Α2 Phase Research Articles

Prospects for anisotropic superfluidity in a Fermi gas of dysprosium

The possibility of obtaining superfluid phases for a Fermi gas of dysprosium with a magnetic dipole – dipole interaction is discussed. The obstacles and possible solutions are shown. The required phases are similar to the A1 phase and the polar β phase in 3He. It is expected that in dysprosium, the macroscopic properties of the phase will be determined by the symmetry of the pair interactions. It is assumed to observe the kinetics of phase formation and spontaneous choice between two energy-degenerate phases with different projections of the orbital angular momentum.

Microstructure and Composition Comparison of Ti‐ and Ta‐Doped Nb3Al Superconducting Wires Fabricated by RHQT Process

Herein, the microstructure and composition of the Ti‐ and Ta‐doped Nb3Al superconductor fabricated by rapid heating, quenching, and transformation (RHQT) process are compared. After rapid heating and quenching (RHQ) process, body‐centered cubic (bcc) phase is formed in all the wires, showing dark stripe and bright contrast morphology in the backscattered electron (BSE) images. After transformation heat‐treatment, all wires consist of A15 phase and show the same morphology as that of the quenched samples. Near‐stoichiometric composition is shown in 3 at% Ti‐ and Ta‐doped Nb3Al samples, but when doping amount is 10 at%, Ti‐doped sample has constant Nb content of 67 at% and Ta‐doped sample has Al + Ta content of 25 at%. Superconducting properties of both 3 at% Ti‐ and Ta‐doped Nb3Al samples are comparable and much superior than that of 10 at% doped samples, including critical temperature (Tc), critical current density (Jc), and irreversible field (Birr). Though 10 at% Ta‐doped Nb3Al has higher Tc than 10 at% Ti‐doped sample, it presents wider ΔTc value and lower Jc performance. When increasing the measurement temperature from 8 to 10 K, the decay ratio of Birr value for Ti‐doped Nb3Al superconductor follows the scaling law; however, the Ta‐doped samples are not the case.

Modulation of the Complex Spherical Packings through Rationally Doping a Discrete Homopolymer into a Discrete Block Copolymer: A Quantitative Study

The Frank–Kasper phase and quasicrystalline phase are an intriguing class of complex crystalline structures, which so far are sporadically observed only in a limited number of block copolymers. Incorporation of a homopolymer into a block copolymer has recently been demonstrated as an effective and robust approach to regulate the formation and evolution of these complex spherical phases. The experimental explorations, however, suffer from inherent chain length distribution of the blending stocks. In this study, we quantitatively assessed the phase behaviors of the block copolymer/homopolymer binary blends using discrete species with a precise chemical structure and uniform chain length, ruling out all interferences associated with chemical heterogeneities. Diverse spherical packings, including σ, A15, C15, and C14 phases, were captured by rationally tuning the chain length and loading content of the homopolymer. The short chains swell the spherical core and drive a transition toward the lattices with a lower interfacial curvature (i.e., σ → A15 → HEX), whereas the long chains localize in the center of the core and prompt the formation of the Frank–Kasper phases with the increasing particle volume asymmetry (C15 and C14). The experimental observation validates the recent theoretical advances, demonstrating that the blending strategy is a robust approach for structural engineering.

Effect of heat input on auxiliary wire GMA-AM TiAl alloys

ABSTRACT In-situ γ-Ti48Al alloys are fabricated using auxiliary wire gas metal arc additive manufacturing in which pure Ti plus Al wires are fused in the arc. This study focuses on the influence of heat input on microstructure and mechanical properties of γ-Ti48Al alloys. As the heat input decreases, the upper part of the homogeneous zone in the middle region always shows α2/γ fully lamellar colonies with the decreased grain size, the dual-phase microstructures in the centric and lower parts gradually transform into fully lamellar colonies, and the volume fraction of α2 phase increases. The average microhardness mainly distributes in the range of 310–490 HV. With a decrease in heat input, the compressive strength in the vertical orientation reduces, and the compression rates in both orientations first decrease and then increase. The fracture morphologies always show a mixed-mode of inter-lamellar and trans-lamellar cleavage fractures.

