Stability Of Bcc Phase Research Articles

Oxidation behavior of lightweight Al2NbTi3V2Zr high entropy alloy at 1000°C

AbstractLightweight high entropy alloy (LWHEA) has great potential in aeroengine hot component applications. Anticorrosion property of LWHEA at high temperatures is one of the key determining factors. In this study, an LWHEA containing Al, Nb, Ti, V, and Zr elements with a molar ratio of 2: 1: 3: 2: 1 was designed and prepared by low energy (LE) and high energy (HE) mechanical milling followed by vacuum hot pressing. The sintered LE alloy consisted of a simple cubic matrix and α intermetallic while the BCC matrix along with α and β intermetallic phases were observed in HE alloy. The oxidation of LE and HE at 1000°C for 48 h was mainly concentrated on α and β phases, and the resultant oxides were ZrO2 and TiO2. Weight gain curve of HE sample within oxidation time of 48 h followed the parabolic law while a two‐stage parabola was found in LE bulk. The steady‐state oxidation of α precipitate was dominant within 12 h and showed a preferential oxidation mode. High temperature induced simple cubic matrix to stable BCC phase transformation and thus LE alloy with longer oxidation time and HE exhibited a similar uniform oxidation. The oxidation rate of the alloy is related to the content of α and β phases.

Computational discovery of ultra-strong, stable, and lightweight refractory multi-principal element alloys. Part II: comprehensive ternary design and validation

Here the discovery of refractory multi-principal element alloys (MPEAs) with high-temperature strength and stability is pursued within a constrained and application-relevant design space. A comprehensive approach is developed and applied to explore all 165 ternary systems in the Al-Ce-Fe-Hf-Mo-Nb-Ta-Ti-V-W-Zr family. A subset of ternary systems that contain large areas in composition–temperature space with high strength and robust BCC phase stability is found. Twelve sets of high-performing alloys are identified, each set optimized for one combination of phase constraint, optimization target, and temperature range. Preliminary mechanical tests support the viability of the method. This work highlights the importance of considering phase stability, exploring non-equiatomic regions of composition space, and applying application-relevant constraints. Parts I and II provide three down-selection techniques for identifying high-performing BCC refractory MPEAs, design guidelines, and many candidates predicted to have BCC phase stability and strengths 2–3 times higher than any reported to date.

Solute trapping and solute drag during non-equilibrium solidification of Fe–Cr alloys

During additive manufacturing (AM), velocities of solid–liquid interfaces are on the order of 0.01−1m⋅s−1. Thus, local interfacial equilibrium may not be present. Molecular dynamics (MD) and Monte Carlo methods are employed to extract thermodynamic properties for a Fe–Cr binary alloy and the parameters that characterize the departure from interfacial equilibrium for velocities present during AM. The Gibbs free energies of the BCC, FCC, and liquid phases are determined and used in a partial-solute-drag model that self-consistently describes non-equilibrium solute trapping and solute-drag behavior. The partial-solute-drag model describes well the interfacial velocities as a function of supersaturation in the liquid and the velocity dependence of the partition coefficient. The maximum possible velocities, v0 of the solid/liquid interfaces are determined, with v0 of the stable BCC phase being higher than the metastable FCC phase, but both values are significantly lower than the reported speed of sound. Additionally, the partial-solute-drag model yields a trans-interface diffusivity much different from that of more classical models and very close to that measured in the MD simulations.

First-principles study of highly-compressed Sb: a stubborn body-centered cubic structure

We searched for plausible crystal structures and the entailing electronic profiles of highly-compressed Sb using an originally developed structure-search method and first-principles calculations based on the density functional theory. We report that the experimentally observed highest-pressure (59 GPa) close-packed body-centered cubic (bcc) structure remains the lowest-enthalpy structure at least up to 1000 GPa within the precision of our calculations. Any possibilities of complex structures with many atoms and distortions within the bcc phase, as in the case of the cI16 structure observed for high pressure P, were also safely ruled out. Careful investigations of the density of states (DOS) and phonon dispersions revealed that the bcc structure becomes more stable with increasing pressure. The DOS and phonon dispersions indicate that the stability of the bcc phase increases with increasing pressure. In understanding the strong stability of this stubborn bcc phase, we discuss the phonon and electronic profiles of Sb.

