Disordered A2 Phase Research Articles

Order-disorder phase transitions in Fe81Ga19-RE ALLOYS (RE = Dy, Er, Tb, Yb) according to neutron diffraction data

New data on the phase compositions and structural transformations in a number of Fe81Ga19 alloys doped with trace amount (≤ 0.2 at.%) of rare earth elements are presented. The data are obtained in neutron diffraction experiments performed with high resolution and in continuous temperature scanning mode when heated to 900 °C and subsequent cooling. It has been established that structural rearrangements proceed in a generally identical manner both in the original Fe81Ga19 alloy and in its doped analogues. Slow heating and subsequent cooling of the alloys (rate ±2 °C/min) leads to the formation of clusters of the D03 phase with sizes in the range (200–300) Å in the matrix of the disordered A2 phase. The sizes and volume fraction of clusters (~0.3 of the sample volume) weakly depend on the specific composition. The degree of ordering of the clusters atomic structure changes with temperature according to a second-order phase transition and is close to unity at room temperature. The search for structural ordering corresponding to the modified D03 phase, discovered in a number of electron diffraction studies, did not lead to a positive result.

Wetting of L10 twin and antiphase boundaries by nanometer-scale L12 in Fe-Pd alloys

This paper reports microstructure associated with the L10 and L12 two-phase coexistence region in magnetic Fe-Pd alloys and analyzes the observed complex nanometer scale wetting layer structures. Fe - 61.8 at% Pd samples were continuously cooled from the disordered A1 phase through the eutectoid isotherm and aged at 650 °C for various times. X-ray diffraction reveals that the samples first order to L12 then transform to L10-dominant L10 + L12 two-phase mixture. It is shown that L10 forms {110} polytwin microstructure with straight {11¯0} antiphase boundaries (APBs), where L12 exists as nanometer-scale wetting layers along the twin boundaries and APBs. The variant selection for L12/L10 wetting layers is discussed, and evidence of closed/open APB structures is shown with high-resolution transmission electron microscopy.

Initial Oxidation Behavior of AlCoCrFeNi High-Entropy Coating Produced by Atmospheric Plasma Spraying in the Range of 650 °C to 1000 °C.

As protective coatings for the thermal parts of aero-engines, AlCoCrFeNi coatings have good application prospects. In this study, atmospheric plasma spraying (APS) was used to prepare AlCoCrFeNi high-entropy coatings (HECs), which were oxidized from 650 °C to 1000 °C. The mechanism of the oxide layer formation and the internal phase transition were systematically investigated. The results show that a mixed oxide scale with a laminated structure was formed at the initial stage of oxidation. The redistribution of elements and phase transition occurred in the HECs' matrix; the BCC/B2 structure transformed to Al-Ni ordered B2 phase and Fe-Cr disordered A2 phase.

Soft magnetic properties of Fe-Ni powder cores in the high frequency range

Fe-Ni cores are excellent candidates for switching regulator inductors, transformers, reactors and in-line noise filters. Herein the influence of the phenol binder content on the microstructure and magnetic properties of Fe-50 wt% Ni powder cores in the high frequency range is investigated. Regardless of the binder content, all cores form a disordered A1 phase. The saturation magnetization (4πMs) and initial permeability increase as the binder content decreases. The core with a binder content of 0.5 wt% shows a relatively high 4πMs of 1.38 T and the highest permeability value of 70.9. Moreover, the total core loss (Pcv) with frequency ranges from 5 kHz to 1 MHz under a magnetic flux density of 10 mT depending on the binder content. A binder content of 3.0 wt% significantly reduces Pcv in the high frequency range above 100 kHz. At a frequency of 500 kHz, the Pcv values are 154.9, 128.7, and 133.8 kW/m3 for the cores with binder contents of 0.3, 0.5, and 1.0 wt%, respectively, but the value drastically decreases to 60.5 kW/m3 for a binder content of 3.0 wt%. Thus, the binder, which acts as an insulator layer between particles, affects the eddy current and consequently, Pcv. These results demonstrate that Fe-Ni powder cores exhibit excellent soft magnetic properties at various frequencies and suggest that a binder content of 3.0 wt% may realize optimal properties for future high frequency applications.