High temperature micro-deformation behavior of continuous TiNb fiber reinforced TiAl matrix composite investigated by in-situ high-energy X-ray diffraction

The continuous TiNb fiber reinforced TiAl matrix composite possesses improved mechanical properties whereas the micro-deformation mechanism remains to be elucidated. This work dynamically analyzed the high-temperature compressive process of TiNbf/TiAl composite by in situ high-energy X-ray diffraction (HEXRD) method. Results indicated β-TiNb and interfacial α2 phase both contained two fiber textures whose formation obeyed the special sequences. Only <110> fiber texture in γ matrix and <0001> fiber texture in α-TiNb existed. The elastic-plastic (E-P) stresses of β-TiNb and α-TiNb grains were higher than macroscopic yield stress σy, especially α-TiNb presented significant strengthening effect. But E-P stresses of γ grains were all lower, especially the [200] and [202] orientated γ grains tended to yield easier along LD direction. Interfacial α2 grains would enter E-P stage earlier due to stress concentration, indicating to share loading effectively. Both fiber and interface can play an important reinforcing role at the initial deformation. However, the fiber degradation at the later stage of work hardening would make it lose the strengthening effect. The load capacity of α2 interface can maintain the strongest until the true strain reaching 22.16%. This work can provide a fundamental understanding of macro-deformation of TiNbf/TiAl composite from the perspective of micromechanical behavior.

Pt accelerated coarsening of A15 precipitates in Cr-Si alloys

The effect of alloying Cr-rich Cr-Si alloys with Pt was investigated by a combination of complementary experimental methods and atomic scale modelling. The investigated Cr-Si and Cr-Si-Pt (Cr ⩾86 at.%) alloys developed a two-phase microstructure consisting of Cr solid solution (Crss) matrix and strengthened by A15 precipitates during annealing at 1200°C. It was found that additions of 2 at.% Pt increase the coarsening rate by almost five times considering annealing times up to 522 h. Pt was found to change the precipitate matrix orientation relationship, despite its low influence on the Crss matrix/A15 precipitate misfit. Through this experimental and modelling approach new insight has been gained into mechanisms of enhanced coarsening by Pt addition. The increased coarsening is principally attributed to a change in interface composition and structure resulting in different thermodynamic stabilities: Pt-containing A15 phase was found to have a broader compositional range if both elements, Pt and Si, are present compared to only Si. Additionally, the Crss phase was found to have a higher solubility of Pt and Si over just Si. Both factors additionally facilitated Ostwald ripening.

Microstructure characterization of gradient composition between TC25G and TiAl alloys prepared by directed energy deposition technique

Graded Materials are now used widely as structural components due to their specific properties. Researching on microstructure evolution of the transition region in graded materials is important to control the entire property of the graded materials. In this study, three alloys with the typical compositions were made by directed energy deposition (DED), which were correlated to the transition zone of the TC25G/TiAl graded material. Results indicate that the DEDed TiAl alloy consists of a duplex microstructure with γm and (α2 + γ) lamellar. For sample AD75 (mixed of 75 wt% TiAl and 25 wt% TC25G powder), the microstructure changes to B2 + α + γm + (α2 + γ). The continuing increase of TC25G alloy content causes the microstructure transfer to β/B2 + α + ω + (α + α2) and β + α + (α + α2) for sample AD50 (mixed of 50 wt% TiAl and 50 wt% TC25G powder) and AD25 (mixed of 25 wt% TiAl and 75 wt% TC25G powder) alloy consequently. Besides, the microstructure of TC25G alloy is β + α + α2. The highest hardness is found in AD75 alloy, followed by AD50, TC25G, AD25 and TiAl alloy, which is attributed to a large amount of B2 and α2 phase formation.