In Situ Constructing the Kinetic Roadmap of Octahedral Nanocrystal Assembly Toward Controlled Superlattice Fabrication.

Crystallization and growth of anisotropic nanocrystals (NCs) into distinct superlattices were studied in real time, yielding kinetic details and designer parameters for scale-up fabrication of functional materials. Using octahedral PbS NC blocks, we discovered that NC assembly involves a primary lamellar ordering of NC-detached Pb(OA)2 molecules on the front-spreading solvent surfaces. Upon a spontaneous increase of NC concentration during solvent processing, PbS NCs preferentially self-assembled into an orientation-disordered face-centered cubic (fcc) superlattice, which subsequently transformed into a body-centered cubic (bcc) superlattice with single NC-orientational ordering across individual domains. Unlike the deformation-based transformation route claimed previously, this solid-solid phase transformation involved a hidden intermediate formation of a lamellar-confined liquid interface at cost of the disassembly (melting) of small fcc grains. Such highly condensed and liquidized NCs recrystallized into the stable bcc phase with an energy reduction of 1.16 kBT. This energy-favorable and high NC-fraction-driven bcc phase grew as a 2D film at a propagation rate of 0.74 μm/min, smaller than the 1.23 μm/min observed in the early nucleated fcc phase under a dilute NC environment. Taking such insights and defined parameters, we designed experiments to manipulate the NC assembly pathway and achieved scalable fabrication of a large/single bcc supercrystal with coherent ordering of NC translation and atomic plane orientation. This study not only provides a design avenue for controllable fabrication of a large supercrystal with desired superlattices for application but also sheds new light on the nature of crystal nucleation/growth and phase transformation by extending the lengths from the nanoscale into the atomic scale, molecular scale, and microscale levels.

Elastic-modulus enhancement during room-temperature aging and its suppression in metastable Ti–Nb-Based alloys with low body-centered cubic phase stability

Changes in the elastic properties during room-temperature aging (RT aging) of metastable Ti–Nb-based alloy single crystals with low body-centered cubic (bcc)-phase stability were investigated. The elastic stiffness components of Ti–Nb–Ta–Zr alloys with different Nb concentrations were measured by resonant ultrasound spectroscopy during RT aging; the results revealed that shear moduli c′ and c44 were increased by RT aging. In the alloy with the lowest Nb concentration, i.e., with the lowest bcc phase stability, shear moduli c′ and c44 were enhanced by the largest amount. The increase rates were ∼5% for 1.1 × 107 s (127 days), whereas the bulk modulus was hardly changed by aging. In Ti–Nb–Ta–Zr–O alloys with different oxygen concentrations, shear moduli c′ and c44 of the alloy with the lowest oxygen concentration increased most significantly. Moreover, the electrical resistivity of Ti–Nb–Ta–Zr and Ti–Nb–Ta–Zr–O alloys was increased by RT aging. Importantly, the enhancements of shear moduli and electrical resistivity were suppressed by increases in the bcc-phase stability (i.e., increase in the Nb concentration) and oxygen concentration; these factors are known to suppress ω (hexagonal) phase formation. However, transmission electron microscopy (TEM) observations revealed that only a diffuse ω structure—an ω-like lattice distortion—was formed after RT aging. On the basis of alloying element effects, TEM observations, and analysis of the changes in elastic properties by using a micromechanics model, it was deduced that the enhancements of shear moduli and electrical resistivity were possibly caused by the formation of a diffuse ω structure.

Metastable bcc phase formation in 3d ferromagnetic transition metal thin films sputter-deposited on GaAs(100) substrates

Co100−xFex and Ni100−yFey (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc{110} slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co100−xFex films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni100−yFey films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co100−xFex single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co100−xFex film with higher Fe content shows a higher saturation magnetization and a lower coercivity.

