M3:2 high-speed steel (HSS) billets with or without Nb addition were prepared by spray deposition. The effects of Nb and post-thermal-mechanical processing (decomposition treatment and hot forging), as well as heat treatment, on the microstructure and properties of M3:2 HSS were investigated. The microstructure of the as-deposited M3:2 HSS consisted of equiaxed grains with a mean size of approximately 25 μm and discontinuous plate-like M2C and irregular MC carbides distributed along grain boundaries. 0.5% Nb addition can refine the M2C plates and spheroidize MC carbides. With 2% Nb addition, the refined grains with a mean size of approximately 12 μm and continuous net of M6C and a uniform distribution of NbC carbides were obtained. The decomposition of metastable M2C carbides can be accelerated with 0.5% Nb addition due to the refined size and lower thermodynamic stability of M2C plates. With the increased degree of decomposition of M2C carbide, the M6C and MC carbides became refined and more uniformly distributed after optimal thermal-mechanical processing and heat treatment, which leads to a significant increase in bend strength and toughness.

High-purity niobium single crystals of five different orientations were compressed at 77 K to 2–4% plastic strain to investigate the mechanisms operative in the initial stage of yielding. The crystals deformed in the direction close to the [001] axis exhibit predominant slip on the high-stressed (101) and a much lower stressed $\left( {0\bar{1}1} \right)$ plane. The expected slip on the $\left( {\bar{1}01} \right)$ plane is nearly homogeneously distributed with only a few sharp slip traces corresponding to localized slip. The samples compressed along center-triangle orientations and those close to the $\left[ {011} \right] - \left[ {\bar{1}11} \right]$ edge deform predominantly by twinning on {112}〈111〉 systems with some contribution from slip on the $\left( {\bar{1}01} \right)\left[ {\bar{1}\bar{1}\bar{1}} \right]$ system with the highest Schmid factor. A majority of twins exhibit internal contrast due to alternating slip on $\left( {\bar{1}01} \right)$ and $\left( {0\bar{1}1} \right)$ planes. No slip traces are observed in the matrix adjacent to the twin, which implies that twin boundaries are impenetrable obstacles for the motion of dislocations.

Magnetite-rich waste from a niobium mine near Araxá, State of Minas Gerais, Brazil, was heated to 500°C and 1000°C under an O2 atmosphere. The original waste and its oxidized products were characterized by energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), room-temperature 57Fe Mössbauer spectroscopy, thermogravimetric analysis (TG) and temperature-programmed reduction (TPR).

Semiquantitative analyses by EDS and quantitative chemical analyses showed the waste to be constituted primarily of Fe with minor amounts of Ti, Ba, Al, Si, Nb, Mn, S and P. Mössbauer and XRD showed the waste to consist predominantly of magnetite and hematite. The magnetite content decreases when the temperature increases due to its direct conversion to hematite. However, at 500°C only 10 wt.% of original magnetite was converted to hematite, confirming the high stability of this magnetite, which could still be detected at 1000°C. The TG profile shows no significant weight gain on heating, indicating a high stability of the magnetite. The TPR profiles show that the hematite in the waste is sintered after treatment at 1000°C and the reduction peaks are consequently shifted to higher temperatures. This high thermal stability is attributed to a moderate isomorphous replacement of Fe by other cations present in the Nb mining waste.

A synthetic analogue, Ca(Ti0.6Al0.2Ta0.16Nb0.04)OSiO4, of an aluminoan tantalian titanite previously described from Craveggia pegmatite (Piemonte, Italy) has been prepared by a ceramic synthesis technique and its crystal structure determined by Rietveld analysis of the powder X-ray diffraction pattern. The synthetic Al-Ta-Nb-rich titanite adopts space group A2/a implying that substitutions at the single octahedral site destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P 21/a → A2/a phase transition. Unit-cell dimensions obtained for the Al-Ta-Nb-rich titanite are: a = 7.0649(1) Å; b = 8.7187(1) Å; c = 6.5701(1) Å; β = 113.755(1)°, V = 370.41(1) Å3. The extensive replacement of Ti by Al, Ta and Nb results in a considerable decrease in the distortion of all coordination polyhedra in the structure of this titanite. These structural data suggest that a solid solution CaTi1-x(Alx /2 [Ta,Nb]x /2)OSiO4 (0 ≤ x ≤ 0.4) adopting the titanite structure might exist.

