Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10⁻⁶ to 1.18 × 10⁻⁴ ms⁻¹ and at a constant temperature gradient at the solid–liquid interface of G_L = 12 × 10³ K m⁻¹. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni₃Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q_a = 401 kJ/mol.     

Structural evolution and grain growth kinetics of the Fe–28Al elemental powder during mechanical alloying and annealing

The structural evolution and grain growth kinetics of the Fe–28Al (28 at.%) elemental powder during mechanical alloying and annealing were studied. Moreover, the alloying mechanism during milling the powder was also discussed. During mechanical alloying the Fe–28Al elemental powder, the solid state solution named Fe(Al) was formed. The lattice parameter of Fe(Al) increases and the grain size of Fe(Al) decreases with increasing milling time. The Fe and Al particles were first deformed, and then, the composite particles of the concentric circle-like layers were generated. Finally, the composite particles were substituted by the homogeneous Fe(Al) particles. The continuous diffusion mixing mechanism is followed, mainly by the diffusion of Al atoms into Fe. During annealing the milled Fe–28Al powder, the order transformation from Fe(Al) to DO₃-Fe₃Al and the grain growth of DO₃-Fe₃Al occurred. The grain growth kinetic constant, K = 1.58 × 10⁻⁹ exp(−540.48 × 10³/RT) m²/s.     

Theoretical and experimental study of photocurrent action spectrum of ITO/sexithiophene/Al structure

We have studied photovoltaic cells using sexithiophene in (ITO/6T/Al) structure, and we have measured the action spectrum obtained by illumination through the aluminum side which is compared to the measured absorption spectrum. Two models were used to interpret this experiment: the Ghosh model using the carriers diffusion property in the bulk, and the kinetic model describing the dynamic behavior of the charge carriers in the device. The confrontation of these models with the experimental spectra allows us to reach the diffusion length of electron–hole pair L=2×10⁻⁶ cm. We also give the mobility of carriers (μ_n≈1.5×10⁻⁴ cm²/V s, and μ_p≈1.5×10⁻⁵ cm²/V s) and their diffusion coefficients (D_n≈4×10⁻⁶ cm²/s, and D_p≈4×10⁻⁷ cm²/s).     

Grain boundary diffusion and segregation of Ni in Cu

Grain boundary (GB) diffusion of ⁶³Ni in polycrystalline Cu was investigated by the radiotracer technique in an extended temperature interval from 476 to 1156 K. The independent measurements in Harrison’s C and B kinetic regimes resulted in direct data of the GB diffusivity D_gb and of the so-called triple product P = s · δ · D_gb (s and δ are the segregation factor and the diffusional GB width, respectively). Arrhenius-type temperature dependencies for both the D_gb and P values were measured, resulting in the pre-exponential factors $D_{\mathrm{gb}}^0=6.93\times {10}^{-7}$ m² s⁻¹ and P₀ = 1.89 × 10⁻¹⁶ m³ s⁻¹ and the activation enthalpies of 90.4 and 73.8 kJ mol⁻¹, respectively. Although Ni is completely soluble in Cu, it reveals a distinct but still moderate ability to segregate copper GBs with a segregation enthalpy of about −17 kJ mol⁻¹.     

Effect of Y2O3 crucible on contamination of directionally solidified intermetallic Ti–46Al–8Nb alloy

