Nb Cr System Research Articles

Comment on Hajra et al.: “High-temperature phase stability and phase transformations of Niobium-Chromium Laves phase: Experimental and first-principles calculation”

This work comments on a recent publication by Hajra et al. (Mater. Design 236 (2023) 112483), which claims to have presented compelling experimental and theoretical evidence in favour of the existence of an equilibrium C14-NbCr2 high-temperature Laves phase in the Cr-Nb system. In the present comment, evidence and conclusions reported in the paper of Hajra et al. are critically put into context of insight from previous works. From this it is concluded here, that the evidence in favour of an equilibrium C14-NbCr2 high-temperature Laves phase is, by far, not that compelling as claimed by Hajra et al.. Instead, the most direct evidence presented in the literature does not support the existence of an equilibrium C14-NbCr2 high-temperature Laves phase. Alternative interpretations of Hajra et al.’s evidence and conclusions are offered, and it is elaborated, which true gaps in knowledge exist concerning the Laves phases in the Cr-Nb system.

Hydride Destabilization in the Ti–Nb–Cr System Through Nb/Ti Ratio Adjustment

Hydride Destabilization in the Ti–Nb–Cr System Through Nb/Ti Ratio Adjustment

A first step towards understanding thermomechanical behavior of the Nb-Cr system through interatomic potential development and molecular dynamics simulations

Utilizing a preliminary interatomic potential, this work represents an initial exploration into the thermomechanical behavior of NbCr solid solutions. Specifically, it examines the effect of different amounts of Cr solute, for which information in the literature is limited. The employed interatomic potential was developed according to the embedded atom model (EAM), and was trained on data derived from density functional theory calculations. While the potential demonstrated reasonable accuracy and predictive power when tested, various results highlight deficiencies and encourage further development and training. Mechanical strength, heat capacities, thermal expansion coefficients, and thermal conductivities were found to decrease with Cr content. Elastic coefficients, too, were observed to be strongly dependent on Cr composition. The Pugh embrittlement criterion was not satisfied for any of the compositions and temperatures explored. Gibbs free energy calculations performed on C14, C15, and C36 NbCr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} allotropes predicted the C36 structure to be the most thermodynamically favorable across all investigated temperatures and it was found that C36 becomes increasingly more stable relative to the other two phases with increased pressure. The inability of this work to accurately capture the stability of the different Laves phases is most likely due to the shortcomings in the developed potential.

Solid-State Hydrogen Storage Properties of Ti–V–Nb–Cr High-Entropy Alloys and the Associated Effects of Transitional Metals (M = Mn, Fe, Ni)

Recently, high-entropy alloys (HEAs) designed by the concepts of unique entropy-stabilized mechanisms, started to attract widespread interests for their hydrogen storage properties. HEAs with body-centered cubic (BCC) structures present a high potential for hydrogen storage due to the high hydrogen-to-metal ratio (up to H/M = 2) and vastness of compositions. Although many studies reported rapid absorption kinetics, the investigation of hydrogen desorption is missing, especially in BCC HEAs. We have investigated the crystal structure, microstructure and hydrogen storage performance of a series of HEAs in the Ti–V–Nb–Cr system. Three types of TiVCrNb HEAs (Ti4V3NbCr2, Ti3V3Nb2Cr2, Ti2V3Nb3Cr2) with close atomic radii and different valence electron concentrations (VECs) were designed with single BCC phase by CALPHAD method. The three alloys with fast hydrogen absorption kinetics reach the H/M ratio up to 2. Particularly, Ti4V3NbCr2 alloy shows the hydrogen storage capacity of 3.7 wt%, higher than other HEAs ever reported. The dehydrogenation activation energy of HEAs’ hydride has been proved to decrease with decreasing VEC, which may be due to the weakening of alloy atom and H atom. Moreover, Ti4V3NbCr2M (M = Mn, Fe, Ni) alloys were also synthesized to destabilize hydrides. The addition of Mn, Fe and Ni lead to precipitation of Laves phase, however, the kinetics did not improve further because of their own excellent hydrogen absorption. With increasing the content of Laves phase, there appear more pathways for hydrogen desorption so that the hydrides are more easily dissociated, which may provide new insights into how to achieve hydrogen desorption in BCC HEAs at room temperature.

Дослідження електричних властивостей та структури аморфних стопів системи Fe–B–P–Nb–Cr

Дослідження електричних властивостей та структури аморфних стопів системи Fe–B–P–Nb–Cr

Thermodynamic re-assessment of binary Cr-Nb system down to 0 K

Based on the recently proposed physically-based segmented model, the descriptions for Gibbs energy of pure Cr and Nb down to 0 K were first established. After that, a thermodynamic re-assessment of binary Cr-Nb system down to 0 K was then performed by taking into account all the experimental phase equilibria and thermodynamic properties. Especially, the experimental heat capacities of Cr2Nb at low temperatures ignored in previous assessments were utilized in the present assessment. The calculated phase equilibria and thermodynamic properties according to the presently obtained thermodynamic descriptions of binary Cr-Nb system agree well with most of the experimental data, and show better agreement than the previous assessments.