Ilmenites from alkaline basic dykes in the Gardar province, south Greenland have Zr contents up to 3850ppm and Nb contents up to 1030ppm. These elements substitute for Ti in the octahedral site. These are amongst the highest recorded Zr and Nb contents for ilmenites, giving distribution coefficients (Kdilmenite/matrix) for Zr up to 11.6 and Nb up to 70.3. Fractionation of ilmenite may thus have a dramatic effect on residual Zr and Nb contents. These effects are discussed in relation to the use of trace element tectonomagmatic discrimination diagrams.

Three new valleriite-group minerals, ekplexite (Nb,Mo)S2·(Mg1−xAlx)(OH)2+x, kaskasite (Mo,Nb)S2·(Mg1−xAlx)(OH)2+x and manganokaskasite (Mo,Nb)S2·(Mn1−xAlx)(OH)2+x are found at Mt Kaskasnyunchorr, Khibiny alkaline complex, Kola Peninsula, Russia. They occur in fenite consisting of orthoclase−anorthoclase and nepheline with fluorophlogopite, corundum, pyrrhotite, pyrite, rutile, monazite-(Ce), graphite, edgarite, molybdenite, tungstenite, alabandite, etc. Ekplexite forms lenticular nests up to 0.2 mm × 1 mm × 1 mm consisting of near-parallel, radiating or chaotic aggregates of flakes. Kaskasite and manganokaskasite mainly occur as flakes and their near-parallel ‘stacks’ (kaskasite: up to 0.03 mm × 1 mm × 1.5 mm; manganokaskasite: up to 0.02 mm × 0.5 mm × 1 mm) epitaxially overgrow Ti-bearing pyrrhotite partially replaced by Ti-bearing pyrite. All three new minerals are opaque, ironblack, with metallic lustre. Cleavage is {001} perfect and mica-like. Flakes are very soft, flexible and inelastic. Mohs hardness is ∼1. D(calc.) = 3.63 (ekplexite), 3.83 (kaskasite) and 4.09 (manganokaskasite) g cm−3. In reflected light all these minerals are grey, without internal reflections. Anisotropism and bireflectance are very strong and pleochroism is strong. The presence of OH groups and an absence of H2O molecules are confirmed by the Raman spectroscopy data. Chemical data (wt.%, electron probe) for ekplexite, kaskasite and manganokaskasite, respectively, are: Mg 6.25, 5.94, 0.06; Al 4.31, 3.67, 3.00; Ca 0.00, 0.04, 0.00; V 0.86, 0.16, 0.15; Mn 0.00, 0.23, 11.44; Fe 0.44, 1.44, 2.06; Nb 18.17, 13.39, 14.15; Mo 15.89, 23.18, 20.08; W 8.13, 7.59, 9.12; S 27.68, 27.09, 24.84; O 16.33, 15.66, 13.36; H (calc.) 1.03, 0.99, 0.89; total 99.09, 99.08, 99.15. The empirical formulae calculated on the basis of 2 S a.p.f.u. are: ekplexite: (Nb0.45Mo0.38W0.10V0.04)S0.97S2· (Mg0.60Al0.37Fe0.02)S0.99(OH)2.36; kaskasite: (Mo0.57Nb0.34W0.10V0.01)S1.02S2· (Mg0.58Al0.32Fe0.06Mn0.01)S0.97(OH)2.32; manganokaskasite: (Mo0.54Nb0.39W0.13V0.01)S1.07S2· (Mn0.54Al0.29Fe0.10Mg0.01)S0.94(OH)2.28. All three minerals are trigonal, space groups Pm1, P3m1 or P321, one-layer polytypes (Z = 1). Their structures are non-commensurate and consist of the MeS2-type (Me = Nb, Mo, W) sulfide modules and the brucite-type hydroxide modules. Parameters of the sulfide (main) sub-lattices (a, c in Å, V in Å3) are: 3.262(2), 11.44(2), 105.4(4) (ekplexite); 3.220(2), 11.47(2), 102.8(4) (kaskasite); 3.243(3), 11.61(1), 105.8(3) (manganokaskasite). Parameters of the hydroxide sub-lattices (a, c in Å, V in Å3) are: 3.066(2), 11.52(2), 93.8(4) (ekplexite); 3.073(2), 11.50(2), 94.0(4) (kaskasite); 3.118(3), 11.62(1), 97.9(2) (manganokaskasite). Ekplexite was named from the Greek word έκπληξη meaning surprise, for its exotic combination of major chemical constituents, kaskasite after the discovery locality and manganokaskasite as a Mn analogue of kaskasite.

