Nb-containing Alloys Research Articles

Effect of Nb, Zr and Zr + Hf on the microstructure and mechanical properties of β-solidifying γ-TiAl alloys

The present work has been devoted to study of β-solidifying γ-TiAl alloys based on Ti–44Al-0.2B and doped with Nb, Zr and Zr + Hf. The phase transformation sequences were established for the alloys. On this basis, upset forging followed by heat treatment was performed that resulted in formation of near the same duplex type microstructures in the alloys. Tensile tests in the temperature range of 20–900 °C and creep tests at 700 °C were carried out for the alloys in the duplex conditions. The alloys doped with Zr and Zr + Hf showed appreciably higher strength, higher temperatures of the brittle-ductile transition (BDT) and enhanced creep resistance while retaining near the same ductility below the BDT temperatures as compared to the Nb-containing alloy. Enhanced creep resistance and higher temperatures of the BDT in the alloys doped with Zr and Zr + Hf in contrast to the Nb-containing alloy were attributed to higher solid solution hardening due to larger atomic radii of Zr and Hf versus Nb and to different partitioning behaviors of Zr and Zr + Hf versus Nb leading to higher concentrations in the γ phase of Zr and Zr + Hf in the Zr- and (Zr + Hf)-containing alloys in contrast to that of Nb in the Nb-containing alloy. Both of these factors contributed to lower diffusivity (especially in γ grains) in the alloys doped with Zr and Zr + Hf. In particular, alloying with Zr and Zr + Hf shifted the development of dynamic recrystallization processes towards high temperatures that slowed down the increase of ductility with increasing the test temperature in the BDT range and led to higher BDT temperatures.

Oxidation thermodynamics of Nb-Ti alloys studied via first-principles calculations

The oxidation thermodynamics of Ti-Nb alloys were studied as a step towards improving our general understanding of the oxidation behavior in complex refractory alloys for high temperature applications. Density Functional Theory (DFT) was utilized and the phonon normal mode frequencies were calculated via the harmonic approximation to analyze the stability of several phases including mixed oxides as a function of both temperature and the oxygen chemical potential. Increasing the chemical potential did not favor the formation of mixed oxides but led to Ti2O3 followed by TiO2, and then Nb2O5. The presence of Ti was shown to favor the formation of TiO2 over Nb2O5 in large regions of the stability phase diagram thereby contributing to enhanced oxidation resistance in Nb-containing alloys. Rutile oxides of the form NbxTi1−xO2 had a larger region of stability than NbxTi1−xO. Finally, the oxygen chemical potential was related to temperature and pressure to facilitate direct application of the results to experiment.

Effects of Nb and Mo additions on thermal behavior, microstructure and magnetic property of FeCoZrBGe alloy* *Project supported by the National Natural Science Foundation of China (Grant No. 51301075) and supported by Sinoma Institute of Materials Research (Guang Zhou) Co., Ltd.

Both Nb and Mo additions play a vital role in FeCo-based alloys and it is crucial to understand their roles and contents on thermal behavior, microstructural feature and magnetic property of alloys. Nanocrystalline alloy ribbons Fe40Co40Zr9 – y M y B10Ge1 (y = 0–4; M = Nb, Mo) were prepared by crystallizing the as-quenched amorphous alloys. The effects of Nb and Mo additions on structures and properties of the Fe40Co40Zr9B10Ge1 alloy are investigated systemically and compared. With increasing Nb or Mo content, the primary crystallization temperature, grain size of α-Fe(Co) phase and coercivity H c all decrease. Moreover, the effect of Mo addition on thermal behavior, microstructure and magnetic properties of the FeCoZrBGe alloy is greater compared to Nb addition. The gap between primary and secondary crystallization peaks of Mo-containing alloys is wider than that of Nb-containing alloys. Both grain size and H c of Mo-containing alloys are smaller than those of Nb-containing alloys. For Fe40Co40Zr9B10Ge1 alloy, high Mo addition proportion is better compared to high Nb addition proportion.

Nb-Ti-Zr alloys for orthopedic implants.