A thermodynamic evaluation of the C–Cr–Nb system

Based on critical evaluation of the literature data, the C–Cr–Nb system has been reviewed and assessed by means of the CALPHAD technique. A reassessment of the Cr–Nb system was conducted in order to better reproduce the invariant reactions in the C–Cr–Nb system. A set of self-consistent thermodynamic parameters for the Gibbs energies of individual phases in the C–Cr–Nb system was then obtained by taking into account the literature data including isothermal sections, liquidus, solidus and invariant reactions. Comprehensive comparisons between the calculated and measured phase diagrams show that all the reliable experimental information is satisfactorily accounted for by the present thermodynamic description. The liquidus projection and reaction scheme over the whole composition and temperature ranges are presented.

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

Abstract In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti3Al base material. And the Ti3Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti3Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni3(Nb, Ti) + (Nb, Ti)Cr2 and TiNi3 phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.

Experimental investigation of phase equilibria in the Zr–Nb–Cr system at 1573 K and 1373 K

Phase equilibria in the Zr–Nb–Cr system at 1573 K and 1373 K were characterized by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) coupled with wavelength-dispersive spectroscopy (WDS). Three single-phase regions and two two-phase regions were identified in the isothermal sections of 1573 K and 1373 K. The single-phase regions are BCC (Cr), C15 and BCC (Zr, Nb), and the two-phase regions are BCC (Cr)+C15 and BCC (Zr, Nb)+C15. With the temperature decreasing from 1573 K to 1373 K, the BCC (Zr, Nb) phase region narrows slightly, while the C15 phase region shrinks noticeably in the Nb-rich direction. The problems concerning sample equilibration and weight loss in the experiments were discussed, and the C15 single-phase region was explored based on the site occupancy modelled by the compound energy formalism (CEF) coupled with first-principles calculations.

Thermodynamic modeling of Cr–Nb and Zr–Cr with extension to the ternary Zr–Nb–Cr system

The total energies of Laves phases in the Cr–Nb and Zr–Cr systems have been calculated by the pseudo-potential VASP code with a full relaxation of all structural parameters. The special quasirandom structures (SQSs) have been constructed and their total energies have been calculated by the VASP code to predict the enthalpies of mixing for bcc and hcp solid solution phases. The phonon calculations for the C14 and C15 Laves phases have been performed to analyze the phase stability at elevated temperatures. The experimental study on the Zr–Cr system has been carried out at different temperatures to determine the phase boundaries. Based on these results, thermodynamic models of Cr–Nb and Zr–Cr with extension to the ternary Zr–Nb–Cr systems have been developed in this work by using the CALPHAD approach.

A New Theoretical Study of the Cr-Nb System

A new thermodynamic assessment of the binary Cr-Nb system was performed using the CALPHAD method based on a review of the available literature and ab initio calculations for the hypothetical end members of the C15 Laves phase. Recent experiments reported in the literature revealed that there is no C14 type high temperature phase in this system, which is in contrast to most of the literature. A thorough analysis of these reports, however, showed that the C14 phase has never been directly observed and that its presence had been derived from ternary phase diagram data, twinning of the C15 low temperature phase and erroneous interpretations of x-ray diffractograms. Due to the lack of clear evidence in favor of its existence, the C14 phase was not considered a thermodynamically stable compound in the present evaluation.

Reply to comments on the absence of a stable hexagonal Laves phase modification (NbCr 2) in the Nb–Cr system

Responding to the preceding comment by Nie, we show that our recently presented experimental evidence for the non-existence of a stable C14–NbCr 2 Laves phase [1] does not contradict previous data from literature, including those brought forward by Nie. Instead, these older data should be reinterpreted.

Comments on “The absence of a stable hexagonal Laves phase modification (NbCr 2) in the Nb–Cr system”

Comments on “The absence of a stable hexagonal Laves phase modification (NbCr 2) in the Nb–Cr system”

WITHDRAWN: Comments on “The absence of a stable hexagonal Laves phase modification (NbCr 2) in the Nb–Cr system”

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.scriptamat.2011.01.032 . The duplicate article has therefore been withdrawn.

The absence of a stable hexagonal Laves phase modification (NbCr 2) in the Nb–Cr system

High-temperature differential thermal analysis and neutron diffraction have been employed to investigate the previously proposed polytypic phase transformation C 14 – NbCr 2 ⇄ C 15 – NbCr 2 at around 1600 °C. It could be shown that the phase transformation does not occur for high-purity alloys and high-purity conditions. However, small amounts of atmospheric contaminations induce the formation of an η-carbide-type phase, which melts (partially) in a temperature range around 1600 °C. This led to the earlier erroneous conclusion of the occurrence of a thermodynamically stable high-temperature modification of NbCr 2 .

