The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720 Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M_3B_2 and Ni_5Zr phases were observed in the U720 Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni_3 Ti phase at the edge of eutectic(γ+γ'). Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following: c matrix, eutectic(γ+γ'), g-Ni_3Ti, M_3B_2 and Ni_5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic(γ+γ') formation; the(γ+γ') formation increased the Ti/Al radio in the liquids and the g-Ni_3Ti was formed in front of the eutectic(γ+γ'); the g-Ni_3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M_3B_2 boride is formed;the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni_5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni_5Zr phase, 1180–1190 °C for M_3B_2 boride and1190–1200 °C for g-Ni_3Ti phase.     

Theory of the nonlinear internal friction associated with bulk diffusion of solute atoms around dislocations in Al–Mg alloys

In this paper the nonlinear (amplitude-dependent) internal friction (P₃ peak) in cold-worked Al–Mg alloys is theoretically studied by solving the bulk diffusion equations of the solute atoms (Mg atoms) under the action of dislocation drag. The results in the case of a constant external stress show that the bow-out distance of the dislocation has an exponential relation with time, which can be well described by an exponential creep function with a Gaussian distribution in τ. With the increasing strain amplitude, the relaxation strength Δ and relaxation time τ decrease, while the distribution parameter increases. Both activation energy H and pre-exponential factor τ₀ deduced from τ through Arrhenius relation are dependent on strain amplitude.     

Electrical characteristics of light-emitting diode based on poly(p-phenylenevinylene) derivatives: CzEH-PPV and OxdEH-PPV

In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole (μ_h) and electron (μ_e) mobility has been investigated. In order to measure the μ_h and μ_e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ_h and μ_e of MEH-PPV sample was measured to be 10⁻⁶ cm²/V s and 10⁻⁸ cm²/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ_h was similar to μ_e with 10⁻¹⁰–10⁻¹¹ cm²/V s. The μ_h and μ_e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was 10 times higher than that prepared from MEH-PPV. The balance of the μ_h and μ_e plays an important role for the quantum efficiency. We analyze the balance of the μ_h and μ_e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.     

Influence of Pre-strain on the Mechanical Properties of A6111-T4P Sheet with Bake Hardening

A6111 is an aluminum alloy, which exhibits good formability and excellent bake hardening property. This study aimed to reveal the influence of strain path, pre-strain orientation(α) as well as pre-strain level on the mechanical properties of A6111-T4 P sheet under bake treatment through uniaxial tension test.(0–5)% pre-strain, 150–170 °C bake temperature and 20–30 min bake time were considered in the study by referring to the actual production process. The results show that both pre-strain level and strain path play significant roles in improving the material properties. In the condition that tensile orientation(β) parallel with pre-strain orientation(β = α), the yield strength can be remarkably improved, and much higher parameter of n in Hockett–Sherby model can be obtained than those when tensile orientation non-parallel with pre-strain orientation(α≠β). In addition, when the pre-strain level, paint bake process were settled and β = α, the curves obtained in five tension orientations are similar in the plastic deformation stage.     

Synthesis and magnetic structure of the YbMn2Sb2 compound

A neutron diffraction investigation has been carried out on the trigonal La₂O₃-type (hP5, space group , No. 164; also CaAl₂Si₂-type) YbMn₂Sb₂ intermetallic. A two-step synthesis route has been tried in this work, and successfully utilised to prepare single phase samples of this compound. This study shows that YbMn₂Sb₂ presents antiferromagnetic ordering below 120 K. The magnetic structure of this intermetallic consists of antiferromagnetically coupled magnetic moments of the manganese atoms, in the Mn1 (1/3, 2/3, Z_Mn) and Mn2 (2/3, 1/3, 1 − Z_Mn) sites; the direction of magnetic moments of manganese atoms forming a φ and a θ angle, respectively with the X- and the Z-axis. At 4 K the magnetic moment of the Mn1 atom is μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 62(4)°, whilst the Mn2 atom has a magnetic moment μ_Mn = 3.6(1) μ_B, with φ = 0° and θ = 242(4)°. On the other hand, in this compound no local moment was detected on the Yb site.     

