The crystal structure of (Nb0.75Cu0.25)Sn2 in the Cu-Nb-Sn system

During the processing of superconducting Nb₃Sn wire, several intermediate intermetallic phases including a previously encountered Cu-Nb-Sn phase show up. The yet unknown crystal structure of this phase is now identified by a combination of different experimental techniques and database search to be of the hexagonal NiMg₂ type with a proposed composition of about (Nb_0.75Cu_0.25)Sn₂. The structure determination started from an evaluation of the lattice parameters from EBSD Kikuchi patterns from quenched material suggesting hexagonal or orthorhombic symmetry. A database search then led to the hexagonal NiMg₂ type structure, the presence of which was confirmed by a Rietveld analysis on the basis of high energy synchrotron X-ray powder diffraction data. Assuming a partial substitution of Nb in orthorhombic NbSn₂ by Cu, the change of the valence electron concentration provokes a structural transformation from the CuMg₂ type for NbSn₂ to the NiMg₂ type for (Nb_0.75Cu_0.25)Sn₂. In the previous literature the (Nb_0.75Cu_0.25)Sn₂ phase described here has occasionally been referred to as Nausite.     

Hf对Zr-Al-Ni-Cu金属玻璃弛豫行为的影响

通过测定快淬态和退火态比热与温度的关系,研究了Hf元素对Zr₇₀Al_7.5Ni₈Cu_14.5（70Zr）、Zr₅₅Al₁₀Ni₅Cu₃₀（55Zr）和(Zr_0.75Hf_0.25)₆₅Al_7.5Ni₁₀Cu_17.5（65Zr_0.75Hf_0.25）厘米级金属玻璃焓和硬度弛豫的影响。结果显示：70Zr和55Zr合金的结构弛豫表现为单峰现象,在退火温度（T_a）接近玻璃转变温度（T_g）时出现弛豫峰;而65Zr_0.75Hf_0.25合金的结构弛豫表现为双峰现象,分别在523和648 K（接近T_g）时出现焓弛豫峰。70Zr和55Zr合金在T_g温度附近出现明显的单峰弛豫行为,表明合金在T_g之前的整个温度范围内具有较高的抵抗退火诱导结构弛豫的能力;另一方面,65Zr_0.75Hf_0.25合金在523 K附近出现一个弛豫子峰,可能是由于Zr-Hf原子对的键合性较弱,混合焓接近于零,随后在T_g附近出现明显的主弛豫。65Zr_0.75Hf_0.25和(Zr_0.5Hf_0.5)₆₅Al_7.5Ni₁₀Cu_17.5（65Zr_0.5Hf_0.5）合金的硬度也表现出类似的双峰现象,导致第一峰的T_a与焓弛豫峰的T_a一致。结果表明,满足非晶形成三原则的Zr、Al、Ni和Cu组成的类二十面体中程有序结构在低T_a温度下保持稳定;只有背离非晶形成三原则的65Zr_0.75Hf_0.25和65Zr_0.5Hf_0.5金属玻璃焓弛豫和硬度弛豫出现了双峰,表明形成大块金属玻璃非必要组元的增加将导致合金结构在低温退火过程中的不稳定性。     

Phase stability of magnesium-rare earth binary systems from first-principles calculations

The comprehensive investigation to cohesive properties of 344 intermetallic compounds in 17 Mg-RE (RE = Sc, Y, and Lanthanide elements) binary system has been carried out systematically in the framework of density-functional theory (DFT) type first-principles with the generalized gradient approximation (GGA). The calculated properties at present work; including total energy, enthalpy of formation, equilibrium volume, bulk modulus, and electronic structure, was consistent with the experimental data. It was convinced that both D0₃-Mg₃RE and B2-MgRE were stable compounds in Mg-RE systems except Mg₃Eu, Mg₃Yb and Mg₃Lu in Mg₃RE series and MgYb for B2-MgRE branch extracting from calculated results.     

