Nb-based Alloys Research Articles

Influence of siliconizing on the oxidation behavior of plasma sprayed MoSi2 coating for niobium based alloy

Plasma spraying combined with halide activated pack cementation (HAPC) was used to deposit silicide coating on Nb-based alloy. X-ray diffraction (XRD) and energy disperse spectrum (EDS) indicate the formation of the siliconized NbSi2 transition layer and the sprayed MoSi2 outer layer. NbSi2 layer prepared with HAPC exhibits relatively uneven surface which could promote the deposition of the sprayed MoSi2. The coating specimen with 5 h siliconizing presented the best oxidation resistance with only 0.18% mass gain after 25 h oxidation at 1200 °C in air. The synergistic protection effect, depending on the continuous silica layer formed on the coating surface and the dispersal silica within the coating and interface, is responsible for the excellent oxidation resistance of the coating.

Automatic Microstructural Characterization and Classification Using Higher-Order Spectra on Ultrasound Signals

During the gas tungsten arc welding of nickel based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as $$\gamma {''}$$ and $$\delta $$ phases can precipitate in the microstructure, during aging at high temperatures. Nevertheless, choosing the appropriate conditions of welding can minimize the formation of the Nb-rich Laves phases and thus reduce the susceptibility to solidification cracking. This study aims at the automatic microstructurally characterizing the kinetics of phase transformations on an Nb-base alloy, thermally aged at 650 and 950 $$^{\circ }$$ C for 10, 100 and 200 h, through backscattered ultrasound signals at frequency of 4 MHz. For this, an automated processing system was designed using the spectrum representation of higher order statistics. The ultrasound signals are inherently non-linear and thus the conventional linear time and frequency domain methods can not reveal the complexity of these signals clearly. Bispectrum (the spectral representation of third order correlation) is a non-linear method which is highly robust to noise. In the proposed system, the bispectrum coefficients are subjected to linear discriminant analysis (LDA) technique to reduce the statistical redundancy and reveal discriminating features. These dimensionality reduced features are fed to the classification and regression tree, random forest and k-nearest neighbor (k-NN) classifiers to automatic microstructural characterization. Bispectrum coupled with LDA and k-NN yielded the highest average accuracy of 95.0 and 78.0 %, respectively for thermal aging at 650 and 950 $$^{\circ }$$ C. Thus, the proposed processing system provides high reliability to be used for microstructure characterization through ultrasound signals.

Automatic microstructural characterization and classification using dual tree complex wavelet-based features and Bees Algorithm

During the gas tungsten arc welding of nickel-based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as γ″ and δ phases can precipitate in the microstructure, during aging at high temperatures. However, it is possible to minimize the formation of the Nb-rich Laves phases and therefore reduce the possibility of solidification cracking by adopting the appropriate welding conditions. This paper aims at the automatic microstructurally characterizing the kinetics of phase transformations on an Nb-base alloy, thermally aged at 650 and 950 °C for 10, 100 and 200 h, through backscattered ultrasound signals at frequency of 4 MHz and dual tree complex wavelet transform (DTCWT)-based feature extraction technique. The feature set comprises of statistical attributes (such as variance, skewness and kurtosis) extracted from the complex wavelet coefficients which are obtained using the DTCWT decomposition of a backscattered ultrasound signal. Also, the performance of the proposed feature extraction technique is compared with the conventional discrete wavelet transform. Finally, these features are fed to the probabilistic neural network (PNN) and radial basis function classifiers to automatic microstructural classification. The training process of these networks depends on the selection of the smoothing parameter of the networks’ activation function in a hidden layer. In this article, we introduce the application of the Bees Algorithm to the automatic adaptation of smoothing parameters. The proposed feature extraction technique coupled with the optimized PNN yielded the highest average accuracy of 96 and 83 %, respectively, for thermal aging at 650 and 950 °C. Thus, the proposed processing system provides high reliability to be used for microstructure characterization through ultrasound signals.

