High Temperature Structural Materials Research Articles

Microstructure and mechanical properties of unalloyed molybdenum fabricated via wire arc additive manufacturing

Molybdenum is an important high-temperature structural material but has poor processability. Additive manufactured unalloyed Mo is generally very small and the mechanical properties is seldomly studied. In this work, wire arc additive manufacturing was adopted, and crack-free molybdenum parts with high-density (99.0%) and characteristic size of 20 mm×20 mm×120 mm were successfully fabricated by short-track scanning. The microstructure and mechanical properties of samples in both the as-deposited and heat-treated states were studied and compared. Large columnar grains were observed, which were basically along the 〈001〉 direction. Heat treatment leads to grain coarsening, and the elimination of some sub-grain boundaries. Due to the weakened effect of grain and sub-grain boundary hardening, the mechanical properties of heat-treated specimens were worse than that of as-deposited specimens at room temperature. Both of them exhibit brittle fracture features. Under high temperature, the ductile fracture is observed, and the as-deposited specimen has similar strength and ductility, compared with the heat-treated specimens, suggesting a weak role of grain and sub-grain boundary at high temperature. A large number of fragments were observed at the fracture surface after high-temperature tests, which was MoO3 by energy dispersive spectroscopy test.

Quasi-quantitative control of the properties at room temperature and 1000°C of NiAl-based composites prepared by hot press sintering

For high-temperature structural materials such as NiAl, the selection of appropriate mechanical properties to meet the room-temperature ductility and high-temperature strength required for practical engineering applications is a pressing issue. To address this issue, novel NiAl-based composites reinforced by 3-dimensional network structure were prepared using pre-alloyed powders sieved into five different particle size ranges. All composites in this paper had excellent mechanical properties as the 3-dimensional network structure could improve both room-temperature ductility and high-temperature strength. Due to almost each powders grew into one grain during the hot pressing sintering process and the grains did not grew further caused by the hindrance of the 3-dimensional network structure, the particle size of the original powders and the grain size of the composites showed an obvious linear relationship. The relationship among original powder particle size, grain size and mechanical properties of the NiAl-based composites was established by fitting method, and the fitted equations were validated with an average deviation of 2.45%, so that quasi-quantitative control of the mechanical properties could be achieved by designing the original powder particle size. This work allowed both room and high temperature mechanical properties of NiAl to be improved and quasi-quantitative control over them, which provided a new idea for the design of composites.

Research progress on microstructure and property regulation of ultra-high temperature oxide eutectic ceramics by high gradient directional solidification technology

With the rapid development of aerospace and other high-tech fields, a new generation of high perfor-mance, high efficiency ultrahigh temperature structural materials and their preparation technologies increasingly becomes the focus of the development of aerospace strategy in the world. Ultrahigh temperature alumina based eutectic ceramics have excellent intrinsic oxidation resistance, corrosion resistance, creep resistance, high strength and other excellent properties. Therefore, they have outstanding advantages and great development prospect in extreme environment such as ultrahigh temperature above 1 400℃, water and oxygen corrosion circumstances. In this paper, the current development status has been firstly introduced for the ultrahigh temperature oxide eutectic ceramics prepared by high gradient directional solidification technologies. Then, the formation mechanism and control method of the defects during the high gradient directional solidification process of the alumina based ultrahigh temperature eutectic ceramics have been summarized. Furthermore, the microstructure characteristics and homogenization control methods, crystal orientation and texture control methods, mechanical properties and strengthening-toughening control methods of oxide eutectic ceramics have been reviewed. Finally, the development trend and breakthrough point have been comprehensively prospected for the preparation of oxide eutectic ceramics with high performance and large size by high gradient directional solidification technologies.

Electrochemical Noise Response of Cr2Nb Powders Applying Mechanical Alloying

Cr2Nb alloys are potential candidates for high-temperature structural materials. The influence of different mechanical alloying parameters (milling time) and sintering processes were studied. After mechanical alloying and observation by scanning electron microscope (SEM), nano powders were characterized and then sintered by spark plasma sintering (SPS). Electrochemical noise (EN) tests were also conducted in order to study the electrochemical behavior. From the current experimental results, it was revealed that ball milling times up to 20 h may explain the influence of Nb–Cr alloys and its association to the Laves phase and corrosion behavior. These insights aimed at improving the samples’ predicted behavior before spending time and resources at high-temperature industrial processes.