Properties of exchange biased Co/Co3O4 bilayer films

Co/Co₃O₄ bilayer films were fabricated by RF sputtering with Co and Co₃O₄ targets. Exchange bias effect in the bilayer films was observed at 80 K by vibrating sample magnetometer. The bias effect disappeared about 240 K slightly lower than the Néel point of CoO and much higher than the Néel temperature of Co₃O₄ about 40 K. To clarify the origin of the exchange bias effect, Auger and X-ray photoelectron spectroscopy were employed and CoO was found at a transition region from Co₃O₄ layer to Co layer due to oxygen diffusion during sputtering. The angular dependence of exchange bias field H_E was obtained to obey function of H_E(θ)=18.06 (kA/m)[−cos θ+0.22 cos 3θ+0.03 cos 5θ−0.01 cos 7θ+].     

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La0.7Ca0.3−xKxMnO3 compounds

The structural and magnetic properties of perovskite oxides La_0.7Ca_0.3−xK_xMnO₃ (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder (σ²). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ₀H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ₀H values. When increasing μ₀H, the structural distortion decreases and for a high applied magnetic field, the M (μ₀H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|ΔS_M|  5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.     

Carbide Precipitation in Austenite of a Titanium-Tungsten-Bearing Low-Carbon Steel

In this study, the carbide precipitation at 925 °C in austenite (γ) of a 0.04C-1.5Mn-0.10Ti-0.39 W (wt%) low-carbon steel was investigated by stress relaxation (SR) high-resolution transmission electron microscopy and atom probe tomography. First-principles calculations were employed to reveal the precipitation mechanism. Results indicate that a high dispersion of W- and Fe-rich MC-type ultrafine carbides (< 10 nm) forms during the very early stage prior to the onset of precipitation determined by SR. These ultrafine carbides possess a B1-crystal structure with a lattice parameter of 3.696 Å, which is quite close to that of γ (3.56 Å). It can significantly decrease the misfit of carbide/γ interface with a cube-on-cube relationship, thus assisting the carbide nucleation. As the time prolongs, a few spherical or polygonal Ti-rich (Ti, W)C particles (18-60 nm) are formed at the expense of the ultrafine carbides by nucleation and growth on them. These (Ti, W)C particles are identified with a “core-shell” structure (Ti-rich core and Ti, W-rich shell), which leads to a better-coarsening resistance compared with pure TiC in Ti steel. Calculation results show that the composition and structure of carbides at certain stage are closely related to a combined effect of W, Fe, and Ti atoms together with interstitial vacancies.     

Subsolidus phase relation and crystal structure of the PrBa2−xSrxCu3O7±δ system

The PrBa_2−xSr_xCu₃O_7±δ solid solution was investigated by means of X-ray powder diffraction in combination with Rietveld analysis. The Sr-doped Pr123 single phase could be synthesized at 950 °C in air. The solubility of PrBa_2−xSr_xCu₃O_7±δ solid solution is 0.2≤x≤0.6. The structure of PrBa_2−xSr_xCu₃O_7±δ is orthorhombic for x=0.2. The structure transforms into tetragonal for 0.3≤x≤0.6. In the PrBa_2−xSr_xCu₃O_7±δ structure, Sr ions can replace Ba ions, the highest value is x=0.6 under our experimental condition. But Sr ions could not replace Pr ions. Furthermore Pr ions could not occupy the sites of Ba ions in the PrBa_2−xSr_xCu₃O_7±δ system. Both ionic radii and chemical properties play an important role in the mutual substitution of Pr, Ba and Sr ions in the Pr123 structure of the PrBa_2−xSr_xCu₃O_7±δ system.     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Effect of Annealing Temperature on the Shape Memory Properties of Cold-Rolled Dual-Phase Ni-Mn-Ga-Gd Alloy

The influence of annealing temperature on the shape memory effect (SME) and recovery ratio of cold-rolled dual-phase Ni₅₈Mn₂₅Ga_16.9Gd_0.1 high-temperature shape memory alloy were investigated. The results showed that the SME can be improved by appropriate annealing. SME of 6.0% and recovery ratio of 98% were observed in the rolled alloy at 550 °C annealing, which were larger than that of the samples annealed at other temperatures. The annealing temperature was in between the recrystallization temperatures of the martensite and γ phase, which leads to the work-hardening state of γ phase kept unchanged during annealing process, and therefore, the critical slip stress of γ phase increased. Thus, the plastic deformation of γ phase reduced during the compression process, and the SME increased.     