Phase evolution and mechanical properties of AlxCoCrFeNi high-entropy alloys by directed energy deposition

Directed energy deposition (DED), as an important branch of additive manufacturing (AM), has been widely used in the design and fabrication of high-entropy alloys (HEAs). Al is a common alloying element, it is critical to understand the effect of Al content on the phase evolution and mechanical properties of AlxCoCrFeNi HEAs. Here, a series of AlxCoCrFeNi (x = 0.2, 0.6 and 1.0) HEAs were manufactured by DED to investigate the alloying effect of Al on the phase evolution and mechanical properties. The phase transformation process was explored in conjunction with thermodynamic calculations. The results showed that the AlxCoCrFeNi HEAs transformed from single FCC phase (Al0.2) to FCC + BCC duplex phases (Al0.6), and then to single BCC phase (Al1.0) with the increase of Al content. Moreover, BCC phase further decomposed into the nanoscale two-phase structure containing ordered B2 phase and disordered A2 phase, which is beneficial to improve microhardness, yield strength and tensile strength. This work provides useful guidelines for the preparation and research of HEA systems with desirable properties.

Enriching Pt3 ensemble with isolated 3-fold hollow site by crystal-phase engineering of Pt3Fe single-nanoparticle for acetylene hydrogenation

Tuning the crystal-phase of bimetallic particles enables a precise control of catalytically active sites but is experimentally challenged by the averaging ensemble effects that are caused by the particle communications occurred during the crystal-phase transformation. Here, H2-treatment at 773–973 K of Pt3Fe single-nanoparticle, confined by a permeable silica shell, achieved crystal-phase transformation from the disordered A1 phase to the chemically ordered L12 phase. Detailed microscopic and spectroscopic characterizations have identified that the disordered A1 particle possessed contiguously connected large Pt domains while the ordered L12 particle afforded regularly populated Pt3 ensembles surrounded by Fe single-atoms. The isolated 3-fold hollow Pt site in the three-atom ensemble efficiently boosted the adsorption of acetylene, yielding a much higher hydrogenation activity. The reaction rate of the L12 particle was twice of that for the A1 particle. Moreover, the charge transfer from the neighboring Fe single-atoms to the Pt atoms on the L12 particle expedited the desorption of ethylene, promoting the selectivity as well. Overall, both geometric and electronic interactions in the ordered crystal-phase synergistically facilitated the selective hydrogenation of acetylene to ethylene.

Mechanical milling/alloying, characterization and phase formation prediction of Al0.1–0.5(Mn)CoCrCuFeNi-HEA powder feedstocks for cold spray deposition processing

In the present work the synthesis of multicomponent nanocrystalline Al0.1- 0.5(Mn)CoCrFeNi HEA powder feedstocks by mechanical alloying (MA) through high energy ball milling to produce HEA coatings with cold spraying were studied. Pure (>99.5) elemental powders of Al (Mn), Co, Cr, Cu, Fe & Ni were mixed in predetermined ratios and mechanically alloyed under Ar gas in a Planetary Ball Mill. MA HEA powder feedstocks were characterized by optical microscopy, SEM+EDAX, XRD & XPS to determine powder particle size, shape & distribution, particle surface condition, chemical composition, crystal & phase structures & microstructures. MA-processed Al-HEA powder feedstocks exhibited tri-modal particle size distribution (PSD) while Mn-HEA had four distinct PSD modes. Fine size fraction of the MA- HEA powders contained A2/B2 (BCC) harder phases with lattice parameters of 2.8831 and .2.8690 Å, respectively and coarse size fraction contained (ductile) softer A1 phase/Cu-rich regions combined (FCC) having a lattice parameter of 3.6181 Å. The ratio of the phases varied depending on the Al and Mn contents in the powder. For higher aluminium contents, there was more disordered A2/B2 phases and less disordered A1 phase in Al0.1–0.5 HEA powder feedstocks. A1 phase contained Fe, Cr, Co and Ni; A2 phase contained Fe, Cr; and B2 phase contained Al, Ni elements in Al0.1–0.5 HEA powder feedstocks. There were passive surface oxide films on the rough surfaces of MA HEA powder feedstocks consisting of a main mixture of surface oxides such as Al2O3, MnO, Cr2O3, Mn2O3, and minor oxides such as CuO, Fe2O3, NiO, Fe3O4, and a few CoCr2O4 spinel oxide. Dual FCC-dual BCC phase prediction of thermodynamic stable phase formation criteria reported in the literature on MA-HEA powder feedstocks were consistent with the experimental results of the present study.

