Increasing Nb Content Research Articles

Microstructure evolution and properties of laser cladding AlFeCrMoNbx refractory high entropy alloy coatings

Refractory high-entropy alloy (RHEA) has superior mechanical properties but low room-temperature ductility. However, eutectic heterostructure alloy has strength-ductility trade-off. In this study, the AlFeCrMoNbx (x = 1, 1.2, 1.4, 1.6, 1.8, and 2.0) RHEA coatings were successfully prepared on the surface of an M2 high-speed steel (HSS) by laser cladding. The increase of Nb content promoted the formation of eutectic microstructure, the coating changed from hypoeutectic structure to hypereutectic structure. When x = 1.8, the coating exhibited a near-eutectic structure consisting of BCC and Laves phases with high hardness and wear resistance. After annealed at 750 °C for 8 h, the phases and microstructure of Nb1.8 REHEA coating did not change, contributing to a high microhardness and wear resistance. With annealing temperature rising, the microhardness and wear resistance decreased. After annealing at 1050 °C, the hardness of the coating still remained 549 HV0.2, indicating a good high-temperature softening resistance of REHEA coating.

Effects of Mo and Nb on the microstructure and high temperature oxidation behaviors of CoCrFeNi-based high entropy alloys

A series of CoCrFeNi(Mo, Nb) high entropy alloys (HEAs) were prepared using vacuum arc-melting. The effect of Nb and Mo on the microstructure and high temperature anti-oxidation properties of HEAs was investigated. The addition of only Mo was found to not cause formation of new phase, but did increase the lattice distortion. After the addition of Nb, as-prepared CoCrFeNiMo0.2Nb0.1 and CoCrFeNiMo0.2Nb0.2 alloys are characteristic of duplex-phase structures of FCC and Laves, in which the Laves phase is semi-coherent with FCC matrix. Our studies also revealed that the oxidation kinetics of the alloys followed parabolic rate law and the corresponding oxidation rate constants were calculated. Due to the synergistic effects of Mo and Nb, the anti-oxidation properties of HEAs improved significantly. It was also discovered that the new Laves phase containing Mo and Nb at the inner oxidation layer hindered the outward diffusion of active metal cations, ergo improving the oxidation resistance of HEAs. With increase of Nb content, it was further recognized that the layered structure of Laves phase was thinner and denser. Therefore, the oxygen ion diffusion process in the CoCrFeNiMo0.2Nb0.2 alloy can be inhibited effectively by the new formation of duplex-phase structures with undulated Laves phase and FCC matrix.

Oxidation behavior of CoCr2−xFeNi2.1Nbx high entropy alloys

High entropy alloys (HEAs) are one of the prominent alloy systems being studied for high-temperature applications. One of the key aspects to consider for high-temperature applications is to understand their oxidation behavior. When Nb is added as the alloying element to base FCC alloy like CoCrFeNi it promotes the formation of dual phase HEA. In the present study, the high-temperature oxidation behavior of annealed CoCr2−xFeNi2.1Nbx (x = 0.25, 0.5 and 1) isothermally at 800 °C in the air is studied. As-homogenized microstructure of these alloys shows a dual phase containing FCC and Laves phase. The amount of Laves phase increases with increasing Nb content. The oxidation kinetics of the three compositions follows parabolic kinetics at 800 °C. The characterization (XRD, Raman, SEM) of the oxide layer reveals the formation of Cr2O3 as the main oxide layer. The cross-sectional SEM images of the oxide samples after 96 h of oxidation confirms absence of internal oxidation and a thin Laves-rich region below the Cr2O3 layer. The preferential formation of Cr2O3 over Nb oxides is explained using the assessment of their thermodynamic activity in the FCC phase.

