Lamellar Eutectics Research Articles

Nanoindentation behaviors of the AlCoCrFeNi2.1 eutectic high-entropy alloy: The effects of crystal structures

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA), as a kind of in-situ composite materials, have achieved good match of strength and plasticity due to its typical eutectic lamella structures. Although many previous studies have investigated the indentation behaviors of the AlCoCrFeNi2.1 EHEA, there are still some controversies about the hardness characteristic of different crystal structures, requiring further clarification. Accordingly, in this study, nanoindentation tests were conducted to investigate the indentation behaviors of different crystal structures (primary L12 phase, primary B2 phase, and the eutectic phase region). The residual indentation and the crystallographic orientations were characterized by the laser scanning confocal microscope, scanning electron microscope, and electron backscatter diffraction. The hardness in different crystal structures were comparatively analyzed and the material deformation mechanisms were discussed correspondingly. The statistic results showed that the eutectic phase region had the lowest hardness, and primary B2 phase had the highest hardness. Furthermore, the slip lines in the primary L12 phase were evenly distributed near the residual indentation, but the slip lines in the eutectic phase region were concentrated near the phase boundaries. These findings are expected to deepen the understanding of the microscale deformation behaviors of EHEAs.

Surface wave phenomenon and microstructure evolution mechanism of electromagnetically levitated ternary Ti-Al-Fe alloy

Electromagnetic levitation technique was performed to investigate the surface wave phenomenon and microstructure evolution mechanism of undercooled ternary Ti50Al30Fe20 alloy. Under the condition of high undercooling, the formation process of surface waves on the levitated alloy surface were directly detected by high-speed camera. The cooling gas provided the external perturbation that promoted surface nucleation, providing the suitable site for surface wave generation. On the rotating semisolid alloy surface, the competition between centrifugal force and surface tension caused the surface wrinkling, leading to the formation of surface waves. The residual liquid phase was solidified in the vortex circulation flow, and the vortex-shape pattern was preserved during rapid solidification. With the increases of undercooling, the microstructure evolved from dendrites and interdendritic lamellar eutectics into uniform anomalous eutectics because of the independent nucleation and competitive growth of (βTi) and TiFe phase. The surface wave was formed only at high undercoolings, indicating this phenomenon was related to the fluidity in eutectic growth. In this case, the uniform microstructure was closer to satisfying the continuum medium hypothesis, and the homogeneous medium reduced the energy damage during wave propagation. In addition, the micromechanical properties were remarkably increased as undercooling rose owing to the microstructural morphology and solute trapping effect.

Significantly Promoting the Thermal Conductivity and Machinability of Negative Thermal Expansion Alloy via In Situ Precipitation of Copper Networks.

Rapid advancements in electronic devices yield an urgent demand for high-performance electronic packaging materials with high thermal conductivity, low thermal expansion, and great mechanical properties. However, it is a great challenge for current design philosophies to fulfill all the requirements simultaneously. Here, an effective strategy is proposed for significantly promoting the thermal conductivity and machinability of negative thermal expansion alloy (Zr,Nb)Fe2 through eutectic precipitation of copper networks. The eutectic dual-phase alloy exhibits an isotropic chips-matched thermal expansion coefficient and a thermal conductivity enhancement exceeding 200% compared with (Zr,Nb)Fe2, along with an ultimate compressive strength of 550MPa. The addition of copper reorganizes the composition of (Zr,Nb)Fe2, which smooths the magnetic transition and shifts it toward higher temperature, resulting in linear low thermal expansion in a wide temperature range. The highly fine eutectic copper lamellae construct high thermal conductivity networks within (Zr,Nb)Fe2, serving as highways for heat transfer electrons and phonons. The in situ forming of eutectic copper lamellae also facilitates the mechanical properties by enhancing interfacial bonding and bearing additional stress after yielding of (Zr,Nb)Fe2. This work provides a novel strategy for promoting thermal conductivity and mechanical properties of negative thermal expansion alloys via eutectic precipitation of copper networks.