Evolution of γ' Particles in Ni-Based Superalloy Weld Joint and Its Effect on Impact Toughness During Long-Term Thermal Exposure

Effects of long-term thermal exposure on γ' particles evolution and impact toughness in the weld joint of Nimonic 263(N263)superalloy were deeply studied at 750℃.Results showed that the precipitates in the weld metal were mainly composed of fine γ' particles,bulky MC carbides,and small M_(23)C_6 carbides.With the thermal exposure time increasing from o to 3000 h,γ' particles in the weld metal grew up from 19.7 nm to 90.1 nm at an extremely slow rate.After being exposed for 1000 h,γ' particles coarsened and some of them transformed into acicular η phase.At the same time,MC carbides decomposed to form η phase and γ' particles.This dynamic transition ensured the slight reduction in impact toughness of the weld metal after the thermal exposure,which indicated the stable serving performance of N263 weld joint.     

Microstructures and hardness of Ti-6Al-4V alloy staging castings under centrifugal field

By means of induction melting technology, Ti-6A1-4V alloy staging casting was made with the same rotation velocity and centrifugal radius. The effects of casting modulus on the grain size, the thickness of lamellar α＋β phase, and the Vickers hardness, as well as the relationships between Vickers hardness, grain size and thickness of lamellar α＋β phase were investigated. The results show that the greater the modulus, the larger the grain size and the thickness of lamellar α＋β phase, and the less the Vickers hardness. The relationship between Vickers hardness and grain size meets the Hall-Petch equation： Hv=353.45＋74.17dG^-1/2. The relationship between the Vickers hardness and the thickness of lamellar α＋β phase is expressed as Hv=2.45d^2α＋β-35.96dα＋β ＋ 476.84.     

Microstructural Evolution of Creep-Induced Cavities and Casting Porosities for a Damaged Ni-based Superalloy Under Various Hot Isostatic Pressing Conditions

The microstructural evolution of casting porosities and creep-induced cavities for a damaged nickel-based superalloy under different hot isostatic pressing(HIP) conditions was investigated in order to understand the effects of HIP parameters on the healing behavior of micropores. A number of small-sized creep cavities formed during long-term service and large-sized porosities formed during the casting process were observed. These microdefects were partially healed after treated at high temperature of 1100 °C combined with 150 MPa pressure for 2 h, together with the formation of the socalled concentrically oriented c0 rafting structure. When HIP temperature was increased to 1150 and 1175 °C, both the amount and the size of the microdefects were decreased. The concentrically oriented c0 rafting around creep cavities became more remarkable, and the primary c0 denuded zone was also formed between the raft structure and the cavity.Energy-dispersive X-ray spectroscopy analysis revealed that the c matrix solute atoms diffused toward the cavity under the concentration gradient, whereas the c0-forming elements diffused in a negative direction. When increasing HIP temperature up to 1200 °C, the micropores were hardly observed, indicating that both casting porosities and creep-induced cavities had almost been healed. Meanwhile, the c0 rafting structure disappeared since HIP temperature was beyond the c0 solvus temperature. It is revealed by the experimental results that the atomic diffusion could mainly dominate the healing process of micropores.     

Creep behavior of ingot and powder metallurgy 6061Al

The creep behavior of 6061Al alloy obtained by ingot metallurgy and powder metallurgy, IM and PM, respectively, has been investigated in the context of published studies on this alloy. The behavior of the IM alloy in a given range of temperatures where the β, Mg₂Si, precipitates are formed, is dominated by dislocation climb-controlled creep and aluminum self-diffusion as rate controlling process. A dependence of the β inter-particle distance, λ, with the applied stress, σ, of the form is found when the creep data are analyzed in the context of the sub-structure invariant model. The superior creep resistance of the PM material can be explained if a threshold stress, σ₀, is brought into the creep equation. This term is the difference between the applied stresses needed to reach a given strain rate in the PM and the IM alloys, and correlates well with a particle–dislocation interaction mechanism according to the Artz–Wilkinson model.     