Electrochemical corrosion behavior and in vitro biocompatibility of Ti-Nb-Sn alloy for dental applications

Ti-Nb-Sn alloy was prepared by mechanical alloying and mold sintering process, and the effects of Nb on the microstructure, corrosion behavior and in vitro biocompatibility were investigated. With the increased Nb content, the microstructure was transformed from Widmannstatten to needle-like structure. Moreover, the stable passivation films of TiO2, Nb2O5 and SnO2 were formed on the surface of the alloy in artificial saliva, and the surface wettability was changed from hydrophilic to hydrophobic, which made the corrosion resistance of the alloy was intensified. The increase of Nb content can promote adhesion and proliferation of mouse osteoblasts, and accordingly improve cell vitality.

Effect of surface characteristics of sputtered titanium and niobium thin alloy films on cell growth and adhesion

Titanium (Ti)–niobium (Nb) thin alloy films were prepared by on-chip sputtering. Their surface properties were characterized, and their cell growth and adhesion properties were investigated. Protrusions were observed on the surface of a thin film made of Ti mixed with a small amount of Nb; these protrusions were smaller and more numerous than those on Ti and Nb thin films. Addition of a small amount of Nb enhanced cell adhesion and cell growth compared to those of Ti or Nb thin films. In addition, cell growth and adhesion decreased with an increase in Nb content. Therefore, the stable morphology, negative zeta potential and suitable protrusion size of these Ti–Nb thin alloy films enhance cell adhesion and cell growth on their surfaces.

High-temperature tribological properties of the novel NiCr(MoNb)x-Ag self-lubricating composite films

The representative FCC structured NiCr(MoNb)x-Ag (x = 0, 0.65 and 1.05) composite films were developed in this work, and the microstructure, mechanical, and tribological properties were studied. It is shown that small amounts of nm-sized Ag particles are dispersed in the thin film matrix. With the increase of Mo and Nb content, the hardness of the films is enhanced while the elastic modulus increases first and then decreases. The NiCr(MoNb)1.05-Ag composite film shows the best tribological performance. The average friction coefficients of the NiCr-Ag, NiCr(MoNb)0.65-Ag and NiCr(MoNb)1.05-Ag films from room temperature to 600 °C are 0.35–0.46, 0.27–0.4 and 0.26–0.32, respectively. NiCr(MoNb)0.65-Ag and NiCr(MoNb)1.05-Ag films show good lubricity at room temperature to 500 °C, and NiCr(MoNb)1.05-Ag film still maintains a low friction coefficient at 600 °C. The wear rate increases with temperature and decreases successively with increasing Mo and Nb content. Moreover, the distribution of elements in the films with higher Mo and Nb content is more stable at high temperatures, which contributes to the improvement of wear resistance.

The microstructure and mechanical performance optimization of a new Fe-Ni-based superalloy for Gen IV nuclear reactor: The critical role of Nb alloying strategy

The effects of Nb alloying on microstructural evolution and mechanical performance of a newly developed solid-solution strengthened Fe-Ni-based superalloy, considered as the candidate material for high-temperature components manufacturing in fourth Generation (Gen IV) advanced nuclear reactor, were systematically investigated. The results show that the increase in the Nb content obviously increased the volume fraction and average size of MC carbides, and the MC carbides more preferentially distributed at grain boundaries. The average grain size and low Σ coincident site lattice (CSL) boundary exhibited a gradually decreased trend with increasing Nb content. Meanwhile, the thermal stability (grain size and grain boundary precipitation) of the alloy can be improved notably via Nb alloying. The tensile strength of Nb-alloyed alloys at 750 °C was enhanced significantly compared to that of Nb-free alloy, mainly due to the synergistic effect of solid-solution strengthening and fine grain strengthening. It was impressive that the variation of Nb content had no obvious influence on the yield strength, but gradually decreased the ultimate tensile strength. The detrimental effects of the decrease in Σ3 boundaries and intergranular precipitates were assumed to restrict the strengthening factors. The ductility of Nb-alloyed alloys decreased remarkably due to the changed dominant deformation mechanism from planar slip to wavy slip; however, it showed an increasing trend in high Nb-content alloys due to the occurrence of dynamic recrystallization and grain rotation. The present work establishes Nb alloying as an effective method to optimize the microstructure and mechanical properties of solid-solution strengthened Fe-Ni-based superalloys, and the appropriate Nb content is recommended to be controlled below 0.5 wt% for this alloy.

