Excellent High-temperature Properties Research Articles

Ultralight Ceramic Fiber Aerogel for High-Temperature Thermal Superinsulation.

Emerging fiber aerogels with excellent mechanical properties are considered as promising thermal insulation materials. However, their applications in extreme environments are hindered by unsatisfactory high-temperature thermal insulation properties resulting from severely increased radiative heat transfer. Here, numerical simulations are innovatively employed for structural design of fiber aerogels, demonstrating that adding SiC opacifiers to directionally arranged ZrO2 fiber aerogels (SZFAs) can substantially reduce high-temperature thermal conductivity. As expected, SZFAs obtained by directional freeze-drying technique demonstrate far superior high-temperature thermal insulation performance over existing ZrO2-based fiber aerogels, with a thermal conductivity of only 0.0663 W·m-1·K-1 at 1000 °C. Furthermore, SZFAs also exhibit excellent comprehensive properties, including ultralow density (6.24-37.25 mg·cm-3), superior elasticity (500 compression cycles at 60% strain) and outstanding heat resistance (up to 1200 °C). The birth of SZFAs provides theoretical guidance and simple construction methods for the fabrication of fiber aerogels with excellent high-temperature thermal insulation properties used for extreme conditions.

Integration of thermal protection and structural health monitoring for carbon fiber reinforced SiBCN composites with SiC coating enhanced interfacial performance

Carbon fiber (CF) reinforced SiBCN composites were generally considered as key candidates for thermal protection under severe aerodynamic heating in aerospace due to their excellent high-temperature properties, but they were also faced with challenges in stably monitoring the structural integrity of themselves under extreme conditions. Herein, with the introduction of SiC coating, multifunctional CF-SiC/SiBCN composites were fabricated which integrated functions of the thermal protection and the structural health monitoring. Compared to the untreated CF/SiBCN composites, the stability of sensing of CF-SiC/SiBCN composites was greatly improved and the sensitivity of CF-SiC/SiBCN composites maintained at a high level with a gauge factor of 652.65. Furthermore, additional researches revealed that the CF-SiC/SiBCN composites enjoyed a high compressive strength (155.33 MPa), a light weight (1.07–1.61 g/cm3), and a relatively low thermal conductivity (4.02 W/(m·K)), which showed a potentiality of CF-SiC/SiBCN composites to be applied as multifunctional structural components in thermal protection systems.

Enhanced helium ion irradiation tolerance in a Fe-Co-Ni-Cr-Al-Ti high-entropy alloy with L12 nanoparticles

• Helium bubbles and large stacking faulted loops are observed as the dominant structural damage in the He ion irradiated HEA reinforced by L1 2 nanoparticles. • The L1 2 nanoparticles provide numerous interfaces for He entrapment and damage elimination, which suppresses He bubble growth. • A correlative TEM/APT characterization reveals that the RIS around He bubbles is dominated by the inverse Kirkendall mechanism. • Irradiation-induced dissolution and re-precipitation of the L1 2 nanoparticles can retain the main microstructure of the L1 2 -strengthened HEA and provide a sustainable irradiation resistance. L1 2 -strengthened high entropy alloys (HEAs) with excellent room and high-temperature mechanical properties have been proposed as promising candidates as structural materials for advanced nuclear systems. However, knowledge about their radiation response is fairly limited. In the present work, a novel HEA with a high density of L1 2 nanoparticles was irradiated with He ion at 500°C. Transmission electron microscope (TEM) and atom probe tomography (APT) were employed to study the evolution of microstructural stability and radiation-induced segregation. Similar to the single-phase FeCoNiCr HEA, the main microstructural features were numerous large faulted dislocation loops and helium bubbles. While the irradiation resistance of the present L1 2 -strengthened HEA is much improved in terms of reduced bubble size, which could be attributed to the considerable He trapping efficiency of the coherent precipitate/matrix interface and the enhanced capability of the interface for damage elimination when the matrix channel width is narrow. APT analysis revealed that an inverse Kirkendall mechanism-dominated radiation-induced segregation (RIS) occurs around bubbles, where a significant Co enrichment and Ni depletion can be clearly observed. In addition, the competing dynamics of ballistic mixing and elemental clustering that raised from the irradiation-enhanced diffusion in a highly supersaturated matrix, along with the low precipitation nucleation barrier due to the small lattice misfit, lead to a dynamical precipitation dissolution and re-precipitation appears under irradiation. Such a promising phenomenon is expected to promote a potential self-healing effect and could in turn provide a sustainable irradiation tolerance over the operational lifetime of a reactor.