Niobium (Nb), Titanium (Ti), and Zirconium (Zr) have attracted much attention as implant materials due to it's excellent mechanical properties and biocompatibility. However, little attention has been paid to high Nb-containing biomedical alloys. Here, the 50 wt.%Nb-XTi-Zr ternary alloy(x = 20wt.%, 30 wt.%, 40 wt.%) with relative density over 90% was prepared by powder metallurgy method. The massive α(Zr) distributed along the grain boundaries and lamellar β(Zr) appeared in the grains of β(Nb) in the 50 wt.%Nb-20wt.%Ti-Zr alloy. The acicular α phase is mainly distributed in the β-grain of 50 wt.%Nb-30wt.%Ti-Zr alloy. And α(Ti)-colonies in the β-grains and continuous α(Ti)GB at β-grain boundary can be observed in the 50 wt.%Nb-40wt.%Ti-Zr alloy. Comparing with Nb-20wt.%Ti-Zr alloy and 50 wt.%Nb-40wt.%Ti-Zr alloy, the 50 wt.%Nb-30wt.%Ti-Zr alloy showed lower Vickers hardness and elastic modulus. Furthermore, the as-sintered 50 wt.%Nb-XTi-Zr alloy promoted the cell proliferation and cell adhesion of MG-63 cells on the surface of alloys. In conclusion, the 50 wt.%Nb-XTi-Zr alloy combines excellent mechanical and biological properties, and the 50 wt.%Nb-30wt.%Ti-Zr alloy with lower elastic modulus (close to the bone) is a more promising candidate for bone implant material.

Effects of annealing temperature on the microstructure, textures and tensile properties of cold-rolled Fe–13Cr–4Al alloys with different Nb contents

In this study, four Fe–13Cr–4Al alloys with different Nb contents (0–1.5 wt%) were prepared by using vacuum induction melting, forging, hot rolling, and cold rolling. The effects of annealing temperature (600 °C–1100 °C) and the Nb contents on the microstructure, textures as well as tensile properties were investigated. The dispersion of fine Fe2Nb-type Laves phase particles was observed in the BCC matrix and along grain boundaries. These particles effectively pinned dislocations and boundaries, resulting in stabilizing the microstructure, delaying the recrystallization temperatures and refining the recrystallized grains. The Laves phase particles were also able to retard the nucleation and growth of the γ fiber grains, therefore Nb-containing alloys had weak recrystallized γ fibers, which was beneficial to improve the processing properties of Fe–13Cr–4Al alloys. As the annealing temperature increased, the strengths decreased while the elongation increased. Because of the effects of solid solution strengthening, precipitation strengthening and fine grain strengthening produced by the addition of Nb element, the strength and hardness of the alloys increased with the increase of Nb content. Comprehensively considering the microstructure and mechanical properties, the suitable addition of Nb content was 1 wt%, and the optimum annealing treatment after cold-rolling with 50% thickness reduction was at 700 °C for 30 min.

Influence of Nb addition on microstructural evolution and compression mechanical properties of Ti-Zr alloys

In order to expand the application horizon of Ti-Zr based alloys, the influence of Nb content on phase composition, microstructural evolution and biomechanical properties is systematically studied. The phase and microstructural characterization of the as-cast alloys is carried out by X-ray diffraction, optical microscopy and transmission electron microscopy. The results reveal that Nb-containing Ti-Zr alloys transformed from single α phase → α + α″ + β phase → single β phase → double β phases with increasing Nb content. In the case of β-type alloys, the addition of Nb improves the bonding energy between atoms, reduces the grain size, increases the elastic modulus, improves the yield strength and renders superior work-hardening behavior. Moreover, the current study provides mechanistic insights into microstructural evolution and strengthening of Nb-containing Ti-Zr alloys with increasing Nb content. Herein, the addition of 5 at.% Nb resulted in an abnormal work hardening during compression deformation under the synergistic influence of stress-induced martensite transformation of β phase and stress-induced twinning of α phase. Moreover, the biomechanical properties are evaluated to demonstrate the potential of Nb-containing Ti-Zr alloys in biological applications.

Short communication: “Effect of Nb on the electrical resistivity of ZrO2 layer formed on Zr alloys”

Although Nb-containing Zr alloys are widely used as the fuel cladding materials in nuclear power plants, the effect of Nb on charge transport in oxide layer is not clear. In this study, it was found that the electrical resistivity of ZrO2 layer formed on Nb-containing Zr alloy (Nb–ZrO2 layer) was approximately 2–3 times higher than that of ZrO2 layer formed on Sn-containing Zr alloy (Sn–ZrO2 layer) between 400 and 600 °C. Mott-Schottky analysis showed lower oxygen vacancy concentration in the Nb–ZrO2 layer compared to Sn–ZrO2 layer.