A new test for evaluating the cracking susceptibility of laser cladding coatings and its validity on assessing the effect of laser power

Cracking is one of the most common and destructive defects and Ni-based superalloys are susceptible to hot cracking during laser cladding. In this study, a new test was developed to evaluate the cracking susceptibility of materials during laser cladding, and the crack rate was used as an evaluation criterion for cracking susceptibility. Three groups of different laser powers (1.2 kW, 1.5 kW, and 1.8 kW) were selected to assess the validity of test in terms of effect of laser power on the cracking susceptibility. The cracking rates of coatings were 15.5 %, 20.4 %, and 22.0 %, respectively. It was observed that the cracking rate increased with increasing laser power, as the crack depth also increased due to the effect of the laser power on the substrate with a processed gap. The reinforcements, which were partially melted WC particles and in-situ generated reinforcements, had a similar impact on crack propagation. If there were no microcracks within the reinforcement, the crack bypassed the reinforcement and continued to propagate along the interface between the reinforcement and the matrix. When microcracks were present within the reinforcement, the stress concentration caused the crack to propagate through the microcracks within the reinforcement, crossing the reinforcement and propagating forward. Additionally, a special cored-eutectic microstructure, consisting of inner ceramic particles and fine lamellar eutectics, was found in coatings created using 1.2 kW.

Microstructure evolution and property regulation of CoCrNiNbx laser cladding coatings

The influence of different Nb on the microstructure evolution, properties of the CoCrNi laser cladding coatings was investigated. With the increase of Nb addition, the eutectic structures showed a transition from planar crystals to cellular crystals to dendritic crystals. Besides, the eutectic lamellae gradually became dense. A large amount of Ni3Nb-HCP structure precipitates were precipitated in the coatings with excessive Nb. The introduction of Nb brought about an increase in heterogeneous nucleation cores, which in turn significantly refined the grain size in the secondary remelting zone of the cladding coating. As the Nb content increased, the hardness of coatings gradually increased, but both wear resistance and corrosion resistance showed a trend of first increasing and then decreasing.

Strong-Yet-Ductile Eutectic Alloys Employing Cocoon-Like Nanometer-Sized Dislocation Cells.

Eutectic alloys (EAs) with superior fluidity are known to be the easiest to cast into high-quality ingots, making them the alloys of choice for making large-sized structural parts. However, conventional EAs (CEAs) have never reached strength-ductility combinations on par with the best in other alloy categories. Via thermomechanical processing of cast Ni-32.88wt%Fe-9.53wt%Al CEAs, a cocoon-like nano-meshed (as fine as 26 nm) network of dislocations (CNN-D) is produced via recovery annealing, through the rearrangement of cold-work-accumulated dislocations anchored by dense pre-existing nanoprecipitates. In lieu of traditional plasticity mechanisms, such as TWIP and TRIP, the CNN-D is particularly effective in eutectic lamellae with alternating phases, as it instigates nanometer-spaced planar slip bands that not only dynamically refine the microstructure but also transmit from the FCC (face-centered-cubic) layers into the otherwise brittle B2 layers. These additional mechanisms for strengthening and strain hardening sustain stable tensile flow, resulting in a striking elevation of both strength and ductility to outrank not only all previous CEAs, but also the state of the art-additively manufactured eutectic high-entropy alloys. The CNN-D thus adds a novel microstructural strategy for performance enhancement, especially for compositionally complex alloys that increasingly make use of nanoprecipitates or local chemical order.

Unraveling the Effect of Elastic Stress on the Destabilization Process of Ni–Sn Eutectic Lamellae at an Elevated Temperature

Unraveling the Effect of Elastic Stress on the Destabilization Process of Ni–Sn Eutectic Lamellae at an Elevated Temperature

The influences of the mechanical equilibrium among interfacial tensions at triple junction on the morphological evolution of eutectic lamella