The effect of Y₂O₃ crucible on contamination of Ti–46Al–8Nb (at.%) alloy directionally solidified (DS) in a Bridgman-type apparatus was studied. Directional solidification experiments were performed in dense Y₂O₃ crucibles using different growth rates, melt temperatures and various reaction time between the melt and the crucible. The main mechanism responsible for the contamination of the DS samples is diffusion controlled dissolution of the Y₂O₃ crucible in the melt which leads to an increase of oxygen and yttrium content in γ(TiAl) + α₂(Ti₃Al) matrix and precipitation of non-metallic particles in interdendritic region. Transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD) showed that these particles are Y₂O₃ phase. The oxygen content and volume fraction of Y₂O₃ particles increase with increasing melt temperature and reaction time. The activation energy for increase of oxygen content is calculated to be Q_O = 412.1 kJ/mol and the kinetics of this process is suggested to be controlled by long-range diffusion with the oxygen content exponent of 3. The activation energy for Y₂O₃ particle formation is calculated to be Q_Y = 421.8 kJ/mol and the time exponent is determined to be m = 0.55. Vickers microhardness measurements in lamellar γ + α₂ matrix without Y₂O₃ particles can be used as an indirect evidence of the level of contamination of DS samples with statistically identical α₂–α₂ interlamellar spacing.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Analysis of electrochemical impedance of polypyrrole|sulfate and polypyrrole|perchlorate films

PPy|SO₄ and PPy|ClO₄ films have been synthesized and investigated in K₂SO₄, ZnSO₄ and NaClO₄ aqueous solutions by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and electron probe microanalysis (EPMA) methods. On the basis of obtained data and calculated impedance parameters as the potential functions, the role of different processes (diffusion of ions, double-layer charging, adsorption and charge transfer) in oxidized, partially reduced and reduced PPy films is estimated. The lowest pseudocapacitance values (from n × 10⁻⁶ to n × 10⁻⁴ μF cm⁻² for 1 μm film), independent of solution concentration, were established for PPy|SO₄ in ZnSO₄. This phenomenon is related with strongly aggravated film reduction process in the solution of double-charged cations. In the case of PPy|ClO₄ in NaClO₄ and PPy|SO₄ in K₂SO₄,where the mono-charged cations participate in redox process, the capacitance values are in the range from: n × 10⁻³ to n × 10⁻² μF cm⁻² and even somewhat higher for PPy|ClO₄ system at oxidized state. The calculated effective diffusion coefficients of ions D remain inside the range from n × 10⁻¹² to n × 10⁻¹⁴ cm² s⁻¹ for PPy|SO₄ in 0.1 M K₂SO₄ and PPy|ClO₄ in 0.1 M NaClO₄ aqueous solution. In the case of PPy|SO₄ film in ZnSO₄ solution the D values are essentially lower.     

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10^?3–5 × 10^?6 s^?1 at 350 °C. The apparent true stress exponent was n_app ～ 7, which can be associated with a deformation process controlled by dislocation mechanisms.     

Grain-boundary diffusion of nickel in Cr-, Fe-, and Zr-modified Ni3Al intermetallic

Tracer grain-boundary diffusion of ⁶³Ni in slightly hypostoichiometric (x_Al < 25 at.%) Ni₃Al alloys containing several percents of Cr, Fe and Zr was studied using both serial sectioning and residual activity methods. Measurements of grain-boundary diffusivity P were carried out in the temperature interval 873–1273 K. It was found that the additions of Cr, Fe and Zr decrease the P and increase the activation enthalpy of ⁶³Ni grain-boundary diffusion.     

Ni radiotracer diffusion in B2 ordered NiFeAl alloys

Ni bulk diffusion was measured in the B2 ordered NiFeAl alloys with a constant Al content of approximately 50 at.%. The ⁶³Ni radioisotope in combination with the radiotracer serial sectioning technique was applied at higher temperatures and the secondary ion mass spectrometry (⁶⁴Ni isotope) was used in a low temperature range. Arrhenius-type temperature dependencies were established for all studied compositions in the temperature interval from 1050 to 1500 K. As the Fe content x in the Ni_50−xFe_xAl₅₀ ternary alloys increases, the Ni diffusivity generally increases along the quasi-binary section between stoichiometric NiAl and FeAl. The activation enthalpy Q of Ni diffusion changes strongly non-monotonically in the ternary alloys between the binary end-members NiAl and FeAl revealing a pronounced minimum at the Ni₄₀Fe₁₀Al₅₀ composition and a conspicuous maximum around the Ni₂₅Fe₂₅Al₅₀ composition.     