An investigation of hollow NiCrAl metal fiber porous material for high-temperature sealing

Porous metal fiber materials are a type of low-density material promising for functional and structural applications. Unlike traditional metal fiber porous materials, this study developed a novel hollow metal fiber porous material based on carbon fibers. Nickel was electroplated on the surface of the chopped carbon fiber, then the carbon core was removed by wet hydrogen sintering, and finally, Cr and Al were infiltrated by gas phase alloying to obtain the finished product. SEM showed that its internal structure was connected by metal fibers interlaced and overlapped to form a three-dimensional network porous structure. TEM and XRD proved that Ni (fcc) is the main phase of the material, and the addition of Cr and Al elements causes Ni lattice distortion. The compressive strength of the material at 900 °C is 4.44 MPa. In addition, the material's strength decreases with the heat treatment temperature increasing. The thermal conductivity of the NiCrAl porous material increases with the temperature, which is 0.143 W/(m·K) at room temperature and 0.416 W/(m·K) at 900 ℃. TG analyses showed that the NiCrAl porous material possessed excellent high-temperature and oxidation-resistance properties. Quartz lamp heating tests showed that the material had good thermal insulation, as the temperature dropped by about 230 °C after passing through the material. The average permeability of the material at room temperature is 5.46 e-6D. This metal fiber porous material has both the inherent properties of metal and the functional properties of porous materials such as lightweight, high-temperature resistance, heat insulation, etc. It is a promising high-temperature sealing material.

Enhancing high-temperature tensile properties of Ni/Ni-W laminated composites for MEMS devices

There is an increasing demand for materials with excellent mechanical performance for Micro-Electro-Mechanical System (MEMS) devices serving at elevated temperatures. However, how to enhance the high-temperature strength of the materials without losing their plasticity is a pending problem. Here, the Ni/Ni-W laminated composites with different monolayer thicknesses and the same ratio of the constituent layer thicknesses were fabricated successfully by using the dual-bath electrodeposition technique. The microstructure stability and tensile properties of annealed Ni/Ni-W laminated composites with different monolayer thicknesses were investigated at 400 °C. The results show that the annealed Ni0.5/Ni-W0.05 laminated composites have both high yield strength (450 MPa) and excellent elongation to failure (25.1%) at 400 °C, being superior to that of the monotonic Ni (179 MPa and 17.7%). Such a high strength of the laminated composite at 400 °C results from the contribution of the intrinsically high strength of the Ni-W layers with excellent thermal stability, the thickness-constrained effect on grain growth of Ni layers and the interface coupling effect of heterogeneous structures. The good plasticity may be derived from the heterogeneous laminated structure and the decrease in the constituent layer thickness, providing a good co-deformation ability. Basic mechanisms for the high tensile strength and good plasticity of the Ni/Ni-W laminated composites at 400 °C were analyzed theoretically. The findings reveal a potential strategy to fabricate MEMS components with excellent high-temperature tensile properties through tailoring the microstructure thermal stability and the constituent layer scale.

Lightweight, ultrastrong and high thermal-stable eutectic high-entropy alloys for elevated-temperature applications

Eutectic high-entropy alloys (EHEAs) that combine the advantages of HEAs and eutectic alloys are promising candidates for high-temperature applications. However, currently developed EHEAs still exhibit high densities and low high-temperature strengths, which limit their usage. Here we propose a strategy to design lightweight, strong, and high thermal-stable EHEAs by introducing an extremely stable Heusler-type ordered phase (L21 phase) containing a high-content of low-density elements and constructing a low lattice misfit eutectic-phase interface, which can lead to generate ultrafine and stable lamellar structures and high-density of coherent nanoprecipitates. As a manifestation of this strategy, a novel bulk Al17Ni34Ti17V32 EHEA was designed to consist of L21 and body-centered-cubic (BCC) phases (interlamellar spacing ∼ 320 nm) with a lattice misfit only 2.4%. This alloy has one of the lowest densities (∼ 6.2 g/cm3) among all EHEAs reported previously and exhibits much higher high-temperature hardness and specific yield strengths than most reported refractory HEAs (RHEAs), lightweight HEAs (LWHEAs), EHEAs, and conventional superalloys. This work paves the way to develop light EHEAs with excellent high-temperature properties.

