Anomalous Eutectic Structure Research Articles

Directional solidification of the Al0.8CrFeNi2.2 eutectic high-entropy alloy

The concepts of high-entropy alloys and eutectic in-situ composites have been applied as an interesting approach to achieve materials with an improved balance between strength and ductility. In this study, the Al0.8CrFeNi2.2 eutectic high-entropy alloy (EHEA), formed by the ordered body-centered cubic (B2) NiAl-rich and face-centered cubic (FCC) CrFeNi-rich phases, was fabricated through arc melting and directionally solidified in a Bridgman setup at translation rates of 1.4–14 μm/s. At the lowest rates, growth occurred with a planar solid–liquid interface, resulting in a highly regular lamellar array. As the translation rate increased, an interface transition from planar to cellular occurred, resulting in eutectic grains surrounded by layers of anomalous eutectic structure. The dependence of the lamellar spacing on the growth rate was established, along with the relationship between them and the Vickers hardness. Finally, the mechanical behavior was assessed by tensile tests, revealing that the translation rate increases the yield strength without sacrificing ductility. The directionally solidified Al0.8CrFeNi2.2 EHEA exhibited a maximum yield strength of approximately 558 MPa with reasonable ductility (total elongation of approximately 6.5 %) under the analyzed conditions.

Thermophysical properties and rapid solidification mechanism of liquid Zr<sub>60</sub>Ni<sub>25</sub>Al<sub>15</sub> alloy under electrostatic levitation condition

In this study, the thermophysical properties and rapid solidification mechanism of highly undercooled liquid Zr<sub>60</sub>Ni<sub>25</sub>Al<sub>15</sub> alloy are investigated through the electrostatic levitation technique. The maximum undercooling of this alloy reaches 316 K (0.25<i>T</i><sub>L</sub>). Both density and surface tension display a linear relationship with temperature, while viscosity is related to temperature exponentially. When alloy undercooling is less than 259 K, two significant recalescence events are observed during solidification, corresponding to the formation of pseudobinary (Zr<sub>6</sub>Al<sub>2</sub>Ni + Zr<sub>5</sub>Ni<sub>4</sub>Al) eutectic and ternary (Zr<sub>6</sub>Al<sub>2</sub>Ni + Zr<sub>5</sub>Ni<sub>4</sub>Al + Zr<sub>2</sub>Ni) eutectic. The growth velocity of the binary eutectic phase gradually increases with further undercooling and reaches a maximum undercooling value of 259 K. In contrast, once undercooling exceeds 259 K, a single recalescence event occurs, leading to the independent nucleation of all three compound phases from alloy melt and the rapid growth of a ternary anomalous eutectic structure. Notably, the growth velocity of the ternary eutectic phase exhibits a gradual decline with further undercooling. This diminishing trend of the growth velocity suggests that further undercooling might entirely suppress crystal growth dynamically at a threshold of 385 K. With classical nucleation theory and the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model, the onsets of crystallization for the three phases are calculated, thereby constructing a time–temperature-transformation (TTT) diagram. This diagram elucidates the competitive nucleation among the three phases in the undercooled melt. Both theoretical and experimental evidence reveal that Zr<sub>6</sub>Al<sub>2</sub>Ni phase is primarily nucleated at lower undercooling levels, whereas under higher cooling condition, it is possible for all three phases to nucleate simultaneously.

Anomalous eutectic formation in undercooled Ni−P eutectic alloy

To investigate the relationship between anomalous eutectic and the remelting of primary phase, thin samples of Ni−19.6at.%P eutectic alloy were solidified over a range of undercoolings with temperature recalescence depressed. Fully regular eutectic structures were obtained. The thin samples were then rapidly heated to certain temperatures between the nucleation and eutectic temperatures, resulting in anomalous eutectic structure. It was found that the volume fraction of anomalous eutectic increased with increasing target temperature. α-Ni particulates were in principle randomly oriented while β-Ni3P phase was well oriented in the anomalous eutectic formed upon heating. Considering that α-Ni is a solid solution while β-Ni3P is a stoichiometric intermetallic compound, it is unambiguous that anomalous eutectic forms due to remelting of the primary regular eutectic.

