Anisotropy in elasticity and thermal conductivity of monazite-type REPO4 (RE = La,Ce, Nd,Sm, Eu and Gd) from first-principles calculations

Starting from theoretical calculations based on LSDA, the authors compute the lattice parameters, cohesive energies and formation enthalpies of monazite-type REPO₄ compounds. The calculated values are satisfactory compared with the experimental results from the elastic constants obtained, the mechanical moduli are evaluated using the strain–stress method. The predicted bulk, Young’s and shear moduli are in good agreement with the experiments. It is shown that the mechanical moduli are low (<200 GPa) and also increase from LaPO₄ to GdPO₄. The three-dimensional contours and their planar projections of Young’s modulus are plotted to illustrate the anisotropy in elasticity. It is found that Young’s moduli of all monazite-type REPO₄ show strong dependence on direction. The linear thermal expansion coefficients are calculated using the empirical method, and the values are in the range 9 × 10^?6–12 × 10^?6 K^?1. Using Clarke’s and Slack’s models, the thermal conductivities of REPO₄ compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO₄ are also explained based on the observed monazite to zircon-type transformation in experiment. Solving the Christoffel equation for monoclinic symmetry, the anisotropy in thermal conductivity is investigated. The results indicate that the total lattice thermal conductivities of monazite-type REPO₄ show weak dependence on direction. Meanwhile, their sound velocities exhibit strong anisotropic properties.     

Dielectric investigation of MM(PO4)2 double orthophosphates (M = Ca, Sr, Ba, Pb; M = Ti, Zr, Hf, Ge, Sn)

M^IIM^IV(PO₄)₂ (M^II = Ca, Sr, Ba, Pb; M^IV = Ti, Zr, Hf, Ge, Sn) ceramics were prepared by solid state reaction method and consolidated by spark plasma sintering in order to study their electrical properties. The dielectric study performed at room temperature was aimed to determine the basic electrical properties of these compounds. Maxwell-Wagner polarization contributions, which are active at low frequencies cause a strong extrinsic increase of both real and imaginary part of permittivity as well of the dc-conductivity for BaZr(PO₄)₂, BaTi(PO₄)₂, BaHf(PO₄)₂, and SrGe(PO₄)₂ double orthophosphates. Moderate real (ε^′r=20-150) and imaginary low-frequency permittivity (ε^′r=7-300) and typical Debye relaxation with relaxation time in the range of ms is typical for: PbHf(PO₄)₂, PbZn(PO₄)₂, and CaGe(PO₄)₂. At high-frequency (f = 10⁹ Hz), the ceramics have permittivities of 2.29÷8.02 and tangent loss of 0.003÷0.153. The compounds SrGe(PO₄)₂, BaGe(PO₄)₂, BaZr(PO₄)₂ and BaSn(PO₄)₂ have excellent high-frequency dielectric characteristics, with losses of 3÷6% and permittivity slightly above 2 and are possible candidate as microwave ceramics.     

Effect of Ti content and nitrogen on the high-temperature oxidation behavior of (Mo,Ti)5Si3

The binary intermetallic compounds Mo₅Si₃ (T1) and Ti₅Si₃ are prone to rapid oxidation below 1000 °C. Recent investigations on (Mo,Ti)₅Si₃, however, revealed that macro-alloying with 40 at.% Ti can result in a very good oxidation resistance in a wide temperature range (750–1300 °C) due to the formation of a duplex layer composed of a silica matrix with dispersed titania. Additionally, Ti decreases density making (Mo,Ti)₅Si₃ a promising key constituent of quaternary Mo-Si-B-Ti alloys considered for ultrahigh temperature structural applications. The aim of this study is to obtain an in-depth understanding of the influence of different Ti concentrations as well as of nitrogen on the oxidation behavior of (Mo,Ti)₅Si₃ at intermediate and elevated temperatures. The microstructure and oxidation mechanisms were analyzed using various experimental techniques. The experimental results were supported by ab initio and thermodynamic calculations.     

Oxygen incorporation in M2AlC (M = Ti,V, Cr)

Oxygen incorporation in the MAX phases Ti₂AlC, V₂AlC and Cr₂AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti₂AlC and V₂AlC, but is incorporated interstitially in the Al layer of Cr₂AlC, even for carbon-deficient Cr₂AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr₂AlC and Cr₂Al were investigated in order to study oxygen incorporation in carbon-deficient Cr₂AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr₂AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation.     

