Excellent High-temperature Properties Research Articles

Regulating the concentration of silica sol to prepare high thermal insulation ceramic nanofiber aerogel composites

Abstract Ceramic aerogels can maintain excellent thermal insulation performance under extreme conditions due to their outstanding pore structure and specific surface area. However, the current application of ceramic aerogels is limited by their brittleness, easy collapse of the structure under external forces, and insufficient mechanical properties. In this study, ceramic nanofiber aerogels prepared by PEO as a polymer were used as building blocks, and silica sol was selected as a high-temperature binder. Through the sol impregnation-atmospheric pressure drying method and calcination treatment, we obtained ceramic nanofiber composites with excellent performance. It is found that when the impregnation concentration is 0.4wt%, the thermal conductivity at room temperature is as low as 27 m W·m-1·K-1, the cold surface temperature at 800°C is as low as 255°C, the tensile strength at 20% tensile strain is as high as 400 kPa, and the compressive strength at 80% compressive strain is as high as 32 kPa. Compared with the traditional ceramic fiber aerogel material, the material exhibits excellent high-temperature insulation performance and mechanical properties, which greatly broadens the application scene of ceramic aerogel.

Towards post-curing parameters optimization of phthalonitrile composites through the synergy of experiment and machine learning

Phthalonitrile composites are highly anticipated in fields such as aerospace, marine, and electronics due to their exceptional heat resistance, excellent high-temperature mechanical properties, and outstanding processability. The conversion of nitrile functional groups to macrocyclic structures during post-curing is essential for performance enhancement. However, the influence of post-curing conditions on the mechanical properties of phthalonitrile composites is complicated, and the underlying mechanism remains unclear. In this study, an experimental and machine learning synergic strategy was employed to optimize the post-curing parameters of the phthalonitrile composites, including temperature, time, and pressure. Through mechanical experiments conducted at both room temperature and 400 °C, scanning electron microscope, and dynamic mechanical analysis, the underlying mechanism of the influence of post-curing parameters on the composite mechanical properties was revealed. The enhancement of the polymerization degree brought about by post-curing is the decisive factor for high-temperature performance, while the room-temperature properties are a tradeoff between the polymerization degree improvement and the defect proliferation. The genetic algorithm-optimized backpropagation (GA-BP) neural network was trained for the efficient and accurate prediction of the optimal post-curing parameters. The tendency of mechanical properties with the post-curing parameters predicted by machine learning is consistent with the mechanism revealed by experiments.

Predictive design of novel nickel-based superalloys beyond Haynes 282

Nickel-based superalloys are in great demand for harsh-service conditions involving high temperatures and oxidative environments. Haynes 282 stands out due to its excellent high-temperature properties and easy fabricability. However, the upper operation temperature of Haynes 282 is limited due to its relatively low liquidus temperature. Equipped with high-fidelity density-functional theory calculations and high-throughput experimentation methodology, we explored new compositional spaces that exhibit higher liquidus temperature and higher strength. While maintaining the manufacturability, the newly designed alloy shows improved strength and ductility at room temperature and better oxidation resistance up to 800 °C. The new compositions showcase a minor change in the refractory and metalloid content can significantly impact the mechanical and oxidation performance of superalloys.

A novel L12-strengthened single crystal high entropy alloy with excellent high-temperature mechanical properties

In this work, a novel and low-cost L12-strengthened single crystal high entropy alloy (HEA) was prepared through the seeding method of directional solidification. The microstructures, tensile properties and deformation mechanisms of the single crystal HEA were systematically investigated. The volume fraction of L12 precipitates after heat treatment was almost 80%, which was higher than that of conventional single crystal superalloys. The maximum yield strength, ultimate tensile strength and elongation, which were separately as 897 MPa, 1038 MPa and 37%, were obtained at the intermediate temperature of 800 °C. Moreover, at higher temperature (900 °C and 1000 °C), the HEA was able to maintain excellent performance, which was comparable to the commercial René N5 second-generation Ni-based single crystal superalloy. Finally, the high performance was examined by the model of strengthening contribution, which proved that the high-volume fraction of L12 precipitates in single crystal HEA was the primary reason for their significant precipitation and solid solution strengthening. Additionally, the formation of Lomer-Cottrell locks and stacking faults could enhance the high-temperature properties of the single crystal HEA.