Critical evaluation and thermodynamic optimization of Fe–Cu, Cu–C, Fe–C binary systems and Fe–Cu–C ternary system

Development of an efficient process for recycling ferrous scrap containing Cu requires reliable thermodynamic knowledge of Fe–Cu based alloy system. It is shown that there still remain discrepancies in existing databases from known experimental data. In order to provide an accurate prediction tool for the process development, a CALPHAD type thermodynamic modeling for the Fe–Cu–C system is presented with re-optimization of its binary sub-systems, Fe–Cu, Cu–C, and Fe–C. Liquid phase was modeled using the Modified Quasichemical Model in the pair approximation which generally gives better results in systems exhibiting positive deviation from ideality (such as in Fe–Cu). Solid solutions such as fcc and bcc were described using Compound Energy Formalism. A supplement experimental work was carried out in order to provide more accurate solid/liquid equilibria in Fe–Cu binary system. The obtained model parameters along with the model equations were shown to reproduce significantly better correspondence to the experimental data, such as the phase equilibria, activity of component in the Fe–Cu–C system, liquidus projections etc., within experimental uncertainty. High temperature stabilization of bcc phase in Fe–C binary system in previous thermodynamic modeling was revisited, and was resolved in the present study.

Nucleation, kinetics and morphology of displacive phase transformations in iron

An extensive, systematic molecular dynamics (MD) study is performed for analysing the nucleation, kinetics and morphology characteristics of thermally-induced, displacive phase transformations from face-centred cubic (fcc) to body-centred cubic (bcc) iron. At the atomic level these transformation characteristics are influenced by a number of factors, including (i) the appearance of free surfaces, (ii) the initial presence of fcc-bcc grain boundaries, (iii) the existence of point defects (i.e., atomic vacancies) near a grain boundary, (iv) the initial thermal velocities of the atoms, and (v) the specific interatomic potential used. Other MD studies that capture the overall transformation behaviour of iron well have often underestimated or ignored the influence by these factors on the transformation response, with the risk of putting the accuracy, generality and physical explanation of the MD results on loose grounds. The present research illustrates the relative contribution of each of the above factors by means of a detailed comparison study for three different interatomic potentials. The accuracy of the interatomic potentials is established by validating for the fcc and bcc phases the calculated elastic moduli, cohesive energy, vacancy formation energy and interfacial energy against experimental and ab initio data reported in the literature. The importance of calibrating material data of both the stable bcc phase and the metastable fcc phase – instead of the stable bcc phase only – is demonstrated. The numerical results call for general caution when interpreting phenomena that start close to instability points and therefore are sensitive to small disturbances; a large spread in the overall transformation time is found under different initial thermal velocities, interfacial lattice incoherence, boundary conditions (free vs. periodic), and interatomic potentials, where for completely transformed atomic systems the discrepancy between the maximum and minimum transformation time appears to be more than a factor of 150. The transformation time is phenomenologically related to the overall activation energy and the cohesive energy difference of the fcc and bcc phases, which, beyond a certain combination of values, may even prevent the transformation process from occurring. Also, the morphology of the bcc product phase is remarkably sensitive to the type of boundary conditions and the choice of interatomic potential, while the influence by both the set of initial thermal velocities and the interfacial lattice incoherence only becomes apparent for specific atomic samples that transform relatively slowly. The presence of fcc-bcc grain boundaries increases the spatial heterogeneity of transformation events, with the appearance of an increasing number of vacancies at the grain boundary giving rise to a larger overall transformation time. The 10 main results following from the present MD study are conveniently summarised at the end of this communication.

Shear Strength Measurement of Gum Metal during High-Pressure Torsion

In the present study, in situ measurements of applied torque and compressive load were conducted during high-pressure torsion (HPT) on Ti-23%Nb-0.7%Ta-2.0%Zr-1.2%O (in at %) , Gum Metal, by using four active strain-gage method. The shear stress was then calculated from the measured torque. The in situ measurements revealed that the maximum shear stress reaches ~2 GPa during HPT. This value is comparable to the ideal shear strength of Gum Metal, which was reported as ~1.8 GPa from experiments using single crystals. The deformation mechanism strongly depends on body-centered cubic (bcc) phase stability at an early stage of HPT straining, where the shear stress is well below the ideal shear strength. On the other hand, the deformation mechanism may be insensitive to the bcc phase stability at a later stage of HPT straining, where plastic deformation occurs at a strength close to the ideal shear strength.

Annealing twins in a multifunctional beta Ti-Nb-Ta-Zr-O alloy

The grain boundary character distribution and annealing twins in a multifunctional β-type Ti-23Nb-0·7Ta-2Zr-O alloy having a stable bcc phase structure are investigated by electron backscattering diffraction (EBSD). The results show that the coincident site lattice (CSL) boundaries, including Σ3 and Σ11 twin boundaries mainly form at the early stage of recrystallization. {112}〈111〉 and {332} 〈113〉 annealing twins are observed to occur in the completely recrystallized Ti-23Nb-0·7Ta-2Zr-O alloy.