Extraction of Metal(Ti,V,Mo)-oxyl Cations by Monoacidic Phosphates and Phosphonates

The extraction of TiO~(2+),VO~(2+),VO_2~+ and MoO_2~(2+)by HDEHP,HEHEHP and HDTMPP was studiedrespectively.The orders of extraction ability for the three extractant systems are as follows:for TiO~(2+),VO~(2+):HDEHP>HEHEHP>HDTMPP;for VO_2~+, MoO_2~(2+):HDEHP< HEHEHP< HDTMPP.The experimental results were discussed from the point of view of extractant structures.     

First principles study of the electronic and magnetic properties of semi-Heusler alloys NiXSb (X = Ti, V, Cr and Mn)

The electronic density of states (DOS), magnetic moments and band structure of semi-Heusler alloy NiXSb (where X = Ti, V, Cr and Mn) has been studied by using the first principles full-potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. From the observations, NiTiSb shows the possibility of half-metallic ferromagnet (HMF) behavior with a band gap of 0.53 eV and an effective moment of 0.35 μ_B. The alloys like NiVSb, NiCrSb and NiMnSb are HMF with band gap of 0.49 eV, 0.38 eV and 0.48 eV and an effective moment of 1.995 μ_B, 3.01 μ_B and 3.99 μ_B respectively. DOS and band structure result shows the 3d states of Ni overlap with 3d states of X atoms suggesting hybridization between them. The exchange-splitting of Ni-3d and X-3d state electrons lead to localized spin moment which determines the HMF behavior of NiXSb. The results obtained are compared and found to be in close agreement with the available data.     

WC-(W,Ti)C-(Ta,Nb)C-Co涂层硬质合金的组织与性能

用控制烧结工艺制备了均质基体结构与梯度基体结构的WC-(W,Ti)C-(Ta,Nb)C-Co硬质合金,采用化学气相沉积工艺在合金表面涂上TiCN/Al203/TiN涂层,制备成相应的涂层硬质合金及刀片.运用金相观察、扫描电镜分析、三点抗弯强度测试及切削性能测试,研究了梯度基体结构对涂层WC-(W,Ti)C-(Ta,Nb)C-Co硬质合金的组织与力学性能影响.结果表明,采用梯度烧结工艺,可以在合金的表层形成梯度韧性区,该韧性区可以提高裂纹扩展阻力.所制备的梯度基体结构合金的抗弯强度明显超过均质基体结构合金,而相应涂层硬质合金刀片在保持耐磨性能的同时能显著提高抗冲击性能.     

Characterization of NbC and (Nb, Ti)N nanoprecipitates in TRIP assisted multiphase steels

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM) we have investigated the size distribution and the structural properties of (Nb, Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb, Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5–20 nm in size) and to have a face centred cubic (fcc) crystal structure with lattice parameter a = 4.36 ± 0.05 Å. In contrast, the (Nb, Ti)N precipitates were found to have a broader size range (5–150 nm) and to have a fcc crystal structure with lattice parameter a = 8.09 ± 0.05 Å. While the NbC precipitates were found to be randomly oriented, the (Nb, Ti)N precipitates have a well-defined Nishiyama–Wasserman orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. Density functional theory calculations were performed for various stoichiometries of NbC_x and Nb_xTi_yN_z phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.     

Optical properties of CeNi5 and CeNi4M (M = Al, Cu) compounds

The experimental study of the optical properties of CeNi₅, CeNi₄Cu and CeNi₄Al compounds was carried out in the 0.083-5.64 eV energy range using the ellipsometry method. The optical constants, dielectric functions and electronic parameters (plasma and relaxation frequencies) were determined. The energy dependencies of the optical interband conductivities are discussed by using the available information on the electronic band structure of these compounds. In the ternary alloys the optical spectra show the presence of peculiarities related to effect of Cu or Al substitution at Ni sites.     