Mechanical properties and microstructure of high-strength and high-ductility steel at elevated temperature

This paper aims to investigate the mechanical behavior and microstructure of a novel high-strength and high-ductility (HSHD) steel at different temperatures ranging from 25 to 600 °C. The testing machine conducted uniaxial tensile tests with seven different temperatures. The true plastic stress-strain curves of HSHD steel were fitted using the Ludwigson model based on the experimental results, and the microstructure of HSHD steel was analyzed using Electron Backscatter Diffraction (EBSD). The experimental results revealed an obvious temperature-mechanical response relationship in HSHD steel, showing a gradual decrease in tensile strength and yield strength with increasing temperature, while the uniform and post-necking elongation exhibit a trend of decreasing and then increasing. Compared to other steels, HSHD steel exhibited significant advantages of strength-ductility balance over a wide range of temperatures. A Ludwigson model coupled with temperature parameters was established, which provided a good fitting effect to the true plastic stress-strain curves of HSHD steel. Microstructure analysis shows significant Brass and Copper textures near the fracture surface of HSHD steel at all temperatures. Additionally, a large number of deformation twinning was observed within the grain of HSHD steel. The excellent high-temperature mechanical properties of HSHD steel can be attributed to the strong thermal stability of deformation twins.

Dynamic strain aging and recrystallization mechanisms in laser powder bed melt printing of highly textured Hastelloy X

Hastelloy X (HX) is used in a wide range of aerospace applications due to its excellent high-temperature properties, Dynamic strain aging (DSA) and Dynamic recrystallization (DRX) are of great interest as they affect the mechanical behavior at high temperatures. In this study, the serrated flow behavior of highly textured Hastelloy X fabricated by laser powder bed fusion (LPBF) is investigated during uniaxial straining over a wide range of temperatures (20–800 °C) and strain rates (5×10−2 s−1 to 5×10−4 s−1). The alloy exhibits different types of tensile serrated flow in the temperature range of 200–600 °C. The alloy has been shown to have a very good tensile flow behavior. The presence of the Portevin-Le Chatelier effect (PLC) due to dynamic strain failure was observed at temperatures lower than 600 °C, respectively, with an average activation energy value of 45 kJ/mol calculated using the quasi-static aging (McCormick) model, and an average activation energy value calculated using the solute resistance (Cottrell) model of 41.8 kJ/mol, which is significantly lower than the 80–130 kJ/mol of conventionally fabricated HX. The effects of temperature and strain rate on fracture morphology and microstructure were analyzed by SEM, and the dynamic recrystallization behavior and nucleation mechanism were characterized by electron backscatter diffraction (EBSD). The changes in the content of recrystallized texture Cube and deformed texture Goss at different deformation temperatures are explained, and the dynamic recrystallization mode changes, from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX) as the deformation temperature increase. Verification of the different DRX mechanisms contributes to a comprehensive understanding of the microstructural evolution of the hot deformation of HX fabricated by LPBF.

Laser additive manufacturing of TiB2-modified Cu15Ni8Sn/GH3230 heterogeneous materials: Processability, interfacial microstructure and mechanical performance

Cu/Ni heterogeneous materials integrate excellent thermal conductivity and high-temperature mechanical properties, enabling them to be widely used in the aerospace domain. Differences in the thermal and physical properties of the Cu and Ni materials, however, make them difficult to be processed using the laser powder bed fusion (LPBF) additive manufacturing process. This study systematically examines the effects of various LPBF process parameters on microstructure, element diffusion, bonding strength and microhardness at the Cu/Ni interface, as well as investigating the mechanisms of defect formation within Cu/Ni heterogeneous materials. The results indicate that a reasonable control of laser energy input (<100 J/mm3) facilitates the Cu/Ni components through strong interfacial metallurgical bonding without pore defect formation. Compared to single-material Cu alloy, the ultimate tensile strength (UTS) of the horizontally bonded Cu/Ni specimen increased by 55.25 %, without significant reductions in elongation. The vertically bonded Cu/Ni tensile specimen fractured in the middle of the Cu region rather than the interfacial region, indicating superb interfacial bonding strength. Another advantage lies in the enhancement of thermophysical properties, with a 109.5 % increase in thermal conductivity achieved in the LPBF-fabricated Cu/Ni heterogeneous materials compared to the single-material Ni alloy. Quasi-static compression experiments indicated that the Cu/Ni lattice structure could absorb more energy when compressed parallel to the build direction (BD), compared to being perpendicular to the BD. This study provides guidance for the design and manufacture of high-performance Cu/Ni heterogeneous components via LPBF.