Thermodynamic reassessment of the Au–Te system

The Au–Te binary system has been thermodynamically reassessed using the CALPHALD method. The excess Gibbs energy of the liquid phase was formulated with the Redlich–Kister expression and the AuTe₂ intermetallic phase was treated as a stoichiometric compound. A set of self-consistent thermodynamic parameters for each phase has been obtained. Good agreement is reached between the calculated and experimental results. Additionally, the standard formation enthalpy and enthalpy of fusion of AuTe₂ intermetallic phase have also been calculated.     

Hydrogen-sorption and thermodynamic characteristics of mechanically grinded TiH1.9 as studied using thermal desorption spectroscopy

Isobaric thermal desorption spectroscopy and X-ray diffraction analysis were used to study the influence of mechanical dispersion during high-energy milling γ-TiH_1.9 hydride upon its hydrogen-sorption characteristics, temperature and enthalpy of the γ → β (TiH_1.9 → Ti[H]_β) phase transition at isobaric heating the sample under hydrogen atmosphere with speed 3°/min. Isobars of hydrogen thermal desorption in the regions of the γ and β phases of the Ti-H₂ system at pressures of 0.1, 0.25, 0.315 and 0.45 MPa of hydrogen in the reactor have been derived. Experimental data obtained for initial titanium hydride and mechanically grinded for 20 min in a planetary ball mill have been used for construction of Van’t Hoff plots and for determination of enthalpy of formation of γ-hydride from solid solution of hydrogen in bcc titanium. Our experimental data reveal that 20 min high-energy influence on titanium hydride powder leads to increasing the specific surface of the samples from 0.13 to 8.58 m²/g and to significant (more than 250°) decreasing the temperature of the beginning of hydrogen release when heating the sample (i.e., to a decrease of thermal stability of mechanically activated TiH_1.9). However, mechanical dispersion does not change the temperature of the γ → β phase transition. It has been established that high-energy milling TiH_1.9 powder causes the effect of a decrease of enthalpy of the formation of γ-hydride from 248 kJ/mole H₂ to 175 kJ/mole H₂.     

Influence of phase transition on the tribological performance of arc-evaporated AlCrVN hard coatings

V-alloyed AlCrN hard coatings were synthesised by reactive arc-evaporation in an industrial-sized deposition system at bias voltages ranging from − 40 to − 150 V. X-ray diffraction analysis has shown that higher energetic growth conditions stabilise the desired metastable face-centered cubic (fcc) crystal structure of Al_0.70Cr_0.05V_0.25N even at very high Al/Cr ratios resulting in hardness values comparable to Al_0.70Cr_0.30N. Ball-on-disc tests were used to assess the friction at 700 °C where Al_0.70Cr_0.05V_0.25N coatings revealed a generally lower coefficient of friction due to the formation of a tribolayer containing the lubricious oxide V₂O₅ as evidenced by Raman spectroscopy. The single-phase fcc-Al_0.70Cr_0.05V_0.25N coating appears to be more oxidation resistant leading to a reduced formation of V₂O₅ and, hence, an increase in friction. The cutting performance was evaluated by conducting side milling tests under dry conditions, where Al_0.70Cr_0.05V_0.25N coatings showed a competitive performance regardless of the growth conditions.     

功率对激光熔覆(Fe0.25Co0.25Ni0.25Cr0.125Mo0.125)86B14涂层组织均质性及磨损性能的影响

目前激光熔覆缺少对涂层组织、相结构纵向均质性与性能关联的研究。采用激光熔覆技术,选取不同的激光功率,制备(Fe_0.25Co_0.25Ni_0.25Cr_0.125Mo_0.125)₈₆B₁₄高熵合金涂层;借助电子探针(EPMA)、扫描电子显微镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)等观察涂层微观组织与物相纵向分布,利用显微硬度计和摩擦磨损试验机测试涂层不同深度部位显微硬度及磨损性能,分析激光功率对熔覆(Fe_0.25Co_0.25Ni_0.25Cr_0.125Mo_0.125)₈₆B₁₄涂层纵向组织、物相分布影响规律及磨损性能。结果表明：三种功率下,涂层均由BCC+FCC相、硬质相Mo2B组成,Mo2B在枝晶间富集。随着功率的增加,涂层中底部显微组织由细枝晶向粗大的柱状晶转变。三种涂层硬...     