In-situ study on γ phase transformation behaviour of γ-TiAl alloys at different cooling rates

γ-TiAl-based alloys are promising lightweight high-temperature structural materials, and the transformation from the α parent phase to γ lamellae during the cooling process has a great influence on the microstructures and mechanical properties of TiAl alloys. In this paper, an in-situ observation technique, high-temperature laser scanning confocal microscopy (HTLSCM), was utilized to investigate the continuous cooling transition (CCT) from the α phase in three compositions. The nucleation and growth behaviors of γ lamellae were studied at several moderate cooling rates. In addition, the processes of helium gas quenching for three alloys were investigated, and the massive transformation was observed in Ti–49Al. CCT diagraphs were concluded for three alloys. The Vickers hardness of TiAl alloys subjected to different cooling rates was tested, and it was found that the hardness of alloys was enhanced with increasing cooling rate due to the refinement of lamellar spacing.

Preparation of Niobium Aluminium Alloy Based on Shock Compression Method

A new method based on a light-gas gun has been proposed to synthesize Nb-Al alloys, and a recovery capsule has been investigated. The copper-coated sample is accessible after shock wave loading. In this paper, we successfully synthesize Nb-Al alloys, which are high-temperature structural materials. X-ray diffraction is employed to clarify the structural characteristics of compounds after impact, and the simulation of X-ray diffractions is employed to clarify the structure. In detail, tetragonal NbAl3 alloys certainly appeared in the recovery capsule; this alloy is considered to be best candidate.

Thermal shock resistance and toughening mechanism of W/Ta and W/TiN/Ta laminated composites

The brittleness of tungsten (W) is a significant limitation on its widespread applications as a high-temperature structural material. Laminated composite technology proved to be an effective method to improve the toughness of W. In this work, W/Ta and W/TiN/Ta laminated composites were prepared by spark plasma sintering (SPS) to overcome the brittleness of W. The toughening properties of the W/Ta and W/TiN/Ta laminated composites were evaluated by electron beam thermal shock and laser thermal diffusivity testing. The toughening mechanism was clarified by scanning electron microscopy, electron probe micro-analysis, and transmission electron microscopy. After the three-point bending test, interfacial debonding was observed predominantly at the incoherent W-TiN rather than at the semi-coherent Ta-TiN interfaces. At 0.30 GW/m2 of electron beam thermal shock, the average width and depth of thermal shock cracks in the W/Ta laminated composite were ~ 9.4 μm and ~ 30.0 μm, respectively, which were much smaller than those in the W/W laminates and W/TiN/Ta laminated composite.

L12-strengthened multicomponent Co-Al-Nb-based alloys with high strength and matrix-confined stacking-fault-mediated plasticity

This study presents the alloy development of a new class of L12-strengthened Co-Al-Nb-based alloys with high γ′-solvus temperatures together with superb strengths at both ambient and elevated temperatures. The L12-Co3(Al, Nb) phase was found to be in equilibrium with the γ-Co matrix and the B2-CoAl phase in the ternary Co-10Al-3Nb alloy after an isothermal aging at 700 °C; however, it transformed into the Laves phase as the aging temperature increased to 800 °C. Alloying additions of Ni helped to suppress the B2 phase formation, resulting in a clean γ-γ′ dual-phase microstructure. Ti and Ta elements further stabilized the L12 structure and increased the γ′-solvus temperature to 1150 °C without inducing the formation of any other deleterious intermetallic phases. The newly developed Co-Al-Nb-Ni-Ti-Ta multicomponent Co-rich alloy has demonstrated outstanding yield strengths at both ambient and elevated temperatures, reaching 1023 ± 27 MPa at 25 °C and 897 ± 53 MPa at 700 °C, respectively. Furthermore, electron microscopy analyses uncovered unique deformation substructures, in which plasticity is predominantly carried out via nanoscale matrix-channel-confined stacking faults. As determined by the first-principle calculations, the absence of particle shearing upon deformation at ambient temperature is ascribed to the ultrahigh planar fault energies of the multicomponent γ′ precipitates. High-density superlattice-stacking-fault shearing and their interactions are responsible for the yield anomaly at 700 °C. These findings not only provide the fundamental understanding of the deformation behavior of the L12-strengthened alloys, but also demonstrate the great potential for developing next-generation high-temperature structural materials based on the multicomponent Co-rich alloy systems.