The corrosion behavior of M5 (Zr–1Nb-0.12O) alloy in 360 °C water with dissolved oxygen

The corrosion behavior of M5 alloy in 360 °C/20 MPa water with 1.5 ppm dissolved oxygen (DO) was studied. The test was conducted by using a circulating autoclave loop system. The results indicated that DO accelerated the corrosion of M5 alloy in high temperature water. The corrosion behavior of β-Nb phase in the barrier layer and the post-transition layer in oxygenated water is completely different from that in deoxygenated water. For materials exposed to oxygenated water, β-Nb phases are sequentially oxidized to crystalline NbO (Nb2+) and crystalline NbO2 (Nb4+), then part of it is completely oxidized to Nb2O5 (Nb5+) in the barrier layer. In the post-transition layer, these crystalline oxidation products of β-Nb phase change to amorphous state, wherein Nb is totally oxidized to Nb5+ state. In the outer layer, the dissolution of β-Nb oxide is not completed when exposed to oxygenated water for even 100 days. Oxygenated water can accelerate corrosion of β-Nb SPPs, which could be responsible for high corrosion rate of Nb-containing Zr alloy in DO water.

On the oxidation mechanism of refractory high entropy alloys

The high-temperature oxidation mechanism of a series of refractory high entropy alloys: TaMoCrTiAl, NbMoCrTiAl, NbMoCrAl and TaMoCrAl at 1000 °C in air was studied. A complex protective oxide layer consisting of Al2O3, Cr2O3 and CrTaO4 oxides was observed for the quinary Ta-containing alloy. The formation of CrTaO4 in this alloy after a short incubation period decreased the oxidation kinetics from a parabolic to a quartic rate law. Ti was found to support the formation of CrTaO4. In the Nb-containing alloys, the formation of different Nb2O5 polytypes near the metal/oxide interface caused a highly porous oxide scale and severe oxide spallation.

Effect of Solute Nb on Grain Growth in Fe-30 Pct Mn Steel

Niobium is known to segregate strongly to grain boundaries in steel. The strong interaction of Nb with grain boundaries results in an important reduction of the grain boundary mobility and strong retardation of recrystallization and grain growth. In this study, the effect of Nb on the mobility of grain boundaries in a second-generation Fe-30 pct Mn TWIP steel is investigated. Grain growth kinetics was measured in a series of Nb-containing Fe-30 pct Mn model alloys. An estimate of the grain boundary mobility was obtained for various temperatures and niobium contents. It was found that Nb slows down the mobility of random high-angle grain boundary segments that are not twin related. The effect of solute Nb on grain growth, in the presence of annealing twins, was modeled using Cahn’s theory of solute drag coupled with a recently formulated twin-inhibited grain growth model. The results are shown to be in very good agreement with the experimentally determined growth kinetics.

FeSiBPNbCu Bulk Nanocrystalline Alloys with High GFA and Excellent Soft-Magnetic Properties

(Fe0.76Si0.09B0.1P0.05)99.3−xNbxCu0.7 (x = 0–1.5 at. %) bulk nanocrystalline alloys were prepared to investigate the alloying effects of Nb on glass forming ability, thermal stability, soft magnetic properties, and crystallization behavior. It was found that the amorphous forming ability was greatly improved with the addition of minor Nb. The thermal stability of Nb-containing alloy was significantly improved because the initial crystallization temperature and crystallization activation of the primary phase were obviously better than that of the Nb-free alloy. Further, the larger intervals of two-phase crystallization temperature and the significantly higher activation energy of crystallization of the second phase in the Nb-containing alloys favor the formation of a single α-Fe(Si) nanocrystalline structure. Moreover, Nb-containing alloys exhibit excellent soft magnetic properties, including high saturation magnetization of 1.42–1.49 T, low coercivity of around 1.0 A/m, and high permeability of about 18,000 at 1 kHz, which makes the alloys promising soft magnetic materials for industrial applications.

