Ordered B2 Phase Research Articles

Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy

Typically, refractory high-entropy alloys (RHEAs), comprising a two-phase ordered B2 + BCC microstructure, exhibit extraordinarily high yield strengths, but poor ductility at room temperature, limiting their engineering application. The poor ductility is attributed to the continuous matrix being the ordered B2 phase in these alloys. This paper presents a novel approach to microstructural engineering of RHEAs to form an “inverted” BCC + B2 microstructure with discrete B2 precipitates dispersed within a continuous BCC matrix, resulting in improved room temperature compressive ductility, while maintaining high yield strength at both room and elevated temperature.

First Principle Study of Ti<sub>50</sub>Al<sub>50</sub> Alloys

The study on the Ti-based materials and its application has been the interest of many research industries. These alloys are known to have an ordered B2 phase at high temperatures and transform to a stable low B19 martensitic phase. First principle approach has been used to study L10, B32, B2 and B19 Ti50Al50 alloys and the results compared well with the available experimental data. The equilibrium lattice constants are in good agreement with the experimental values (within 3% agreement). Furthermore, the elastic constants of these alloys are calculated, and revealed stability for L10 and B19 structures, while B2 and B32 gave C′<0 (condition of instability).

Al-Ti-Containing Lightweight High-Entropy Alloys for Intermediate Temperature Applications.

In this study, new high-entropy alloys (HEAs), which contain lightweight elements, namely Al and Ti, have been designed for intermediate temperature applications. Cr, Mo, and V were selected as the elements for the Al-Ti-containing HEAs by elemental screening using their binary phase diagrams. AlCrMoTi and AlCrMoTiV HEAs are confirmed as solid solutions with minor ordered B2 phases and have superb specific hardness when compared to that of commercial alloys. The present work demonstrates the desirable possibility for substitution of traditional materials that are applied at intermediate temperature to Al-Ti-containing lightweight HEAs.

Antiphase domains or dispersed clusters? Neutron diffraction study of coherent atomic ordering in Fe3Al-type alloys

A combination of high-resolution and in situ real-time neutron diffraction was used to collect the experimental data concerning coherent atomic ordering in Fe3Al-type alloys in a wide temperature range. The analysis of the obtained data was carried out within the framework of two models: antiphase domains and dispersed clusters embedded in matrix. The second model, for which the atomic ordering is organized in the form of clusters of mesoscopic sizes (from ∼200 to ∼400 Å) randomly distributed inside a less ordered matrix, provides a better fit of the data. For two chemically identical Fe3Al samples - the first was in the as-cast polycrystalline bulk state, and the second was grown as a single crystal - the initial state can be described as a partially ordered B2 structure (matrix) with dispersed clusters of the D03 ordered phase. The initial (quenched) state of the (Fe0.88Cr0.12)3Al polycrystalline sample is the disordered A2 phase with clusters of the partially ordered B2 phase. After heating and subsequent slow cooling, the structure of both binary and ternary alloys is B2 matrix with dispersed D03 clusters, whose dimensions are increased up to ∼900 Å.

The effect of Ti addition on phase selection of CoCrCu0.5FeNi high-entropy alloys

ABSTRACTWith the increasing titanium, the volume fraction of face-centred cubic-structured dendrites decreased, and ordered B2 phase, σ, and Laves phase appeared in CoCrCu0.5FeNiTix alloy when x > 0.5. With the increase of Ti content, ΔSmix, ΔHmix, valence electron concentration (VEC), and Λ (=ΔSmix/δ2) of CoCrCu0.5FeNiTix alloys decreased, while atomic size difference (δ) and electronegativity difference (Δχ) increased. Topologically close-packed (TCP) structures are favoured when ΔSmix > 14.53 J K−1 mol−1. TCP phases can be found when δ > 5.05 and Δχ ≥ 0.124. Simple solid solution phase is favoured when VEC ≥ 8.26 but multi-phases including TCP phase coexist when VEC < 8.26. Single-disordered solid solution forms only when Λ > 0.637, solid solution and TCP phases coexist when 0.531 < Λ < 0.637, and TCP phases are favoured when Λ < 0.531.

Cu–Fe–Co System: Verification of the High-Temperature Phase Equilibria and Thermodynamic Modeling of the Low-Temperature Phase Relations Involving Ordered Phase

The thermodynamic assessment of the Co–Cu–Fe system has been updated. The critical review of the literature data on the ordered B2 phase formation has been provided. For the Co–Fe and Co–Cu–Fe systems, the parameters of the bcc solution and ordered phase have been adjusted to describe the thermodynamic properties and to predict the phase equilibria at low temperatures. For the ordered B2 phase, the two-sublattice model has been adopted. The high-temperature phase equilibria have been verified using the newly available literature information on liquidus and solidus. The available data on the ordered B2 phase have been analyzed using the developed thermodynamic assessment. The results of the calculations are in satisfactory agreement with the experimental data.

