On the stability of the V5B6-phase (original) (raw)
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Boride morphology in A (Fe, V, B) Ti-alloy containing B2-phase
Acta Metallurgica et Materialia, 1995
The morphology of borides grown from the melt in a Ti-A1 (Fe, V, B) alloy has been investigated. Flake-like dispersoids are shown by energy dispersive X-ray spectroscopy and electron energy loss boron maps to consist of interleaved titanium boride and B2 grains, with selected area diffraction analysis identifying the boride phase as TiB2. High resolution electron microscopy reveals that the TiB2 and B2 are very high aspect ratio plates, reflecting the overall morphology of the dispersoids, with the interfacial and orientational relationship being { 1010}x~B2//{001 } B2 and < 0001 > r,B:// < 010 > nz. Growth methods of the TiB2/B2 dispersoids are discussed in light of the surprisingly fine nano-scale of the interleaving plates. R6sum6-La morphologie des borures primaires form6s fi partir d'un alliage Ti-A1 (Fe, V, B) fondu a 6td 6tudi6e. Par spectrom6trie des rayons X en s61ection d'6nergie et par des cartes chimiques obtenues en spectroscopie des pertes d'6nergie, les dispersoides, ayant une forme de plaquettes, sont compos6s d'un empilement de grains lamellaires de borure de titane et de la phase B2. Avecla diffraction des ~lectrons en s61ection d'aire, le borure a 6t6 identifi6 comme ~tant TiB2. La microscopie 61ectronique en haute r6solution revile que les phases TiB2 et B2 ont un facteur de forme 61ev6 repercutant la morphologie g6n6rale des dispersoides. Les relations d'orientations et d'interface entre ces deux phases sont { 10T0}Tm2 // {001 }s2 et < 0001 > TiB2 // < 010 > SZ. Les m~canismes de croissance des dispersoides TiB2/B2 sont discut6s/l la lumi~re de la surprenante structure tr6s fine (/l l'6chelle nanom6trique) et al ternante des plaquettes. Zusammenfassung-Die Morphologie von prim~iren, aus der Schmelze gewachsenen Boriden in einer Ti-A1 (Fe, V, B)-Legierung ist untersucht worden. Die Analyse flockenartiger Ausscheidungsphasen mit energiedispersiver R6ntgenspektroskopie und Bor-spezifischer Elektronenenergieverlustspektroskopie zeigte, dab diese aus ineinander geschobenen Titanborid-und B2-Phasen bestanden, wobei durch Elektronenbeugungsbilder die Boride als TiB2 identifizert wurden. Hochaufl6sende Elektronenmikroskopie offenbarte die Gestalt der TiB2-und B2-Phasen als lang-gestreckte Platten mit Orientierungen { 10T0}r~B2// {001 }B2 und < 0001 > T~B2 // < 010 > B2, die Morphologie der Ausscheidungspartikel reflektierend. Das Wachstum der TiB2/B2-Ausscheidungsphasen wird im Hinblick auf deren strukturelle Feinheit im Nanometer-Bereich diskutiert.
Recent Advances in Boron-containing Materials [Working Title]
Mechanical alloying in the transition IV-V group metal-boron systems runs by the two following mechanisms: mechanically induced reaction of self-propagating synthesis determined by the enthalpy of refractory compound formation and capability to form substitutional solid solution through replacement of a metal atom by boron atoms; and diffusion-controlled process when a supersaturated interstitial solid solution prevails and its bcc lattice gradually transforms to the hexagonal lattice of the MeB 2 phase at a critical boron content. The domination of one of the above mechanisms is determined by capability of boron to form substitutional or interstitial solid solution. In the case of formation of combined (SSS and ISS) solid solutions, domination of a mechanism is determined by the interatomic bond strength as well as by the intensity of mechanical alloying. The method for calculation of the free Gibbs energy of the interstitial and substitutional solid solutions on the basis of the regular solution model was developed. It was shown that during milling tantalum and boron in a planetary mill, at first the formation of a combined solid solution occurs where two boron atoms replace one tantalum atom. Both the mechanisms of solid solution formation decrease the solution Gibbs energy. When a SSS dominates over the formation of an ISS, the Gibbs energy acquires a minimum value at a concentration of boron in tantalum of 50 at%, which leads to the solution decomposition.