In this work, the influence of the mechanical equilibrium among interfacial tensions at three-phase triple junctions on the morphological evolution of growing eutectic lamella was quantitatively investigated through the phase-field simulations. We found that, when eutectic lamella grows at the steady-state, the orientations of the three interfacial tensions under mechanical equilibrium state are highly coincident with the tangent lines of the contour of the corresponding interfacial morphologies at the three phases’ triple junction. It is argued that the eutectic lamellar growth is unable to reach the steady state when the interfacial tensions fail to achieve mechanical equilibrium at the triple junctions. Moreover, a strong dependence of interfacial morphology on the state of the mechanical equilibrium between interfacial tensions is determined, and quantified through the change of lamella spacing and local curvature radius of lamella with the interfacial tensions. Furthermore, the underlying mechanism about the effects of mechanical equilibrium state on eutectic morphological evolution is also explained. We conclude that the final steady-state of eutectic growth is determined by the complex interplay among solute diffusion, eutectic morphology and the mechanical equilibrium of interfacial tensions.

Misoriented Lamellar Microstructures in Mo‐Si‐Ti Alloy Due to Asymmetrical Nucleation Distance and Interfacial Energies: A Phase‐Field Analysis

The material properties are crucially affected by the microstructure formed during solidification, which is typically divided into three stages: (I) Early stage of nucleation in molecule scale, which is beyond the scope of the mean‐field model; (II) Middle stage of nucleation, where the dispersed nuclei have formed; (III) Late stage of nucleation, where the nuclei contact with each other. In previous studies, the formation of the stable eutectic lamellae is mostly based on the assumption that a stable solid–solid interface has already been established, corresponding to the stage (III), and the growth stage (II) is often overlooked. In the current work, by varying the nucleation density and distance, an alternative mechanism for the misoriented microstructure formation in Mo‐Si‐Ti alloy is proposed, which considers nucleation stage (II). Furthermore, the misoriention angle as a function of the nucleation density, distance, and the interfacial energies is quantified by systematic phase‐field simulations. The simulated composition distribution reveals the mechanism for the misorientation of eutectic lamellar pairs, which becomes more pronounced when the solids‐fluid interfacial energies are unequal. It is expected that the present work provides a potential perspective for the fundamental understanding of misoriented microstructures in solidification.

Microstructure evolution in undercooled Ag-50at. %Cu hypereutectic alloy

Ag–Cu alloys, as one type of typical eutectic solders, have been widely used in microelectronic packaging. The welding performance of Ag–Cu solders is closely associated with their microstructures. However, the microstructure evolution of Ag–Cu alloys during solidification is still poorly understood. In this work, a new melt fluxing technique was proposed to investigate the microstructure evolution of undercooled Ag-50at.%Cu hypereutectic alloy. The maximum undercooling of Ag-50at.%Cu alloy can reach up to 135 K. The obtained results show that the microstructures consist of primary dendrite and lamellar eutectic at low undercooling (ΔT – 10 K). With the increase of undercooling, dendrites undergo refining (ΔT < 65 K) and then disappear completely (ΔT = 78 K) and appear again (ΔT = 119 K). It is also discovered that cellular lamellar eutectics formed with ΔT within range of 65 K–78 K. Unexpectedly, irregular microstructures are observed when ΔT > 135 K. Theoretical calculation results indicate that the formation of irregular eutectic is associated with the dendritic fragmentation and eutectic coarsening under larger undercooling (ΔT = 135 K). These results not only elucidate the relationship between the solidification microstructure and undercooling of Ag-50at. %Cu hypereutectic alloys but also has positive significance for other hyper-eutectic systems.

Transition from micro-rod to nano-lamella eutectics and its hardening effect in niobium/silicide in-situ composites

To improve the mechanical properties of niobium/silicide in-situ composites via rapid solidification, the evolution of eutectic geometry and the corresponding hardening effect in a prototype Nb–18Si (at.%) composite upon electron beam surface melting (EBSM), i.e., a rapid remelting and solidifying sequence, were studied. Results show that rod-like Nb solid solution (Nbss)/Nb3Si eutectics prevail in the arc-melted state, yet evolve into lamellar arrangements after EBSM. Atomic scale scanning transmission electron microscopy (STEM) and near-atomic scale atom probe tomography (APT) were employed to characterize the three-dimensional stacking of nano-laminated Nbss/Nb3Si eutectics and their compositions. Compared with the rod-like eutectics, the lamellar eutectics via EBSM demonstrate a prominent eutectic refinement (39.5 nm in spacing) and an increased volume fraction of Nbss (~41%). Nano-indentation testing reveals that with the microstructural transition from micro-rod to nano-lamella eutectics, a significant increment in hardness up to 13.9 GPa is achieved.