Alloy Design of Nb-Si Based High Temperature Alloys by Phase Stability Control

In order to spheroidize -Nb5Si3 strengthening phase embedded in Nb matrix for attaining a good room temperature toughness of Nb-Si alloy, the authors have proposed a microstructure control technique by combining eutectic and eutectoid reactions. Nb3Si intermetallic compound formed during solidification is a key phase for the microstructure control, but its stability is very sensitive to the alloying elements. Nb3Si disappears by adding as small as 3 at% of W and Mo, while these elements are very effective for the solid solution strengthening of Nb phase. For a further alloy development, establishment of an alloy design concept based on the control of phase stability of Nb3Si is needed. Similarly to ferrous alloys such as stainless steels where Cr and Ni are added to control the stability of bcc phase and fcc phase, two alloying elements (one is a stabilizing element and the other is a destabilizing element for Nb3Si phase) are added to a Nb-Si binary master alloy and their microstructure is investigated using SEM. The stabilizing element Ta is found to enlarge the composition area where Nb3Si exists even with the destabilizing element Mo, and it is confirmed that the phase stability concept is useful for designing Nb-Si based alloys.

Processing and characterization of rapidly quenched Ti-based alloys: Influence of solidification rate on the as-quenched structures

Abstracts From technological viewpoint, icosahedral Ti–Zr–Ni phase may give challenging opportunities as novel hydrogen storage and hydride battery. Nevertheless, from the structural standpoint, the phase selection criterion as function of solidification route remains unclear. A series of Ti 45 Zr 38 Ni 17 alloys, known to provide stable and well-ordered quasicrystals, was produced by planar flow casting using different processing parameters. From structural characterization, it appears that, at high cooling rate, i.e. high wheel speeds, or for the highest temperature of the melt, the as-quenched sample exhibits a mixture of nanoscale β and amorphous phases. The icosahedral phase precipitates when decreasing the wheel speed, from lower temperatures of the melt or after annealing a crystallized amorphous matrix. The reproducible formation of quasicrystals, observed in the amorphous matrix containing β particles, corroborates the competition between icosahedral and body-centered cubic (bcc) phases during the solidification process. These results are promising since it could be possible to control volume fractions of these two phases by choosing well-fitted thermal treatments so that quasicrystals mechanical properties should be improved by dispersion of a stable bcc phase.

Polymorph Selection during the Crystallization of Softly Repulsive Spheres: The Inverse Power Law Potential

Using hybrid Monte Carlo molecular simulations, we study crystallization from the melt of softly repulsive spheres interacting through an inverse power law potential. We work at fixed supercooling (i.e., at a temperature 25% below the melting temperature) and consider three systems, defined by different values for the inverse power exponent n: n = 5, n = 6.67, and n = 10. Modifying the value of n allows us to study the onset of crystallization in the domain of stability of the body-centered cubic (bcc) phase (n = 5 and n = 6.67) and in the domain of stability of the face-centered cubic (fcc) phase (n = 10). We show that, for the three systems, polymorph selection does not take place during crystal nucleation since the structure of the critical nuclei obtained for the three systems is not well defined. However, our results demonstrate that polymorph selection takes place during the growth step since growth proceeds either into the stable bcc phase for the two smaller values of n (n = 5 and n = 6.67) or into the stable fcc phase for the larger value of n (n = 10). We also show that we did not achieve complete control of polymorphism for n = 10. The growth step gives rise to either slowly growing crystallites composed of two blocks of different structures (the stable fcc form and the metastable bcc form) or rapidly growing crystallites of the metastable bcc form.

EBSPによるゴムメタルの変形組織解析

EBSP (electron backscattering pattern) analysis was performed on microstructure of three β titanium alloy specimens after cold working to study peculiar plastic behavior of the multifunctional alloy, Gum Metal. The specimens employed were Ti-36%Nb-2%Ta-3%Zr-0.3%O (mass%) alloy (Gum Metal) and two reference alloys having lower and higher bcc phase stability than that of Gum Metal. Deformation twinning of {332} was observed in the specimen with lower bcc stability, and dislocation glide in the specimen with higher bcc stability. Orientation boundaries with rotation angle of 10-30 degrees were observed in Gum Metal specimen, and they are considered to be identical to the giant planar faults which were observed in TEM in the previous study. It seems that the giant faults act as grain boundaries and numerous subgrains are generated in the later stage of plastic deformation. Amount of crystallographic rotation in deformed Gum Metal specimens was very large, which implies that huge elastic energy was stored during plastic deformation.