The transient liquid phase bonding process of a γ-TiAl alloy with brazing solders containing Fe or Ni

The high Nb-containing γ-TiAl alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %) was successfully brazed by transient liquid phase (TLP) bonding using two brazing solders with different melting point depressing elements (MPD): Ti-24Ni and Ti-29Fe (in at. %). The brazing process was executed for 24 h at 1110 °C. An additional annealing was performed for 168 h at 1000 °C for a better homogenization. The joints were characterized with high-energy X-ray diffraction (HEXRD) and scanning electron microscopy (SEM) using energy dispersive X-ray spectroscopy (EDX). Depending on the MPD and if the specimens were annealed two to three symmetrically arranged transition zones developed between the substrate and the middle of the joint with different microstructures, phase compositions, and chemical compositions. Beside α2 and γ which have been present in the substrate additional phases as β, βo, ωo, τ3, and τ2 were identified in the joints. We discuss the different solidification paths and the following solid-state transformations. The Fe-containing joint solidified via a single βo phase field leading to large βo grains. In contrast, the Ni-containing joint solidified via a two-phase field resulting in a finer grained microstructure.

Effect of Nb and Fe on damage evolution in a Zr-alloy during proton and neutron irradiation

Detailed analysis was carried out on proton and a neutron irradiated Nb-containing Zr-alloy to study the evolution of dislocation loop size and densities as well as the formation and evolution of irradiation-induced precipitation/clustering. The results obtained here have been contrasted against previously published work on a Nb-free Zr-alloy [1, 2] to investigate the mechanistic reason for the improved resistance to irradiation-induced growth of Nb-containing Zr alloys. The combined use of bright field scanning transmission electron microscopy, ultra-high-resolution energy dispersive spectroscopy and atom probe tomography analysis provides evidence of evenly distributed radiation-induced Nb clusters that have formed during the early stage of proton irradiation and Fe-rich nano-rods near Fe-containing second phase particles. The former seems to have a profound effect on <a> loop and subsequent <c> loop formation, keeping <a> loop size small but number density high while <c> loops seem to initially form at similar dose levels compared to a Nb-free alloy but <c> loop line density does not increase during further irradiation. It is hypothesized that the formation of the Nb nano-precipitates/clusters significantly hinders mobility and growth of <a> loops, resulting in a small size, high number density and limited ability of <a> loops to arrange along basal traces compared to Nb-free Zr-alloys. It is suggested that it is the limited <a> loop arrangement that slows down <c> loop formation and the root cause for the high resistance of Nb-containing Zr-alloys to irradiation-induced growth.

Formation and temporal evolution of modulated structure in high Nb-containing lamellar γ-TiAl alloy

The formation and temporal evolution of the modulated structure in a lamellar γ-based Ti−45Al−8.5Nb alloy have been investigated by transmission electron microscopy (TEM) in combination with first-principles theory in this work. The results show that the Nb-rich O phase as a constituent of the modulated structure is thermodynamically stable below 650 °C in the α2 lamellae. The morphology of the O phase variants changes from thin plate-like shape with a low volume fraction at initial annealing to rectangle/square shape with a high volume fraction after a prolonged annealing, and the retransformed α2, named as α2-ІІ hereafter, emerges at intersections of the variants with two orthogonal habit planes due to their elastic interactions. The partitioning coefficient of Nb between the O phase and α2 is about 2 at 600 °C. The diffusion coefficient of Nb derived from growth kinetics of the O phase is about (1.3 ± 0.2) × 10−22 m2s−1 in the α2 lamellae. Significant precipitation hardening effect of the O phase has been revealed for the α2 lamellae and γ/(α2+O) lamellar microstructure, which is supposed to be attributed to refining the α2 lamellae associated with elastic strain energy from the α2→O phase transformation and introducing the interface between the modulated lamella and adjacent γ phase.

Microstructural examination of zirconium alloys following in-pile creep testing in the HALDEN reactor

Post-irradiation examination (PIE) in the form of transmission electron microscopy (TEM) was used to characterize the microstructure of several specimens of Zircaloy-2 and Zircaloy-2 plus 1% Nb that had previously underwent in-pile creep testing in the HALDEN reactor. The purpose of the examination was to explore a microstructural basis for an apparent substantially increased rate of hardening over that observed with similar materials in the higher fast-flux environments of the ATR or HFIR, and to investigate the increased creep strength exhibited by the Nb-containing alloy relative to pure Zircaloy-2. The analysis of irradiation-induced defects indicated a higher than expected density, which was consistent with the observed high hardening rate. Modeling based on mean field rate theory suggests the lower neutron flux in the HALDEN reactor results in a higher fraction of irradiation-induced defects being available for sink (loop) nucleation and growth.