Thermodynamic Analysis of Nb-Cr Powders of Mechanical Alloying

In this study, Nb-Cr binary mixtures were prepared by mechanical alloying. And mechanical alloying process of mixed Nb-Cr(1:2) powders was studied, and the phases of milling powders analyzed by XRD and SEM. It has been found that the powders form solid solution first, then amorphous phase appeared gradually and crystallization of amorphous takes place with the different milling time. A thermodynamic model of MA of Nb-Cr system was developed based on Miedema Semi-experimental theory to explain the phase transformations of Nb-Cr system during milling. Using Miedema semi-empirical model the free energies of the ordered Laves-phase NbCr2 intermetallics, solid solution and amorphous alloy were calculated. From XRD and SEM analysis of Nb/2Cr powders prepared by mechanical alloying in a planetary ball milling, it was found that the calculated results were in accordance with the experiments.

Thermodynamic description of the Cr–Nb–Si isothermal section at 1473 K

In this paper, an assessment of the ternary Cr–Nb–Si system is presented. The thermodynamic evaluation is based on the description proposed by Costa-Neto et al. for the Cr–Nb system and by Du and Schuster for the Cr–Si system. A minor revision of the description of the Nb–Si system due to Yang et al., by considering the solubility range of the two compounds αNb 5Si 3 and NbSi 2, is introduced in order to obtain a better agreement with the experimental data of diffusion. A thermodynamic modeling of the isothermal ternary section at 1473 K based on data available in the literature (essentially diagrammatic data) is proposed. The optimized set of coefficients and the calculated isothermal section are provided.

Polytypic transformations in Laves phases

The Cr 2X (X=Ti, Zr, Hf, Nb or Ta) Laves phases have the hexagonal (2H or 4H) structure at high temperatures and the cubic (3C) structure at room temperature. Their compositional ranges are dictated by their e/ a ratios. Experimental observations of two-phase alloys in several of these systems demonstrate that the hexagonal structure is often retained at room temperature. Subsequent annealing at elevated temperature transforms the metastable hexagonal structure to the stable cubic structure. In single-phase Cr 2Ti and Cr 2Ta, the transformation is extremely sluggish whereas in Cr 2Nb, it is rapid. In two-phase alloys, the transformation is almost always sluggish (except in the Cr–Nb system) and is accompanied by compositional changes. Transmission electron microscopic observations are consistent with the transformations being achieved by synchroshear behind glissile synchroshockley dislocations. The synchroshear mechanism is described and the transformation kinetics are discussed in terms of long-range diffusion in two-phase alloys and in terms of the mobility of synchroshockley dislocations in single-phase alloys.

Oxidation resistance and mechanical properties of Laves phase reinforced Cr in-situ composites

Two-phase Cr(X)–Cr2X (X=Nb, Ta) in-situ composites are of interest for high-temperature applications due to their high melting points and potential for high-temperature strength. A six cycle, 120 h, 1100°C cyclic oxidation screening test was used to evaluate potential for high-temperature oxidation resistance of several Cr(X)–Cr2X in-situ composites. Alloys based on the Cr–Ta system near the Cr(Ta)–Cr2Ta eutectic exhibited superior oxidation resistance compared to corresponding alloys based on the Cr–Nb system. The binary Cr–Ta alloys were also found to exhibit a moderate degree of room-temperature fracture toughness, in the range of 9–10 MPa√m. It was concluded that the Cr(Ta)–Cr2Ta alloys are a promising base for future high-temperature intermetallic alloy development efforts.

Correlation of alloy microstructure with oxidation behavior in chromia-forming intermetallic-reinforced Cr alloys

The oxidation behavior of the Cr(X) solid solution and intermetallic Cr2X Laves phases (X = Nb, Ta) was studied individually and in combination at 1,100°C in humid air. The two-phase Cr(Ta) + Cr2Ta alloys exhibited superior oxidation resistance to either the single-phase Cr(Ta) or the single-phase Cr2Ta alloys. However, the two-phase Cr(Nb) + Cr2Nb alloys exhibited lower oxidation resistance than either the single-phase Cr(Nb) or Cr2Nb alloys. The two-phase Cr(Ta) + Cr2Ta alloys were also found to exhibit superior oxidation resistance to the two-phase Cr(Nb) + Cr2Nb alloys. This was hypothesized to be related to the lower amount of the Cr2X phase in the Cr(Ta)–Cr2Ta eutectic and to the lower solubility of Ta in the Cr(X) phase, relative to the Cr–Nb system. Both of these factors resulted in significantly less oxidation and nitridation in the Cr(Ta) + Cr2Ta alloys.