Influence of Strengthening Elements and Heat Treatment on Microstructure and Fracture Toughness of NiAl-Cr(Mo)-Based Eutectic Alloy.

Due to their potential improvement of high-temperature properties, the refractory metal hafnium (Hf) and the rare earth holmium (Ho) have attracted much attention. In the present research, NiAl-Cr(Mo) eutectic alloys with different Ho and Hf additions were fabricated by conventional smelting method and heat-treated to study the synergetic influence of strengthening elements and heat treatment. The samples were characterized using XRD, SEM, and TEM, and the three-point bending test was performed to obtain fracture toughness. The results demonstrate that Hf addition leads to the formation of Ni2AlHf Heusler phase and that Ho promoted the formation of Ni2Al3Ho phase. The microstructure of the alloy is affected by thermal treatment, with the coarsening of eutectic lamellae after heat treatment. The mechanical properties are improved by Hf and Ho additions, with increased fracture toughness. Overall, this study provides insights into the microstructure and properties of NiAl-Cr(Mo) eutectic alloys and highlights the potential of Hf and Ho addition to improve room-temperature properties. Specifically, the as-cast NiAl-Cr(Mo)-Hf eutectic alloy contains a relatively fine NiAl/Cr(Mo) eutectic lamella but coarse eutectic cell with Ni2AlHf phase embellished along the cell boundary. Minor Ho addition induces the formation of Ni2Al3Ho phase, which leads to the coarsening of the intercellular region but contributes to the refining of eutectic cell. In addition, the synergetic effect of Ho and Hf promotes the precipitation of Ni2Al3Ho and Ni2AlHf phases in the intercellular zone and increases the interface dislocations. Heat treatment benefits the solid solution of Ni2Al3Ho and Ni2AlHf phases, which improves their size and distribution by secondary precipitation. The Ni2AlHf phase in the NiAl-Cr(Mo)-Hf eutectic alloy becomes fine and uniformly distributed, but the NiAl/Cr(Mo) eutectic lamella in the eutectic cell becomes coarse. In comparison, heat treatment mainly optimizes the size and distribution of the Ni2Al3Ho and Ni2AlHf phases in the NiAl-Cr(Mo)-Hf-Ho eutectic alloy. Furthermore, heat treatment helps to eliminate the interface dislocations in the large NiAl precipitates and the NiAl/Cr(Mo) phase interfaces, which also contributes to fracture toughness by decreasing stress concentration. Minor Ho addition decreases the fracture toughness of as-cast NiAl-Cr(Mo)-Hf eutectic alloy from 6.7 to 6.1 MPa·m1/2, which should be ascribed to the coarsened intercellular region including aggregated Ni2Al3Ho and Ni2AlHf phases. However, minor Ho-doped NiAl-Cr(Mo)-Hf eutectic alloy obtained the highest fracture toughness of 8.2 MPa·m1/2 after heat treatment. This improved fracture toughness should be mainly attributed to the refined and well-distributed Ni2Al3Ho and Ni2AlHf phases in the heat-treated NiAl-Cr(Mo)-Hf-Ho eutectic alloy.

Magnetic Field-Dependent Microstructure Evolution of Solidified Co39.2Ni39.2Al21.6 Eutectic Medium-Entropy Alloy