Preparation of high purity α-Al2O3 from industrial aluminum hydroxide

Aluminum oxide (alumina; Al2O3) has advantages such as its thermal, chemical, and physical properties when compared with several ceramics materials. It is widely used in ceramics such as firebricks, abrasives and integrated circuit (IC) packages; catalysts; catalyst supports; ion exchange and other fields. Alumina has a complex structure and many crystalline polymorphic phases such as α-Al2O3, βAl2O3, γ-Al2O3, δ-Al2O3, θ-Al2O3, η-Al2O3, κ-Al2O3, χAl2O3 and ρ-Al2O3. Among these phases, α-Al2O3 is widely used and studied as high temperature structural material, electronic packaging, corrosion resistance ceramics and translucent ceramics. This study focus on the preparation of high purity α-Al2O3 from industrial aluminum hydroxide. The results show that after removing Na2O and SiO2 with distilled water, the sample was dissolved in NaOH solution to form NaAlO2 solution. Precipitating of Al(OH)3 from NaAlO2 solution with one of the following substances: HCl (4N), H2SO4 (4N), CH3COOH (4N) or CO2 (45% Vol). The precipitated Al(OH)3 was filtered and washed with distilled water and then dried. Finally, thermally decompose Al(OH)3 at 11000C in 2 hours to form pure α-Al2O3.

Oxidation Protection of High-Temperature Coatings on the Surface of Mo-Based Alloys—A Review

Molybdenum and its alloys, with high melting points, excellent corrosion resistance and high temperature creep resistance, are a vital high-temperature structural material. However, the poor oxidation resistance at high temperatures is a major barrier to their application. This work provides a summary of surface modification techniques for Mo and its alloys under high-temperature aerobic conditions of nearly half a century, including slurry sintering technology, plasma spraying technology, chemical vapor deposition technology, and liquid phase deposition technology. The microstructure and oxidation behavior of various coatings were analyzed. The advantages and disadvantages of various processes were compared, and the key measures to improve oxidation resistance of coatings were also outlined. The future research direction in this field is set out.

Dense additive-free bulk boron nitride ceramics developed by self-densification of borazine

Boron nitride (BN) ceramics are promising candidates for high-temperature structural and functional materials. However, it is difficult to sinter dense additive-free bulk BN monoliths because of the covalent bond and flake structure. Here, we report dense bulk BN with relative density higher than 95% without sintering additives achieved via a self-densifying approach of borazine within a wide temperature range from 800 °C to 1800 °C. A polyborazylene with a controlled degree of cross-linking was synthesized and press-molded into shaped green bodies, which were pressureless pyrolyzed into porous frameworks and then densified through repeated borazine infiltration and pyrolysis method. The microstructural and crystalline evolution of the bulk BN ceramics, as well as the corresponded mechanical, thermal and dielectric properties were investigated.

Effect of electroless nickel plating on high temperature oxidation behavior of carbon nanotube reinforced René104 superalloy composites

In this study, multi-walled carbon nanotubes (MWCNTs) reinforced René104 superalloy laminated composites with sub-micron grain size were prepared by ball milling and spark plasma sintering (SPS). MWCNTs did not change the composition and distribution of oxide scale of René104 alloy, but the micropores caused by MWCNTs formed oxygen molecular channels, which accelerated the oxidation at 900 °C. Electroless nickel plating on MWCNTs can improve the compatibility between MWCNTs and René104 matrix, thus significantly reducing the influence of MWCNTs on high temperature oxidation. This work will pave the way to develop the high-temperature structural materials reinforced by carbon nanomaterials.