Uniaxial deformation of face-centered-cubic(Ni)-ordered B2(NiAl) bicrystals: atomistic mechanisms near a Kurdjumov–Sachs interface

Creating tailored interfaces between soft and hard materials is a promising route to simultaneously enhance ductility and strength of multicomponent materials. Here, we study deformation mechanisms in a model bicrystal, with a Kurdjumov–Sachs (KS) interface, between face-centered-cubic Ni and ordered-B2 NiAl slabs using molecular dynamics simulations. The bicrystals were uniaxially deformed by strain rates of $$10^7$$ and $$10^9\,\hbox {s}^{-1}$$ by holding temperatures constant at 300, 500, 700, and 900 K for each strain rate. Our simulations reveal atomistic processes that create sessile and glissile dislocations, and their reactions during high-strain rate deformation. At $$10^9\,\hbox {s}^{-1}$$ strain rates, dislocation processes enhance ductility and cause large-scale atomic rearrangements in the KS interfacial region. This subsequently causes nucleation, growth, and coalescence of nano-voids into cracks inside the harder B2-ordered phase bordering the interface. Our results suggest that interfaces between “soft”–“hard” materials likely withstand high-strain rates better.

Microstructure and mechanical properties of heat treated Al1.25CoCrCuFeNi high entropy alloys

The influence of cooling rates and heat treatments on the microstructure and mechanical properties of Al1.25CoCrCuFeNi high entropy alloys (HEA) were investigated using a variety of experimental techniques including DTA, XRD, SEM, TEM, microhardness, and compression tests. It was found that Al1.25CoCrCuFeNi solidifies dendritically with the majority of the dendrite core (DC) regions composed of modulated spinodal microstructure consisting of disordered Cr+Fe rich BCC nanosize lamellae, and Al+Ni-rich ordered B2 nanosize lamellae, along with elongated Cu-rich FCC precipitates at the interfaces. The ordered B2 phase solidified first and underwent spinodal decomposition to create the modulated structure. The interdendritic regions (ID) consisted of a bulk FCC Cu-rich phase with nanosize precipitates. Heat treatments at 650 and 850°C caused the disordered BCC lamellae to transform to σ phase accompanied with soft FCC phases. Reheating to 1040°C caused the σ phase to transform back to the disordered BCC as well as nucleation of another FCC phase. Depending on the heat treatment temperature, the DC regions transformed from the fine modulated microstructure to grains at 1040°C and back to a coarse modulated microstructure after heat treatment at 1200°C. The Cu-rich phases in the ID regions remained unchanged during heating up to 1040°C however, there were several changes in the morphology of the nanosized precipitates in the Cu-rich phase. The interdendritic phase was found to dissolve at around 1200°C. The influence of these changes on the mechanical properties are discussed.

Tunable ferromagnetic resonance behavior in Co2FeSi film by post-annealing

Co2FeSi film is potential in the spintronics applications, due to its low damping factor, which is reflected in the ferromagnetic resonance behavior. In this work, we demonstrate that the ferromagnetic resonance behavior in Co2FeSi film can be well engineered by post-annealing. After 450 °C post-annealing for 1 hour, the Gilbert damping factor decreases drastically from 0.039 at as-deposited state to 0.006, and the inhomogeneity contribution of ferromagnetic resonance linewidth decreases to 60.5 Oe. These decreases are ascribed to the crystallization of film from amorphous state to an ordered B2 phase. Higher annealing temperature, however, leads to the formation of the A2 phase with higher atomic disorder, instead of B2 phase, and brings about the increase of Gilbert damping.

Strain-tuning of the magnetocaloric transition temperature in model FeRh films

The chemically ordered B2 phase of equiatomic FeRh is known to absorb or evolve a significant latent heat as it traverses its first-order phase transition in response to thermal, magnetic, and mechanical drivers. This attribute makes FeRh an ideal magnetocaloric material testbed for investigation of relationships between the crystalline lattice and the magnetic spins, which are especially experimentally accessible in thin films. In this work, epitaxial FeRh films of nominal 30 nm and 50 nm thicknesses with out-of-plane c-axis orientation were sputter-deposited at high temperature onto (0 0 1)-MgO or (0 0 0 1)-Al2O3 substrates and capped with Al, Au, Cr, or W after in situ annealing at 973 K to promote CsCl-type chemical order. In this manner a controlled strain state was invoked. Experimental results derived from laboratory and synchrotron x-ray diffraction combined with magnetometry indicate that the antiferromagnetic (AF)—ferromagnetic (FM) magnetostructural phase transformation in these films may be tuned over an ~50° range (373 K–425 K) through variation in the c/a ratio derived from lattice strain delivered by the substrate and the capping layers. These results supply fundamental information that might be used to engineer the magnetocaloric working material in new system designs by introducing targeted values of passive strain to the system.

Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification

The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with <001> oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys.

Structures and corrosion properties of the AlCrFeNiMo0.5Tix high entropy alloys

The AlCrFeNiMo0.5Tix (x = 0, 0.25, 0.4, 0.5) high entropy alloys were prepared by copper mold suction casting, and the structures and aqueous corrosion behavior of the alloys were investigated in the present study. It shows that all of these alloys possess BCC + B2 structures at various Ti contents, while Ti element is enriched in ordered B2 phase. With increasing Ti content, the passive film becomes more protective. The charge transfer resistance (Rct) value increases from 229 kΩ · cm2 for the AlCrFeNiMo0.5 alloy to 635 kΩ · cm2 for the AlCrFeNiMo0.5Ti0.5 alloy. Meanwhile, the pitting potential (Epit) increases from 90 mV for the AlCrFeNiMo0.5 alloy to 410 mV for the AlCrFeNiMo0.5Ti0.5 alloy. The present results indicate that the corrosion resistance of the AlCrFeNiMo0.5Tix high entropy alloys has been significantly enhanced by Ti additions.

Phase and thermal study of equiatomic AlCuCrFeMnW high entropy alloy processed via spark plasma sintering

The present study describes the synthesis and preliminary characterization of a novel nanocrystalline equiatomic AlCuCrFeMnW high entropy alloy (HEA) via mechanical alloying (MA) followed by spark plasma sintering. A structural property of present HEA was investigated using X-Ray diffractometry (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analysis. XRD of this sintered alloy revealed the formation of ordered B2 phase (AlFe type), sigma phase (Cr rich), FeMn type phase and BCC phase. The particle morphology and composition of present HEA was investigated by scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS). Differential Scanning Calorimetry (DSC) of this alloy confirmed that there is phase transformation occurs at 918.17 °C, 925.23 °C and 936.11 °C with three different heating rates of 10 K/min, 20 K/min and 30 K/min respectively and activation energy corresponding to this transformation is 160.2 KJmol-1. The microhardness of AlCuCrFeMnW HEA is 891 HV. The phase evolution in this alloy has been considered using thermodynamic parameters, and the structure-property relationship has also been proposed by conventional strengthening mechanisms.

Orientation Relationships in Al0.7CoCrFeNi High-Entropy Alloy.

A detailed microstructural evaluation was executed on the crystallographic texture as well as the mechanisms for nucleation, phase transformation, and grain growth in a Al0.7CoCrFeNi high-entropy alloy. The microstructure and crystallographic orientations were characterized by electron backscatter diffraction, and the chemical composition variations by energy-dispersive X-ray spectroscopy. The cast Al0.7CoCrFeNi alloy started in the BCC phase and partially transformed into the FCC phase. It was found that the Pitsch orientation relationship (OR) dominates the nucleation mechanism of the FCC phase; however, deviations with respect to the Pitsch OR are observed and are attributed to the differently sized atoms forming an ordered B2 phase in the alloy causing lattice distortions. The dual phase BCC-FCC microstructure contains FCC Widmanstätten plates oriented parallel to the {110}BCC planes of the parent grain. It was found that the crystal orientation distribution after the BCC-FCC phase transformation is confined and is explained as a product of the governing mechanisms.

Effect of B2-ordered phase on the deformation behavior of Ti-Mo-Al alloys at elevated temperature

The microstructures and mechanical properties of as-cast Ti-Mo-Al alloys with various compositions were systematically investigated in this study, aiming to explore the potentials of B2-ordered phase (β2) in high temperature applications. According to the experimental results, these as-cast Ti-Mo-Al alloys were mainly composed of β2 phase and contained a large number of dislocation walls and sub-grain boundaries. Overall they exhibited a high level of compressive strength at 1073 K, with the maximum value approaching ∼879 MPa. The impressive mechanical properties of β2-dominating Ti-Mo-Al alloys are related to the excellent stability of B2 structure and extensive dislocation entanglements at elevated temperature.