Effect of Aluminium and Molybdenum on the microstructural evolution and mechanical properties of hypoeutectic Nb-Si-Cr Alloys

Among the refractory metals, Nb-based alloys are considered the most promising potential materials in the field of high temperature applications (⩾1150°C). In this regard, novel alloy design strategies are needed to be adopted to obtain a balance between strength and creep resistance at elevated temperature and ductility and fracture toughness at low temperature. The present investigation is aimed at developing the Nb-rich hypoeutectic quaternary (Nb-Si-Cr-Al) and quinary (Nb-Si-Cr-Al-Mo) alloys and study microstructural evolution and mechanical properties and thereby studying the effect of addition of Al and Mo. The composition of the quaternary (Nb-Si-Cr-Al) and quinary (Nb-Si-Cr-Al-Mo) alloys are chosen using intelligent guess and the binary and ternary phase diagrams among the elements. The mechanical properties of alloys were analyzed and correlated with the microstructure. Precipitates of hard and brittle silicide (βNb5Si3) as well as Laves (Cr2Nb) phase contribute the strength whereas (Nb)ss imparts ductility to the alloy.

Hydrothermal growth of TiO2-CaP nano-films on a Ti–Nb-based alloy in concentrated calcium phosphate solutions

The Ti–Nb-based TLM alloy (Ti–25Nb–3Zr–2Sn–3Mo) was subjected to hydrothermal treatment in the concentrated Ca3(PO4)2, CaHPO4, and Ca(H2PO4)2 solutions for the purpose of calcification. The treated samples are covered by films consisting of Ca-rich nano-crystallites (100–500 nm) and small nano-grains. XPS and XRD analyses reveal the formation of hydroxyapatite, TiO2, and Nb2O5 at the sample surface. The sample hydrothermally treated in CaHPO4 solution exhibits bioactivity by inducing the formation of apatite layer after soaking test in the simulated body fluid for 15 days. The work would provide a good bioactive surface modification method for Ti–Nb alloy implants with complex shapes and even pores.

Elastic-modulus enhancement during room-temperature aging and its suppression in metastable Ti–Nb-Based alloys with low body-centered cubic phase stability

Changes in the elastic properties during room-temperature aging (RT aging) of metastable Ti–Nb-based alloy single crystals with low body-centered cubic (bcc)-phase stability were investigated. The elastic stiffness components of Ti–Nb–Ta–Zr alloys with different Nb concentrations were measured by resonant ultrasound spectroscopy during RT aging; the results revealed that shear moduli c′ and c44 were increased by RT aging. In the alloy with the lowest Nb concentration, i.e., with the lowest bcc phase stability, shear moduli c′ and c44 were enhanced by the largest amount. The increase rates were ∼5% for 1.1 × 107 s (127 days), whereas the bulk modulus was hardly changed by aging. In Ti–Nb–Ta–Zr–O alloys with different oxygen concentrations, shear moduli c′ and c44 of the alloy with the lowest oxygen concentration increased most significantly. Moreover, the electrical resistivity of Ti–Nb–Ta–Zr and Ti–Nb–Ta–Zr–O alloys was increased by RT aging. Importantly, the enhancements of shear moduli and electrical resistivity were suppressed by increases in the bcc-phase stability (i.e., increase in the Nb concentration) and oxygen concentration; these factors are known to suppress ω (hexagonal) phase formation. However, transmission electron microscopy (TEM) observations revealed that only a diffuse ω structure—an ω-like lattice distortion—was formed after RT aging. On the basis of alloying element effects, TEM observations, and analysis of the changes in elastic properties by using a micromechanics model, it was deduced that the enhancements of shear moduli and electrical resistivity were possibly caused by the formation of a diffuse ω structure.