Microstructure and high-temperature mechanical properties of Inconel 718 superalloy weldment affected by fast-frequency pulsed TIG welding

Inconel 718 (In-718) alloy has excellent high-temperature mechanical properties and is a strategic material in the military industry, but its high-temperature mechanical properties may decrease due to the comprehensive negative effects of grains and Laves phase coarsening during processing and application. In this paper, a fast-frequency pulsed tungsten inert gas (FFP-TIG) welding process was proposed, and two different waveforms were introduced: fast-frequency double-pulsed (FFDP) waveforms and fast-frequency single-pulsed (FFSP) waveforms. By studying the weld morphology, microstructure and mechanical properties of In-718 alloy weldments, the results showed that FFP-TIG welding reduces heat input, promotes molten pool flow, refines grains and other effects, thereby improving the high-temperature tensile strength of In-718 alloy weldments. FFDP waveforms have a better effect on grain refinement than FFSP waveforms due to the reduction in average heat input and the enhancement of stirring molten pool. The minimum average grain size and Laves phase size of the FFP-TIG weldment were 38.14 μm and 1.133 μm, respectively. Similarly, the high-temperature tensile strength of FFP-TIG weldments reaches a maximum of 685.54 MPa, which was 90.9 % of the base metal strength.

Effect of Mo Content on Microstructure and Mechanical Properties of Laser Melting Deposited Inconel 690 Alloy

Inconel 690 alloy is widely used in nuclear power, petrochemical, aerospace, and other fields due to its excellent high-temperature mechanical properties and corrosion resistance. The Inconel 690 alloy with different Mo content was fabricated by laser melting deposition (LMD). The effects of Mo content on the microstructure and mechanical properties were investigated. The microstructure of as-deposited Inconel 690 is composed of columnar dendrites grown epitaxially, and M23C6 carbides are precipitated in the grain boundaries. With the increase of Mo content, the amount of precipitated carbide increases gradually. At the same time, the grain boundary becomes convoluted. The tensile test at room temperature shows that the high Mo content in the as-deposited Inconel 690 increases the ultimate strength but decreases the ductility. Compared with low Mo content, the alloy with high Mo deposition has better mechanical properties. The present study provides a new method to achieve the preparation of Inconel 690 alloy with excellent integrated mechanical properties.

Preparation and Properties of MAX Phase High-Temperature Antioxidant Coatings on C/C Composite Surfaces by Ultra-High Speed Laser Cladding

Prefabricated MAX (Ti3SiC2, Ti3AlC2) phase coating materials with coefficient of thermal expansion Si and SiC as transition layers and excellent high-temperature antioxidant properties were used for ultra-high-speed laser melting on the surface of carbon/carbon composites, so that the produced coatings have good interfacial bonding and high-temperature antioxidant properties. In this study, we characterised and analysed the microscopic morphology and composition of the sample coatings using X-ray diffraction and scanning electron microscopy, investigated their interfacial bonding properties by scratch tests and studied the high temperature oxidation resistance of the samples at 1773 K for 1 h. The results show that the bonding strength between the coating and the substrate is good, the surface is flat without flaking, the coating thickness can be controlled below 60 μm, the microstructure is dense, and there is no obvious deformation or failure of the substrate, and the dense coating formed by laser melting has strong oxidation resistance, so that the substrate carbon/carbon composite is well protected at high temperature.

Effects of pyrocarbon interphase on microstructure and properties of C/SiBCN composites

C/SiBCN composites are expected to be widely used in aerospace applications because of their excellent high-temperature stability. However, the interfacial reactions have significantly limited their practical application. A pyrocarbon (PyC) interphase can improve the interfacial reactions of C/SiBCN composites. In this study, PyC interphases of different thicknesses (0.1 μm, 0.25 μm, and 0.5 μm) were introduced via chemical vapor deposition (CVD) process. The interface bonding of C/SiBCN composites with 0.1 μm and 0.25 μm thick interphases was relatively weak and the composites with 0.5 μm interphase exhibited strong interface bonding. After heat treatment at 1600 °C, the mechanical properties of the C/SiBCN composites with the 0.5 μm thick interphase was maintained at 131 MPa, and it was maintained at 105 MPa even after heat treatment at 1900 °C, indicating their excellent high-temperature mechanical properties. In short, 0.5 μm thick PyC interphase can effectively improve the interfacial reaction of the C/SiBCN composites, facilitating their application in high-temperature environments.

