Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy

Precipitation behavior of Heusler phase (Ni₂AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni₂AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni₂AlHf phase was slightly reduced. In addition, the spherical Ni₂AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni₂AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni₂AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl↔Ni₂AlHf phase transformation.     

Failure analysis of Fe–Co–Ni alloy,cracked during processing

Cracks were observed in Fe–Co–Ni cast alloy bars. The fracture surfaces were observed under stereo, optical and scanning electron microscopes (SEM). The chemical composition details were studied using SEM equipped with energy dispersive spectrometer (EDS). The study revealed segregation of titanium which subsequently formed the brittle phases leading to cracking in castings. Some experiments were also designed to confirm various observations.     

Transition from ferro- to ferrimagnetic ordering via Mn-disorder in (Ni,Co)MnGa quaternary Heusler alloy

An inclusive investigation of electronic and magnetic properties of ordered and disordered (Ni,Co)MnGa quaternary Heusler alloy has been undertaken using the density functional theory-based full potential linearized augmented plane wave method within generalized gradient approximation as exchange–correlation potentials. We observe that the increasing Mn concentration in Ni_1?x CoMn_1+x Ga (x = 0, 0.25, 0.50 and 0.75) lowers the ferromagnetic (FM) character of the parent (Ni,Co)MnGa alloy. This change creates two inequivalent Mn sites aligning antiparallel to each other and generates the ferrimagnetic (FiM) ordering in all resultant disordered alloys. Further, this replacement sets off a structural change from Y- to X_a-structure. The highest magnetic moment has been found to be 4.95 μ_B for ordered (Ni,Co)MnGa alloy, whereas it decreases with increase in Mn-concentration for disordered systems. The variation of half-metallicity with increasing Mn concentration is analysed. The stability of FM and FiM state for the present ordered and disordered alloys, respectively, is also examined.     

Magnetic properties of thin-film Co/Fe/Ni magnetic systems

The magnetic properties of Co/Fe/Ni thin-film structures grown by magnetron sputtering have been studied using magnetooptical techniques. The results of x-ray diffraction measurements showed that all samples possessed a nanocrystalline structure. The magnetization curves and hysteresis loops were measured using the equatorial Kerr effect for two orientations of the external magnetic field. It is established that the Co/Fe/Ni thinfilm structures exhibit a planar magnetic anisotropy. The magnetic behavior of each layer in the initial inhomogeneous Co/Fe/Ni structure is substantially influenced by stray fields of the adjacent layers. This circumstance accounts for the complex shapes of hysteresis loops. The annealing in vacuum at T = 500°C renders Co/Fe/Ni thin-film structures magnetically hard compared to the initial state. The experimental results are explained by certain features of the microstructure of samples.     

New Heusler Compounds Cr2YSb (Y=Fe, Co and Ni): Magnetic and Electronic Properties

The electronic and magnetic properties of the Cr₂YSb (Y=Co, Fe, and Ni) Heusler alloys with both CuHg₂Ti-type and AlCu₂Mn-type structures have been investigated using first-principles calculations based on density functional theory (DFT). Two nearly half-metallic ferrimagnets (HMFs), Cr₂CoSb, and Cr₂FeSb in CuHg₂Ti-type structure, are predicted. The energy gap lies in the minority spin band for both alloys. The calculated total spin magnetic moments are 2μ _B and 1μ _B per unit cell for Cr₂CoSb and Cr₂FeSb alloys, respectively, which are in good agreement with the Slater–Pauling relation. For these alloys, the magnetic moments of Y and Cr(B) are antiparallel to that of Cr(A) and all of these moments increase with increasing lattice constant. It was also found that the half-metallic properties of Cr₂CoSb and Cr₂FeSb are unaffected to the lattice distortion and the half-metallicity can be obtained within the wide range of 5.52–6.07 Å and 5.96–6.16 Å for Cr₂CoSb and Cr₂FeSb alloys, respectively.     

Aspects of phase equilibria in Fe/Co/2.5 to 3.0% V alloys

Remendur alloys, one of which is used as the base material in remanent reed sealed contacts for the communications industry, are ternary alloys containing approximately equal iron and cobalt with 2 to 4 wt % vanadium. The available equilibrium diagram for this system does not provide precise positioning of the phase boundaries in the regions of commercial interest and, consequently, does not permit accurate determination of the amounts of the phases present or their compositions. This paper reports on the precise determination of five tie lines in theα ₁+γ two-phase field in the region of interest, by metallographic and microprobe techniques. In the 900 to 950° C range, this field was found to be narrower than expected from published data. Submicron fcc (γ) particles form during annealing at 600° C by decomposition of a non-equilibrium bcc (α₂) phase into a secondary ordered bcc phase (α′₁) and stabilizedγ. Deformation, by drawing and by stamping, enhances coercivity in these alloys by promoting a more uniform, more finely divided dispersion ofγ particles. Annealed microstructures are especially sensitive to vanadium content, annealing temperature, and annealing time.     