A (Fe, Cr)-free Co39.2Ni39.2Al21.6 eutectic medium-entropy alloy (EMEA) was designed and fabricated to study the microstructure and its evolution during slow solidification under different intensities of high static magnetic field (0 T, 5 T and 10 T). It was found that the original microstructure was characterized by FCC/BCC mixed herringbone eutectics consisting of two types of lamellar structures: a curved and wormy anomalous eutectic in the fringe, and a straight and long regular eutectic in the center. Nano-sized L10-type martensite layers are also distributed on the BCC lamellar as the martensitic transformation product. The FCC and BCC phases were enriched in Co and Al elements, respectively, while Ni element was distributed homogenously in both phases. With increasing magnetic field intensity, the herringbone eutectic structures remained stable, without the formation of a primary phase, while the phase constitution and the orientation relationships in the eutectic structures remained unchanged, with no obvious magnetically induced alignments. However, the lamellar spacing of the regular lamellar eutectic decreased significantly from 3.3 μm (0 T) to 1.93 μm (10 T); by contrast, the volume fraction of the anomalous eutectics increased considerably from 28.35% (0 T) to 55.14% (10 T), and the assumption that the imposed convection and destabilization of lamellar eutectics is controlled by the magnetic field is discussed in depth. Our results show a great potential for tailoring microstructures and properties by applying a strong magnetic field during the solidification process of EMEAs.

Revealing the phase-transformation path in a FeCoNiSnx eutectic high entropy alloy system by crystallographic orientation relationships

High entropy alloys are the focus of current research. An accurate description of their phase-transformation path, however, is a challenge when their phase constituent and transformation process are complex. In this study, a FeCoNiSnx eutectic high entropy alloy (EHEA) system was investigated and a novel FeCoNiSn EHEA composed of BCC + HCP phases was reported. The transition from the hypoeutectic to the fully eutectic and then to the hypereutectic microstructure with the Sn addition was characterized by the electron backscatter diffraction (EBSD) technology, and the phase-transformation path was clarified by crystallographic orientation relationships. The studies reveal that the primary phase of FeCoNiSnx (x = 0.2, 0.4) is FCC structure, and a further Sn addition induces an obvious phase-transformation from FCC to BCC in both the primary phase and eutectic lamellar, which satisfies the Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) variant orientation relationship. The mechanical results confirm that the phase structure and microstructure transition caused by Sn addition do significantly improve the strength and hardness of FeCoNiSnx EHEAs, but have serious adverse effects on plasticity. This study would be of significance to understanding the phase-transformation process in HEAs and preparing the HEAs with aimed mechanical properties.

Dual-phase synergistic deformation characteristics and strengthening mechanism of AlCoCrFeNi2.1 eutectic high entropy alloy fabricated by laser powder bed fusion

Eutectic high entropy alloy (EHEA) possesses promising prospects for industrial application due to its controllable and near-equilibrium dual-phase structure. Due to the advantages of high material utilization, efficient production, and design freedom, the laser powder bed fusion (LPBF) technique provides a new path to prepare EHEA components with complex structure and excellent performance. In this study, near fully dense AlCoCrFeNi2.1 samples were obtained by adjusting the process parameters of LPBF. Considering the balling phenomenon and powder splashing during the LPBF process, laser remelting was selected as an optimized scanning strategy to further improve the forming quality of AlCoCrFeNi2.1. The microstructure of remelted AlCoCrFeNi2.1 sample exhibited regular eutectic lamellae consisting of nano-scale face-centered-cubic (FCC) and B2 phases, in which the FCC phase accounted for a higher proportion. By investigating the tensile behavior and deformation mechanism, it was revealed that the ultrafine eutectic lamellae could induce a strong dual-phase synergistic strengthening, thereby significantly improving the strength of the sample. Compared with the vacuum induction melted (VIM) sample, the remelted sample showed a 54% increase in ultimate tensile strength (UTS -1518 MPa) and a 130% increase in yield strength (YS -1235 MPa) with reasonable plasticity. This study indicates that by combining the design and manufacturing freedom of LPBF with the EHEA, it is expected to fabricate high-property 3D EHEA parts, expanding the application field of EHEA.

On the growth kinetics of lamellar ternary eutectics of the αβγ-kind

On the growth kinetics of lamellar ternary eutectics of the αβγ-kind

A correlation of eutectic growth mechanism with resultant microhardness enhancement for undercooled Ni-47.7%Mo alloy

Liquid Ni-47.7 % Mo alloy was undercooled by 328 K (0.21 TE) and solidified under microgravity condition to investigate the correlation of microhardness enhancement with eutectic growth mechanism. The microstructural morphology transformed from (Ni) + NiMo lamellar eutectic to a duplex mixture of lamellar and anomalous eutectics, and finally into anomalous eutectic. With the increase of alloy undercooling, the eutectic interphase spacings decreased significantly, leading to a remarkable structural refinement and a conspicuous rise of Vickers hardness. The dependence of microhardness upon eutectic spacing was revealed for lamellar and anomalous eutectics, respectively.