Adjusting the γ/α2 and Ti2AlC phase on microstructure evolution and improving mechanical properties at room and elevated temperatures with the addition of Ta

Alloys of Ti-46Al-6Nb-2.5C-xTa (at. %) were prepared to adjust the γ/α2 phase, refine microstructures, and improve mechanical properties at room and elevated temperatures simultaneously. Phase contents, lamellar colony size, Ti2AlC morphology, compressive and tensile properties, and related mechanisms were discussed. The results show that the relative content of the γ phase increases and the relative content of the α2 phase decreases when Ta content increases from 0 to 2.0 at. %. The solid solution effect of Ta in reinforcing phases contributes to the absence of brittle B2 phase. The content of small length-diameter ratios of Ti2AlC particles increases. The lamellar colony size decreases from 29.9 to 21.6 μm. The constitutional undercooling and heterogeneous nucleation are increased by Ta during the solidification process. Compressive strength increases from 1974 to 2330 MPa when Ta content increases to 0.8 at. %, and the strain increases from 19.8 to 28.2% when Ta content increases to 1.2 at. %. The tensile strength of the Ti-46Al-6Nb-2.5C-2.0Ta alloy increases from 460 to 503 MPa and its strain increases from 4.1 to 8.3%, when temperature increases from 750 to 950 °C. The refinement of lamellar colonies and Ti2AlC, solid solution of Ta, and the ratio of γ/α2 are the main reasons for improving mechanical properties of Ti-46Al-6Nb-2.5C-2.0Ta alloy.

Interfacial microstructure and shear performance of TiAl to Nb–Si alloy diffusion bonded with pure Ti interlayer

Diffusion bonding of high niobium β-γ TiAl (HNBG) to Nb–Si alloy was carried out using Ti foil as interlayer. The interfacial microstructural evolution, growth kinetics and mechanical properties of HNBG/Ti/Nb–Si joint were investigated in detail. The microstructure observations indicate that the interfacial microstructure of the joint was composed of β-(Nb, Ti)5Si3, (Nb,Ti)ss//α-Ti, β-Ti//α2//β/B2 from zone I to zone IV. The β-Ti phase and α-Ti phase in zone II obey the orientation relationship, {110}β || {0002}α and <111>β|| <11–20>α. The γ phase in HNBG alloy and β/B2 phase in zone IV obey the orientation relationship, {111}γ || {110}β/B2 and <110>γ || <111>β/B2. The growth activation energies of β-(Nb, Ti)5Si3 and β/B2 phases were 166 kJ/mol and 507 kJ/mol respectively. The content of β phase in zone II and the average grain size of α2 phase in zone III increased with bonding temperature or bonding time. The shear test results showed that the ultimate shear strength was 385 MPa at 1000 °C-10 min-10 MPa. Fracture mainly occurred beween the interface of zone III and IV, and between the interface of zone IV and HNBG alloy. The fracture mode was mainly characterized by brittleness rupture along the phase boundaries of α2 and β/B2, β/B2 and HNBG alloy with slight amount of cleavage fracture in zone IV.

Temperature-Induced Phase Transitions of Fe-Ga Alloys for Sensors

Slow cooled arc melted Fe-Ga alloys with different compositions were characterized. GI-XRD confirms the BCC structure and existence of single phase A2 in 17 at.% of Ga and multiphase A2 and L12 in 30 at. % of Ga. The Ga defects migrates during annealing and reduces the lattice. The Galfenol alloy with the 30 at.% of Ga has an A2 phase around 400 oC and an L12 phase around 560 oC. The formation of the L12 structure is due to an exposure in high temperature region. The magnetostriction of Fe83 - Ga17 and Fe70 - Ga30 alloys seems to be 64 and 27 ppm. Decrement of magnetostriction is due to change in anisotropy coefficient.