Effects of V on the microstructure and mechanical properties of HfNbTaTiVx refractory multi-principle element alloys: A combined experimental and computational study

Refractory multi-principle element alloys (RMPEAs) represent a promising class of high-temperature structural materials due to their excellent high-temperature mechanical properties. An important challenge in the application of RMPEAs is achieving a suitable balance between high strength and ambient ductility. Previous investigations have suggested that vanadium (V) element plays a key role in maintaining the high strength and ductility of RMPEAs. Nevertheless, the fundamental mechanisms by which V influences the strength and ductility of RMPEAs remain unclear. To address this issue, a series of HfNbTaTiVx (x = 0.00–1.00) alloys were designed to investigate the effects of V content on the microstructural and mechanical properties of RMPEAs. The results demonstrate that the prepared HfNbTaTiVx alloys are single-phase body-centered cubic (BCC) solid solutions. V addition helps to improve the tensile yield strength of the alloys while concurrently preserving their ductility. The tensile yield strength increases from 729 MPa (x = 0.00) to 997 MPa (x = 1.00) with the increase of V. Impressively, the HfNbTaTiV0.25 alloy exhibits a tensile fracture plastic strain of 21.9%, which surpasses most existing RMPEAs. The increase in yield strength mainly comes from the intensification of local lattice distortion and solid solution strengthening caused by the introduction of V. Furthermore, the addition of V helps to decrease the elastic anisotropy of the alloys, which is advantageous for their processing quality. Overall, this study not only elucidates the role of V in conferring strength and ductility to RMPEAs but also provides valuable insights for the development and industrial application of RMPEAs.

Microwave absorbing performance of reduced graphene oxide @ alumina fabricated by atomic layer deposition method

In this work, a series of alumina coating reduced graphene oxide (RGO@Al2O3) composites with core-shell structure was synthesized by the atomic layer deposition (ALD) method. The alumina (Al2O3) thickness of the RGO@Al2O3 could be controlled by changing the number of ALD cycles. The microstructure, high-temperature resistance, and microwave absorbing properties of the composites were investigated in detail. The composite could provide excellent high-temperature resistance and microwave absorbing properties. The composite started decomposition at 620 °C. The maximum reflection loss value of the composite obtained by applying 200 Al2O3 deposition cycles is −14.2 dB at 15.7 GHz, and its effective absorption bandwidth reaches 5.0 GHz (13.0–18.0 GHz). The results show that the RGO@Al2O3 composite has the potential application in high-temperature microwave absorbing fields, due to excellent high-temperature resistance, reflection loss, and effective absorption bandwidth.

Ablation and mechanical behaviour of C/C composites under an oxyacetylene flame and tensile loading environment

Carbon/carbon (C/C) composites impress with their excellent high-temperature mechanical properties. In this paper, in order to simulate the complex service conditions of aerocraft thermal protection systems, an experimental platform for simultaneous ablation and tension was designed. The mechanical and ablation properties of the C/C composites were tested under an oxyacetylene flame at heat flux of 2185 kW/m2. Meanwhile, the mechanism of mutual influence of ablation and tensile loading was revealed by microstructural characterisation. By comparing the tension-only and ablation-only conditions, the results show that the strength of the C/C composites under ablation conditions decreased by 52.0%, which was associated with the crack propagation and porosity increase caused by flame. The tensile loading increased the linear and mass ablation rates by 21.8% and 82.9%, respectively, which was due to the interfacial damage caused by tensile stresses provides more channels for flame to enter the material interior, thus accelerating oxidative ablation.

