Ultrahigh Temperature Alloy Research Articles

Effects of TiB2 contents on the microstructure and oxidation behavior of Mo-Si-B composite coatings

A novel Mo-Si-B-TiB2 composite coating was prepared on Nb-Si based ultrahigh temperature alloy by slurry sintering. The effects of TiB2 contents (1, 3, 5 and 10 wt%) on the microstructure and oxidation behavior of Mo-Si-B composite coatings were revealed. The results show that TiB2 modified Mo-Si-B composite coatings display a bilayer structure, in which the outer layer is composed of MoSi2, Mo5Si3 and TiB2 phases, and the inner layer is composed of (Ti,Nb)5Si4, (Nb,X)5Si3 and (Nb,Ti)B2 phases. The added TiB2 particles are evenly distributed in the outer layer of the coatings. By introducing TiB2 into the coating, the service life of Mo-Si-B composite coatings is extended from 50 h to 100 h at 1250 ℃, the oxidation rate of the coating is reduced from 1.62 to 0.02 mg2 cm−4 h−1. This indicates that the addition of appropriate TiB2 effectively slows down the oxidation rate and significantly improves the oxidation resistance of Mo-Si-B composite coatings. The Mo-Si-B composite coating with 3 wt% TiB2 had the lowest mass gain (3.9 mg/cm2) after oxidation at 1250 ℃ for 100 h, and it exhibited the best oxidation resistance. This is attributed to the formation of composite oxide scale with crack-free, good healing ability, suitable levels of differential thermal expansion and low oxygen permeability.

Metastable Liquid Properties and Surface Flow Patterns of Ultrahigh Temperature Alloys Explored in Outer Space.

The metastable liquid properties and chemical bonds beyond 2000 K remain a huge challenge for ground-based research on liquid materials chemistry. We show the strong undercooling capability, metastable liquid properties and surface wave patterns of refractory Nb-Si and Zr-V binary alloys explored in space environment. The floating droplet of Nb82.7Si17.3 eutectic alloy superheated up to 2338 K exhibited an extreme undercooling of 437 K, approaching the 0.2TE threshold for homogeneous nucleation of liquid-solid reaction. The microgravity state endowed alloy droplets with nearly perfect sphericity and thus ensured the high accuracy to determine metastable undercooled liquid properties. A special kind of swirling flow was induced for liquid alloy owing to Marangoni convection, which resulted in the spiral microstructures on Zr64V36 alloy surface during liquid-solid phase transition. The coupled impacts of surface nucleation and surface flow brought in a novel olivary shape for these binary alloys. Furthermore, the chemical bonds and atomic structures of high temperature liquids were revealed to understand the liquid properties in outer space circumstances.

Theoretical study of mechanical and thermodynamic properties of W–Fe alloys: Promising ultra-high temperature alloy materials

In this paper, three kinds of W–Fe alloy models are constructed, namely W15Fe1, W14Fe2 and W13Fe3 alloys. By investigating the phonon spectra, it is found that W13Fe3 alloy is thermodynamically unstable. Therefore, the mechanical and thermodynamic properties of W, W15Fe1 and W14Fe2 alloys are investigated based on first principles method. The investigation demonstrates that the bulk modulus of W15Fe1 alloy increases rapidly with increasing temperature, exhibits high-temperature alloy characteristics. However, the bulk modulus of W14Fe2 alloy decreases rapidly with increasing temperature, and is lower than that of W. The thermal expansion coefficient of W15Fe1 alloy decreases with increasing temperature, and is significantly lower than that of W at 1400K, exhibits low expansion behavior at high temperatures. The thermal expansion coefficient of W14Fe2 alloy increases rapidly with increasing temperature, and is obviously higher than that of W. The thermal conductivity of W15Fe1 alloy exceeds that of W at 2300K, which indicates that doping an appropriate Fe in W can improve the thermal conductivity of W at high temperatures. However, the thermal conductivity of W14Fe2 alloy is significantly lower than that of W and W15Fe1 alloy. At 0K, doping Fe in W decreases the mechanical strength of W, but improves its ductility.