Unveiling the structure-property relationship in metastable Heusler compounds by systematic disorder tuning

Heusler compounds represent a unique class of materials that exhibit a wide range of fascinating and tuneable properties such as exotic magnetic phases, superconductivity, band topology, or thermoelectricity. An exceptional, but for Heusler compounds common, feature is that they are prone to antisite defects and disorder. In this regard, the $\mathrm{Fe}{}_{2}\mathrm{VAl}$ Heusler compound has been a particularly interesting and disputed candidate. Even though various theoretical scenarios for the interplay of physical properties and disorder have been proposed, the metastable disordered A2 phase hitherto precluded experimental investigation in bulk samples. Here, we report experimental results on disorder-tuned $\mathrm{Fe}{}_{2}{\mathrm{VAl}}_{0.9}{\mathrm{Si}}_{0.1}$ alloys all the way toward the A2 phase, which we realized via rapidly quenching our samples from high temperatures. We measured the thermoelectric properties of these materials in a wide temperature range (4 to 700 K); they suggest a gradual semimetal/narrow-gap semiconductor $\ensuremath{\rightarrow}$ metal transition upon increasing the disorder. We also find a large anomalous Hall effect in the disordered A2 phase, arising from the side-jump scattering of charge carriers at the antisite magnetic moments. This is corroborated by measurements of the temperature- and field-dependent magnetization, which increases dramatically up to $\ensuremath{\approx}2.5\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$ as compared to the ordered compound ($<0.1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\text{B}}/\mathrm{f}.\mathrm{u}.$). This study provides an experimental realization of the metastable A2 structure in bulk $\mathrm{Fe}{}_{2}\mathrm{VAl}$-based alloys and grants insight into the structure-property relationship of these materials. Our work confirms that temperature-induced antisite disorder, occurring during thermal heat treatment, can be a precisely tuneable parameter in the family of Heusler compounds.

Preparation of Fe‐Pt Powders from Ingot by a Simple and Cost‐Effective Spark Erosion Device

AbstractFePt powders with 1 µm average particle size have been synthesized from bulk ingot with no need for cryogenic temperatures or conventional powder processing. This is achieved with a compact, cost‐effective spark erosion device which is simple to assemble, with no need of a mechanical workshop. The as‐synthesized powders show a disordered A1 phase with curie temperature, Tc = 300 °C. The effect of annealing on the transition from the disordered to the ordered phase and on the magnetic properties are investigated in situ by magnetic susceptibility measurements. The phase transition to the L10 phase (Tc = 445 °C) is complete at 630 °C where the coercivity increases from 0 to 0.3 T. A urther increase of the heating temperature has no effect on the magnetic and crystallographic properties.

Effect of boron addition on the phase-transition temperature of CoPt-B nanoparticles synthesized by sol–gel autocombustion using sago starch as a chelating agent

In this research, we reported the effect of boron addition on the phase-transition temperature of CoPt-B nanoparticles using a sol–gel autocombustion method. Sago starch was used as a chelating agent. From the results, the suitable values of the sago starch concentration, boron concentration, and annealing temperature were determined to be 10% (w/v), 0.20 mol/L, and 500 °C, respectively. All the synthesized samples were characterized by the SEM, TEM, EDS, XRD, and VSM techniques. The XRD patterns of the synthesized CoPt-B nanoparticles demonstrated that the phase-transition temperature of CoPt-B changes from the disordered A1 phase to the ordered L10 phase at 500 °C. It can be concluded that the addition of boron could reduce about 100 °C of the ordering temperature of CoPt-B. Moreover, the VSM data confirmed that the obtained samples exhibited their ferromagnetism properties, indicating that the synthesized CoPt-B nanoparticles can be applied as ultrahigh-density magnetic recording media.

Structural and electronic properties of the spin-filter material CrVTiAl with disorder

The effects of chemical disorder on the electronic properties of the spin-filter material CrVTiAl are investigated experimentally and theoretically. Synchrotron X-ray diffraction experiments on bulk CrVTiAl and the associated Rietveld analysis indicate that the crystal structure consists primarily of a mixture of a partially ordered B2 phase, a fully disordered A2 phase, and a small component of an ordered L21 or Y phase. High temperature resistivity measurements confirm the existence of a bandgap. First-principles, all-electron, self-consistent electronic structure computations show that the chemically disordered A2 and B2 phases are metallic, while the spin-filter properties of the ideal Y-type phase are preserved in the presence of L21 disorder (Cr and V mixing). The Hall coefficient is found to decrease with increasing temperature, similar to the measured increase in the conductivity, indicating the presence of thermally activated semiconductor-like carriers.