Synthesis of (Ti,W, Mo,V)(C,N) nanocomposite powder from novel precursors

(Ti, W, Mo, V)(C, N) nanocomposite powders with globular-like particle of ∼10–100 nm were synthesized by a novel method, namely carbothermal reduction–nitridation (CRN) of complex oxide–carbon mixture, which was made initially from salt solution containing titanium, tungsten, molybdenum, vanadium and carbon elements by air drying and subsequent calcining at 300 °C for 0.5 h. Phase composition of reaction products was discussed by X-ray diffraction (XRD), and microstructure of the calcined powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that the synthesizing temperature of (Ti, W, Mo, V)(C, N) powders was reduced greatly by the novel precursor method. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N) is at only 1200 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the complex oxide–carbon mixture and its unusual honeycombed structure.     

摩擦速度对原位自生钢基复合材料Fe(Ti,W,Cr,V,Nb)Cp磨损性能的影响

用铸造的方法制备了原位自生复合碳化物(Ti,W,Cr,V,Nb)Cp增强钢基复合材料(In-situMMCs),并对该复合材料的高速磨损性能及磨损机理进行了研究。结果表明,原位自生复合材料中自生碳化物颗粒细小、圆整、分布均匀,自生碳化物体积分数达到42.8%;在低速150N载荷下,自生复合材料的耐磨性能随着自生碳化物体积分数的增加而提高;摩擦系数随摩擦速度的增大先减少后增大,自生碳化物体积分数大的自生复合材料的摩擦系数先快速减小后慢速增大,磨损率先减小后迅速增加。     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Formation Kinetics of Unsaturated (Ti, W)C Solid Solution

ＦｏｒｍａｔｉｏｎＫｉｎｅｔｉｃｓｏｆＵｎｓａｔｕｒａｔｅｄ（Ｔｉ，Ｗ）ＣＳｏｌｉｄＳｏｌｕｔｉｏｎＬｉｎＣｈｅｎ’ｇｕａｎｇ，ＤｅｎｇＦｅｎｇｘｉａｎｇ，ＨｅＣｏｎｇｘｕｎａｎｄＧａｏＺｈａｏｚｕ（林晨光）（邓凤翔）（贺从训）（高兆祖）（Ｇｅｎｅｒ...     

Mechanical behaviors of NiAl-Cr(Mo)-based near eutectic alloy with Ti, Hf, Nb and W additions

Effects of Ti, Hf, Nb and W alloying elements addition on the microstructure and the mechanical behaviors of NiAl-Cr(Mo) intermetallic alloy were investigated by means of XRD, SEM, EDX and compression tests. The results show that Ni-31Al-30Cr-4Mo-2(Ti, Hf, Nb, W) alloy consists of four phases: NiAl, α-Cr solid solution, Cr2Nb and Ni2Al(Ti, Hf). The mechanical properties are improved significantly compared with the base alloy. The compression yield strength at 1 373 K is 467 MPa and the room temperature compression ductility is 17.87% under the strain rate of 5.56×10-3 s-1, due to the existence of Cr2Nb and Ni2Al(Ti, Hf) phases for strengthening and Ti solid solution in NiAl matrix and coarse Cr(Mo, W) solid solution phase at cellular boundaries for ductility. The elevated temperature compression deformation behavior of the alloy can be properly described by power-law equation: ε =0.898 σ8.47exp[-615/(RT)].     

Investigation of the corrosion for Mg-xGd-3Y-0.4Zr (x = 6, 8, 10, 12 wt%) alloys in a peak-aged condition

The corrosion behaviour of Mg-6Gd-3Y-0.4Zr (GW63K), Mg-8Gd-3Y-0.4Zr (GW83K), Mg-10Gd-3Y-0.4Zr (GW103K) and Mg-12Gd-3Y-0.4Zr (GW123K) were investigated in peak-aged condition using immersion and salt spraying tests and electrochemical measurements in 5% NaCl solution. The corrosion resistances of Mg-xGd-3Y-0.4Zr alloys were found to decrease with the increase of Gd content from 6% to 10% and then increase from 10% to 12%. The corrosion products of Mg-xGd-3Y-0.4Zr alloys were discovered to be composed of voluminous tiny erect flakes by FE-SEM, and the compactness of the corrosion films decreases with the increase of Gd content. The results of AES revealed that the corrosion film is enriched with Gd and Y. The potentiodynamic polarization curves showed that the overpotentials of cathodic hydrogen evolution on GW63K and GW83K are much higher than on GW103K and GW123K. The results of EIS are in good agreement with the morphologies of the corrosion products and the corrosion rates of the corrosion tests.     