Mechanical and corrosion properties of biomedical (TiZr)90-xNbxTa5Mo5 medium entropy alloys

High- or medium-entropy alloys with single phase body centered cubic (BCC) structure have attracted significant scientific interests recently, due to their great potentials for biomedical applications. However, BCC high entropy alloys generally do not have good tensile ductility at room temperature. Herein, we utilize the valence electron concentration (VEC) theory to design three (TiZr)90-xNbxTa5Mo5 (x = 5,10,15) biomedical medium entropy alloys (BioMEAs). The BioMEAs showed a single phase BCC structure, a low elastic modulus of 79.2 ± 1.6–82.9 ± 1.2 GPa, a good tensile ductility of 24.4%, and a high specific yield strength of 141.2–137.4 10−3 MPa∙m3/kg. The good tensile ductility of the alloys was found attributed to the cross-slip of mixed dislocations and the formation of kink bands, avoiding the excessive entanglement of dislocations and relieving stress concentration. The increase of Nb content (5–15 at.% Nb) contributed little to the yield strength of the BioMEAs. In addition, the (TiZr)90-xNbxTa5Mo5 alloys showed excellent corrosive resistance in modified Fusayama saliva (MFS) solution, in which passive films composed of a variety of oxides spontaneously developed on the exposed surfaces. Our work has cast new light to develop lightweight and economical BioMEAs with excellent comprehensive properties.

Effect of Nb addition on the microstructure and martensitic transformation of NiTiHfScNb high temperature shape memory alloys

In the present work, the effects of Nb addition and aging on the microstructure evolution and martensitic transformation of (Ni50.3Ti28.7Hf20.0Sc1.0)100-xNbx (x = 0, 5, 10, 15 at.%) high temperature shape memory alloys were investigated. The addition of Nb results in the formation of the eutectic β-Nb phase along with martensite and Sc2O3 at room temperature. Aging at 550 °C for 3 h leads to the formation of nanoscale H-phase. The surface area fraction of the H-phase decreases with increasing Nb content to 10 at.%. As the Nb content increases, the transformation temperatures of the solution-treated NiTiHfScNb alloy increase and then decrease due to the variation of Ni content in the matrix. The aged samples show a smaller transformation hysteresis in comparison with the solution-treated samples. The difference of thermal hysteresis between the solution-treated and the aged samples increases with increasing Nb content because of the H-phase precipitation and Nb solution in matrix.

Effect of Nb on the Hydrogen-induced Cracking of X80 Pipeline Steel

Nb is a key alloying element added into X80 pipeline steel, and it can enhance mechanical properties by refining grains and introducing precipitates. However, HIC resistance is also one of the most significant performances for pipeline steel served in H2S environment. An all-inclusive recognition of the response of niobium to HIC refusal is important for the appropriate design of the Nb addition amount, the rolling and cooling parameters, and protective techniques to obtain excellent mechanical properties, including strength and ductility and HIC refusal. So in the paper, many test methods, such as EBSD, SEM, and HIC, were adopted to research the response of niobium addition on resistance of hydrogen-induced cracking performance of X80. As the increase in Nb content, grain size decreased and had the strongest trapping hydrogen atom ability. Besides, the percentage of low-angle grain boundaries went up, and more precipitates greatly raised the amount of irreversible hydrogen pitfalls. Nb could greatly promote the HIC resistance, inhibiting crack initiation, and propagation.