International Journal of Refractory Metals & Hard Materials, 2000
Intermetallic aluminides including those phases of the Nb-Al system are of interest for high-temperature structural applications. Through aluminothermic reduction (ATR) of Nb 2 O 5 different alloys of the Nb-Al system can be produced by varying the amount of aluminum (excess aluminum) in the thermit charge. In this work, various Nb-Al alloys were produced by varying Nb 2 O 5 and Al powder blends. The resulting alloys were characterized by chemical analysis (Al, O, and C), X-ray diffraction and scanning electron microscopy. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60% over the stoichiometric amount to reduce the Nb 2 O 5 . The carbon content was lower than 300 wt-ppm. The oxygen content decreases with increasing excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al 2 O 3 ) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nb ss (niobium solid solution) and Nb 3 Al (alloy produced with 10% excess Al); Nb 3 Al (alloys produced with 15 and 20% excess Al); Nb 3 Al, Nb 2 Al, and NbAl 3 (alloy produced with 30% excess Al); and Nb 2 Al and NbAl 3 (alloys produced with 40, 50, and 60% excess Al).
Refinement of the eutectic composition in the LaB6–VB2 system
Izvestiya vuzov. Poroshkovaya metallurgiya i funktsional’nye pokrytiya
The LaB6–VB2 alloy with the eutectic structure was obtained by cold crucible induction melting followed by crystallization. The mole ratio of components in the initial powder mixture was 35 : 65. The structure and composition of the LaB6–VB2 material were studied by X-ray diffraction, scanning electron microscopy, and X-ray microanalysis. The composition of the alloy is represented by two boride phases — cubic LaB6 and hexagonal VB2. Two-phase eutectic regions up to 500 μm in size represent a LaB6 matrix filled with 0.8–2.0 μm thick VB2 fibers (filamentary, rod crystals). VB2 fibers are predominantly oriented along the direction of the temperature gradient that appeared when cooling the melt, i.e. from the outer surface of the sample to its center. The integrated phase area analysis was used to determine the eutectic composition: 42 Ѓ} 1 mol% LaB6 and 58 Ѓ} 1 mol% VB2.
Intermetallics, 2002
The microstructures of three Nb-Al-V alloys with nominal compositions Nb-10Al-20V, Nb-15Al-20V and Nb-25Al-40V (in at.%) have been investigated. It is shown that the alloys each exhibit an A2 or B2 matrix and often contain A15 and/or s phase precipitates depending on thermal history. Both the A15 and s phase precipitates exhibit two different well-defined orientation relationships and for the former these correspond to minimisation of elastic strain energy. ALCHEMI data from the B2 phase indicate that this is more stable for higher Al concentrations, and this is consistent with measurements of A2/B2 order-disorder transformation temperatures. In the alloy Nb-15Al-20V, the precipitation of the A15 phase in a supersaturated B2 matrix is preceded by the separation of the B2 phase into Al-rich domains in an Al-lean matrix. #
First principles calculation of phase diagrams of V-Nb, V-Ta and Nb-Ta alloys
2012
We report the solid state phase diagram of V-Nb, V-Ta and Nb-Ta alloys computed by combining the density functional theory total energies with the cluster expansion and Monte Carlo techniques. From the computed phase diagrams, we find that V-Nb and Nb-Ta alloys form continuous series of solid solutions and the solid solution phase is stable down to ambient temperatures, consistent with experiments. The calculated bcc V-Ta phase diagram exhibits complete miscibility. Since the current cluster expansion ignore V
First-principles calculation of phase equilibrium of V-Nb, V-Ta, and Nb-Ta alloys
Physical Review B, 2012
In this paper, we report the calculated phase diagrams of V-Nb, V-Ta, and Nb-Ta alloys computed by combining the total energies of 40-50 configurations for each system (obtained using density functional theory) with the cluster expansion and Monte Carlo techniques. For V-Nb alloys, the phase diagram computed with conventional cluster expansion shows a miscibility gap with consolute temperature T c = 1250 K. Including the constituent strain to the cluster expansion Hamiltonian does not alter the consolute temperature significantly, although it appears to influence the solubility of V-and Nb-rich alloys. The phonon contribution to the free energy lowers T c to 950 K (about 25%). Our calculations thus predicts an appreciable miscibility gap for V-Nb alloys. For bcc V-Ta alloy, this calculation predicts a miscibility gap with T c = 1100 K. For this alloy, both the constituent strain and phonon contributions are found to be significant. The constituent strain increases the miscibility gap while the phonon entropy counteracts the effect of the constituent strain. In V-Ta alloys, an ordering transition occurs at 1583 K from bcc solid solution phase to the V 2 Ta Laves phase due to the dominant chemical interaction associated with the relatively large electronegativity difference. Since the current cluster expansion ignores the V 2 Ta phase, the associated chemical interaction appears to manifest in making the solid solution phase remain stable down to 1100 K. For the size-matched Nb-Ta alloys, our calculation predicts complete miscibility in agreement with experiment.