Euhedral crystals of complexly zoned niobian titanite (up to 0.3 mm) are enclosed in hedenbergite (Hd53—81Di15—43Jh3-5) and quartz from a hedenbergite vein skarn at Kamenne doly near Pisek, Czech Republic. They are associated with minor clinozoisite-epidote (Ps3—22), calcite, plagioclase (An95). scapolite (Me80—82), scheelite, pyrrhotite, fluorapatite, arsenopyrite, native bismuth and Bi,Te-minerals. The following textural and compositional subtypes were recognized: (I) Nb-rich titanite, (II) Nb-moderate titanite in the central zone, (III) Nb-poor, Sn-enriched titanite and (IV) Nb-poor, Al,F-rich titanite in the outer zone. The substitution Al(Nb,Ta)Ti—2 is dominant in subtypes I and II, the titanite subtype I being characterized by elevated contents of Al ≤ 0.257 atoms per formula unit (a.p.f.u.), Nb (≤ 0.161 a.p.f.u.) and Ta (≤ 0.037 a.p.f.u.). Amounts of Al, Nb and Ta in subtype II are smaller and more variable. The minor substitution SnTi—1 occurs chiefly in titanite subtype III with a content of Sn ≤ 0.039 a.p.f.u.. The substitution Al(F,OH)(TiO)_i is typical for titanite subtype IV exhibiting elevated contents of Al (s£ 0.221 a.p.f.u.), F (≤ 0.196 a.p.f.u.) and Fe (≤ 0.039 a.p.f.u.).

The negative relationship of substitutions Al(F,OH)(TiO)_i vs. SnTi—1 and Al(Nb,Ta)Ti—1 is constrained chiefly by crystal structure rather than by the composition of parent medium alone. Textural relations suggest that the Nb-moderate titanite in the core zone and entire outer zone are products of fluids-induced dissolution-reprecipitation processes. The studied niobian titanite represents a new F-enriched type from a medium-grade, calc-silicate rock.

Pyrochlore group minerals have the general formula A16–xB16O48Z8–y·nH2O, with A mainly Na, Ca, Sr, REE, Th, U; B mainly Nb, Ta, Ti and Z being O, F, OH. In this study, pyrochlore specimens were examined from carbonatites at Argor, Carillon Dam, Chilwa Island, Fen, Lueshe, Oka, Mbeya, Meech Lake and Verity. Primary features include a background with little compositional variation, from core to rim, upon which are commonly superimposed narrow oscillatory zones, parallel to {111}. These zones are usually characterized by high Ta, in many cases coupled with U (here Argor, Meech Lake and Verity specimens), but Chilwa Island and Fen pyrochlores have little Ta and zonation is mainly by enrichment in Ce and Nb. Primary zonation may persist through high-temperature metamorphism (Meech Lake and Verity) and metamictization (Meech Lake). Oscillatory zones were generated by a disequilibrium system that cooled under tranquil conditions, signalling absence of magma turbulence and, in many cases, the end of crystal growth. Some fresh crystals (Oka, Fen) have no oscillatory zones, possibly the product of magma turbulence in space or time. Low-temperature effects may mimic those of primary high temperature and are especially characterized by replacement rims, pyrochlore-inpyrochlore veinlets and low A-ion totals (Carillon Dam, Lueshe, Myeba).

Idioblastic, mm-scale brookite is reported from SiO2-Ba-Fe-rich metasomatic rocks from the carbonatitic Gross Brukkaros Volcanic Field, Southern Namibia. Electron probe microanalysis shows significant concentrations of Nb (12–17 wt.% Nb2O5), coupled with Fe, substituting for Ti in the structure. Calculation of the analytical data indicates evolving Fe2+/(Fe2++Fe3+) ratios during crystallization and two distinct substitution mechanisms in operation: 2Ti4+ ⇋ Nb5+ + Fe3+ and 3Ti4+ ⇋ 2Nb5+ + Fe2+. The compositional patterns observed are explained in terms of initial growth in an environment which evolved from oxidizing to reducing. The outermost 10 µm rims of the crystals are characterized by oxidizing conditions, resulting from a subsequent metasomatic overprint.