Thermodynamic modeling of the Ti–Al–V system over the entire composition and a wide temperature range

Ti–Al–V alloys have received considerable attention owing to their excellent high-temperature mechanical properties. After conducting a critical review of the available experimental data for the ternary Ti–Al–V system, a thermodynamic assessment of the system was carried out over the entire composition and a wide temperature range utilizing the CALPHAD method. The extensive homogeneity ranges of the α2 and γ phases were satisfactorily reproduced. Of particular significance was the first accurate description of the phase equilibria at 1573 K. The model parameters obtained in this study well represented the thermochemical properties and phase equilibria of the experimental data. Furthermore, the enthalpies of the widely used Ti alloy Ti–6Al–4V were reproduced using the thermodynamic description established in this study. Thus, this study provides a reliable basis for guiding the development and design of Ti–Al–V alloys.

Fast epitaxial brazing of DD6 single crystal superalloy using novel NiCoCrAlNbTi high entropy filler alloy

In recent years, Ni-based single crystal superalloys are widely applied in the fabrication of blades and vanes in aero-engine industry due to the excellent high temperature properties. In this study, a novel high entropy filler without Si and B was proposed. The melting point of the newly developed Ni-Co-Cr-Al-Nb-Ti filler was 1155ºC and fast epitaxial brazing DD6 single crystal superalloy was realized under the brazing condition of 1200ºC for 30 min. Furthermore, the influence of aging heat treatment on the interface microstructure and joint strength was investigated. The as-brazed joint interface mainly consisted of γ, γ’, γ’’-(Ni,Co,Cr)3Nb and NiAl phase. After aging treatment, element distribution became more uniform and the size of intermetallic phase was refined. The γ+γ’ phase in brazing seam exhibits same [001] orientation with DD6 substrate. The orientation relationship between γ and γ’’ was also analyzed. The joint tensile strength tested at 980ºC was 559 MPa.

Effect of sintering temperature on microstructure and mechanical properties of Ti2AlNb-based composites

Ti2AlNb-based composites are a great choice for achieving high-temperature properties by introducing reinforcement to meet engineering application requirements. In this study, Ti–22Al–25Nb (at.%) alloys and nano-Al2O3/Ti–22Al–25Nb composites were prepared by mechanical alloying (MA) and hot pressing (HP). The effects of sintering temperature on the microstructure and mechanical properties of Ti–22Al–25Nb alloys and Ti2AlNb-based composites were investigated. The sintering temperature effectively regulated the morphology and size of the primary O phase. The addition of Al2O3 particles refined the grain size of the matrix and precipitates, and it induced the phase transformation. When the sintering temperature reached 1350 °C, Ti–22Al–25Nb alloys and Ti2AlNb-based composites obtained a fine and uniform equiaxed microstructure. The hardness and ultimate compressive strength of the composite first increased and then decreased with increasing sintering temperature. Since the addition of Al2O3 particles promoted the precipitation of the ultrafine acicular O phase from the B2 phase, the composite sintered at 1350 °C had the optimal mechanical properties. The hardness of the Ti–22Al–25Nb alloy and Ti2AlNb-based composite was 4.12 GPa and 5.1 GPa, respectively. The ultimate compressive strength of the composite at room temperature (RT) and 650 °C was 2642.3 MPa and 2462.5 MPa, which was 25.8% and 9.6% higher than that of the Ti2AlNb alloy. The strengthening mechanism of the composites mainly includes dispersion strengthening and Orowan strengthening of the nano-Al2O3 particles, as well as precipitation strengthening of the acicular O phase. This study provides a theoretical guidance for designing Ti2AlNb-based composites with excellent high-temperature properties.

Tailoring Multiple Strengthening Phases to Achieve Superior High-Temperature Strength in Cast Mg-RE-Ag Alloys.