Multi-wire arc additive manufacturing of TC4-Nb-NiTi bionic layered heterogeneous alloy: Microstructure evolution and mechanical properties

Additive manufacturing of TC4-NiTi alloy components posed many challenges due to the significant differences in their material properties. No previous literature has been found to study bonding characteristics of TC4-NiTi interface fabricated via multi-wire arc additive manufacturing (MWAAM). This paper introduces the preparation of TC4-Nb-NiTi bionic layered heterogeneous structure alloy components using MWAAM with Nb as the interlayer. The results indicated that phase composition from TC4 region to NiTi region as follows: α-Ti + β-Ti → α-Ti + β-Ti + (Nb, Ti) → NiTi + NiTi2 + Ni3Ti + α-Ti + (Nb, Ti) → NiTi + Ni3Ti. The microstructure of TC4 alloy side gradually refined with the increase of Nb content, while Nb and NiTi formed a eutectic structure at the interface. The maximum microhardness of MWAAM TC4-Nb-NiTi alloy component was 393.3 HV. The ultimate compressive strength (1419.87 MPa) and fracture strain (22.12%) for MWAAM TC4-Nb-NiTi sample was obtained. The ultimate tensile stress and the elongation of MWAAM TC4-Nb-NiTi sample were 378.8 MPa and 3.09%, respectively. The fracture mode of tensile samples was a mixed fracture dominated by brittle fracture.

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

Corrosion behavior of NbxCu0.3Cr2Fe2Ni3Mn2 high-entropy alloys in HNO3 solution

NbxCu0.3Cr2Fe2Ni3Mn2 (x = 0, 0.2, and 0.4; named Nb0, Nb0.2, and Nb0.4, respectively) high-entropy alloys (HEAs) were prepared using arc melting process. The effect of Nb content on the microstructure and corrosion behavior of HEAs in HNO3 solution was studied. Nb0 and Nb0.2 alloys are composed of a single FCC phase, whereas Nb0.4 alloy is composed of FCC + Laves phase. In 10% and 30% HNO3 solutions, the immersion corrosion rate and potentiodynamic polarization corrosion rate of the HEAs showed a trend of first increasing and then decreasing with the increase in Nb content. After 30 days of immersion corrosion, Nb0.4 alloy exhibited the lowest average corrosion rate in 10% and 30% HNO3 solutions, with values of 4.6 × 10−3 and 2.8 × 10−3 mm/yr, respectively. Potentiodynamic polarization corrosion showed that Nb0.4 alloy also had the lowest corrosion current density and average corrosion rate in 10% and 30% HNO3 solutions, with values of 1.74 × 10−5, 5.56 × 10−6 A/cm2 and 0.18, 0.06 mm/yr, respectively. In addition, The Rpo of Nb0.4 alloy are 91 times and 8737 times higher than those of Nb0 alloy, respectively, and has the largest relative thickness of the passivation film. Therefore, Nb0.4 alloy exhibited the best resistance to HNO3 corrosion, mainly because adding Nb reduced the segregation of Cu, thereby weakening the corrosion effect of the primary battery. Meanwhile, Nb could play a stabilizing role in the passivation film of the alloy. The formation of corrosion-resistant Laves phase also correspondingly improved the HNO3 corrosion resistance of the alloy.

Effect of Niobium on Sintering Process of Gamma Phase Uranium Alloys Fuel Pellets

Abstract As a candidate material for metallic fuel, U-Mo metal fuel pellets are the most promising. U-Mo and U-Mo-Nb alloy pellets with a certain porosity were successfully prepared by the process of hydrogenation/dehydrogenation—compression molding—argon liquid-phase sintering. In order to study the effect of Nb addition on γ phase uranium alloy fuel pellets, microstructure and thermophysical properties of the samples were investigated using scanning electron microscope (SEM), in situ synchrotron X-ray diffraction (XRD), and differential scanning calorimeter (DSC) measurements. Results showed that with the increase of Nb content in the pellets from the nonadd to micro-adding, Nb can facilitate the diffusion of Mo into the U matrix, resulting in the formation of a metastable γ-U phase. Meanwhile, during the same liquid phase sintering process of U-Mo fuel pellets, with the increase of Nb content, the number of secondary phases in U-Mo fuel pellets gradually decreased, while the size and number of voids of the secondary phases decreased. The specific heat capacity and thermal diffusivity of porous γ phase uranium alloys fuel pellets with different density were measured and thermal conductivity from 100 °C to 600 °C were calculated according to the experiment results. It is suggested that the thermal conductivity will increase with the density of pellets increasing.