Synthesis and characterization of Co2FeAl Heusler alloy nanoparticles

Heusler alloy Co₂FeAl (CFA) nanoparticles have been synthesized by reducing a mixture of the precursors: CoCl₂·6H₂O, Fe(NO₃)₃·9H₂O and AlCl₃·6H₂O under H₂ atmosphere. XRD, SEM and TEM techniques have been used for the characterization of the prepared material. XRD and SAED data from TEM show the formation of mixed phases of L2₁, B2 and A2 type crystal structure of the alloy. The estimated particle size from XRD data and TEM micrograph has been found in the range of 10 nm to 50 nm. The saturation magnetization has been found of 115 emu/g from M-H characteristics which is close to its bulk value of saturation magnetization. Chemical composition of the elements has also been estimated from EDAX, which shows a ratio of Co:Fe:Al as 2.12:1.06:0.81.     

Effect of Fe Substitution on Ni2MnGe Heusler Alloys: A First Principles Study

The effect of Fe substitution on electronic structure and magnetic properties of Ni₂MnGe alloy has been investigated by using full-potential local-orbital minimum-basis method, which is used in the VCA form. The theoretical lattice parameters are obtained from the dependence of the total energy on the lattice parameter. A decreasing trend of total magnetic moment with Fe substitution is found for Ni₂Mn_1?x Fe _x Ge for studied concentrations x. The pressure–magnetization behavior is obtained for Ni₂Mn_1?x Fe _x Ge for applied pressure from 0 to 2000 GPa, and the changes of total magnetic moment gradually approach linear with the increase of x concentration. The value of equilibrium bulk modulus of Ni₂Mn_1?x Fe _x Ge (for x=0.0–1.0) obtained from Murnaghan equation of state (EOS) shows that the studied compounds are less compressible than Ni₂MnGe.     

Structure and magnetic properties of mechanically synthesized nanocrystalline Co52Fe26Ni22 alloy

Mechanical alloying method was used to prepare nanocrystalline Co₅₂Fe₂₆Ni₂₂ alloy. X-ray diffraction was applied for determination of the structure of the alloy. During milling Co-based solid solution with f.c.c. lattice was formed. After 80 h of synthesis the lattice parameter was equal to 0.3575 nm while the average grain sizes and the mean level of internal strains were about 24 nm and 0.72%, respectively. Mössbauer spectroscopy was adopted to characterize the local atomic order of the Co₅₂Fe₂₆Ni₂₂ alloy. In the nearest neighbourhood of ⁵⁷Fe isotopes there are at least six Co atoms, three Ni atoms and three Fe atoms giving the hyperfine magnetic field equal to 32.45(1) T. Magnetization measurements allowed to determine the effective magnetic moment of the Co₅₂Fe₂₆Ni₂₂ alloy to be equal to 1.63 μ_B per formula unit. Curie temperature of the obtained alloy is equal to 1000 K.     

Heusler合金Co2MnAl1-xGex的电子结构、磁性和半金属性

基于密度泛函理论(DFT),使用广义梯度近似(GGA)和局域密度近似(LDA)研究了Heusler合金Co2MnAl1-xGex(x=0、0.25、0.5、0.75、1)的磁性和电子结构。随着掺杂浓度x的增加,合金的晶格常数和磁矩都线性增加,分别很好地满足Vegard和S-P规律。对于整个合金系列,随x的增加,自旋向下带带隙宽度略有减少,费米面向高能移动。由于Co2MnAl0.5Ge0.5合金的费米面恰好居于其自旋向下带带隙中部,此时合金具有最好的半金属稳定性,预期在磁隧道结器件中能实现高的自旋极化率。     

Silicon nitride-metal joints: phase equilibria in the systems Si3Ni4-Cr,Mo, W and Re

Phase diagrams in the ternaries Cr-Si-N, Mo-Si-N, W-Si-N and Re-Si-N are established. No ternary phase is found. Si₃N₄ coexist under argon at 1273 K with all binary Cr-silicides but not with chromium, with MoSi₂ and Mo₅Si₃ but not with Mo₃Si or molybdenum, with WSi₂, W₅Si₃ and tungsten, and with ReSi₂, Re₁₇Si₉ and rhenium. At 1673 K, Si₃N₄ is found in coexistence with Cr₅Si₃, MoSi₂, Mo₅Si₃, WSi₂, W₅Si₃, ReSi₂ and Re₁₇Si₉. The implications of these phase equilibria for joining silicon nitride with low thermal expansion metals are discussed.     