Influence factors and microstructure evolution during preparation of nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders

Nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders with crystalline size of about 35 nm were synthesized at 1300 °C from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation techniques. Influence of main technological parameters was investigated by X-ray diffraction, and microstructure of the milled powders and reaction products was studied by scanning electron microscopy. The results show that the phase evolution of TiO₂ follows TiO₂ → Ti₃O₅ → Ti(C, N), and (Ti, W, Mo, V)(C, N)–Ni composite powders with higher nitrogen content and smaller crystalline size can be produced by introducing high nitrogen pressure. By contrast with high nitrogen pressure, high synthesizing temperature and long isothermal time can contribute to dissolution of W, Mo and V atoms into Ti(C, N). In addition, synthesizing temperature has a significant effect on the microstructure evolution of (Ti, W, Mo, V)(C, N)–Ni composite powders.     

First-principles investigation of electronic,mechanical and thermodynamic properties of L12 ordered Co3(M,W) (M = Al,Ge, Ga) phases

Studies were carried out on the equilibrium structural, temperature-dependent mechanical and thermodynamic properties of the Co₃(M, W) (M = Al, Ge, Ga) phases in terms of first-principles calculations. The results of the ground-state elastic constants revealed that Co₃(M, W) phases are mechanically stable and possess intrinsic ductility. It was found that the elastic heat-resistant properties of Co₃(Ge, W) phase are inferior to those of Co₃(Al, W) and Co₃(Ga, W). Analyzing the charge density difference provides the explanation that the sharp decrease in mechanical properties is mainly due to the weakening of Co–Ge bonding at elevated temperatures for Co₃(Ge, W). The elastic anisotropy as a function of temperature is discussed using a universal index. It is observed that Co₃(M, W) phases show a high degree of elastic anisotropy. The degree of elastic anisotropy could be significantly decreased by an increase in temperature for Co₃(M, W). The lattice vibration is treated with the quasiharmonic phonon approach, considering both the vibrational and thermal electronic contributions. The thermodynamic properties as a function of temperature are computed without any adjustable parameters, including heat capacity, entropy, enthalpy and thermal expansion coefficient.     

Investigation on temperature dependence of photoluminescence in Sr1.7Eu0.3MxCeO4.15+x/2 (M = Li, Na, K, x = 0, 0.3) red phosphors

The temperature-dependent luminescence of Sr_1.7Eu_0.3M_xCeO_4.15+x/2 (M = Li⁺, Na⁺, K⁺, x = 0, 0.3) samples was investigated and discussed in the temperature range from 303 to 573 K. It is found that the thermal quenching temperature of samples decreases with Li⁺-/Na⁺-doping but increases with the incorporation of K⁺. We suggest that these observations are resulted from two factors. One is that the incorporation of Li⁺/Na⁺/K⁺ ions reduces the strength of potential field at the O²⁻ sites, and then results in a red-shift of the Eu-O charge transfer band. The other is that Δr expands with Li⁺-/Na⁺-doping but shrinks with K⁺-doping. We consider that it is a feasible way to adjust the temperature-dependent luminescence properties of materials by introducing appropriate impurities.     

In situ STM study of the duplex passive films formed on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (ECSTM) investigations of the anodic Cu(I)/Cu(II) duplex passive layers grown on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH are reported. The outer Cu(II) part of the duplex film formed on both substrates is crystalline with a terrace and step topography. The observed lattices are consistent with a bulk-like terminated CuO(0 0 1) surface on both substrates. This common crystallographic orientation is explained by the hydroxylation of the otherwise polar and unstable oxide surface at the passive film/electrolyte interface. The epitaxy of the oxide layers is governed by the parallel alignment of the close packed directions of the CuO outer layers and Cu₂O inner layers on both substrates. A granular and amorphous layer covering the crystalline CuO(0 0 1) oxide has been observed on Cu(0 0 1) but not on Cu(1 1 1). It is assigned to a film of copper hydroxide corrosion products formed by a dissolution-precipitation mechanism. Its absence on the passivated Cu(1 1 1) surface is explained by the higher stability of the Cu₂O(1 1 1) precursor oxide formed on this substrate in the initial stages of growth of the duplex passive film, resulting in a lower amount of dissolved copper.