Molecular Dynamics Simulation of the Evolution of Interfacial Dislocation Network and Stress Distribution of a Ni-Based SingleCrystal Superalloy

The evolution of misfit dislocation network at γ/γ' phase interfaces and the stress distribution characteristics of Ni-based single-crystal superalloys under different temperatures of 0,100 and 300 K are studied by molecular dynamics(MD) simulation.It was found that a closed three-dimensional misfit dislocation network appears on the γ/γ' phase interfaces,and the shape of the dislocation network is independent of the lattice mismatch.Under the influence of the temperature,the dislocation network gradually becomes irregular,all[110]dislocations in the y matrix phase emit and partly cut into the γ′ phase with the increase in temperature.The dislocation evolution is related to the local stress field,a peak stress occurs at γ/γ' phase interface,and with the increase in temperature and relaxation times,the stress in the γ phase gradually increases,the number of dislocations in the y phase increases and cuts into γ' phase from the interfaces where dislocation network is damaged.The results provide important information for understanding the temperature dependence of the dislocation evolution and mechanical properties of Ni-based single-crystal superalloys.     

A model of migrating grain-boundary grooves with application to two mobility-measurement methods

Grain-boundary migration controls grain growth and is important in materials processing and synthesis. The mobility of grain boundaries is usually measured by the “quarter-loop” and Sun–Bauer methods. In these methods, a grain boundary migrates and its tip position along a free surface is recorded to infer the mobility. At the tip, a groove develops to reduce the combined surface energy. The groove is small and adjusts quickly. Thus, in both methods, the groove can be treated at each instant as migrating at constant speed. We study this quasi-steady groove formed via surface diffusion, and find that the groove turns the grain boundary (by angle θ) away from being perpendicular to the free surface. We add this tilting effect into both measurement methods by solving the migrating grain-boundary profiles for arbitrary θ. Computed profiles agree well with two Sun–Bauer experiments in which θ=18 and 30°.     

LaMg2NiH7, a novel quaternary metal hydride containing tetrahedral [NiH4] complexes and hydride anions

A new ternary compound of composition LaMg₂Ni has been found and investigated with respect to structure and hydrogenation properties. It crystallizes with the orthorhombic MgAl₂Cu type structure (space group Cmcm, a=4.2266(6), b=10.303(1), c=8.360(1) Å; V=364.0(1) ų; Z=4) and absorbs hydrogen near ambient conditions (<200 °C, <8 bar) thereby forming the quaternary metal hydride LaMg₂NiH₇. Neutron powder diffraction on the deuteride revealed a monoclinic distorted metal atom substructure (LaMg₂NiD₇: space group P2₁/c, a=13.9789(7), b=4.7026(2), c=16.0251(8) Å; β=125.240(3)°, V=860.39(8) ų; Z=8) that contains two symmetry independent tetrahedral [NiD₄]⁴⁻ complexes with Ni–D bond lengths in the range 1.49–1.64 Å, and six D⁻anions in tetrahedral metal configuration with bond distances in the ranges 1.82–2.65 Å (Mg) and 2.33–2.59 Å (La). The compound constitutes a link between metallic ‘interstitial’ hydrides and non-metallic ‘complex’ metal hydrides.     

Structural, thermodynamic, and electrochemical properties of TixZr1−x(VNiCrMnCoAl)2 C14 Laves phase alloys

The crystal structure of the monoclinic phase η-Al₁₁Cr₂ of the space group C2/c, a ≈ 1.76 nm, b ≈ 3.05 nm, c ≈ 1.76 nm, β ≈ 90° [L.A. Bendersky, R.S. Roth, J.T. Ramon, D. Shechtman, Metall. Trans. A 22A (1991) 5] has been determined by single-crystal X-ray diffraction. The structure model, refined to a final R value of 0.0441, has the composition of Al_83.8Cr_16.2. a = 1.77348(10) nm, b = 3.04555(17) nm, c = 1.77344(10) nm, monoclinic angle β = 91.0520(12)°. There are 80 (66Al + 14Cr) independent atomic positions in a unit cell, of which all Cr atom sites and 8 Al atom sites have icosahedral coordination. These icosahedra are interconnected forming icosahedral chains along , (1 0 1) icosahedral layer blocks as well as a three-dimensional icosahedral structure.     

Sub-solvus Cellular Recrystallization and P Phase Formation in a Single-Crystal Superalloy Containing Re

Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.