Phase stability and microstructures of high entropy alloys ion irradiated to high doses

The microstructures of AlxCoCrFeNi (x = 0.1, 0.75 and 1.5 in molar ratio) high entropy alloys (HEAs) irradiated at room temperature with 3 MeV Au ions at the highest fluence of 105, 91, and 81 displacement per atom, respectively, were studied. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses show that the initial microstructures and phase composition of all three alloys are retained after ion irradiation and no phase decomposition is observed. Furthermore, it is demonstrated that the disordered face-centered cubic (FCC) and disordered body-centered cubic (BCC) phases show much less defect cluster formation and structural damage than the NiAl-type ordered B2 phase. This effect is explained by higher entropy of mixing, higher defect formation/migration energies, substantially lower thermal conductivity, and higher atomic level stress in the disordered phases.

A combinatorial assessment of AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys: Microstructure, microhardness, and magnetic properties

This article discusses a novel combinatorial approach for assessing composition-microstructure-microhardness-magnetic property relationships, using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys as a candidate system. The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ∼25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. With increasing Al content, there was a gradual change from a fcc-based microstructure (including the ordered L12 phase) to a bcc-based microstructure (including the ordered B2 phase), accompanied with an increase in microhardness. Interestingly, with increasing paramagnetic Al content, saturation magnetization as well as coercivity increases and reaches a maximum value when x = 1.3, indicating the tunability of magnetic properties by a paramagnetic element in this alloy system. Such graded alloys are highly attractive candidates for investigating the influence of systematic compositional changes on microstructural evolution and concurrent physical and mechanical properties in complex concentrated alloys or high entropy alloys.

Size-Dependent Disorder-Order Transformation in the Synthesis of Monodisperse Intermetallic PdCu Nanocatalysts.

The high performance of Pd-based intermetallic nanocatalysts has the potential to replace Pt-containing catalysts for fuel-cell reactions. Conventionally, intermetallic particles are obtained through the annealing of nanoparticles of a random alloy distribution. However, this method inevitably leads to sintering of the nanoparticles and generates polydisperse samples. Here, monodisperse PdCu nanoparticles with the ordered B2 phase were synthesized by seed-mediated co-reduction using PdCu nanoparticle seeds with a random alloy distribution (A1 phase). A time-evolution study suggests that the particles must overcome a size-dependent activation barrier for the ordering process to occur. Characterization of the as-prepared PdCu B2 nanoparticles by electron microscopy techniques revealed surface segregation of Pd as a thin shell over the PdCu core. The ordered nanoparticles exhibit superior activity and durability for the oxygen reduction reaction in comparison with PdCu A1 nanoparticles. This seed-mediated co-reduction strategy produced monodisperse nanoparticles ideally suited for structure-activity studies. Moreover, the study of their growth mechanism provides insights into the size dependence of disorder-order transformations of bimetallic alloys at the nanoscale, which should enable the design of synthetic strategies toward other intermetallic systems.

Characterization of the microstructure of the compositionally complex alloy Al1Mo0.5Nb1Ta0.5Ti1Zr1

Electron microscopy, X-ray energy dispersive spectroscopy, and tomographic reconstructions were used to characterize a high entropy alloy predominantly composed of refractory elements. The intragranular microstructure was found to consist of a periodic two phase mixture, where a disordered bcc phase is aligned orthogonally in an ordered B2 phase. The phases were found to exhibit continuous lattice registry and an orientation relationship given by <100>bcc//<100>B2, {001}bcc//{001}B2. X-ray energy dispersive spectroscopy was used to determine the compositions of the phases, and spectral images were used in the tomographic reconstruction of the alloy to reveal the morphology and the elemental partitioning between phases.

Experimental investigation and thermodynamic re-assessment of the Al–Mo–Ni system

NiAl-based alloys have been investigated, because they are promising alternative materials for high temperature structural applications. Recent advancements in directional solidification offer the opportunity to manufacture metal-matrix composites of NiAl strengthened by embedded fibers of refractory metals such as Mo. The mechanical properties of these composites can be considerably improved, compared to NiAl, at least in fiber direction. The Al–Mo–Ni system has been thermodynamically assessed by several authors. However, none of them can reproduce a satisfactory description along the section NiAl–Mo. In the present work, liquidus and solidus temperatures of the NiAl–Mo system were measured by a laser heating-fast pyrometry apparatus. The results agree well with literature data. The thermodynamic descriptions of the Al–Ni and Al–Mo systems from the literature were refined and a new thermodynamic dataset of the Al–Mo–Ni system was established. The ordered B2 phase and its disordered A2 (bcc) parent phase were described by a single Gibbs energy function. Very good agreement between the calculated phase diagram and the experimental data is obtained. The description of the section NiAl–Mo is considerably improved and we conclude that this section does not represent a quasi-binary phase diagram.