Evolution of iron-rich intermetallics and its effect on the mechanical properties of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing

Si addition is commonly used to modify the iron-rich intermetallics in Al–Cu–Mn–Fe alloys, which is beneficial to increasing the use of recycled aluminum. Most of the available research has focused on the effect of Si content on the room-temperature mechanical properties of Al–Cu–Mn–Fe alloys. To expand the application of Al–Cu–Mn–Fe–Si alloys as light heat-resistant structural components in the automotive and aerospace industries, it is of great importance to investigate the evolution of iron-rich intermetallics and its effect on the fracture behavior of Al–Cu–Mn–Fe–Si alloys after thermal exposure and high-temperature tensile testing. In this work, the evolution of iron-rich intermetallics and the high-temperature mechanical properties of heat-treated Al-6.5Cu-0.6Mn-0.5Fe alloys with different Si contents after thermal exposure and high-temperature tensile testing were assessed by tensile tests, image analysis, scanning electron microscopy, X-Ray diffraction, transmission electron microscopy, and atomic probe tomography. The results indicate that the Al-6.5Cu-0.6Mn-0.5Fe alloys with 0.1Si and 0.5Si additions have excellent and stable high-temperature mechanical properties after long thermal exposure, which are better than those of most heat-resistant Al alloys. The high performance of the high-temperature mechanical properties is attributed to the high heat resistance of secondary intermetallics and precipitated particles. The addition of Si is detrimental to the strength of Al-6.5Cu-0.6Mn-0.5Fe alloys after long thermal exposure. This can be attributed to the solid-state phase transformation of iron-rich intermetallics from α-Fe to β-Fe, which results in the increase of needle-like Fe-rich phases and Si particles, the agglomeration of secondary intermetallics, and the consumption of Al2Cu phases.

Deformation Mechanisms in Compositionally Complex Polycrystalline CoNi-Base Superalloys: Influence of Temperature, Strain-Rate and Chemistry

Recent studies revealed the excellent high temperature properties of polycrystalline CoNi-base superalloys. However, their underlying deformation behavior has been reported only scarcely so far. In this work, the deformation mechanisms of four polycrystalline compositionally complex CoNi-base superalloys with slightly varying chemical compositions were investigated by compression and creep experiments at temperatures between 750 °C and 850 °C and strain-rates between 10–3 and 10–8 s−1. In the two (Ta + Ti)-rich alloys, a transition of the deformation mechanism from shearing by APB-coupled dislocation pairs to stacking fault shearing and finally also to microtwinning is observed with decreasing strain-rate and increasing temperature. In contrast, APB-based shearing mechanisms represent the dominant mechanism in both (Al + W)-rich alloys in all conditions. At high temperatures and low strain-rates, dislocation glide-climb processes also contribute to plastic deformation in all alloys. By correlating the underlying defect structures with the mechanical properties of these alloys, it becomes evident that a transition to stacking fault shearing and microtwinning leads to a lower strain-rate dependency and superior high-temperature strength in comparison with APB-based mechanisms. Reasons for the different deformation mechanisms, the influence of segregation processes, the consequences for mechanical properties and implications for a mechanism-based alloy design are discussed.

Ultrasonic-Assisted Electrochemical Milling of SLM-Printed Hastelloy X Based on Analysis of Material Passivation Characteristics

Hastelloy X (HX) is widely used in the aerospace field for its excellent corrosion resistance and high-temperature mechanical properties that can be fabricated into complex structures directly by the selective laser melting (SLM) technique. However, SLM-printed (SLM-ed) HX with high strength and hardness is challenging to process using conventional manufacturing techniques and may result in machining flaws that don’t fulfill engineering standards. Therefore, an ultrasonic-assisted electrochemical milling (UAECM) method using a tube electrode is proposed to fabricate high aspect ratio structures on SLM-ed HX with high-quality. Firstly, the passivation characteristics of SLM-ed HX before and after solid solution treatment (SST) were investigated using polarization curves and electrochemical impedance spectroscopy (EIS). Secondly, the electrochemical milling process and the electrode gap flow field were simulated. Then, the effect of processing parameters such as ultrasonic amplitude, electrical parameters, and mechanical parameters on the groove width and stability was investigated by the orthogonal and single factor experiments. Finally, the cavity and bump structures were machined by layered milling with an average groove width of 960 ± 15 μm, a groove depth of 4.4 mm, an aspect ratio of 4.5, and surface roughness of 1.524 μm and 1.622 μm, respectively, demonstrating the adaptability and machining accuracy of the UAECM method.

