al–Si eutectic bonding is used to achieve thick AAO-on-Si, with templates up to 90 μm thick bonded to the wafer. Carbon nanotubes (CNTs) are grown in the AAO templates bonded to Si to create proof-of-concept on-chip electrochemical double layer capacitors (EDLC) devices. Our devices show a promising value of 0.44 mF cm−2, much higher than previous AAO capacitors on Si chips. The structure of the CNT/AAO/Al/Si stack is examined using SEM and TEM, and the device performance is tested through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) measurements.

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.

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.

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.

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.