Creep behaviour of a new air-hardenable intermetallic Ti–46Al–8Ta alloy

Creep behaviour of a new cast air-hardenable intermetallic Ti–46Al–8Ta (at.%) alloy was investigated. Constant load tensile creep tests were performed at initial applied stresses ranging from 200 to 400 MPa in the temperature range from 973 to 1073 K. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of the minimum creep rate is n = 5.8 and the apparent activation energy for creep is calculated to be Q_a = (382.9 ± 14.5) kJ/mol. The kinetics of creep deformation of the specimens tested to a minimum creep rate (creep deformation about 2%) is suggested to be controlled by non-conservative motion of dislocations in the γ(TiAl) matrix. Besides dislocation mechanisms, deformation twinning contributes significantly to overall measured strains in the specimens tested to fracture. The initial γ(TiAl) + α₂(Ti₃Al) microstructure of the creep specimens is unstable and transforms to the γ + α₂ + τ type during creep. The particles of the τ phase are preferentially formed along the grain and lamellar colony boundaries.     

Z-scan measurements of third-order optical non-linearities in poly(phenylacetylenes)

Poly(phenylacetylene) (PPA) and poly(p-methoxyphenylacetylene) (PMOPPA) were synthesized by catalytic polymerization of monosubstituted alkynes and were fully characterized by conventional techniques. Linear and third-order non-linear optical properties of polymers solutions were investigated at λ=780 nm using a Z-scan set-up equipped with a femtosecond laser source at different operating regimes. A high repetition rate (76 MHz) regime was used to measure the linear and non-linear absorption coefficients by means of thermo-optical effects. Values of the linear and non-linear absorption coefficients were α=4.9×10⁻³ cm⁻¹ and β=1.1×10⁻¹¹ cm W⁻¹ for PPA and α=2×10⁻² cm⁻¹ and β=2.1×10⁻¹⁰ cm W⁻¹ for PMOPPA. Very low repetition rate (14 Hz) excitation was used to evidence the purely-optical non-linear refractive indexes, that were γ=6×10⁻¹⁸ cm² W⁻¹ and γ=11×10⁻¹⁸ cm² W⁻¹, respectively, for PPA and PMOPPA.     

A first-principles study of the diffusion of atomic oxygen in nickel

In this study, the coefficients of diffusion of oxygen in nickel-based alloys are determined by atomistically modeling the oxygen diffusion process using a vacancy-mediated diffusion model. Density functional theory is used to calculate the energy of the system. The activation barrier energy for the diffusion of atomic oxygen in nickel is quantified by determining the most favorable path, i.e., the minimum-energy path, for diffusion. Phonon analysis is performed using the direct force-constant method. The calculated pre-exponential factor for the lattice diffusion of oxygen in nickel is 5.45 × 10⁻⁷ m²/s and the activation energy is 158.65 kJ/mol.     

Conductive complexes of [Ni(dmid)2] with TTF,TMTTF, and ET

Three charge-transfer salts of [Ni(dmid)₂]⁻ (dmid: 1,3-dithiol-2-one-4,5-dithiolate)—TMTTF[Ni(dmid)₂], TTF_x[Ni(dmid)₂] and ET_x[Ni(dmid)₂] (TMTTF: tetramethyl-tetrathifulvalene, TTF: tetrathifulvalene, ET: bis(ethylenedithio)-tetrathiafulvalene) are prepared and characterized. The TMTTF[Ni(dmid)₂] complex has a structure with mixed packs is formed by cations and anions, which alternate each other with subsequent shift. This compound is semiconductor with the room-temperature conductivity of 2.2 × 10⁻³ Ω⁻¹ cm⁻¹. The TTF_x[Ni(dmid)₂] and ET_x[Ni(dmid)₂] complexes have a highest conductivity: σ_RT 1.86 and 0.54 Ω⁻¹ cm⁻¹, respectively.     