Mg alloys with excellent high-temperature mechanical properties are urgently desired to meet the design requirements of new-generation aircraft. Herein, novel cast Mg-10Gd-2Y-0.4Zn-0.2Ca-0.5Zr-xAg alloys were designed and prepared according to the advantages of multi-component alloying. The SEM and XRD results revealed that the as-cast microstructures contained α-Mg grains, β, and Zr-containing phase. As Ag rose from 0 wt.% to 2.0 wt.%, the grain size was refined from 40.7 μm to 33.5 μm, and the β phase significantly increased. The TEM observations revealed that the nano-scaled γ' phase could be induced to precipitate in the α-Mg matrix by the addition of Ag. The stacking sequence of lamellar γ' phases is ABCA. The multiple strengthening phases, including β phase, γ' phases, and Zr-containing particles, were effectively tailored through alloying and synergistically enhanced the mechanical properties. The ultimate tensile strength increased from 154.0 ± 3.5 MPa to 231.0 ± 4.0 MPa at 548 K when Ag was added from 0 to 2.0 wt.%. Compared to the Ag-free alloy, the as-cast alloy containing 2.0 wt.% Ag exhibited a minor reduction in ultimate tensile strength (7.0 ± 4.0 MPa) from 498 K to 548 K. The excellent high-temperature performance of the newly developed Mg-RE-Ag alloy has great value in promoting the use of Mg alloys in aviation industries.

Variety of yield strength caused by precipitations of L12-Ni3Al and B2-NiAl phases in alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel exhibits excellent high-temperature properties and has attracted significant attention in thermal power generation industry. Microstructure evolutions and yield strengths of 20Ni-AFA steel during isothermal aging are investigated. The results show that five types of phases are found in 20Ni-AFA steel during isothermal ageing: NbC, L12-Ni3Al, M23C6, B2-NiAl and Laves phases. Addition of Cu causes precipitation of L12-Ni3Al phase in 20Ni-AFA steel at the early stage of isothermal aging. L12-Ni3Al phase grows and gradually transforms into B2-NiAl phase which possesses a more stable structure with the extension in isothermal aging time. The transformation leads to yield strengths of 20Ni-AFA steel to decrease anomalously during 24-120 h isothermal aging. Yield strengths of 20Ni-AFA steel increase again as isothermal aging time continues to increase.

Solidification microstructures of V-Nb-Mo-Ta-W alloys: Insights from non-equiatomic alloys

We have investigated the solidification microstructures of the non-equiatomic and equiatomic V-Nb-Mo-Ta-W alloys, which are one of the first refractory high entropy alloys, and found intriguing dual body-centered cubic (BCC) microstructures. There were two issues to be addressed in these alloys: an unidentified strengthening mechanism and intense microsegregation in the as-cast state. We determined that the as-cast microstructure of the non-equiatomic V14Nb19Mo20Ta24W23 and V22Nb24Mo24Ta24W10 alloys consist of two BCC phases with different lattice constants, and that the equiatomic alloy of the V-Nb-Mo-Ta-W system consists of two BCC phases with different compositions but the same lattice constants. The two BCC phases form lattice-matched interfaces, suggesting that precipitation-hardening with high lattice-matching can be one of the reasons for the excellent high-temperature mechanical properties of this alloy system. In addition, we also found that the alloys that were poor in V and Nb, and rich in Ta and W compositions solidified without precipitating of secondary BCC phases. The alloys in this compositional range have partition coefficients of the elements closer to 1 than those with near-equiatomic compositions.

Preparation, microstructure and properties of cordierite-mullite-corundum porous ceramics for high-temperature gas-solid separation

In order to obtain ceramic materials for high-temperature gas-solid separation, the cordierite-mullite-corundum porous ceramics were prepared by particle stacking and pore-forming agent method, with bauxite as aggregate and in-situ synthesized cordierite as binder. Designed four-factor four-level orthogonal experiments for the first time to improve the porosity of the samples. The phase composition and microstructure of the samples were studied by XRD, SEM and other testing methods. The results show that: X6 sample (firing temperature of 1340 °C, 50 wt% binder, aggregate coarse, medium and fine mass percentage 30:50:20, plus 20 wt% charcoal powder, 10 wt% graphite) has the best comprehensive performance, with porosity of 41.51 % and bending strength of 17.88 MPa. Pore size distribution range from 10 to 40 μm. Corundum and mullite, with excellent high-temperature properties formed by the aggregate, are tightly bonded together by cordierite, contributing to the strength of the porous ceramics. The addition of pore-forming agent improves the porosity and forms a stable and uniform pore structure in the porous ceramics. The high porosity and large connected pore size reduce the viscous resistance of the porous ceramics to the gas, improve the filtration performance of the material, and make it expected to be applied in the field of hot gas clean-up.