Formation and Morphological Transition of Diversified Eutectic Structures in Ag-Based Brazing Filler Metals with Sn Addition

Eutectic structures in Ag-based brazing filler metals play an important role in improving the properties of alloys and brazed joints. The typical microstructures and morphological evolution of Ag-Cu eutectic structures in Ag-based brazing filler metals solidified at various cooling rates and with the addition of different Sn contents were investigated. The formation mechanisms of anomalous eutectic structure (AES) and the morphological transition from lamellar eutectic structure (LES) to AES were elucidated. The microstructures in alloys solidified at a slow cooling rate and with low Sn content consisted of the full AES, which was attributed to the remelting of the LES during recalescence. Increasing the Sn content and cooling rate promoted the formation of developed Ag dendrite, eutectic dendrite with branches, and AES with large dimensions. The AES in rapidly solidified filler metals with a high Sn content was ascribed to the decomposition of the Ag-rich dendrite skeleton and the growth of Cu-rich between the ternary dendrite arms.

Designing lightweight dual-phase refractory VNbTiSi-based eutectic high-entropy alloys for use at elevated temperatures

The emergence of high-entropy alloys (HEAs) provides a vast compositional space for designing high temperature materials for next generations of advanced propulsion systems. In this study, novel lightweight (about 6.0 g/cm3 in density), refractory (melting point higher than 1630 °C) VNbTiSi-based dual-phase eutectic high entropy alloys (EHEAs) that consist of body-centered cubic (BCC) solid solution and tetragonal-structured silicide phase are designed. The as-cast compressive strength at room temperature (RT) can be as high as 2.4 GPa and the RT fracture toughness achieves 24.58 MPa∙m1/2, higher than many of the as-cast and even directionally solidified Nb–Si based alloys. The tensile yield strength and elongation at elevated temperature (1100 °C) can be above 300 MPa and 10%, respectively. The anomalous eutectic structure with rod-like silicide phase dispersed in the continuous BCC network is revealed, and two kinds of orientation relationships between the two phases are identified: {100}BCC//{110}Silicide (or <100>BCC//<110>Silicide), and <111>BCC//[011]Silicide or <111>BCC//[101]Silicide with no paralleled planes observed. The BCC phase provides ductility both at room and elevated temperatures, where plastic deformation is dominated by the multiplanar glide of long straight screw dislocations, whereas the silicide phase undergoes no plastic deformation and acts as the strengthening phase at elevated temperatures. This study evidences the feasibility of phase-constituent simplification in a complex composition space based on the high entropy strategy and the possibility of optimizing materials on demand.

Microstructure and mechanical property evolution of additive manufactured eutectic Al-2Fe alloy during solidification and aging

This study presents the microstructure and mechanical properties of an additive manufactured recyclable Al-2Fe alloy fabricated by laser powder bed fusion (LPBF). The microstructure evolution of the Al-2Fe alloy aged at elevated temperatures and its impact on the mechanical properties were systematically investigated. Firstly, the single melt scanning experiment verified that Al-Al6Fe anomalous eutectic structure was initially formed from the liquid phase. Secondly, the as-built Al-2Fe contains nano-size and rod-shaped Al6Fe particles which uniformly dispersed in a supersaturated Al matrix. The Al6Fe particles with an average diameter of ~89 nm shows a coherent crystallographic relationship with the matrix as (046)Al6Fe // (2¯02¯)Al, and [032¯]Al6Fe // [1¯11]Al. The ultimate tensile strength and total elongation of 287.0 MPa and 12.0% were obtained in the ultrafine as-built Al-2Fe, respectively. Aging at elevated temperatures made the Al6Fe particles coarsened and transformed into Al13Fe4 particles at 350 °C. The amount of the total Al-Fe intermetallic compounds (Al6Fe+Al13Fe4) increased with the aging temperature due to the precipitation of the supersaturated Fe atoms. It exhibits a slow increase in grain size and a decrease in dislocation density in the Al-2Fe samples with the increase of aging temperature. All Al6Fe particles were transformed into coarse Al13Fe4 particles at 550 °C. Thirdly, the quantitative analysis indicates that the dominant contribution to the yield strength derives from the dispersion strengthening mechanism by creation of fine Al6Fe and Al13Fe4 particles. The results in this work demonstrate that ultrafine microstructure can be obtained in a simple binary Al-Fe alloy as a result of the extremely high cooling rate during LPBF process, which offers a design strategy towards high-strength recyclable aluminum alloys.

Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy

The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields. However, the desired fine primary lamellar eutectic structure (PLES) is usually replaced by a coarse anomalous eutectic structure (AES) when the undercooling prior to solidification exceeds a certain value. The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue. In this work, the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting. The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth (ECG) to a rapid eutectic-dendritic growth (EDG) above a undercooling of 72 K, accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures. Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence, respectively. The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations. Below 72 K, the destabilization of PLES formed by slow ECG is caused by the mechanism of “termination migration” driven by interfacial energy. While above 72 K, the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.

In Situ and Ex Situ Characterization of the Microstructure Formation in Ni-Cr-Si Alloys during Rapid Solidification-Toward Alloy Design for Laser Additive Manufacturing.

Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this work, the phase formation of four Ni-Cr-Si alloys was studied as a function of cooling rate and chemical composition using a liquid droplet rapid solidification technique. Post mortem x-ray diffraction, scanning electron microscopy, and in situ synchrotron microbeam X-ray diffraction shows the present and evolution of the rapidly solidified microstructures. Furthermore, the obtained results were compared to standard laser deposition tests. In situ microbeam diffraction revealed that due to rapid cooling and an increasing amount of Cr and Si, metastable high-temperature silicides remain in the final microstructure. Due to more sluggish interface kinetics of intermetallic compounds than that of disorder solid solution, an anomalous eutectic structure becomes dominant over the regular lamellar microstructure at high cooling rates. The rapid solidification experiments produced a microstructure similar to the one generated in laser coating thus confirming that this rapid solidification test allows a rapid pre-screening of alloys suitable for laser beam-based processing techniques.

Microstructural refinement and anomalous eutectic structure induced by containerless solidification for high-entropy Fe–Co–Ni–Si–B alloys

The mechanical properties of high-entropy alloys (HEAs) strongly depend on their microstructural features. In this work, the phase formation and morphologies of Fe26.7Co26.7Ni26.7Si8.9B11.0 HEAs fabricated under different undercooling levels by the drop tube technique together with subsequent quenching were investigated in detail. It was found when the droplet size decreases (i.e. the increase of the undercooling level), the grain refinement is easily achieved without any phase transformation for the present hypoeutectic HEA which is composed of the fcc dendrites and eutectic structure. After quenching under large undercooling levels, the liquid should be deeply undercooled into an asymmetric couple zone, leading to the primary formation of eutectic structure and the subsequent precipitation of the fcc dendrites. The direct product of rapid solidification after deep undercooling and subsequent recalescence may induce the decoupled eutectic growth, resulting in a transition from the regular into anomalous eutectic structures. Furthermore, the volume fractions of the eutectic structures and the fcc dendrites increase and decrease with decreasing droplet size, respectively. As a result, the Vickers hardness gradually increases with decreasing droplet size, which exhibits a linear relationship with the volume fraction of eutectic matrix. The present study may provide a simple way to tailor microstructures and mechanical properties of HEAs.