Dispersed clusters in (Fe,Cr)3Al alloys: Neutron time-of-flight diffraction study

The structure and microstructure of $\mathrm{Fe}\ensuremath{-}x\mathrm{Al}\ensuremath{-}y\mathrm{Cr}$ alloys with $x=25$ and 27 and 0 \ensuremath{\le} $y\ensuremath{\le}15$ have been studied by neutron diffraction at several fixed temperatures with high $\mathrm{\ensuremath{\Delta}}d/d$ resolution and with high-intensity continuous scanning in a wide temperature range. The joint analysis of the obtained data established that the microstructure of these compounds is organized in the form of nanoscopic-sized clusters with an ordered atomic structure coherently embedded in a disordered (or less ordered) matrix. In the quenched samples, the matrix is a disordered A2 phase with clusters of a partially ordered B2 phase, whereas, in the annealed samples, the clusters of the ordered $\mathrm{D}{0}_{3}$ phase are dispersedly distributed in the B2 matrix. The characteristic size of the clusters depends on the state of the alloy, the chromium content, and the temperature and ranges in size from 100 to 1000 \AA{}. For the matrix, the coherent scattering domain size exceeds 3500 \AA{}. Therefore, the state of the alloys is actually a two-phase state, $\mathrm{A}2+\mathrm{B}2$ or $\mathrm{B}2+\mathrm{D}{0}_{3}$, although these phases are indistinguishable in elemental composition. One of the factors stabilizing the phase-separated state in a compositionally homogeneous medium can be local composition fluctuations at the level of elementary cells, which in turn lead to fluctuations in the interactions of neighboring pairs of atoms.

Thermodynamic assessment and experimental investigation of the Al–Mn–Ni system

The ternary Al–Mn–Ni system plays an important role in the development of several structural and functional materials. To accomplish a purposeful development of alloys related to this system, an assessment of the thermodynamic parameters was performed. The assessment is based on literature data, as well as experimental investigations regarding the phase equilibria at 800 and 1000 °C. Moreover, the decomposition of the disordered A2 phase in the Mn-rich corner of the Al–Mn–Ni system was investigated by means of transmission electron microscopy and in-situ X-ray diffraction. Calculations based on the present description are in excellent agreement with the experimental data available in literature and reproduce the experimental results of the present work.

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.

Correlation of structural ordering with magnetic properties of pulsed laser deposited Co2FeGa Heusler alloy thin films

In the present contribution, structural and magnetic properties of Pulsed laser deposited (PLD) Co2FeGa (CFG) thin films have been studied as a function of substrate temperature. Structural investigation carried out by X-Ray Diffraction (XRD) measurement reveals mixed phase cubic structure of the films which changes from a nearly ordered to highly disordered phase with an increase in substrate temperature. Grazing Incidence X-Ray Diffraction (GIXRD) study shows the presence of disordered A2 phase similar to the bulk target. Grazing Incidence X-Ray Reflectivity (GIXR) measurement reveals the formation of bi-layer structure in the films with different density and thickness though the overall thickness remains same. Field Emission Scanning Electron Microscopy (FESEM) study shows the formation of a droplet-like morphology of the films. Extended X-ray Absorption Fine Structure (EXAFS) study serves a major role in complementing the XRD results and also shows a strong hybridization of Co with Ga maintaining the half metallicity. A novel approach in analyzing the EXAFS data additionally gives quantitative estimation of the different kinds of disorders present in the samples at the atomic level. In addition to this, magnetization result suggests that the films grown at lower substrate temperatures acts as proper ferromagnet following the well known spin wave theory with higher value of spontaneous magnetization in comparison to other samples. A lesser value of saturation magnetization in the films compared to bulk also supports the presence of antisite disorders.

Calorimetric study and microstructure analysis of the order-disorder phase transformation in silicon steel built by SLM

Innovative Additive Manufacturing (AM) technologies like Selective Laser Melting (SLM) could prove to be efficient for the processing of brittle silicon steel (Fe-Si) with high silicon content. This research elucidates the effects of heat-treatment on the microstructure of SLM-built high silicon steel, with particular emphasis on the formation of ordered phases, which are known to cause undesired material embrittlement.Silicon steel with 6.9 wt % Si is produced by SLM and investigated performing Differential Scanning Calorimetry (DSC), microstructure analysis and hardness measurements. As-built Fe-Si parts are found to consist primarily of disordered A2 phase as the high cooling rates typical of SLM suppress the ordering phase transformations. It is shown how heat treatments can be applied to modify the state of ordering and morphology of the rapidly solidified microstructure. By shedding light on the phase-ordering mechanisms and the effect of heat treatments on microstructure of high-Si steel built by SLM, the present study paves the way towards the optimisation of the mechanical and magnetic properties of this alloy.