Thermal-Resistance Effect of Graphene at High Temperatures in Nanoelectromechanical Temperature Sensors.

Graphene membranes act as temperature sensors in nanoelectromechanical devices due to their excellent thermal and high-temperature resistance properties. Experimentally, reports on the sensing performance of graphene mainly focus on the temperature interval under 400 K. To explore the sensing performance of graphene temperature sensors at higher temperature intervals, micro-fabricated single-layer graphene on a SiNX substrate is presented as temperature sensors by semiconductor technology and its electrical properties were measured. The results show that the temperature coefficient of the resistance value is 2.07 × 10-3 in the temperature range of 300-450 K and 2.39 × 10-3 in the temperature range of 450-575 K. From room temperature to high temperature, the "metal" characteristics are presented, and the higher TCR obtained at higher temperature interval is described and analyzed by combining Boltzmann transport equation and thermal expansion theory. These investigations provide further insight into the temperature characteristics of graphene.

Microstructure and high-temperature mechanical properties of new-type heat-resisting aluminum alloy Al6.5Cu2Ni0.5Zr0.3Ti0.25V under the T7 condition

Making use of the low-pressure sand mold casting preparation Al6.5Cu2Ni0.5Zr0.3Ti0.25V alloy. After T7 heat treatment, microstructure observation shows that the average grain size of the Al matrix is 150 μm, and the second phase is θ'-Al2Cu, Al7Cu4Ni, Al3M (M = Ti, Zr, V). The tensile strength is 127.5 MPa, the yield strength is 108Mpa, and the elongation is 16.11 %. The θ'-Al2Cu phase in the grain, a small amount of Al3M (M = Ti, Zr, V) thermal stable phase, and the Al7Cu4Ni phase at the grain boundary make the alloy exhibit excellent high-temperature mechanical properties at 350℃.

Formation of lamellar microstructure in Ti-48Al-7Nb-2.5V-1Cr alloy

The reasonable design and application of phase transformation is an efficient approach to coordinate the microstructure and properties of alloys. The sequence of compositional and structural transformation determines the type and mechanism of phase transformation. Because of their low density and excellent high-temperature properties, TiAl alloys that contain fully lamellar microstructure have been applied to low-pressure turbine engine blades. However, there is no consensus on the mechanism of lamellar formation. Here, we propose a new mechanism that is dominated by spinodal decomposition, in which compositional transformation occurs prior to structural transformation during lamellar formation. Transitional lamellae with fcc-based long-period superstructures are introduced to prove the mechanism of sequentially spinodal decomposition, shear transformation, compositional modulation and ordering.

Tribological performance of a TiZrNbMo0.6 refractory high entropy alloy at elevated temperatures

Refractory high-entropy alloys (RHEAs) have attracted extensive attention in the field of high-temperature for their excellent high-temperature resistance, mechanical properties, and wear performance. One kind of RHEAs, e.g., Ti-Zr-Nb-Mo, is a potential candidate for wear-resistant materials due to its high strength and low modulus. In this study, the microstructure, oxidation resistance, tribological properties and mechanisms of TiZrNbMo 0.6 RHEA were studied. SEM and EDS were used to characterize the morphology and compositions of the wear surfaces and cross-sections, as well as the Si 3 N 4 counterparts. The results indicate that the TiZrNbMo 0.6 alloy has the optimal wear resistance due to forming a compacted and continuous glaze layer on the surface at the temperature of 500 °C. The evolution of the wear mechanism at different temperatures has been discovered and the oxidation test has been conducted to explain the mechanisms of tribological properties at high temperatures. The dominate wear mechanism changes from abrasive wear and adhesive wear at room temperature to oxidative wear and slight abrasive wear at 500 °C. With the wear temperature increasing to 800 °C, the dominate wear mechanism is severe oxidative wear, and followed partly abrasive wear, which results in a higher wear rate at 800 °C than that at 500 °C. • The microstructure, oxidation resistance and tribological properties of a novel TiZrNbMo 0.6 RHEA were investigated. • The wear behavior and mechanisms at ambient and elevated temperatures have been discussed. • The TiZrNbMo 0.6 RHEA is the excellent wear resistance at 500 ℃ against Si 3 N 4 ball. • The good wear performance at 500 ℃ results from the formed continuous and protective glaze layer on the worn surface.