Design of Refractory High Entropy Alloys TiZrHfNbX (X=Cr/Ta) system, using thermodynamic calculations

Refractory high entropy alloys (RHEAs) are materials with novel concept that do not use a specific element as a base material and mixture of multiple elements in near equal amounts. This concept is expected to be adopted for use in aerospace heat-resistant materials that can be used at higher temperatures than currently available. We focused on the TiZrHfNbCr system as a new high-temperature structural material. We carried out thermodynamic calculation using the CALPHAD method and proposed compositions of TiZrHf(NbCr)2 and TiZrHf(NbCr)7 in addition to the equimolar composition (TiZrHfNbCr). These alloys are fabricated by arc melting, and microstructures in TiZrHf(NbCr)7 alloys, predicted by CALPHAD were observed in SEM images. Although some agglomeration of Nb is recognized by observation, microstructures of TiZrHf(NbCr)2 were also good agreement with the prediction by CALPHAD. From the results of TiZrHfNbCr, TiZrHf(NbCr)2 and TiZrHf(NbCr)7, complex oxides were formed and small weight gains were observed compared to other alloys (e.g., ZrTi and NbTa). The effect of formation of complex oxide to the oxidation behavior of these alloys will be discussed in the presentation.

Investigation on thermoelectric materials of Ni-Al-M system Heusler alloy

In this study, we investigated the electronic structure and DOS of Ni-Ti-Al Heusler alloys, which have use in research and development as high-temperature structural materials, for high thermoelectromotive force by using first-principles calculations. Based on these analyses, we aim to create and analyze materials with non-stoichiometric compositions and nano- and composite structures in material experiments. The goal is to realize practical high-performance Ni-Ti-Al Heusler alloy thermoelectric materials by searching for materials that exhibit synergistic effects from electronic structure and DOS to nano- and micro-composites on a multiscale and by elucidating the mechanisms.

Excellent strength-ductility synergy of NiAl-based composites achieved by a 3-dimensional network structure

NiAl intermetallic compounds have attracted the attention of researchers in the field of high-temperature structural materials, but their poor room-temperature ductility and insufficient high-temperature strength crying out for solutions have hindered their practical application. To solve these problems, a novel 3-dimensional network structure-reinforced NiAl-based composite was designed by hot presing sintering sinter with pre-alloyed powders. The 3-dimensional network structure was composed of discrete nearly spherical HfO 2 and rod-shaped HfRe 2 . The mechanical properties of the composites were closely correlated with the distribution of the 3-dimensional network structure in the matrix, which was controlled by the sintering temperature and holding time. A microstructure with a desirable 3-dimensional network structure distribution and the best comprehensive mechanical properties was produced at 1375 °C/60 min. The microstructural evolution along with the strengthening and toughening mechanisms were discussed in detail. The increase in the high-temperature strength was mainly attributed to the pinning effect of the 3-dimensional network structure on dislocations and the restraining effect on the sliding and rotation between the matrix grains, while the increase in room-temperature ductility was mainly due to the grain refinement and hindering of crack propagation by the 3-dimensional network structure. In this paper, the development of a novel low-density NiAl-based composite with a 3-dimensional network structure improved both the strength and ductility, providing design ideas for the application of NiAl-based materials.

Insights into high thermal stability of laser additively manufactured Al2O3/GdAlO3/ZrO2 eutectic ceramics under high temperatures