DFT studies of hydrogen storage properties of Mg0.75Ti0.25

Absorption energies of hydrogen in Mg_0.75Ti_0.25 alloys as a function of the hydrogen concentration were calculated using Density Functional Theory. Four types of structures of alloys and their hydrides including TiAl₃, ZrAl₃, AuCu₃, and segregated types of structures were considered. The stability of the configurations, and the structural and electronic bonding properties were studied. The hydrogenation properties depend highly on the structure of the alloys. The ordered alloys have very similar properties to that of pure Mg. For segregated alloys, the hydrogenation properties can be divided to Ti-like, ordered alloy-like and Mg-like from low to high hydrogen concentration. The formation energies show that for the four structures, segregated Mg_0.75Ti_0.25 is favored for alloys, whereas TiAl₃ type of Mg_0.75Ti_0.25H₂ are favored for hydrides. Therefore hydrogen induced structural rearrangement of the intermetallic structures of the Mg_0.75Ti_0.25 might occur upon hydrogen cycling. For the non-homogenous Mg-Ti-H system, further phase segregation of Ti in Mg might occur. Partial dehydrogenation with some hydrogen remaining in the Ti-rich region may improve reversibility.     

Microstructure,phase transformation and mechanical property of Nb-doped Ni–Mn–Ga alloys

This study investigated the microstructure, phase transformation and mechanical property of (Ni_49.8Mn_28.5Ga_21.7)_100-xNb_x (x = 1, 3, 6, 9) alloys. The Nb1 alloy exhibited a single austenite phase at room temperature. With increasing Nb content for Nb3, Nb6 and Nb9, the alloy changed to a dual phase consisting of austenitic matrix and Nb-rich second phase with a hexagonal structure, and the amount of the second phase increased with the increase of Nb content. The martensitic transformation temperature and Curie temperature were changed and the transformation enthalpy was gradually reduced with increasing Nb content. The change of martensitic transformation temperature and Curie temperature was related to the introduction of Nb in the Ni–Mn–Ga structure that decreased valence electron concentration (e/a), increased unit cell volume and reduced magnetic exchange of the alloys. The decrease of transformation enthalpy was mainly attributed to the formation and increase of the Nb-rich second phase that reduced volume fraction of the matrix taking part in phase transformation. All the alloys presented a similar compression behavior with progressively fracturing characters (occurrence of several stress drops before complete fracturing). The fracture strength was slightly enhanced with increasing Nb content from Nb0 to Nb9, but the ductility has no apparent improvement.     

Enthalpy of formation of Au-Sn intermetallic phases. Part II

The technique of solution calorimetry with liquid Sn has been used for the determination of enthalpy of formation of intermetallic compounds from the Au-Sn system. The intermetallic phases were prepared and homogenized in a glow-box operated in high purity argon. They were analyzed by an X-ray diffraction method before the calorimetric investigations to confirm their crystallographic structure. The obtained experimental values of enthalpy of formation were −5.8 ± 0.3 and −1.3 ± 0.4 kJ/mole of atoms for ζ′-Au₅Sn, and ζ-Au₅Sn of the composition Au₈₉Sn₁₁, respectively. Using the DTA technique the transition temperatures were measured for the following phases: η-AuSn₄, ?-AuSn₂, δ-AuSn, ζ′-Au₅Sn and ζ-Au₅Sn (Au₈₉Sn₁₁). It was found that the obtained data in the study agreed very well with the results previously presented in literature, except for the ζ-Au₅Sn (Au₈₉Sn₁₁) intermetallic phase, for which a difference of about 50° was observed for the liquidus temperature.     