Improved toughness and oxidation resistance of Nb[sbnd]Si based ultra-high temperature alloys by in-situ generated La2O3 phase

The effects of in-situ rare earth oxide phase La2O3 on the microstructure and properties of NbSi based ultrahigh temperature alloys were investigated. The NbSi based alloys were doped with varying amounts of La (0.05, 0.1, 0.2, 0.4 and 0.8 at.%). The La2O3 phase forms in situ during the melting process. The fracture toughness is greatly improved by the La2O3 phase, and the KQ value of the 0.4La alloy is 27.1% higher than that of the base alloy. The improvement of fracture toughness is due to these three aspects: first, the high-melting-point La2O3 phase (TmLa2O3= 2588 K) precipitates first and effectively refines the microstructure. As a result, the frequency of secondary cracking and crack bridging increases. Second, the formation of the La2O3 phase significantly reduces the issue of oxygen contamination. Third, the high-hardness La2O3 phase separates during the fracture process, reducing the constraints on the deformation of NbSi based alloys. Additionally, La2O3 has the lowest Gibbs free energy among all possible formed oxides. During the oxidation process, the continuous oxidation behavior of the La2O3 phase reduces the degree of oxidation inside the alloy. Therefore, the oxidation resistance is significantly improved.

A novel composite Si-Mo-Ti coating with Ti concentration gradient for Nb-Si based ultrahigh temperature alloy prepared by slurry sintering

Composite Si-Mo-Ti coatings with Ti concentration gradient have been successfully prepared on Nb-Si based ultrahigh temperature alloy by slurry sintering. The gradient structure has a higher content of Ti and a lower content of Mo in the outermost layer, which can inhibit the formation of volatile MoO3 and cracks in the coating and guarantee the rapid formation of a dense Si-O-Ti layer during oxidation. After oxidation at 1380 ℃ for 100 h in air, a continuous and compact (Ti,X)O2 layer and a dense duplex Si-O-Ti/(Ti,X)O2 layer formed, which dramatically reduces inward diffusion of O2- and effectively improves the oxidation resistance.

GENERATION OF EFFUSION HOLES ON ULTRA-HIGH TEMPERATURE ALLOY BY MICRO ELECTRO-DISCHARGE MACHINING PROCESS

Fabrication of surface features like effusion holes on ultra-high temperature (UHT) alloys with micro electro-discharge machining ([Formula: see text]-EDM) is a very convolute strategy. Due to the exceptional mechanical properties and resistance to ease brittle transformation at elevated temperatures, UHT alloys are being widely used in hot sections of aero engines. This research tailored the existing capabilities of [Formula: see text]-EDM setup by employing a copper-tungsten (Cu-W) hybrid tool electrode which has been rarely reported. This research addresses investigations, predictive modeling, and optimization during [Formula: see text]-EDM of UHT alloy, namely Inconel 718. For experimentation, the factors like inter-electrodes gap (IEG), current ([Formula: see text]), and pulse on/off time ([Formula: see text]on, [Formula: see text]off) were considered as input variables to analyze the responses like overcut (OC), tool wear rate (TWR), material removal rate (MRR), and diametric alterations in between the top and bottom shape of holes’ feature (DE). The multi-objective desirability-based artificial bee colony optimization (MOABC) projected optimal parametric settings of 5.276 A, 16.389 [Formula: see text]s, 1.77 [Formula: see text]s, and 50 [Formula: see text]m for [Formula: see text], [Formula: see text]on, [Formula: see text]off, and IEG, respectively. These settings provide MRR, TWR, OC, and DE solutions of 0.001479, −0.00038, 0.030239, and 0.047048 mm, respectively. Further, the confirmation test has been performed to validate the optimal solution. The results supported the ideal settings with error values of 0.03177, 1.31578, 0.05863, and 0.04704 for MRR, TWR, OC, and DE, respectively. Distinctive diagnostic tests assessed by several statistical parameters ([Formula: see text], [Formula: see text], [Formula: see text]2, AD-P values) confirmed that the accuracy of developed models for prediction of various performance characteristics has high degree of resemblance with experimental results and the adequacy as well as accuracy of models is also demonstrated. Moreover, the surface morphology has also been scrutinized by microscopical observation to perceive the consequences of the electro-discharge phenomenon on both electrodes. Overall, the research comprises experimentation, surface integrity study, and parametric optimization followed by a confirmation test to perform the betterment of the current [Formula: see text]-EDM approach.