Effects of Nb on hot tensile deformation behavior of cast Haynes 25 Co-Cr-W-Ni alloy

Effect of Nb on the hot tensile deformation behavior of cast Haynes 25 Co-Cr-W-Ni alloy was investigated in temperature range of 950–1200 °C. In the base alloy, thermodynamic calculations showed that M23C6 and M6C are the stable forms of carbides at temperatures below 1000 °C and over 1050 °C, respectively. For Nb-containing alloy, however, WC and NbC are observed below 1050 °C. At higher temperatures, W carbides dissolve and tungsten diffuses into NbC particles to form complex carbides. The yield point phenomena (YPP) were observed in both alloys at temperatures over 1050 °C due to the immobilization of matrix dislocations by W, Cr and C. Addition of Nb increased the strength and elongation due to the precipitation of NbC carbides. Absorbing the solute atoms and favoring dynamic recrystallization (DRX) seems to be the possible mechanisms. Loss of hot ductility occurred in both alloys at 1050 °C. It was attributed to the dissolution of carbides into matrix and their negative effects on dynamic recovery (DRV) and recrystallization. DRX was the major microstructural mechanism at high temperature, e.g. 1150 °C. At low temperatures, e.g. 950 °C, only DRV occurred.

High-temperature rotary-bending fatigue characteristics of a high Nb-containing beta-gamma TiAl alloy

The rotary-bending fatigue tests of a high Nb-containing beta-gamma TiAl alloy based on microstructure control have been conducted at 800 °C. A fine nearly lamellar microstructure with the lamellar colony of 80 µm and slight content of B2 phase of 2% is obtained by an improved two-step heat treatment. The rotary-bending fatigue results perform the scattered S-N data, flat S-N fitting curve and an excellent fatigue limit reached at 355 MPa. SEM results exhibit an obvious transition between stable crack growth and fast fracture surface. Massive cracks initiate at lamellar interface, colony boundary and B2 phase on the specimen surface, which coalesce and propagate from the margin to the center of specimen. TEM analysis indicates that dislocation glide and mechanical twinning are the mainly deformation mechanism. The interlamellar and translamellar fracture result from dislocation pile-up and the inclined slip band or mechanical twinning, respectively.

On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr2O3 scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.

Effects of Laves phase particles on recovery and recrystallization behaviors of Nb-containing FeCrAl alloys

The microstructures and mechanical properties of deformed and annealed Nb-containing FeCrAl alloys were investigated. Fine dispersion of Fe2Nb-type Laves phase particles was observed in the bcc-Fe matrix after applying a thermomechanical treatment, especially along grain/subgrain boundaries, which effectively stabilized the recovered and recrystallized microstructures compared with the Nb-free FeCrAl alloy. The stability of recovered areas increased with Nb content up to 1 wt%. The recrystallized grain structure in Nb-containing FeCrAl alloys consisted of elongated grains along the rolling direction with a weak texture when annealed below 1100 °C. An abnormal relationship between recrystallized grain size and annealing temperature was found. Microstructural inhomogeneity in the deformed and annealed states was explained based on the Taylor factor. Annealed Nb-containing FeCrAl alloys showed a good combination of strength and ductility, which is desirable for their application as fuel cladding in light-water reactors.

High-resolution electron microscopy characterization of modulated structure in high Nb-containing lamellar γ-TiAl alloy

The microstructure in a lamellar γ-based Ti-45Al-8.5Nb alloy has been investigated by transmission electron microscopy (TEM). The results show that there exists a nano-scale modulated structure with tweed contrast within α2 laths and the orthorhombic phase as a constituent of the modulated structure has the O1 structure with a random occupancy of Ti and Nb atoms on the Wyckoff sites 8 g and 4c2. The orthorhombic phase has a thin plate-like morphology with orientation relationships of {001}O//{0001}α2 and <110>O//<112¯0>α2. The habit plane for orthorhombic phase variants lies at {350}O//{134¯0}α2, and the O1/α2 interface is not atomic flat. The results further exhibit an enrichment of Nb in the orthorhombic phase in comparison with the α2 parent phase. Based on the TEM evidences, the formation of the orthorhombic phase arises from a phase separation reaction in the α2 parent phase, which is dominated by a diffusion mechanism.