Eutectic-reaction brazing of Al0.3CoCrFeNi high-entropy alloys using Ni/Nb/Ni interlayers

• A lamellar C14 Laves/γ phase eutectic structure formed in the brazing seam. • The distribution and morphology of Laves/γ phase eutectic in the brazing seam were changed by tailoring the brazing temperature. • The maximum shear strength of brazed joints was up to 592 MPa, reaching 95 % of Al 0.3 CoCrFeNi base metal. In this study, a novel eutectic-reaction brazing of Al 0.3 CoCrFeNi high-entropy alloys (HEAs) 1 1 high-entropy alloys (HEAs); 2 base metal (BM); 3 Proeutectic Zone (PEZ); 4 Eutectic Zone (EZ) was investigated with a design of laminated Ni/Nb/Ni interlayers. The typical Al 0.3 CoCrFeNi brazing seam consisted of proeutectic γ, the lamellar eutectic structure composed of face-centered cubic (FCC) phase (eutectic γ) and C14 Laves, as well as a few amounts of Nb based solid solution. A high density of nanoscale ordered L1 2 (γ′) phase was precipitated within γ phase matrix. With the brazing temperature raised from 1200 to 1320 °C, the dissolution volume of Al 0.3 CoCrFeNi alloy into Ni-Nb liquid was increased. The microstructure of the brazing seam changed from hypereutectic (blocky Laves + γ phase) to eutectic (Laves + eutectic γ) to hypoeutectic structures (proeutectic γ + Laves/γ eutectic structure). The shear strength of joints was increased gradually due to the formation of eutectic lamellae and the cellular growth of proeutectic γ phase. The maximum shear strength of the joint brazed at 1320 °C for 10 min was up to 592 MPa, reaching 95% of the base metal. The joint mainly ruptured in the soft proeutectic γ phase in a ductile fracture mode.

Numerical investigation of eutectic growth dynamics under convection by 3D phase-field method

Control of eutectic growth trajectory is a technologically important subject in metallurgy and applied physics. The interaction between solute transport and interface capillary determines eutectic growth under convection. By employing a multiphase-field lattice-Boltzmann method, together with a parallel-adaptive mesh refinement algorithm, the effect of convection on 3D eutectic growth is quantified including lamellar and rod eutectics. The flow-induced solute distribution changes eutectic growth dynamics by establishing a new solute transport equilibrium. The growth behaviors of different eutectic patterns are investigated, and the eutectic growth dynamics is largely dependent on the constitutional undercooling and the curvature undercooling ahead of the solidification front. The natural convection changes the width ratio of coexisting solid phases, while the forced convection causes a tilt band for both lamellar and rod eutectics. Rod-to-lamellar transition under larger convection intensity, which is first predicted by numerical techniques, is discussed and good agreement with previous works is achieved. The deep understanding of the convection effect can help guide how to regulate eutectic patterns through changing the solidification condition.

Microstructure and mechanical properties of wrought Al0.2CoCrFeNiMo0.5 high entropy alloy using gas tungsten arc welding process

In the present study, wrought processed Al0.2CoCrFeNiMo0.5 high entropy alloy (HEA) was welded using gas tungsten arc welding (GTAW) to determine its potential as a structural material. The microstructural changes during welding were analyzed and correlated with the mechanical properties. The welded HEA specimen has shown similar microstructural constituents, a typical FCC matrix with BCT secondary phase (sigma). The microstructure changed from irregular and heterogeneous secondary phase in base metal (BM), to eutectic broken lamellae of sigma phase in FCC matrix at heat affected zone (HAZ) and as-cast structure (dendritic and interdendritic regions) of sigma phase in FCC matrix at fusion zone (FZ). Upon tensile testing, the welded specimens fractured in the FZ with comparable tensile properties.