Formation and oxidation behavior of Mo-Si-B composite coatings with B-rich interlayer prepared by slurry sintering method

The Mo-Si-B composite coatings with a B-rich interlayer were prepared on Nb-Si based ultrahigh temperature alloys by slurry sintering method, which exhibited a three-layer structure. The effect of B content (0, 5, 10 and 15 at%) on the microstructure and oxidation resistance of the coatings was investigated. The 10B coating had the lowest mass gain (5.9 mg/cm2) after oxidation at 1250 °C for 100 h. Its good oxidation resistance is attributed to the formation of oxide scales with crack-less, high fluidity and low oxygen permeability, which effectively prevents the inward diffusion of oxygen and prompts the transition from internal oxidation to external oxidation.

Microstructure and fracture behaviours of Nb–16Si ultra-high temperature alloy fabricated by laser directed energy deposition

Microstructure and fracture behaviours of Nb–16Si ultra-high temperature alloy fabricated by laser directed energy deposition

Microstructure and oxidation resistance of Ti modified Si-Mo coating on Nb-Si based ultrahigh temperature alloy

Ti modified Si-Mo coatings have been successfully prepared on Nb-Si based ultrahigh temperature alloy by slurry sintering method. The results show that the addition of Ti as well as its content has an obvious influence on the density, grain size and oxidation resistance of the coatings. The 5 at% Ti modified coating possesses the best oxidation protection performance with a multi-layered structure consisting of a C11b MoSi2/C40 (Mo,Ti)Si2 outer layer and a dense (Ti,Nb)5Si4/(Nb,X)5Si3 inner layer. After oxidation, the formation of SiO2 and TiO2 can quickly seal the holes in the coating and form a dense SiO2/TiO2 scale.

Microstructure, Mechanical Properties and Oxidation Resistance of Nb-Si Based Ultrahigh-Temperature Alloys Prepared by Hot Press Sintering.

Nb-Si based ultrahigh-temperature alloys with the composition of Nb-22Ti-15Si-5Cr-3Al (atomic percentage, at. %) were prepared by hot press sintering (HPS) at 1250, 1350, 1400, 1450 and 1500 °C. The effects of HPS temperatures on the microstructure, room temperature fracture toughness, hardness and isothermal oxidation behavior of the alloys were investigated. The results showed that the microstructures of the alloys prepared by HPS at different temperatures were composed of Nbss, βTiss and γ(Nb,X)5Si3 phases. When the HPS temperature was 1450 °C, the microstructure was fine and nearly equiaxed. When the HPS temperature was lower than 1450 °C, the supersaturated Nbss with insufficient diffusion reaction still existed. When the HPS temperature exceeded 1450 °C, the microstructure coarsened obviously. Both the room temperature fracture toughness and Vickers hardness of the alloys prepared by HPS at 1450 °C were the highest. The alloy prepared by HPS at 1450 °C exhibited the lowest mass gain upon oxidation at 1250 °C for 20 h. The oxide film was mainly composed of Nb2O5, TiNb2O7, TiO2 and a small amount of amorphous silicate. The formation mechanism of oxide film is concluded as follows: TiO2 forms by the preferential reaction of βTiss and O in the alloy; after that, a stable oxide film composed of TiO2 and Nb2O5 forms; then, TiNb2O7 is formed by the reaction of TiO2 and Nb2O5.