Isothermal Section of the V-Si-B System at 1600 °C in the V-VSi2-VB Region
Journal of Phase Equilibria and Diffusion, 2009
In recent years, the Me-Si-B (Me-metal) ternary systems have received considerable attention aiming at the development of high-temperature structural materials. Assuming that any real application of these materials will rely on multicomponent alloys, as is the case of Ni-base superalloys, phase equilibria data of these systems become very important. In this work, results are reported on phase equilibria in the V-Si-B system, and are summarized in the form of an isothermal section at 1600°C for the V-VSi 2-VB region. Several alloys of different compositions were prepared via arc melting and then heat-treated at 1600°C under high vacuum. All the materials in both as-cast and heat-treated conditions were characterized through x-ray diffraction, scanning electron microscopy, and selected alloys via wavelength dispersive spectroscopy. A negligible solubility of B in the V 3 Si, V 5 Si 3 (T 1), and V 6 Si 5 phases as well as of Si in V 3 B 2 and VB phases was noted. Two ternary phases presenting the structures known as T 2 (Cr 5 B 3prototype) and D8 8 (Mn 5 Si 3-prototype) were observed in both as-cast and heat-treated samples.
The pack-boronizing of pure vanadium under a controlled atmosphere
Applied Surface Science, 2010
Pack boronizing of pure vanadium was performed at 1100 • C for 4, 8, 12 and 16 h under a controlled atmosphere. Characterization of the boride formed on the surface of pure vanadium was carried out by metallographic techniques, profilometry, SEM-EDS, XRD and microhardness measurements. The metallographic studies revealed that a single boride layer with dense, compact and relatively smooth morphology was formed on the surface of pure vanadium. The interface between boride layer and base metal was wavy in nature. The formation of only the VB 2 phase on pure vanadium was confirmed by surface and crosssectional XRD analysis. The microhardness of the boride layer was approximately 3700 HV for all boriding times. Fracture toughness of the boride layer was evaluated using Vickers indentation, giving the value of 2.1-5.9 and 1.7-3.4 MPa m 1/2 for Palmqvist and median/radial approaches, respectively. Thickness of the boride layer increased almost parabolically from about 23 to 50 m with boriding time. Surface roughness of the coating was relatively increased from approximately 0.58 to 2.25 m by boriding duration.
Development of solidification microstructure in boron-modified alloy Ti–6Al–4V–0.1B
Hypoeutectic boron addition (0.1 wt.%) to Ti-6Al-4V is known to cause significant refinement of the cast microstructure. In the present investigation, it has been observed that trace boron addition to Ti-6Al-4V alloy also ensures excellent microstructural homogeneity throughout the ingot. A subdued thermal gradient, related to the basic grain refinement mechanism by constitutional undercooling, persists during solidification for the boron-containing alloy and maintains equivalent b grain growth kinetics at different locations in the ingot. The Ti-6Al-4V alloy shows relatively strong texture with preferred components (e.g. ingot axis||[0 0 0 1] or [1 0 1 0]) over the entire ingot and gradual transition of texture components along the radius. For Ti-6Al-4V-0.1B alloy, significant weakening characterizes both the high-temperature b and room-temperature a texture. In addition to solidification factors that are responsible for weak b texture development, microstructural differences due to boron addition, e.g. the absence of grain boundary a phase and presence of TiB particles, strongly affects the mechanism of b ! a phase transformation and consequently weakens the a phase texture. Based on the understanding developed for the boron-modified alloy, a novel mechanism has been proposed for the microstructure and texture formation during solidification and phase transformation.