Effect of TiC on the high-temperature oxidation behavior of WMoTaNbV refractory high entropy alloy fabricated by selective laser melting

The refractory high-entropy alloys possess both high melting points and excellent high-temperature properties. As a result, they are expected to become a new generation of high-temperature materials. Along these lines, in this work, selective laser melting (SLM) was used to prepare WMoTaNbV and TiC/WMoTaNbV-based alloys, while the influence of the SLM process parameters on the surface morphology, internal defects distribution and microstructure of the alloy was thoroughly analyzed. On top of that, the long-term high-temperature oxidation behavior of the two alloys at 600°C was studied. The extracted results showed that the process parameters had a serious impact on the surface quality and internal defects of the refractory high-entropy alloys. By optimizing the process parameters, the final surface roughness value of the WMoTaNbV-based alloy was 9.148 μm (Sa). On the contrary, the surface roughness value of the TiC/WMoTaNbV compound was 14.218 μm (Sa). It is also interesting to notice that the incorporation of TiC could significantly reduce the number of cracks within the TiC/WMoTaNbV alloy. Moreover, the WMoTaNbV-based alloy was mainly composed of dendritic structures, while the TiC/WMoTaNbV-based alloy consisted of cellular structures. The main oxidation products of the WMoTaNbV-based alloy were the Ta16W18O94 and Nb14W3O47 ternary oxidation products. After the TiC particles were added, the Ti-containing oxidation products such as TiO2, Ti2Nb10O29. etc. within the TiC/WMoTaNbV-based alloy increased significantly, preventing thus effectively the alloy from being seriously oxidized. The cellular structures, carbides and TiO2 phase are helpful in improving the oxidation resistance of the TiC/WMoTaNbV-based alloy.

Remarkably high fracture toughness of HfNbTaTiZr refractory high-entropy alloy

• The fracture behavior of the Hfnbtatizr refractory HEA was investigated. • The HEA exhibits a remarkably high fracture toughness of K JIC = 211 MPa•m 1/2 . • Fully-ductile fracture mode with the formation of kink bands was observed. • The excellent damage-tolerance plus high strengths in a wide temperature range endow this HEA a promising structural material. High-entropy alloys (HEAs) with face-centered cubic (FCC) phase such as CoCrFeMnNi or CoCrNi generally exhibit ultra-high fracture toughness but relatively low strength. In contrast, body-centered cubic (BCC) HEAs often display higher strengths, but the few reports on fracture toughness measurement demonstrate low toughness values. Here we show that the BCC HfNbTaTiZr refractory HEA, which possesses a combination of high strength, good tensile ductility and excellent high-temperature properties, also exhibits a remarkably high fracture toughness. By using the "single specimen" compliance method for J -integral measurement according to the ASTM E1820–17 standard, its fracture toughness K JIC was experimentally determined to be 210 MPa m 1/2 , which renders this HEA among the toughest metallic materials. The excellent damage tolerance makes the HEA promising for applications as high-temperature structural materials such as in aerospace field.

The Observation of Cellular Precipitation in an Ni36Co18Cr20Fe19Al7 High-Entropy Alloy after Quenching and Annealing.

High-entropy alloys (HEAs) comprise a minimum of five major elements. These alloys show some special characteristics, such as excellent mechanical and high temperature properties. The development of the HEAs requires a knowledge of phase transformations during alloy making procedures. The phase transformations of an Ni36Co18Cr20Fe19Al7 HEA were studied in this research. The alloy underwent hot forging, cold rolling, annealing at and quenching from 1323 K, and isothermal holding at 873 K. The alloy is a single face-centered cubic (FCC) phase in the as-quenched condition. After annealing at 873 K, not only fine coherent L12 particles precipitated homogeneously in the FCC matrix, but lamellae of FCC and L12 phases also developed from the grain boundaries. Both lamellar FCC and L12 grains have a cubic-on-cubic orientation relationship (OR). The composition of the lamellar L12 phase is Ni60Co8Cr6Fe6Al20, and that of the lamellar FCC phase is Ni31Co15Cr28Fe21Al4. Cellular precipitation occurs in the HEA, and the high-temperature FCC (γ) transforms to a lamella of low-temperature FCC (γ1), and an L12 phase, i.e., γ → γ1+L12.