Multiple origins of anomalous eutectic microstructure in rapidly solidified Mg-Al alloy

Mg-33.3 wt.%Al eutectic alloys were fabricated by melt spinning at wheel speeds from 5 to 50 ms−1. SEM and S/TEM analyses showed the formation of anomalous eutectic structures at all wheel speeds. Four distinctive eutectic microstructures were observed, depending on the wheel speed, each with a unique formation mechanism. At low wheel speeds, solute trapping and high interfacial energy resulted in the remelting of primarily grown regular eutectic microstructures and was responsible for the formation of the anomalous eutectic Type A. A coupled-to-decoupled-growth transition occurred at intermediate wheel speeds giving rise to the anomalous eutectic Type B. The decoupled nucleation and growth of the eutectic phases followed by a coupled dendritic growth lead to the formation of flower-like microstructures at anomalous eutectic Type C. At sufficiently high wheel speed, no sign of regular eutectic was found and a unique cellular microstructure of ϒ phase along with inter- or intragranular α phase grains was formed, anomalous eutectic Type D, as a result of the decoupled nucleation and growth of the eutectic phases. The observed microstructures were examined thoroughly, and the correlated formation mechanisms were discussed on the basis of the SEM images and S/TEM and EDS analysis.

Modeling remelting induced destabilization of lamellar eutectic structure in an undercooled Ni-18.7 at.% Sn eutectic alloy

Achieving a high melt undercooling (ΔT) is expected as an effective strategy to significantly reduce lamellar spacing of bulk eutectic alloys. However, the undercooled eutectic alloys usually exhibit coarse anomalous eutectic structure (AES) when ΔT exceeds a certain value. In our recent work (Scripta Materialia, 177 (2020) 123–127), we showed that in an undercooled Ni-18.7 at.% Sn eutectic alloy the remelting induced destabilization of primary lamellar eutectic structure (PLES) during the post-recalescence period can lead to the formation of the AES using high-temperature laser scanning confocal microscopy technique. In this work, an analytical model within the framework of interface stability analyses is established to quantitatively describe the destabilization of PLES during the post-recalescence period. The model calculations show good agreement with the experimental results. The analyses based on the derived model indicate that the destabilization of PLES is driven by solute supersaturation and capillary force. The former amplifies the interface perturbation of the eutectic lamellae and initiates the destabilization, while the latter dominates the subsequent morphological transition process. We further showed that our model is consistent with the reported experimental results of other undercooled eutectic alloys.

In situ observation of remelting induced anomalous eutectic structure formation in an undercooled Ni-18.7 at.% Sn eutectic alloy

The forming mechanism of anomalous eutectic structure (AES) in undercooled eutectic alloys has been a controversial issue. In this work, we conducted in situ observation of the AES forming process in an Ni-18.7 at.% Sn eutectic alloy using a high-temperature laser scanning confocal microscope. Our results provide an evidence of the AES formation induced by remelting of primary eutectic lamellae constituted by Ni3Sn and α-Ni lamellae. During recalescence, a pronounced remelting of the Ni3Sn phase into liquid occurs, while the α-Ni lamellae subject to a rapid morphological transition into granules. Eventually, the completion of solidification leads to the AES formation.

Metastable solidification of hypereutectic Co2Si-CoSi composition: Microstructural studies and in-situ observations

Metastable solidification of undercooled Co60Si40 melts was investigated by microstructural studies and in-situ high-energy X-ray diffraction. Five solidification paths were identified. Three of them were observed at low undercoolings, which show uncoupled and coupled growth of stable β-Co2Si and CoSi compounds. The other paths were observed at high undercoolings, which show peritectic and primary crystallization of a metastable Co5Si3 compound. The β-Co2Si and Co5Si3 compounds crystallize into a hexagonal crystal structure and experience solid-state decomposition. Microstructure formation depends on solidification path. The coupled and uncoupled growth of the stable compounds produces a regular lamellar eutectic structure and an anomalous eutectic structure, respectively. The crystallization and solid-state decomposition of the metastable Co5Si3 compound brings about a fine-grained two-phase mixture, which represents another type of anomalous eutectic structure. The results provide proof of two rare mechanisms of anomalous eutectic formation and shed light onto metastable phase relations in the undercooled region of the CoSi system.