Application of Linear Discriminant Analysis to Ultrasound Signals for Automatic Microstructural Characterization and Classification

During the gas tungsten arc welding of nickel based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as ?^" and ? phases can precipitate in the microstructure, during aging at high temperatures. Nevertheless, choosing the appropriate conditions of welding can minimize the formation of the Nb-rich Laves phases and thus reduce the susceptibility to solidification cracking. This study proposed and evaluated the performance of an automated processing system to microstructurally characterizing the kinetics of phase transformations on a Nb-base alloy, thermally aged at 650 and 950 °C for 10, 100 and 200 h, using Linear Discriminant Analysis (LDA) on Background echo and Backscattered ultrasound signals at frequencies of 4 and 5 MHz. The main goal of this work is to design a more practical processing system in terms of the accuracy and the speed of processing. This system is composed of three methodologies: the first methodology uses LDA coefficients of normalized ultrasound signals, the second methodology uses LDA coefficients of error signals of the third-order linear prediction model of normalized ultrasound signals and the third methodology uses LDA coefficients of Discrete Cosine Transform. In all three methods, the Probabilistic Neural Network was used as a classifier. The highest accuracies were provided by the third methodology with average classification accuracies of 94.50 and 75.50 %, respectively for thermal aging at 650 and 950 °C. Indeed, LDA proved to be an efficient processing step for microstructural classification tasks.

Effect of microstructure and addition of alloying elements on hydriding kinetics of Zr–Nb-based alloys

The addition of Al, Cu and Mn in Zr–Nb-based alloys was evaluated through corrosion and hydrogen absorption tests in cold rolled and annealed conditions. For the studied alloys, the heat treatment resulted in an α-Zr matrix with β-Nb precipitates and X-ray diffraction (XRD) confirmed the formation of hydrides after hydrogenation in all conditions evaluated. It was observed that the alloy with Al addition absorbed the largest amount of hydrogen and the alloy containing Cu took more time to begin the absorption. The alloy with Mn absorbed little hydrogen but the onset of absorption occurred in a short time. The results indicate that the alloy with Cu addition is the most promising and the alloy with Al addition is inadequate for nuclear application.

Origin of shear induced β to ω transition in Ti–Nb-based alloys

Ti–Nb-based alloys are essential materials for biomedical implant and aerospace applications. They reveal complex phase transformation behavior. Here, a {211}β〈111〉β twinning induced β (body-centered cubic phase) to ω (hexagonal phase) transition in Ti–Nb-based alloys is demonstrated by transmission electron microscopy and analyzed employing ab initio calculations and the linear elastic inclusion theory. Our theoretical results reveal a distinct energy barrier for the β to ω transition, where the contribution from lattice rearrangement, rather than the elastic contribution associated with lattice parameter mismatch, plays the major role. It is shown that this energy barrier can be overcome by {211}β〈111〉β shear, explaining why {211}β〈111〉β twinning or, alternatively, the β to α″ (orthorhombic phase) transition promotes local formation of the ω phase.

Analysis of hydrogen mobility in Nb-based alloy membranes in view of new description of hydrogen permeability based on hydrogen chemical potential

Hydrogen permeability of Nb-based alloy membranes have been analyzed in view of the new description of hydrogen permeation based on hydrogen chemical potential in order to investigate the alloying effects on the mobility of hydrogen atom, B. There is a liner relationship between the normalized hydrogen flux, J⋅L, and the PCT factor, fPCT. The mobility of hydrogen atom is expressed in a format of the Arrhenius equation. Then, the alloying effects on the activation energy, E, and the pre-exponential factor, B0, have been investigated. It is found that logarithm of the pre-exponential factor, lnB0, is proportional to the activation energy, E. In other words, when one factor decreases by alloying, the other factor also decreases linearly. As a result, the addition of ruthenium, tungsten and molybdenum into niobium enhances the hydrogen diffusivity at low temperature below the intersection temperature, Tint.

Phase equilibrium between bcc solid solution matrix and B2 aluminides in Nb-based alloy

ABSTRACTB2 aluminides have a role of Al reservoir for Al2O3 surface and it is expected to increase oxidation resistance of (Nb,Mo)-bccss substrate. For the accumulation of the basic information to design the alloy composed of B2 coating on bccss matrix, bccss - B2 two-phase field was investigated in the Nb-Mo-Ni-Pd-Al system at 1273 K. It is found that Pd-rich B2-(Ni,Pd)Al phase is in equilibrium with Nb-rich bccss phase, while without Pd, the composition range of bccss coexisting with B2-NiAl phase is limited to be low Nb.