Er2O3 for high-gain waveguide amplifiers

Following the demonstration of room-temperature luminescence, Er2O3 has been explored as a high-gain medium for ultra-compact waveguide amplifiers. With sputtered and annealed films, we measure three radiative lifetimes (7 ms, 0.8 ms, and 0.5 ms) and upconversion coefficients at 4.2 K. We have correlated these measurements with three crystalline phases: the thermodynamically stable bcc phase and the metastable fcc and hcp phases. The 7-ms lifetime is correlated with the fcc phase, implying the metastable crystal state has a profound influence on inhibiting upconversion interaction between neighbor Er atoms. Measurements indicate optical gain >3 dB/cm is possible.

Erratum: “A comparison of pseudomorphic bcc phase stability in Zr/Nb and Ti/Nb thin film multilayers” [J. Mater. Res. 19, 707 (2004)

This article was originally published in [J. Mater. Res. 19, 707 (2004)] and was incorrectly printed. The correct version appears below.

A Comparison of Pseudomorphic Bcc Phase Stability in Zr/Nb and Ti/Nb Thin Film Multilayers

A series of Nb-rich Zr/Nb and Ti/Nb multilayers were sputter deposited. Upon a reduction in thickness, a pseudomorphic bcc phase was stabilized in the Zr and Ti layers. X-ray and electron diffraction techniques were used to confirm these phase transformations. The change in phase stability was modeled by the competition between volumetric and interfacial components of the total free energy of a unit bilayer representing the multilayer. An outcome of this model is the ability to plot phase stability diagrams for multilayers, referred to as biphase diagrams, as a function of bilayer thickness and volume fraction. A comparison of the phase stability boundary between hcp/bcc and bcc/bcc for these two systems has shown that the bcc Ti’s pseudomorphic phase stabilization is maintained for a much larger layer thickness as compared to Zr. Atom probe compositional profiles of the Ti/Nb multilayers have indicated that the Nb layers interdiffused into the Ti layers thus helping to facilitate the bcc Ti phase stability in the Ti/Nb multilayers.

A Comparison of Pseudomorphic Bcc Phase Stability in Zr/Nb and Ti/Nb Thin Film Multilayers

A series of Nb-rich Zr/Nb and Ti/Nb multilayers were sputter deposited. Upon a reduction in thickness, a pseudomorphic bcc phase was stabilized in the Zr and Ti layers. X-ray and electron diffraction techniques were used to confirm these phase transformations. The change in phase stability was modeled by the competition between volumetric and interfacial components of the total free energy of a unit bilayer representing the multilayer. An outcome of this model is the ability to plot phase stability diagrams for multilayers, referred to as biphase diagrams, as a function of bilayer thickness and volume fraction. A comparison of the phase stability boundary between hcp/bcc and bcc/bcc for these two systems has shown that the bcc Ti’s pseudomorphic phase stabilization is maintained for a much larger layer thickness as compared to Zr. Atom probe compositional profiles of the Ti/Nb multilayers have indicated that the Nb layers interdiffused into the Ti layers thus helping to facilitate the bcc Ti phase stability in the Ti/Nb multilayers.

Phase diagram of an empirical potential: The case of Fe-Cu

Molecular dynamics simulations are used to calculate the Gibbs free energy in the entire compositional range of Fe-Cu alloys described with a set of embedded atom potentials available in the literature. Thermodynamic integration and switching Hamiltonian techniques are used to obtain the phase diagram at high temperatures (neglecting phonon quantum effects and electronic contributions) with no further approximations. Limitations of the model were confirmed, such as the absence of the \ensuremath{\gamma} and \ensuremath{\delta} phases, a bcc to fcc transformation before melting for pure Fe, the unexpected existence of a stable bcc phase in pure Cu at high $T,$ and consequently complete solid solubility of Fe in Cu in the bcc phase in some temperature range. This work seeds light on the power and limitations of the empirical description of complex systems.