Prediction of structure and elastic properties of AlCrFeNiTi system high entropy alloys

The effect of alloy compositions on structure and elastic properties of single-phase AlCrFeNiTi and AlCrFeNiTiX (X = V, Mn, Co, Zn, Zr, Nb, Mo and La) high entropy alloys was investigated by calculating solid solution characteristics of alloys combining with the first principles method. The obtained formation enthalpy, cohesive energy and mechanical stability indicated AlCrFeNiTi and AlCrFeNiTiX (X = V, Mn, Co, Nb and Mo) HEAs adopt the body centered cubic structure instead of face centered cubic structure. There is a rather good agreement between theoretical structure predictions and the classic criteria of valence electron concentration VEC. A detailed investigation on electronic structure of AlCrFeNiTi and AlCrFeNiTiX alloys revealed the bonding behavior of alloys. In addition, the calculated results of polycrystalline elastic parameters confirmed the ductility of AlCrFeNiTiX alloys would weaken with increasing mixing enthalpy ΔH_mix (or decreasing atomic size difference δ), and the increase in ΔH_mix is predicted to decrease the elastic anisotropy. Furthermore, we predicted that the present high entropy alloys should be more elastically isotropic when the corresponding the mixing enthalpy ΔH_mix has a larger value.     

Point defect structures of YAl2 and ZrCo2 Laves phase compounds by first-principles calculations

In Laves phase alloys with prominent size mismatch between constituent atoms and/or large negative enthalpy of formation, the existence of vacancies as the dominant point defect type is often suggested. However, there are not enough experimental data to prove or disprove these arguments. Employing first-principles calculations, we study the point defect structures of YAl₂ and ZrCo₂ C15 Laves phases, as both compounds exhibit large size mismatch between constituent atoms, and large negative enthalpy of formation. We find that one must go beyond the simple geometrical or enthalpy arguments in determining the point defect structures of these alloys. In both compounds, the point defect structure is found to be dominated by the anti-site defects on the larger atom-rich side of the stoichiometry.     

Dielectric properties and point defect behavior of antimony oxide doped Ti deficient barium strontium titanate ceramics

The microstructures and dielectric properties of Sb₂O₃-doped Ti deficient barium strontium titanate ceramics prepared by solid state method were investigated with non-stoichiometric level and Sb₂O₃ content by SEM, XRD and LCR measure system. It is found that with the increase of δ, (Ba_0.75Sr_0.25)Ti_1–δO_3–2δ ceramics transform from single phase solid solutions with typical cubic perovskite structure to multiphase compounds while (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics remain to be single-phase with the increasing Sb₂O₃ content. The distortion of the ABO₃ perovskite lattice caused by V_Ti″″ and V_o^?? induces the drop of Curie temperature and the rise of relative dielectric constant in (Ba_0.75Sr_0.25)Ti_1–δO_3–2δ ceramics with increasing δ value. The orientation of V_o^?? elastic dipoles results in the domain-wall pinning and thus the reduction of the dielectric loss. With increasing Sb₂O₃ content, the relative dielectric constant, dielectric constant maximum and Curie temperature of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics decrease dramatically while the dielectric loss increases.     

First-principles study on a new type of quaternary carbonitride VWCN with outstanding mechanical properties

Ceramics with outstanding mechanical properties have great potential applications in industrial fields ranging from cutting tools to scratch and wear resistant films. Here, a new type of transition-metal carbonitride VWCN is proposed by introducing the fourth element W into the ternary V₂CN system. The value of valence electron concentration (VEC) per two atoms was tuned to enhance the hardness and the corresponding ideal strengths as well as the electronic stability. By an improved structure searching method, an orthorhombic Pmm2 structure of VWCN was firstly uncovered and verified as its ground-state phase. The relative formation enthalpy calculations show that it is the most energetically favorable phase in the pressure range of 0–300 GPa. The lattice dynamical stability was also checked by the phonon spectrum calculations. The systematic study of mechanical property showed this Pmm2-VWCN not only has a great improvement of hardness value from 23 GPa for V₂CN to 35 GPa, but also has a large pure shear strength of 31 GPa. Meanwhile, it also exhibited surprisingly high ductility and toughness relative to V₂CN. Electronic analysis revealed that the mechanical enhancement induced by the introduction of the element W mainly stems from the variation of structure and the consequent presence of hybridizations between TM-d and LE-p orbitals, which is also responsible for the minimized E_F DOS by exactly separating the bonding and antibonding states.     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