The major and trace element composition of over 40 wolframites from the Barroca Grande mine of the Panasqueira W-Sn-Cu quartz-vein hosted orebody have been determined by electron probe microanalysis.

The hübnerite content of wolframite is largely uniform (12 to 20 mol. %) throughout most of the mine, suggesting relative uniformity in composition of the main stage ore-forming fluids. However, wolframites show a striking pattern of enrichment in niobium adjacent to major sub-vertical fault zones. Typical wolframite Nb2O5 contents are below 1000 ppm, whilst values greater than 1000, 2000 and 4000 ppm are generally only found in wolframites less than 50, 30, and 10 metres respectively from major sub-vertical faults. This strongly suggests that some of these fault zones acted as major pathways for hydrothermal fluid migration during ore formation. The idea that the greisened cupola at Panasqueira was the sole major pathway for fluid movement is discounted.

Rutile grains occur extensively in host phases of biotite and quartz-feldspar aggregate in high-temperature paragneisses of the Mogok metamorphic belt of Myanmar. They occur as an isolated phase and sometimes show intergrowth texture with ilmenite. Most rutile grains contain up to 3.7 wt.% Nb2O5, which shows positive correlations with Fe and trivalent elements. Niobium substitutes for Ti by a coupled substitution with the trivalent cations (M3+) of Nb5+M3+Ti4+-2. Fine-grained rutile grains included in ilmenite are distinctly poor in Nb (<0.1 wt.% as Nb2O5) and contain Fe of 1.7–3.2 wt.% as Fe2O3, suggesting vacancybearing substitution of Fe3+4 Ti4+-3□–1, where □ indicates a vacancy. The rutile grains in the felsic phases contain high Zr contents of up to 4200 ppm, suggesting equilibrium temperatures over 800°C using the Ti-in-rutile geothermometer. These high-temperature conditions are consistent with those estimated by conventional methods reported in the literature and suggest widespread occurrences of the upperamphibolite and granulite facies metamorphic rocks in the middle segment of the Mogok metamorphic belt. In contrast, the Zr contents of rutile grains in biotite are usually <1000 ppm, implying equilibrium temperatures lower than 750°C. Most of the rutile grains poorer in Zr might have been included in biotite and were isolated from the zircon-bearing system during an early stage of prograde metamorphism. Some other rutile grains poorer in Zr might have been an exsolved phase from Ti-rich biotite during retrograde metamorphism, which was furthered by the infiltration of metamorphic fluid under lower-amphibolite facies conditions.

Atomic-scale characterization of hydrogen and formation of niobium hydrides, using ultraviolet (wavelength=355 nm) picosecond laser-assisted local-electrode atom-probe tomography, was performed for ultrahigh purity niobium utilizing different laser pulse energies, 10 or 50 pJ/pulse or voltage pulsing. At 50 pJ/pulse, hydrogen atoms migrate onto the 110 and 111 poles as a result of stimulated surface diffusion, whereas they are immobile for <10 pJ/pulse or for voltage pulsing. Accordingly, the highest concentrations of H and NbH were obtained at 50 pJ/pulse. This is attributed to the thermal energy of the laser pulses being transferred to pure niobium specimens. Therefore, we examined the effects of the laser pulse energy being increased systematically from 1 to 20 pJ/pulse and then decreasing it from 20 to 1 pJ/pulse. The concentrations of H, H2, and NbH and the atomic concentration ratios H2/H, NbH/Nb, and Nb3+/Nb2+ were calculated with respect to the systematically changing laser pulse energies. The atomic concentration ratios H2/H and NbH/Nb are greater when decreasing the laser pulse energy than when increasing it, because the higher residual thermal energy after decreasing the laser pulse energy increases the mobility of H atoms by supplying sufficient thermal energy to form H2 or NbH.

New-type Ni-based superalloys with and without Nb were designed in this study. Their hot corrosion behaviors were investigated at 800 °C with the deposition of a mixture of Na2SO4 and NaCl. The corrosion kinetics was studied by thermogravimetry. Microstructure of the corrosion scales was studied by SEM and the phase constituent was analyzed by XRD. Results showed that the corrosion kinetics followed approximately parabolic law. The corrosion scales on the two Ni-based alloys were comprised of Cr2O3, Al2O3, TiO2, and NiCr2O4. NiO was only detected in the scale on alloy without Nb. Nb2O5 appeared with the addition of 2.0 wt% Nb. No sulfide emerged in the scales. The corrosion scales both exhibited a layered structure. With Nb addition, the hot corrosion resistance of the alloy was notably improved. The action mechanism of Nb was investigated extensively in this study.