Phase formation sequence, magnetic and structural development during solid-state reactions in 72Pt/28fcc-Co (001) thin films

Abstract The phase formation sequence during the thermally induced solid-state reaction between polycrystalline Pt and epitaxial fcc-Co (001) films in the Pt/fcc-Co(001) bilayers are systematically examined using X-ray diffraction and magnetic measurements. The films have nominal atomic ratio Co:Pt = 28:72 and total thickness 300–400 nm. Annealing to the temperature of 375 °C does not change the structural and magnetic properties of the films; this is indicative of the absence of considerable mixing at the Co/Pt interface. With the subsequent increase of the annealing temperature, the phase formation in the Pt/fcc-Co(001) bilayers has been found to have two temperature (375 °C–575 °C and 575 °C–825 °C) intervals. Solid-state reaction between Pt and Co starts above 375 °C, and nanoclusters containing the ordered L10 phase epitaxially intergrow with the disordered A1 phase of the composition CoPt3 form and exist in the first temperature interval. The distinctive feature of the first interval is the formation of in-plane rotatable magnetic anisotropy. In the second temperature interval, the (L10 + A1) two-phase mixture grows into the ordered L12-CoPt3 phase leading to the disappearance of rotatable anisotropy. Possible origin of the rotatable magnetic anisotropy is discussed. The first magnetocrystalline anisotropy constant of L12-CoPt3 has the maximum value of −5.0·105egr/cm3 and order parameter 0.55 at 675 °C. A careful analysis of thin film solid-state reactions implies the existence of low-temperature transformation (∼375 °C) on the Pt-rich side of the Co-Pt system.

Growth of L1-ordered crystal in FePt and FePd thin films on MgO(001) substrate

Formation of L10-oredered structure from disordered A1 phase has been investigated for FePt and FePd films on MgO(001) substrates employing a two-step method consisting of low temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. L10-(001) variant crystal with the c-axis perpendicular to the substrate grows preferentially in FePd films whereas L10-(100), (010) variants tend to be mixed with the L10-(001) variant in FePt films. The structure analysis by X-ray diffraction indicates that a difference in A1 lattice strain is the influential factor that determines the resulting L10-variant structure in ordered thin films. Misfit dislocations and anti-phase boundaries are observed in high-resolution transmission electron micrographs of 10 nm-thick Fe(Pt, Pd) film consisting of L10-(001) variants which are formed through atomic diffusion at 600 °C in a laterally strained FePt/PeFd epitaxial thin film. Based on the experimental results, a nucleation and growth model for explaining L10-variant formation is proposed, which suggests a possibility in tailoring the L10 variant structure in ordered magnetic thin films by controlling the alloy composition, the layer structure, and the substrate material.

Influence of chemical ordering on the thermal conductivity and electronic relaxation in FePt thin films in heat assisted magnetic recording applications

We report on the out-of-plane thermal conductivities of tetragonal L10 FePt (001) easy-axis and cubic A1 FePt thin films via time-domain thermoreflectance over a temperature range from 133 K to 500 K. The out-of-plane thermal conductivity of the chemically ordered L10 phase with alternating Fe and Pt layers is ~23% greater than the thermal conductivity of the disordered A1 phase at room temperature and below. However, as temperature is increased above room temperature, the thermal conductivities of the two phases begin to converge. Molecular dynamics simulations on model FePt structures support our experimental findings and help shed more light into the relative vibrational thermal transport properties of the L10 and A1 phases. Furthermore, unlike the varying temperature trends in the thermal conductivities of the two phases, the electronic scattering rates in the out-of-plane direction of the two phases are similar for the temperature range studied in this work.

Magnetization reversal and microstructure in polycrystalline Fe50Pd50 dot arrays by self-assembling of polystyrene nanospheres

Nanoscale magnetic materials are the basis of emerging technologies to develop novel magnetoelectronic devices. Self-assembly of polystyrene nanospheres is here used to generate 2D hexagonal dot arrays on Fe50Pd50 thin films. This simple technique allows a wide-area patterning of a magnetic thin film. The role of disorder on functional magnetic properties with respect to conventional lithographic techniques is studied. Structural and magnetic characteristics have been investigated in arrays having different geometry (i.e. dot diameters, inter-dot distances and thickness). The interplay among microstructure and magnetization reversal is discussed. Magnetic measurements reveal a vortex domain configuration in all as-prepared films. The original domain structure changes drastically upon thermal annealing performed to promote the transformation of disordered A1 phase into the ordered, tetragonal L10 phase. First-order reversal magnetization curves have been measured to rule out the role of magnetic interaction among crystalline phases characterized by different magnetic coercivity.