Effect of Heating Temperature on Microstructure of Directionally Solidified Ni-43Ti-7Al Alloy

By liquid metal cooling (LMC) process, the Ni-43Ti-7Al (at.%) alloy has been directionally solidified (DS) at different heating temperatures (1450°C, 1550°C, 1650°C) and a constant withdrawal rate of 100μm/s. The results showed that anomalous eutectic structures which consisted of Ti2Ni and TiNi phases were formed at the grain boundaries of as-cast sample and similar structures were also observed in the intercellular regions of DS samples. However, the microstructure changed from the equiaxial structure to the cellular structure due to the axial thermal gradients imposed. After DS, the NiTi and Ti2Ni phases preferentially grew along certain orientation, but the preferred crystallographic orientations of them changed as the heating temperature increased to 1650°C, which might be related to the change of melt structure. As expected, the volume fraction of Ti2Ni increased from 3.3% to 5.2% and the cellular spacing decreased from 47.8μm to 27.0μm as heating temperature increased. In addition, the stability of solid/liquid interface decreased, resulting from the coupling effects of G and ΔT- with the heating temperature increasing.

Rapidly solidified Ag-Cu eutectics: A comparative study using drop-tube and melt fluxing techniques

A comparative study of rapid solidification of Ag-Cu eutectic alloy processed via melt fluxing and drop-tube techniques is presented. A computational model is used to estimate the cooling rate and undercooling of the free fall droplets as this cannot be determined directly. SEM micrographs show that both materials consist of lamellar and anomalous eutectic structures. However, below the critical undercooling the morphologies of each are different in respect of the distribution and volume of anomalous eutectic. The anomalous eutectic in flux- undercooled samples preferentially forms at cell boundaries around the lamellar eutectic in the cell body. In drop-tube processed samples it tends to distribute randomly inside the droplets and at much smaller volume fractions. That the formation of the anomalous eutectic can, at least in part, be suppressed in the drop-tube is strongly suggestive that the formation of anomalous eutectic occurs via remelting process, which is suppressed by rapid cooling during solidification.

Three-dimensional reconstruction of anomalous eutectic in laser remelted Ni-30 wt.% Sn alloy

Laser remelting has been performed on Ni-30 wt.% Sn hypoeutectic alloy. An anomalous eutectic formed at the bottom of the molten pool when the sample was remelted thoroughly. 3D morphologies of the α-Ni and Ni3Sn phases in the anomalous eutectic region were obtained and investigated using serial sectioning reconstruction technology. It is found that the Ni3Sn phase has a continuous interconnected network structure and the α-Ni phase is distributed as separate particles in the anomalous eutectic, which is consistent with the electron backscatter diffraction pattern examinations. The α-Ni particles in the anomalous eutectic are supersaturated with Sn element as compared with the equilibrium phase diagram. Meanwhile, small wavy lamella eutectics coexist with anomalous eutectics. The Trivedi–Magnin–Kurz model was used to estimate undercooling with lamellar spacing. The results suggest that the critical undercooling found in undercooling solidification is not a sufficient condition for anomalous eutectic formation. Besides, α-Ni particles in the anomalous eutectic do not exhibit a completely random misorientation and some neighboring α-Ni particles have the same orientation. It is shown that both the coupled and decoupled growth of the eutectic two phases can generate the α-Ni + Ni3Sn anomalous eutectic structure.

Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

The drop-tube technique was used to solidify droplets of the Ni-25.3 at. pct Si alloy at high cooling rates. XRD, SEM, and TEM analysis revealed that the metastable phase, Ni25Si9, formed as the dominant phase in all ranges of the droplets, with γ-Ni31Si12 and β 1-Ni3Si also being present. Three different microstructures were observed: the regular and anomalous eutectic structures and near single-phase structure containing small inclusions of a second phase, termed here as heteroclite structure. Both eutectic structures comprise alternating lamellae of Ni25Si9 and β 1-Ni3Si, which, we conjecture, is a consequence of an unobserved eutectic reaction between the Ni25Si9 and β 1-Ni3Si phases. The matrix of the heteroclite structure is also identified as the metastable phase Ni25Si9, in which twined growth is observed in the TEM. As the cooling rate is increased (particle size decreased), the proportion of droplets displaying the entire heteroclite structure tends to increase, with its fraction increasing from 13.91 pct (300 to 500 µm) to 40.10 pct (75 to 106 µm). The thermodynamic properties of the Ni25Si9 phase were also studied by in-situ heating during XRD analysis and by DTA. This showed the decomposition of Ni25Si9 to β 1 and γ-Ni31Si12 for temperatures in excess of 790 K (517 °C).

Effect of exposure to alkaline solution on Sn–9Zn solder joints

The microstructural, phase and mechanical behaviors of Sn–9Zn solder bulk and joint in 6M potassium hydroxide solution were investigated by means of open circuit potential measurement. The anomalous eutectic structure of Sn–9Zn solder was completely replaced by sheet-like and flake-like corrosion products after immersion. Post-immersion phase analysis revealed that the mixture of Sn and Zn oxides dominated the surface. The preferential removal of Zn was determined to control the initial electrochemical reactions of the solder. Relatively poorer ultimate tensile strength than that of the as-prepared counterpart was obtained after immersion, revealing that the mechanical integrity of the Cu/Sn–9Zn/Cu solder joint was damaged by the preferential removal of Zn. The weak intermetallic compounds formed between the interfaces of the Sn–9Zn/Cu further reduced the interfacial strength of the Cu/Sn–9Zn/Cu solder joint.

The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

A melt encasement (fluxing) method was used to undercool Ag–Cu alloy at its eutectic composition. The recalescence of the undercooled alloy was filmed at a high frame rate. For undercoolings <60K, a microstructure consisting of mixed anomalous and lamellar eutectic is observed. Analysis of eutectic spacing in the lamellar eutectic reveals little dependence upon the undercooling of the bulk melt and is consistent with growth at an undercooling of 1.5K. Depending upon undercooling, the progress of the recalescence front may be either continuous or spasmodic, wherein periods of rapid growth are separated by significant interludes in which growth totally arrests. Analysis of spot brightness profiles reveals that, during continuous growth, the recalescence is characteristic of the advancement of a planar, space-filling front, while a double recalescence occurs during spasmodic growth, the first of which is characteristic of the propagation of a dendritic, or non-space-filling, front. It is concluded that, during spasmodic growth, the propagation of two-phase, or eutectic, dendrites is observed, which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling.

Microstructure evolution of directionally solidified Ni–43Ti–7Al alloy during heat treatment

The microstructure change of directionally solidified Ni–43Ti–7Al alloy after heat treatment has been investigated by transmission electron microscopy and back scattered electron imagings in this paper. After solution and aging treatment, the NiTi + Ti2Ni anomalous eutectic structure nearly cannot be observed and Ti2Ni phases become spheroidized. The β′-Ni2TiAl precipitates are nearly spherical at the early stage of aging at 800 °C (0.1–1 h); however, they become aligned along 〈100〉 directions and change to cubic shape after aging for 20 h. In the course of further aging, the coarsened semi-coherent β′ precipitates occur preferentially in intercellular regions. Then the coarsened β′ precipitates begin to occur in intracellular regions with the increasing aging time and aging temperature. These β′ precipitates change the shape to sphere and plate, accompanied with loss of their coherency by introducing interface dislocations surrounding them. Finally, the formation mechanism of β′ precipitates are discussed compared to other studies in the Ni–Ti–Al system.