Effect of Nb on the Damping Property and Pseudoelasticity of a Porous Ni-Ti Shape Memory Alloy.

In order to develop novel high damping materials with excellent pseudoelasticity (PE) properties to meet the application requirements in aerospace, medical, military and other fields, porous Ni50.8Ti49.2 shape memory alloy (SMA) was prepared by the powder metallurgy method. Different contents of Nb element were added to regulate the microstructures. It was found that after adding the Nb element, the number of precipitates significantly decreased, and the Nb element was mainly distributed in the Ni-Ti matrix in the form of β-Nb blocks surrounded by Nb-rich layers. Property tests showed that with the increase in Nb content, the damping and PE increased first and then decreased. When the Nb content reached 9.0 at.%, the highest damping and the best PE could be achieved. Compared with the porous Ni-Ti SMA without Nb addition, the damping and PE increased by 60% and 35%, respectively. Correlated mechanisms were discussed.

Effect of Nb Content on Phase Transformation and Comprehensive Properties of TiNb Alloy Coating

As a β phase transformation promoting element of titanium alloy, the Nb element can induce different phase transformations of the alloy, improving the comprehensive properties of the alloy. However, the research on the effect of Nb content on the properties of TiNb alloy coating during laser cladding is not comprehensive. Herein, TixNb (x = 32.5~62.5 wt.%) alloy coatings were prepared by laser cladding technology, and their mechanical properties, corrosion resistance, and biocompatibility were analyzed. The results show that the Nb element promotes the precipitation of different phase components in the phase transformation-induced coating of titanium alloy. The grain refinement during the laser cladding process effectively improves the microhardness of the TiNb alloy coating. At the same time, the wear resistance of the α″ + β dual-phase Ti32.5Nb alloy is enhanced. In addition, with the increase in Nb content, the coating exhibits better corrosion resistance. In vitro cell experiments showed that the TiNb alloy coating had excellent biocompatibility compared to the TC4 substrate. Therefore, the laser-clad TiNb alloy coating has high comprehensive performance and has reference value in the field of biological implantation.

Mechanical, wear and corrosion behavior of Ti–6Al–xNb (x = 3.5–21 wt%) alloys manufactured by powder metallurgy

In this study, Ti–6Al–xNb alloys with different Nb percentages (x = 3.5–21 wt%) were produced by the conventional powder metallurgy technique. After mixing and pressing the alloy powders in appropriate proportions, the green samples were sintered in a tube furnace under a vacuum atmosphere at 1200 °C for 2 h. The X-ray diffraction analysis (XRD) showed that all the alloys consist of α–Ti, TiNb (β–Ti) and AlNb2 phases. The results showed that the Ti–6Al–14Nb alloy among all alloys has the best mechanical properties, and the microhardness, tensile and flexural strength are 276.4 Vickers hardness (Hv0.2), 453 MPa and 1682 MPa, respectively. Tribological tests were carried out in both dry and wet conditions with Hanks' Balanced Salt Solution. While generally, the specific wear rates of the samples increased with Nb content up to 17.5 wt% under dry conditions, on the other hand, they increased with increasing Nb content under wet conditions. On the other hand, the specific wear rate increased with increasing the sliding distance. The best corrosion resistance alloy among all the samples was Ti–6Al–7Nb with −0.0968 V corrosion potential (Ecorr), 0.015 μA/cm2 corrosion current density (Icorr) values.