Preparation of Ti-46Al-8Nb Alloy Ingots beyond Laboratory Scale Based on BaZrO3 Refractory Crucible

The high Nb-containing TiAl-based alloy ingot beyond laboratory scale with a composition of Ti-46Al-8Nb (at.%) was prepared by a vacuum induction melting process based on a BaZrO3 refractory crucible. A round bar ingot with a diameter of 85 mm and a length of 430 mm was finally obtained, and the chemical composition, solidification pathway, microstructure and tensile properties of the ingot were investigated. The results show that the deviations of Al and Nb content along a 430 mm long central part of the ingot are approximately ±0.39 at.% and ±0.14 at.%, and the oxygen content in the ingot can be controlled at around 1000 ppm. The structure of the alloy ingot is a full lamellar structure composed of γ and α2 phases, and the thickness of the lamellae is approximately 0.53 μm. In case of the α2 phase, the surface content of the ingot is higher than the middle region and the centrical region; also, it indicated a decreasing trend. During cooling, the alloy solidified from a peritectic reaction (L + β→α) rather than the solidified via β phase (β→α). In addition to Al segregation and Nb segregation, β-phase particles associated with γ phase at the triple junction of the colonies were observed. Moreover, the tensile properties of the longitudinal-cut sample in the ingot is significantly better than those of the transverse-cut sample, with a tensile strength of up to as high as 700 MPa and a corresponding fracture elongation of 1.1%. However, the tensile strength of the transverse-cut sample is only 375 MPa, and the fracture elongation is 0.52%.

Mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy compressed at elevated temperature

The mechanism of stress- and thermal-induced fct → hcp → fcc crystal structure change in a TiAl-based alloy was investigated via experiments and molecular dynamics simulations. According to the experimental results, laths occurring in γ-phase grains were identified as γ-phase twins, and increasing strain promoted the γ → α2 (fct → hcp) phase transformation when the alloy was deformed at 1150 °C. A large number of ultrafine γ lamellae precipitated when deformed at 1200 °C, which was attributed to the thermal-mechanical coupling. The atomic mechanisms of the α2(α) → γlamella phase transformations were investigated using molecular dynamics simulations. The results showed that the generalised stacking fault energy along different directions (<1‾010>) exhibited remarkable anisotropy on the basal plane (0001) of the hcp crystal structure, regardless of the ordering or disordering phase. The disordering of the hcp crystal structure significantly affected the formation of the γ lamellae. Regardless of the ideal stoichiometric or off-stoichiometric composition, the stress-induced hcp → fcc crystal structure transformation occurred more easily in the disordered phases. Grain-boundary nucleation dominated the precipitation of the ultrafine γ lamellae. In addition, intragranular nucleation of γ lamellae was observed, which was mainly attributed to the activation and movement of the Shockley partial dislocation loop with a Burgers vector of 1/6<1‾010> whose formation was not directly related to the dissociation of the superpartial dislocation 1/6<112‾0>.

Structural-Phase Сhanges During the D1a→A1 Phase Transition in the Ni4Mo Alloy

The results of studying the structural and thermal effects in the region of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy are presented. It is found that the D1a→A1 transition occurs in a narrow temperature range of 16 °C while the long-range order parameter п in this range decreases from 1 to 0.85 and, upon reaching ТК, decreases abruptly to 0. Studying the thermal effects allows obtaining the temperature dependence of the specific heat Ср(Т) is obtained and determination of the enthalpy and entropy of the D1a→A1 order-disorder phase transition in the Ni4Mo alloy. It is found that the main contribution to the entropy of transformation is configurational. The nonconfiguration contribution is not large and contributes to the ordering processes. It is established that the beginning of the deviation on the temperature dependence of Cp(T) from the linear one correlates with the beginning of the formation of the two-phase state D1a +A1 and the beginning of a decrease in the atomic long-range order in the D1a. Based on calculations based on the first a principles of thermodynamic parameters of compounds, the enthalpy of formation in the Ni-Mo system is obtained. It is found that the most stable compound in this system is Ni3Mo. Theoretical calculations of the compound formation enthalpy give a value of -8.3 kJ/mol.