ОПРЕДЕЛЕНИЕ ПРИМЕСЕЙ В СПЛАВЕ НА ОСНОВЕ Nb МЕТОДОМ ИСП-МС. Часть I

ОПРЕДЕЛЕНИЕ ПРИМЕСЕЙ В СПЛАВЕ НА ОСНОВЕ Nb МЕТОДОМ ИСП-МС. Часть I

High temperature deformation of Ti–Al–Nb–Cr–Mo alloy with ultrafine microstructure

Ti–Al–Nb-based alloys have shown promise for high temperature applications, however limited research has been conducted in γ(TiAl) + σ(Nb2Al) alloys that are aimed towards increasing strength and operating temperatures through microstructural control. Alloys with less than 30% σ-phase have been investigated, exploring relationships between microstructure and deformation mechanisms. An alloy with composition Ti–45Al–14Nb–5Cr–1Mo has been produced, characterized, and tested at high temperature under compression at strain rates ranging from 10−3 to 10−5 s−1. The flow behavior of this alloy shows strain rate and temperature dependence with constant strain rate sensitivity at each temperature in the testing regime. Microstructural analysis reveals the γ-phase is primarily responsible for the alloy's ability to accommodate deformation to large strains under compression.

Design, strength prediction of Ti35NbxSnyZrzMo alloys with low elastic moduli and experimental verification on their mechanical properties

Based on the d-electron alloy design method and the average valence electron concentration theory, three kinds of metastable β-type Ti–35Nb-based alloys containing different quantities of Sn, Zr, and Mo alloying elements were designed, whose $$ \overline{Md} $$ = 2.45 eV and e/a = 4.24, and $$ \overline{Bo} $$ values = 2.869, 2.866, and 2.860, respectively. Meanwhile, a formula for predicting the solid solution strength of Ti–Nb-based alloys was developed using JMatPro software. The experiments show that the designed alloys are all composed of single β phase after solid solution treatment, the tensile strength of the alloys coincides well with that predicted by the formula (the relative error <4 %), and the elastic moduli decrease with $$ \overline{Bo} $$ values increasing. The Ti35Nb3.7Zr1.3Mo alloy has the biggest $$ \overline{Bo} $$ value (2.869) and the lowest elastic modulus (54 GPa) among the three designed alloys.

Rational design of Nb-based alloys for hydrogen separation: A first principles study

We have investigated the effect of alloying metal elements on hydrogen solubility and mechanical integrity of Nb-based alloys, Nb15M1 (where M = Ca–Zn, Ge), using first principles-based calculations. In general, the chemical interaction between the interstitial H and metal is weakened as the alloying element is changed from an early to a late transition metal, leading to lower H solubility and higher resistance to H embrittlement. This effect becomes more pronounced when a smaller alloying element is used due to stronger elastic interaction between interstitial H and metal atoms. These finding may provide scientific basis for rational design of Nb-based hydrogen separation membranes with tailored H solubility to effectively suppress H embrittlement while maintaining excellent hydrogen permeation rate.

Morphology of sintered silicide coatings remelted by high frequency electron beam

Silicide coatings were prepared on Nb-based alloy Nb-10W by a fused slurry method. Such sintered silicide coatings have defects such as pores, cracks, and uneven top surfaces, which have negative influence on the high temperature oxidation resistance of the coatings. Electron beam remelting is considered one of the most convenient processes to reduce these disadvantages and improve coating oxidation resistance. The effect of high frequency electron beam irradiation on surface remelting and microstructural modification of silicide coatings was investigated in this study. Scanning electron microscopy, X-ray diffraction, and atomic force microscopy were used to characterize phase and morphology before and after EB surface treatment. The surface roughness of the silicide coatings was reduced by two orders of magnitude and the average grain size was reduced by about one order of magnitude after treatment. This resulted in improvement of the high-temperature oxidation resistance of the EB treated silicide coatings, which offered about 50h protection under isothermal oxidation in air at 1700°C.