Laser additive manufacturing techniques have been considered to be the most promising methods to fabricate melt-grown Al 2 O 3 -based eutectic ceramics which are potential candidates for high temperature structural applications. In this paper, Al 2 O 3 /GdAlO 3 /ZrO 2 ternary eutectic ceramic was additively manufactured based on one-step melt-growth by laser directed energy deposition, and its thermal stability under high temperatures was studied. The results indicate that the as-solidified eutectic composite consists of ultrafine phases of α-Al 2 O 3 , GdAlO 3 and t-ZrO 2 , and presents an irregular eutectic morphology with amounts of terminations and branches. The 3D-printed eutectic ceramic exhibits a superior thermal stability that no mass variation and no phase transition occur even after annealed at 1500 °C for 200 h. In addition, the submicron-scaled eutectic microstructure and the polished surface topography can maintain stability up to 1400 °C which is higher than 80% of its melting point (>0.8 T m ). With further increasing the temperature, obvious microstructure coarsening, and surface holes formation occur, leading to the severely deterioration of the mechanical property. The hardness of the as-deposited eutectic ceramic rapidly decreases from 15.84 ± 0.61 GPa to 13.01 ± 0.40 GPa after 200 h annealed at 1500 °C. The results indicate that the additively manufactured Al 2 O 3 -based eutectic ceramics have great potential to be long-time high-temperature structural materials applied at elevated temperatures. • Highly dense Al 2 O 3 /GdAlO 3 /ZrO 2 eutectic ceramic is 3D-printed based on melt growth. • Thermal stability of the additively manufactured eutectic ceramic is studied under high temperatures. • Critical transition temperature from stability to instability is determined, and the instability mechanism is revealed.

Microstructure and Oxidation Behavior of Nb-Si-Based Alloys for Ultrahigh Temperature Applications: A Comprehensive Review

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys.

A ductile Nb40Ti25Al15V10Ta5Hf3W2 refractory high entropy alloy with high specific strength for high-temperature applications

Refractory high entropy alloys (RHEAs) with good high-temperature softening resistance have been revealed as promising candidates for high-temperature structural materials. In this work, a low-density ductile RHEA, Nb40Ti25Al15V10Ta5Hf3W2, has been developed. The RHEA with a BCC matrix and B2 nanoprecipitates exhibits excellent specific yield strength. The compressive specific yield strength (σ0.2/ρ) at 1073 K is as high as 83.2 MPa g−1 cm3. The different deformation behaviors during compression at 1073 K and 1273 K are also identified. The dislocation-dominated deformation provides the initial strain hardening capability, and then microcracks and dynamic recovery accelerate the transition from strain hardening to softening at 1073 K. While the diffusion-controlled dislocation annihilation and continuous dynamic recrystallization (DRX) are the dominant reasons for persistent strain softening at 1273 K. Our work not only reports a promising RHEA with excellent high-temperature properties, but also promotes the development of RHEAs for high-temperature applications.

Microstructure and mechanical properties of multi-phase reinforced Hf-Mo-Nb-Ti-Zr refractory high-entropy alloys

In this paper, Hf-Mo-Nb-Ti-Zr alloy system with single BCC solid solution phase was selected as the matrix alloy. In order to design a new high temperature structural material with excellent comprehensive properties, the multiphase reinforced refractory high entropy alloys was prepared by adding metallic elements of Al or Cr and nonmetallic elements of B, C or Si into Hf-Mo-Nb-Ti-Zr alloys at the same time. The results show that the type and content of newly formed phases are closely related to the mixing enthalpy between alloying elements and the constituent elements of the matrix alloy. This provides a valuable reference for the prediction of phase formation when adding alloying elements to improve the properties of high entropy alloys.

High-temperature mechanical properties and deformation behavior of carbides reinforced TiNbTaZrHf composite

Refractory high entropy alloys (RHEAs) have broad prospects in the field of high-temperature structural materials because of their outstanding mechanical properties at high temperatures. In this work, a novel TiNbTaZrHf based composite was fabricated by powder metallurgy in-situ method. By introducing carbides, the TiNbTaZrHf based composite exhibits an ultra-high yield strength of 2620 MPa at room temperature. Meanwhile, the composite still maintains a high strength of 508 MPa at 1000 °C. The significant strength improvement can be attributed to the load-bearing effect caused by the dispersed carbides with high volume fraction, while the intergranular fracture of carbides is responsible for the plastic instability. When the temperature rises to 1200 °C, the yield strength decreases significantly. The weakening of load-bearing effect and the activation of grain boundary sliding are the main reason for the strength reduction, while interfacial debonding between the matrix and carbides becomes the main failure behavior.