Phase formation and enhanced performance of Nb3Al prepared by spark plasma sintering and subsequent annealing

Although the Nb3Al wire prepared by the rapid heating and quenching (RHQ) technique has achieved excellent superconducting critical current density (Jc), various new preparation techniques have still been continuously tried to improve its performance. In this paper, the phase formation and superconducting properties of Nb3Al prepared with spark plasma sintering (SPS) was studied. It is found that the SPS in the temperature range of 1200–1400 °C can quickly produce high-density, nano-structured Nb3Al superconductor, which has a transition temperature around 16 K. Due to its tight and clean grain connection, the sample shows a high-performance current-carrying capacity in self-field. After post-annealing, the flux pinning force and the superconducting performance are significantly improved. The study clarified that the phase formation mechanism of the SPSed Nb3Al is similar to that directly formed from high temperature liquid phase rather than the transformation from bcc phase to A15 phase. The high temperature liquid phase may originate from the local high temperature induced by the plasma heating in the early stage of SPS process. Such a unique phase formation mechanism results in very few structural defects existing inside the sample. The results of this study suggest that chemical doping or other means which can introduce high density structure defects in Nb3Al should be considered to further improve the in-field Jc for the SPSed Nb3Al.

Highly complex magnetic behavior resulting from hierarchical phase separation in AlCo(Cr)FeNi high-entropy alloys

SummaryMagnetic high-entropy alloys (HEAs) are a new category of high-performance magnetic materials, with multicomponent concentrated compositions and complex multi-phase structures. Although there have been numerous reports of their interesting magnetic properties, there is very limited understanding about the interplay between their hierarchical multi-phase structures and the resulting magnetic behavior. We reveal for the first time the influence of a hierarchically decomposed B2 + A2 structure in an AlCo0.5Cr0.5FeNi HEA on the formation of magnetic vortex states within individual A2 (disordered BCC) precipitates, which are distributed in an ordered B2 matrix that is weakly ferromagnetic. Non-magnetic or weakly ferromagnetic B2 precipitates in large magnetic domains of the A2 phase, and strongly magnetic Fe-Co-rich interphase A2 regions, are also observed. These results provide important insight into the origin of coercivity in this HEA, which can be attributed to a complex magnetization process that includes the successive reversal of magnetic vortices.

Effect of stoichiometry and film thickness on the structural and magnetization dynamics behavior of Co2MnAl thin films cosputtered on Si(1 0 0)

Obtaining low gilbert damping (α) and high spin polarization (P) in Heusler alloy thin films is one of the key requirements from the device application point of view in spintronics. Both α and P are correlated, closely related to the density of states (DOS) at the Fermi level and need to be controlled for engineered material. We investigated the effect of stoichiometry (different Co to Mn atomic percent ratios, viz. 2.21, 4.53, 5.43, and 9.19) and film-thickness {(t) in the range of 24–90 nm} on the structural and magnetization dynamics behavior in co-sputtered Co2MnAl thin films grown over Si(100) substrates. While the stoichiometric (∼50 at.% of Co) films with t ≥ 38 nm were found to exhibit B2 order with low α, the off-stoichiometric films (t ∼ 50 nm) were found to stabilize in partially ordered A2 phase and exhibited higher α. The change in structural ordering due to change in stoichiometry and film-thickness in polycrystalline Co2MnAl (CMA) thin films helped in tuning α in the range of (4.11 ± 0.06) × 10-3 – (11.95 ± 0.28) × 10-3. The minimum value of α ∼ (4.11 ± 0.06) × 10-3 was achieved for stoichiometric CMA (t = 70 nm) thin films which is close to the literature value of epitaxially grown L21 and B2 ordered CMA thin films. The study shows that we can tune and attain low damping in polycrystalline CMA Heusler thin films by controlling the stoichiometry.

Achieving high ductility of Ti2AlNb sheet without sacrificing the tensile strength without post heat treatment

So far, one main shortcoming restricting the wide application of Ti2AlNb sheet is the poor ductility. To solve this problem, we hot packed rolled the Ti2AlNb compact prepared by spark-plasma-sintering the pre-alloyed powders at the α2+B2+O phase field in this work. The sheets were rolled by different thickness reductions and passes. Results showed that the packs made of 304 stainless steel effectively protected the Ti2AlNb sheet from cracking, and the obtained sheets had good surface formation quality. B2 grain size and the texture intensity increased with increasing the rolling pass. The sheets rolled by all schemes had excellent ductility and relative high strength without applying any post heat treatment. An extreme high elongation of 13.91% with a tensile strength of 1063.27 MPa was obtained after four rolling passes. The nano O phases had excellent ability to coordinate transformations and account for the excellent ductility of the sheet. The α2 phases mainly strengthen the strength of the sheet; maintaining its high strength. This work may provide a new insight to develop commercial high ductility Ti2AlNb alloys.