Atom probe is a powerful technique for studying the composition of nano-precipitates, but their morphology within the reconstructed data is distorted due to the so-called local magnification effect. A new technique has been developed to mitigate this limitation by characterizing the distribution of the surrounding matrix atoms, rather than those contained within the nano-precipitates themselves. A comprehensive chemical analysis enables further information on size and chemistry to be obtained. The method enables new insight into the morphology and chemistry of niobium carbonitride nano-precipitates within ferrite for a series of Nb-microalloyed ultra-thin cast strip steels. The results are supported by complementary high-resolution transmission electron microscopy.

A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N2,3, Nb-M4,5, Nb-M2,3, Nb-M1, and Nb-L2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO2, and +5 for Nb2O5). Chemical shifts of the Nb-N2,3, Nb-M4,5, Nb-M2,3, and Nb-L2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb2O5. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were performed on electrochemically anodized niobium and niobium oxide. Sintered anodes of Nb and NbO powders were anodized in 0.1 wt% H3PO4 at 10, 20, and 65 V to form surface Nb2O5 layers with an average anodization constant of 3.6 ± 0.2 nm/V. The anode/dielectric interfaces were continuous and the dielectric layers were amorphous except for occurrences of plate-like, orthorhombic pentoxide crystallites in both anodes formed at 65 V. Using EELS stoichiometry quantification and relative chemical shifts of the Nb M4,5 ionization edge, a suboxide transition layer at the amorphous pentoxide interface on the order of 5 nm was detected in the Nb anodes, whereas no interfacial suboxide layers were detected in the NbO anodes.

In this article, the secondary electron-emission properties of both vertically and laterally inhomogeneous samples are discussed. To study the effect of surface coverage, the total electron-emission yield of tungsten and niobium samples was measured as a function of primary electron energy and oxide thickness. A method is suggested to avoid charging difficulties during AES measurements of samples that consist of both metal and various insulator parts.

Nb–silicide in situ composites have great potential for high-temperature turbine applications. Nb–silicide composites consist of a ductile Nb-based solid solution together with high-strength silicides, such as Nb5Si3 and Nb3Si. With the appropriate addition of alloying elements, such as Ti, Hf, Cr, and Al, it is possible to achieve a promising balance of room-temperature fracture toughness, high-temperature creep performance, and oxidation resistance. In Nb–silicide composites generated from metal-rich binary Nb-Si alloys, Nb3Si is unstable and experiences eutectoid decomposition to Nb and Nb5Si3. At high Ti concentrations, Nb3Si is stabilized to room temperature, and the eutectoid decomposition is suppressed. However, the effect of both Ti and Hf additions in quaternary alloys has not been investigated previously. The present article describes the discovery of a low-temperature eutectoid phase transformation during which (Nb)3Si decomposes into (Nb) and (Nb)5Si3, where the (Nb)5Si3 possesses the hP16 crystal structure, as opposed to the tI32 crystal structure observed in binary Nb5Si3. The Ti and Hf concentrations were adjusted over the ranges of 21 to 33 (at.%) and 7.5 to 33 (at.%) to understand the effect of bulk composition on the phases present and the eutectoid phase transformation.

This article reviews the most recent progress in the development of Nb-silicide-based in situ composites for potential applications in turbine engines with service temperatures of up to 1350°C. These composites contain high-strength Nb silicides that are toughened by a ductile Nb solid solution. Preliminary composites were derived from binary Nb-Si alloys, while more recent systems are complex and are alloyed with Ti, Hf, W, B, Ge, Cr, and Al. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, fatigue-crack-growth behavior, high-temperature creep performance, and oxidation resistance over a broad range of temperatures. Nb-silicide-based composites are described with emphasis on processing, microstructure, and performance. Nb silicide composites have been produced using a range of processing routes, including induction skull melting, investment casting, hot extrusion, and powder metallurgy methods. Nb silicide composite properties are also compared with those of Ni-based superalloys.