Study of energy bands and magnetic properties of Co2CrSi Heusler alloy

The electronic and magnetic properties of Co₂CrSi is calculated by using full-potential linearized augmented plane wave (FP–LAPW) method based on density functional theory (DFT). Density of states (DOS), magnetic moment and band structures of the system are presented. For the exchange and correlation energy, local spin density approximation (LSDA+U) with the inclusion of Hubbard potential U is used. Our calculation shows indirect bandgap of 0·91 eV in the minority channel of DOS. This is supported by band structures and hence favoured the half metallic ferromagnetic (HMF) nature of the system. The effective magnetic moment of 4·006 μB also supported our conclusion with a near integral value. The DOS of Co and Cr were found to hybridize and was also responsible for the ferromagnetic nature of the system.     

Microstructure and giant baro-caloric effect induced by low pressure in Heusler Co51Fe1V33Ga15 alloy undergoing martensitic transformation

Solid-state refrigeration based on the magneto-or mechano-caloric effect,including elasto-and barocaloric in ferroic phase transition materials is promising to replace the current vapor compression refrigeration in consideration of environmental-friendliness and energy-saving.However,both high driven field and small thermal changes in all of these caloric materials hinder the development of solid-state refrigeration.Here we report a giant baro-caloric effect near room temperature induced by a low hydrostatic pressure in Co-based Co51Fe1V33Ga15 Heusler alloy.The maximum adiabatic temperature change under the applied pressure change of △p=0.1-100MPa can be as high as △TMaxad=7.7 K (△TMaxad / △preaches up to ～7.7 K kbar 1),surpassing the △TMaxad/△pvalue reported hitherto in baro-caloric alloys.In addition,the microstructure is also studied by using the electron microscopes.Along with the austenite and martensite,the submicron V-rich particles are precipitated in this alloy,which are believed to account for enhancing mechanical properties.     

氮杂碳包Fe、Co、Ni材料的合成及介电特性

采用交流电弧法,在氮气氛下合成了氮杂碳包金属Fe、Co、Ni纳米晶.采用热重分析(TG)法测量了合成材料中Fe、Co、Ni等金属的含量.借助于透射电子显微镜(TEM)、粉末衍射法(XRD)、X光电子能谱法(XPS)等分析手段进行了合成材料的形貌观察和结构表征.采用反射-传输网络参数法在HP8510B矢量网络分析仪上进行了材料复介电常数测量.结果表明氮杂碳包金属Fe、Co、Ni纳米晶在8.2～12.4GHz频率范围内具有复介电特性,其损耗角正切tgδε=ε"/ε′平均值分别达到0.9,0.31和0.30,显示出氮杂碳包金属Fe、Co、Ni纳米晶对微波具有较好的介电损耗特性.     

Modeling phase equilibria in polymer-solvent systems for process design

An excess Gibbs energy model is presented for calculating phase equilibria in multicomponent systems containing polymers and solvents. The model represents a combination of a physical contribution obtained from a lattice model and a chemical contribution that accounts for association and solvation effects. The lattice model is based on a revised version of Freed's latticc-field theory developed by Hu, Prausnitz, and co-workers. The model accurately represents solvent activities and liquid-liquid equilibria in binary and ternary polymer solutions over wide ranges of temperature and polymer molecular weight. It is capable of reproducing liquid-liquid equilibria with upper and lower critical solution temperatures as well as closed-loop and hourglass-shaped phase diagrams. Because of its numerical simplicity, a reasonably small number of binary parameters, and its applicability to multicomponent systems, the model cm be useful for modeling industrial processes involving polymers.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Mechanical properties of Cr, Mn, Fe, Co, and Ni modified titanium trialuminides

To increase gas turbine efficiency, new high-temperature-resistant and lightweight materials are needed. Titanium trialuminides are an interesting option, but they are currently too brittle at room temperature to be used as turbine blades. Brittleness of TiAl₃ can be reduced by alloying with several transition metal elements. The effect of ternary additions of Cr, Mn, Fe, Co, or Ni at various stoichiometries (3 to 9 at.%) to the intermetallic TiAl₃ is studied by metallography, X-ray diffraction and three-point-bend testing. These methods indicate that the 6 at.% Cr or Co additions give the highest flexural strength with only a small trade-off in failure strain when compared to the 9 at.% additions of chromium and manganese.