Phase transformation behavior of Ti–40Al–8Nb alloys with a submicron (ω0+γ) microstructure during tempering at 1000 °C

In this work, the phase transformation behavior of Ti–40Al–8Nb (at. %) alloys with a submicron (ω0+γ) microstructure during tempering at 1000 °C was investigated, and special attention was paid on the ω0-assisted α2 nucleation. Results indicated that the ω0+γ→B2→α2 transformation dominated the precipitation of α2 phase. The ω0 and γ phases first dissolved into the B2 phase within a short time, and then the B2→α2 transformation occurred. The α2 phase could nucleate in the interior of B2 matrix and precipitate at the B2/γ interfaces by following the Burgers orientation relationships (ORs), <11–20>α2//<111>B2 and (0001)α2//{110}B2. With the increase of isothermal holding time, the γ phase reprecipitated through two phase transition routes, including B2→γ and α2→γ transformations by following the Kurdjumov-Sachs ORs, <111>B2//<110>γ and {110}B2//{111}γ, and Blackburn ORs, i.e., (0001)α2//{111}γ and <11–20>α2//<011>γ, respectively. Finally, a (α2+γ+B2) microstructure was obtained after isothermal holding at 1000 °C for 300 min.

Microstructure and sliding wear behavior of HVOF sprayed Al(1−x)CoCrFeNiTix high-entropy alloy coatings

Al(1−x)CoCrFeNiTix high-entropy alloy (HEA) coatings were prepared by HVOF technology with atomic ratio of 0 ∼ 0.25 (x = 0, 0.125, 0.25). The microstructure, phase composition and sliding wear behavior of the coatings were studied. The results show that the Al(1−x)CoCrFeNiTix HEA coatings have compact structure. The coatings include B2 phase and A2 phase. Besides, the coatings have good sliding wear resistance. The wear failure mechanism of the AlCoCrFeNi coating is mainly three body abrasive wear, oxidative wear and fatigue wear. With the increase of replacement Ti, the degree of abrasive wear decreases gradually and the wear resistance and the effect of oxidation wear increases.

Improved critical current density of Cu-doped Nb3Al wire through optimized Rapid-Heating Quenching and Transformation method

We have prepared 2.5% Cu-doped Nb3Al wire by the Rapid-Heating Quenching and Transformation (RHQT) method with optimized annealing. XRD results show that A15 and supersaturated bcc phase are formed after high current RHQ process, and the supersaturated bcc phase is transformed into A15 phase after conventional annealing (800 °C\\10 h). We observed the microstructures at different annealing time by field-emission scanning electronic microscope (FESEM) and found that σ phase is generated from the decomposition of the metastable A15 phase after prolonged annealing due to its lower free energy at 800 °C, and gradually accumulates Cu to form the Cu-rich σ phase after 20 h. The low-field superconducting properties were also given through magnetic measurement. Both Tc and Bc2 decreased slightly along with the increase of σ phase volume when the annealing is prolonged, while Jc reached its peak after 20 h\\800 °C annealing, which were 1836 A/mm2 at 4.2 K and 5 T and 822 A/mm2 at 8 K and 5 T for 800 °C\\20 h annealing, about 34% and 36% higher than that of the conventional transformed sample, respectively. The calculated Fpmax of the samples with optimized annealing condition also increased accordingly. The fitting analysis of fp using the Dew-Hughes model showed that the pinning mechanism was surface pinning. The additional diffusion channels for Cu provided by newly generated interfaces of σ and Nb(Al) particles enhance the Cu segregation on the A15 boundaries and may be the source of the pinning force improvement.