Mechanical properties of the novel L12-Ir3Z ultra-high temperature alloys (Z =Ti, V, Zr, Nb, Hf and Ta)

Ir 3 Z ( Z = Ti, V, Zr, Nb, Hf and Ta) ultra-high temperature alloys, well known as their high melting point and excellent mechanical properties, are promising candidates for high temperature structural materials beyond nickel-based materials. Nevertheless, knowledge of their mechanical properties and the mechanism of tensile/shear deformation still remains poor. In this work, the fundamental mechanical properties and mechanical behavior under different tensile/shear loading directions of Ir 3 Z alloys were studied using DFT calculations. All Ir 3 Z alloys have very high mechanical moduli (404.7–561.3 GPa for Young’s modulus). For both mechanical and tensile/shear strength, Ir 3 V, Ir 3 Nb and Ir 3 Ta are distinctly superior to Ir 3 Ti, Ir 3 Zr and Ir 3 Hf, in which Ir 3 V shows the most outstanding performance. Furthermore, these alloys exhibit similar tensile/shear stress-strain relationship and all of them show anisotropic mechanical behavior under strain, i.e., [100] direction shows the strongest resistance to tensile stress and (111)/[ 1 ( _ ) 10] turns out to be the most likely slip system. Meanwhile, the results of average charge density as well as the density of states diagram suggest that the degree of interatomic bonding mainly determines the ideal strength of different structure and tensile/shear direction of Ir 3 Z alloys. • Ir 3 (V, Nb, or Ta) are distinctly superior to Ir 3 (Ti, Zr or Hf) for tensile/shear strength. • (111))/[− 110] turns out to be the most likely slip system for studied six Ir 3 Z alloys. • The interatomic bonding mainly determines the ideal strength of Ir 3 Z alloys.

A study on fine Nbss/γ-(Nb,X)5Si3 eutectic with significantly improved room-temperature fracture toughness by added ZrB2

Four NbSi based ultrahigh temperature alloys with compositions of Nb-16Si-20Ti-1ZrC-xZrB2 (x = 0, 0.5, 1, 3) (at.%) were prepared by vacuum non-consumable arc melting. The microstructure evolution, fracture toughness, and fracture behavior of four alloys were investigated. Results showed that four alloys consisted of Nbss, γ-(Nb,X)5Si3, and (Nb,X)3Si phases. The low content of ZrB2 facilitates the disintegration of (Nb,X)3Si, while the high content of ZrB2 stabilizes (Nb,X)3Si. The maze-like Nbss/γ-(Nb,X)5Si3 eutectic structure forms in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy. With the increase of ZrB2 content, the room-temperature fracture toughness of Nb-Si-Ti-ZrC-based alloy increases and decreases. When the addition amount of ZrB2 is 0.5 at.%, the room-temperature fracture toughness is the largest of 18.4 MPa·m1/2, and the fracture mode changes from quasi-cleavage fracture to dimple fracture. Compared with the base alloy, it is increased by 28%. It is the highest value of as-cast arc melting that has been reported so far. The maze-like eutectic of the high content of Nbss (54.9 ± 5.9%) phase promotes the formation of microcracks and inhibits the propagation of cracks in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy.

Effect of Re addition on microstructure and mechanical properties of Nb-Si-Ti-Al-Cr-Hf ultrahigh temperature alloys

In order to improve the mechanical performance of multicomponent NbSi based alloy, the effects of rare metal element Re addition on microstructure evolution and mechanical performance of Nb-16Si-20Ti-2Al-2Cr-2Hf (at.% as follows) alloys are studied systematically. With the high melting point alloying element Re addition, the primary Nbss phase of Nb-16Si-20Ti-2Al-2Cr-2Hf-xRe transforms from equiaxed structure to dendritic structure due to the component supercooling, and the silicide phase transforms from α-Nb5Si3 to β-Nb5Si3. Moreover, with the increasing Re addition, the microstructure has been refined firstly, then coarsened and finally formed developed dendrite. The element Re preferentially solubilizes in the Nbss phase, when the Re content exceeds 1.0%, it can be detected in silicide phase. The Vickers microhardness and compressive strength of the multicomponent NbSi based alloys have been improved significantly because of the solid solution strengthening by Re addition. And the most obvious improvement is the room temperature fracture toughness increases from 10.33 MPa m1/2 to 14.40 MPa m1/2, as the Re content increases from 0 at.% to 0.5 at.%, which is 39.4% higher than the Re free NbSi based alloy.