Alloying Effect on Elastic and Mechanical Properties of Refractory Medium-Entropy Alloys from First-Principles Calculations

There is still a challenge to rapidly and accurately acquire the composition dependence of mechanical properties of multi-principal element alloys due to their huge composition space and complex electronic structures. Aiming to straightforwardly tune the mechanical properties through alloying, we systematically calculate the elastic and mechanical properties for four selected body-centered-cubic (bcc) Ti–Nb–M (M = Zr, Mo, Sn, Ta) refractory medium-entropy alloys (RMEAs) using a first-principles method. It is shown that the elastic stability of the bcc RMEAs can be significantly (weakly) improved with the increase in Nb, Mo, and Ta (Sn) content, but it is insensitive to Zr. The elastic stability, elastic modulus, hardness, and strength generally enhance, whereas Pugh’s ratio and Zener anisotropy (AZ) decrease with the increase in valence electron concentration. Additionally, there is a correlation between the magnitude of the AZ and the shape of the single-crystal Young’s modulus (E[hkl]). The largest (smallest) E[hkl] appears in the <111> (<100>) direction. Particularly, the most isotropic Ti60Nb20Mo20 has the highest hardness and strength among all the Ti–Nb–M alloys. Moreover, the change of elastic properties induced by alloying is related to the change of density of states. This work sheds deep light on the design of high-performance Ti-based multi-principal element alloys.

Determination of Corrosion Resistance of High-Silicon Ductile Iron Alloyed with Nb

In this study, the effects of Nb on the microstructural characteristics, hardness, and corrosion resistance of high-silicon ductile cast iron (HSDI)-3.6 wt.% Si were investigated. Samples from different castings with 0–0.9 wt.% Nb were obtained and compared to a commercial ductile iron. Microstructures showed that the amount of ferrite in the matrix increased with increasing Nb content, from 34% for unalloyed HSDI to 88% for HSDI-0.9 wt.% Nb. The presence of randomly distributed NbC carbides was identified by EDX for all the samples alloyed with Nb, and the hardness of the HSDI increased with the Nb content. To evaluate the influence of the Nb content on the corrosion resistance of HSDI, potentiodynamic tests were carried out in a solution of H2SO4. The highest corrosion rate on HSDI was obtained for the HSDI-0.3 wt.% Nb sample, with 2802 mills per year, due to the amount of pearlite present and the lowest presence of NbC carbides, compared to the HSDI-0.9 wt.% Nb, with 986 mills per year. This behavior was attributed to the ferrite matrix obtained because of a high Si content in the DI, which delayed the anodic dissolution of the alloy and suppressed the pearlitizing effect of Nb for contents greater than 0.3 wt.%, as well as to the effect of NbC carbides, which acted as inhibitors.

Particles, Microstructures, and Impact Toughness of CGHAZ of Ca Deoxidation Shipbuilding Steel Plates with Different Nb Contents

Herein, the effects of the Nb content on the particles, microstructure evolution, and mechanical properties of base metal (BM) and high‐heat input coarse‐grained heat‐affected zone (CGHAZ) of shipbuilding steel plates with Ca deoxidation are studied. With increasing Nb content, the strength and microhardness of the BM increase without significant loss of impact toughness. However, in CGHAZ, the average grain size increases from 168 to 244 μm, and the brittle phase fraction increases from 0.33% to 11.76%. The high‐angled grain boundary (HAGB) density in microstructures reduces from 0.35 to 0.19 μm−1, and the average effective grain sizes increase from 10.97 to 12.48 μm. The particles in 40 Nb‐BM and 40 Nb‐CGHAZ are mainly cubic TiN particles. In 170 Nb, except for cubic particles, there are approximately ellipsoid particles with high value of Nb content, which tend to dissolve into matrix at the elevated temperature. With increasing Nb content, the average size of particles in CGHAZ increases from 18.33 to 32.29 nm, and the number density decreases from 13.44 to 10.35 μm−2. Therefore, the impact toughness of CGHAZ decreases from 197 to 96 J at −20 °C under the high‐heat input welding of 400 kJ cm−1.