Mössbauer investigation of Sn diffusion and segregation in grain boundaries of polycrystalline Nb

The parameters of nuclear γ resonance (Mössbauer) spectra on ^119mSn nuclei inserted into grain boundaries of polycrystalline Nb have been determined at the annealing temperatures of 680 to 994 K [0.25–0.36 melting temperature (T _m)]. Two components are observed in the nuclear emission γ resonance spectra at all of the annealing temperatures considered. One of them (component 1) is suggested to result from the ^119mSn atomic probes localized in grain boundary cores, while the other one (component 2) corresponds to that located in the near-boundary areas. Diffusion annealing temperature dependences of both components parameters have been analyzed. The coefficients and enthalpy of Sn grain-boundary segregation are determined based on this analysis, as well as the ratio of the diffusion pumping zone extension to the grain boundary width and the enthalpy of the atomic probes “pumping” from grain boundaries into the bulk.     

Thermodynamic assessment of the Fe-U binary system

The Fe-U system is one of the binaries of the U-Fe-Zr-O quaternary system that is important for a safe nuclear program. A new thermodynamic assessment of Fe-U is presented, taking into account some recent thermodynamic measurements: enthalpy of formation and of melting as well as heat capacities for both intermetallic compounds Fe₂U and FeU₆. The calculated phase diagram and thermodynamic data generally agree very well with the experimental values. The calculated temperature of the peritectic transformation [FeU₆ ↔ liquid + bcc-U] is equal to 1104 K, which is higher than in the previous assessments, but in agreement with Labroche’s value. However, the experimental melting enthalpy of FeU₆ is not reproduced by the present set of thermodynamic parameters.     

Thermodynamic Modeling of the Succinonitrile-Water System

Succinonitrile-water (SCN-H₂O) system is a widely used transparent metallic alloy system due to its analog solidification behavior to metals. In the present work, Gibbs energy of pure succinonitrile was derived utilizing temperature as well as enthalpy of transformations, and temperature dependencies of heat capacity available in the literature. The phase diagram for the binary SCN-H₂O system was assessed via the CALPHAD approach using phase equilibrium data available in the literature. Self-consistent thermodynamic parameters were obtained. A good agreement between the experimental and calculated data for the phase diagram has been achieved. The present work contributes to the development of the thermodynamic database of the SCN-H₂O system that can be incorporated into thermodynamic and kinetic codes for computational simulations.     

Effect of Fe2O3 on Sm-doped ceria system solid electrolyte for IT-SOFCs

The effect of Fe₂O₃ addition (0–2.5 mol%) on the densification, crystal structure, ionic conductivity, and aging behavior of Ce_0.8Sm_0.2O_1.9 (SDC) was studied. The addition of Fe₂O₃ promotes densification, reducing sintering temperature by ～100–150 °C. X-ray diffraction showed that these materials exhibit a fluorite structure; a second phase of Fe₂O₃ is identified when Fe₂O₃ content was ≥1.5 mol%. Impedance spectroscopy measurements indicated that SDC with 0.25 mol% Fe₂O₃ has the highest conductivity, about 10% higher than that of SDC at 700 °C. Conversely, a reduction in conductivity is observed in all samples after aging in air at 800 °C for 120 h. Because of the harmful effect of aging, conductivity rapidly decreases as Fe₂O₃ content in the samples exceeded 0.25 mol%. However, SDC with 0.25 mol% Fe₂O₃ shows the same magnitude of decrease in conductivity compared with that of SDC after aging. This indicates that SDC with 0.25 mol% Fe₂O₃ continues to present the best conductivity even after high-temperature aging.