Mechanical Properties of the Novel L12-Ir3z Ultra-High Temperature Alloys (Z = Ti, V, Zr, Nb, Hf and Ta)

Mechanical Properties of the Novel L12-Ir3z Ultra-High Temperature Alloys (Z = Ti, V, Zr, Nb, Hf and Ta)

Improving primary Nbss morphology and fracture toughness in hypoeutectic Nb-Si-Ti ternary alloy by ultrasonic treatment

Nb-Si ultra-high temperature alloys have been a hot topic in aviation materials, but low fracture toughness limits the application. Ultrasound treatment has a significant advantage on the grain refinement and properties improvement, successfully induced into hypoeutectic Nb-Si-Ti ternary alloys to improve primary morphology and fracture toughness. In this work, Nb-16Si-22Ti alloy was treated by ultrasound of 0 s, 80 s, 120 s, 160 s and 200 s. Meanwhile, the coupled physical fields were simulated in Comsol Multiphysics to reveal the mechanism of ultrasonic action. Results show that γ-Nb5Si3 appears with ultrasound induced, and the solidification path is close to the equilibrium transition. Primary Nbss phases were coarsened and spheroidized after ultrasound treatment, and fracture toughness improved by 69.16%. Simulation results show that the strong acoustic active zone is at the height of 15 mm in the ingot, with maximum sound pressure of 36.3 MPa. The high sound pressure can break dendrites. Maximum fluid velocity reaches 0.03 m/s in the longitudinal section and 0.02 m/s in the transversal section with several circulations. Acoustic streaming and cavitation effects break dendrites and spheroidize Nbss phase, benefit to elements distribution uniformly, equilibrium transition and fracture toughness improvement.

Microstructure and properties of Mo-Si-B alloy fabricated via novel technology: Plasma Oscillatory Pressure Sintering

Molybdenum silicon boron alloy is regarded as a new generation of ultra-high temperature alloys and is expected to replace the existing nickel-based alloys. However, the molybdenum silicon boron alloys prepared by powder metallurgy exhibited low density and had many pore defects, which were left hanging in the air. Here, a novel oscillatory sintering method is used to replace the traditional atmospheric sintering. It is found that the addition of the oscillatory frequency improved the density and mechanical properties of the alloy, and made the crystal grains refined to a large extent. It did have certain reference value for the preparation and development of molybdenum silicon boron alloy.

Composite alloying effects of V and Zr on the microstructures and properties of multi-elemental Nb–Si based ultrahigh temperature alloys

Abstract Four multi-elemental Nb–Si based ultrahigh temperature alloys with compositions of Nb–22Ti–15Si–5Cr–3Al-2Hf-xV-yZr ((x, y) = (0, 0), (3, 0), (0, 4) and (3, 4)) (at.%) were prepared by vacuum non-consumable arc melting. The effects of single and combined additions of V and Zr on the microstructure, room temperature fracture toughness, microhardness and compressive yield strength and oxidation resistance at 1250 °C of the alloys have been investigated. The results show that the 0V–0Zr alloy is eutectic structure of NbSS/(Nb,X)5Si3, while the other three alloys are hypereutectic structure, mainly consisting of primary γ(Nb,X)5Si3 blocks and NbSS/(Nb,X)5Si3 eutectic. Alloying with V and Zr can suppress the formation of α(Nb,X)5Si3 and promote the formation of γ(Nb,X)5Si3. The room temperature fracture toughness of the alloys is improved by single addition of V or Zr, and is markedly improved by the composite additions of V and Zr. The microhardness of γ(Nb,X)5Si3 is enhanced by the solid solution strengthening of V and Zr addition, while that of NbSS is reduced by the solid solution softening of V addition. The compressive yield strength at 1250 °C of the alloys reduces with V addition while increases with Zr addition, and the alloy with composite additions of V and Zr shows the highest compressive yield strength. The single additions of V or Zr can ameliorate, but the composite additions of V and Zr degrade the oxidation resistant performance at 1250 °C.