Preparation of Li4SiO4 from lithium-ion battery cathode waste and diamond wire saw silicon powder using a two-step process

Li4SiO4 materials have excellent high-temperature CO2 adsorption properties. In this thesis, Li4SiO4 was produced by a two-step process by using Li+ from waste lithium-ion battery cathodes as a partial lithium source. The diamond wire saw silicon powder generated by the photovoltaic industry, was used as the silicon source. The reduction melting process of LiMeO2 (Me = Ni, Co, or Mn) in the battery cathode material was analyzed, and the effect of the Li: Si molar ratio in the melting system on the reduction of metal oxides was investigated. The effects of roasting temperature and roasting time on the adsorption properties of Mn-doped Li4SiO4 were also studied. The results showed that the maximum adsorption capacity of Mn-doped Li4SiO4 reached 29.44 wt% under a pure CO2 atmosphere, which was about 2.6 times higher than that of pure Li4SiO4. The saturated adsorption capacity reached 34.2 wt%. This study achieved the resource utilization of solid waste and may reduced the preparation costs of Li4SiO4 adsorbent, which may promote carbon capture technologies.

Insights into orientation-dependent plasticity deformation of HfNbTaTiZr refractory high entropy alloy: An atomistic investigation

HfNbTaTiZr refractory high entropy alloy has emerged as a prospective structural material suitable for high-temperature applications owing to its remarkable combination of high strength, good tensile ductility and excellent high-temperature properties. However, the insufficient understanding of the mechanical responses of this refractory high entropy alloy has hindered the improvement of performance in alloy design and potential for engineering applications. Therefore, the orientation-dependent tensile behaviors of HfNbTaTiZr refractory high entropy alloy are investigated from nanoscale using molecular dynamics simulations. The simulation results show that the mechanical responses under uniaxial tension are strongly correlated to the crystallographic orientation with respect to the loading direction, and the highest Young's modulus and yield strength are achieved along [111] direction. In addition, the deformation twinning and phase transformations are charactered. The tensile behavior oriented along [001] direction is dominated by the BCC-FCC phase transformation, while that oriented along [110] or [111] direction by the BCC-HCP phase transformation. The results reveal the critical role of tensile direction in generating specific crystalline microstructures under high-strain-rate loading conditions, which can enlighten new design strategy in engineering refractory high entropy alloys with specific orientations for extreme environments.

Dual coating layers to enhance oxidation resistance for refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) possess excellent high-temperature mechanical properties, making them have the promising potential for applications at temperatures higher than Ni-base superalloys. However, their development has been limited by their poor oxidation resistance, which demands an urgent and effective improvement. In particular, the oxides of RHEAs generally suffer from severe pest phenomenon between 500 °C and 800 °C. In this study, the low-density, strong and ductile BCC Mo0.3Nb1.3TiZr RHEA was designed and prepared, and the isothermal oxidation mechanism was studied at mainly 800 °C.To effectively improve the oxidation resistance of the alloy, dual coating layers with inner-layer aluminide and outer-layer silica protection were designed and investigated. Firstly, pack cementation was used to prepare aluminide coating on RHEA. Secondly, the sol-gel method was used to deposit silica films over the aluminide coating. With single aluminide coating deposited on the base metal, the weight gain was reduced from 92.2 mg/cm2 of uncoated specimen to 4.5 mg/cm2 at 800 °C for 24 h. With the dual coating layers, no pest phenomenon was observed between 700 °C and 900 °C and the weight gain dropped sharply from 88.4 mg/cm2 of aluminized specimen to 2.8 mg/cm2 at 800 °C for 48 h. The aluminide coating and SiO2 film successfully adhered to the surface of the RHEA. In addition, the SiO2 film reduced the consumption of aluminide coating. Cracking, accelerated oxygen penetration, and pest phenomenon were not observed. This dual coating layers are very promising in solving the poor oxidation resistance of RHEAs.