Controlling reversible martensitic transformation in titanium alloys with high strength and low elastic modulus

This paper reports on β-type titanium alloys with high β transus temperature and low martensite start temperature that display high strength and low elastic modulus. The strategy is based on Zr and Sn additions to Ti–Nb-based alloys. The α″ martensite in Ti–24Nb–4Zr–8Sn alloy is structurally much closer to the β phase than other alloys in terms of transformation strain and tetragonality, allowing homogeneously nucleated reversible mechanisms to be triggered before the formation of nanoscaled equiaxed α″ martensite by strain glass transition.

The mechanical properties and strengthening mechanism of Nb-based super-alloys

In the present paper, the mechanical properties and strengthening mechanism of Nb-based alloy were studied. The scanning electronic microscope, transmission electron microscope, Olympus optical microscope and vacuum high-temperature tensile engine were used to characterize the alloy. The Nb-based super alloy can be strengthening by two approaches. One is solid solution strengthening,i.e.W and Mo in the Nb based alloy act as alloy elements to form solid solution, improving high temperature creep properties.The other one was aging strengthening,i.e.C and Zr were added to form carbide particles during the aging. The effect of C and Zr content and heat treatment on aging strengthening was also investigated.

Elastic properties of Nb-based alloys by using the density functional theory

A first-principles density functional approach is used to study the electronic and the elastic properties of Nb15 X (X = Ti, Zr, Hf, V, Ta, Cr, Mo, and W) alloys. The elastic constants c11 and c12, the shear modulus C′, and the elastic modulus E〈100〉 are found to exhibit similar tendencies, each as a function of valence electron number per atom (EPA), while c44 seems unclear. Both c11 and c12 of Nb15X alloys increase monotonically with the increase of EPA. The C′ and E〈100〉 also show similar tendencies. The elastic constants (except c44) increase slightly when alloying with neighbours of a higher d-transition series. Our results are supported by the bonding density distribution. When solute atoms change from Ti(Zr, Hf) to V(Ta) then to Cr(Mo, W), the bonding electron density between the central solute atom and its first neighbouring Nb atoms is increased and becomes more anisotropic, which indicates the strong interaction and thus enhances the elastic properties of Nb—Cr(Mo, W) alloys. Under uniaxial 〈100〉 tensile loading, alloyed elements with less (more) valence electrons decrease (increase) the ideal tensile strength.

Low Young’s modulus in Ti–Nb–Ta–Zr–O alloys: Cold working and oxygen effects

The origin of the low Young’s modulus of cold worked Ti–36Nb–2Ta–3Zr– xO mass% polycrystals with a body-centered cubic (β-phase) structure, referred to as gum metal, was investigated with a focus on the roles of oxygen concentration, the electron–atom ( e/ a) ratio, and the cold working process. Analysis of the temperature dependence of the microstructures and elastic properties of single crystals at x = 0.09, 0.36, 0.51% O using transmission electron microscopy and an electromagnetic acoustic resonance method, respectively, revealed that the shear moduli c′ and c 44 of the 0.36 and 0.51% O alloys softened upon cooling near room temperature (RT) and exhibited low values at RT. This was because suppression of the α″ martensitic transformation by oxygen addition led to retention of the low stability single β-phase state at RT. The Hill approximation indicated that the low c′ and c 44 values caused by softening gave rise to the low Young’s modulus, which is common to some Ti–Nb-based alloys with an e/ a ratio of ∼4.24. Analysis of the microstructures and elastic properties of solution-treated and cold worked x = 0.06, 0.30, 0.47% O alloy polycrystals at RT revealed that the Young’s modulus increased upon 90% cold working due to formation of the α″ martensite phase (0.09% O) and ω phase (0.09, 0.30, and 0.47% O) with a high elastic modulus in the β-phase matrix. However, increasing the oxygen concentration suppresses the increase in Young’s modulus because oxygen addition decreases the amount of α″ and ω phases formed while retaining the low stability β phase. Therefore, cold working combined with oxygen addition produces a low Young’s modulus compatible with high strength.