Effects of Mo and Zr composite additions on the microstructure, mechanical properties and oxidation resistance of multi-elemental Nb-Si based ultrahigh temperature alloys

• Adding Mo and/or Zr suppresses α(Nb,X) 5 Si 3 formation whilst promotes γ(Nb,X) 5 Si 3 formation. • Zr addition ameliorates room temperature fracture toughness, while further Mo addition degrades it. • Adding Mo/Zr enhances, and cooperatively adding Mo and Zr further increases compressive strength at 1250 ℃. • The alloy containing Mo and Zr shows the best oxidation resistance at 1250 °C. The Nb-Si based alloys adding Mo, Zr and Mo-Zr respectively were prepared by vacuum non-consumable arc melting. The alloys’ microstructure and comprehensive performances including microhardness, room temperature fracture toughness as well as compressive strength and oxidation resistance at 1250 °C were evaluated systematically. The results show that adding Zr in multi-elemental Nb-Si based alloys changes the microstructure from eutectic to hypereutectic, while further adding Mo in Zr-containing alloy decreases the content of primary silicide blocks. Both the single and composite additions of Mo and Zr in the alloys suppress the formation of α(Nb,X) 5 Si 3 whilst promote the formation of γ(Nb,X) 5 Si 3 . Both the dissolved Mo (mainly in Nbss) and Zr (primary in γ(Nb,X) 5 Si 3 ) have the effect of solid solution strengthening and improve the microhardness of phases. The room temperature fracture toughness of the alloys is ameliorated by Zr addition, while is reduced by further Mo addition. Alloying with Mo or Zr alone can enhance, and composite alloying with Mo and Zr continues to increase the compressive strength at 1250 °C of the alloys. The alloy with composite additions of Mo and Zr shows the best oxidation resistance at 1250 °C due to the formation of a denser and well adhesive inner layer of scale and the improvement of the oxidation resistance of silicides under the synergistic effects of Mo and Zr.

Oxidation and hot corrosion behaviors of Nb−Si based ultrahigh temperature alloys at 900 °C

Abstract Oxidation and hot corrosion behaviors at 900 °C of Nb−Si based ultrahigh temperature alloys were investigated. Both oxidation and hot corrosion kinetics curves of the alloy involve an initial parabolic stage and a later rapid linear stage. In the initial oxidation stage (1−50 h), a thin and continuous scale is formed on the alloy surface, while severe pest degradation phenomenon is observed in the linear oxidation stage. Compared with oxidation of the alloy in static air, a linear hot corrosion stage happens earlier and catastrophic scale disintegration occurs after hot corrosion for 20−100 h, demonstrating that molten salts (Na2SO4 and NaCl) could significantly accelerate the oxidation process of the alloy. STEM results indicate that the corroded scale consists mainly of TiO2, Nb2O5, TiNb2O7, amorphous silicate and NaNbO3.

Influence of molybdenum contents on the microstructure, mechanical properties and oxidation behavior of multi-elemental Nb–Si based ultrahigh temperature alloys

The mechanical properties, including room-temperature fracture toughness, microhardness and compressive strength at 1250 °C, and oxidation resistance at 1250 °C of Nb–Si based alloys with different Mo contents (0, 2, 5 and 10 at.% Mo additions) were systematically evaluated. The roles of Mo content additions in the relationships between these properties and microstructures in a multi-elemental system of Nb–Ti–Si–Cr–Al–V-Hf-Zr-Mo were revealed. The results show that the four alloys are all composed of primary γ(Nb,X)5Si3 blocks, three types of Nbss/γ(Nb,X)5Si3 eutectic and a small amount of Nbss/Cr2Nb eutectic. Alloying with Mo obviously decreases the amount and size of primary γ(Nb,X)5Si3 blocks, and correspondingly increases the concentration of Si in the remaining liquid during solidification, which results in preferable formation of regular eutectic. Both the microhardness of microstructural constituents at room temperature and the compressive strength at 1250 °C of the alloys are obviously improved due to the solid solution strengthening effect by Mo addition in the alloys. Adding 2 at.% Mo in the alloys simultaneously ameliorates the room-temperature fracture toughness as well as the oxidation resistance at 1250 °C of the alloys, while the additions of 5 and 10 at.% Mo in the alloys deteriorate them. Moreover, the adhesion of oxide scales is significantly improved by alloying with Mo.