Synthesis,characterization and evaluation of cation-ordered LnBaCo2O5+δ as materials of oxygen permeation membranes and cathodes of SOFCs

LnBaCo₂O_5+δ (Ln = La, Pr, Nd, Sm, Gd, and Y) was synthesized via an EDTA–citrate complexing process. The particular Ln³⁺ dopant had a significant effect on the oxide’s phase structure/stability, oxygen content, electrical conductivity, oxygen permeability, and cathode performance. Stable, cation-ordered oxides with layered lattice structures were obtained with medium-sized Ln³⁺ ions over a wide range of oxygen partial pressures, a property essential for applications as oxygen separation membranes and solid oxide fuel cell (SOFC) cathodes. PrBaCo₂O_5+δ demonstrated the highest oxygen flux (∼5.09 × 10⁻⁷ mol cm⁻² s⁻¹ at 900 °C), but this value was still significantly lower than that of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ perovskite (∼3.1 × 10⁻⁶ mol cm⁻² s⁻¹ at 900 °C). The observed difference was attributed to the much longer diffusion distance through a polycrystalline membrane with a layered lattice structure than through cubic perovskite because bulk diffusion was the rate-limiting step of permeation. An area-specific resistance of ∼0.213 Ω cm² was achieved at 600 °C with a PrBaCo₂O_5+δ cathode, suggesting that the layer-structured oxides were promising alternatives to ceramic membranes for SOFC cathodes.     

The effect of a novel organic compound chiral macrocyclic tetraamide-I interfacial layer on the calculation of electrical characteristics of an Al/tetraamide-I/p-Si contact

The Al/tetraamide-I/p-Si Schottky barrier diode (SBD) has been prepared by adding a solution of a novel nonpolymeric organic compound chiral macrocylic tetraamide-I in chloroform on top of a p-Si substrate and then evaporating the solvent. It has been seen that the forward-bias current–voltage (I–V) characteristics of Al/tetraamide-I/p-Si SBD with a barrier height value of 0.75 eV and an ideality factor value of 1.77 showed rectifying behaviour. The energy distribution of the interface state density determined from I–V characteristics increases exponentially with bias from 5.81 × 10¹² cm⁻² eV⁻¹ at (0.59-E_v) eV to 1.02 × 10¹³ cm⁻² eV⁻¹ at (0.40-E_v) eV. It has showed that space charge limited current (SCLC) and trap charge limited current (TCLC) are the dominant transport mechanisms at large forward-bias voltages.     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

Cr effects on magnetic and corrosion properties of Fe–Co–Si–B–Nb–Cr bulk glassy alloys with high glass-forming ability

Effects of the Cr addition on glass formation, magnetic and corrosion properties of {[(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]_0.96Nb_0.04}_100−xCr_x (x = 1, 2, 3, 4 at.%) alloys have been investigated. It was found that the addition of Cr element slightly decreases the glass-forming ability (GFA), but is very effective in increasing corrosion resistance and improving soft magnetic properties for this Fe–Co–B–Si–Nb bulk glassy alloy within the composition range examined. The Fe–Co–B–Si–Nb–Cr alloys exhibit high GFA. Full glassy rods with diameters up to 4 mm can be synthesized by copper mold casting. The Fe-based bulk glassy alloys (BGAs) exhibit a high saturation magnetization of 0.81–0.98 T as well as excellent soft magnetic properties, i.e., extremely low coercive force of 0.6–1.6 A/m and super-high initial permeability of 26,400–34,100. Furthermore, corrosion measurements show that corrosion rate and corrosion current density of these Fe-based BGAs in 0.5 M NaCl solution decrease from 7.0 × 10⁻¹ to 1.6 × 10⁻³ mm/year and 3.9 × 10⁻⁶ to 8.7 × 10⁻⁷ A/cm², respectively, with increasing Cr content from 0 to 4 at.%. The success of synthesizing the new Fe-based BGAs exhibiting simultaneously high GFA as well as excellent good soft magnetic properties combined with high saturation magnetization and enhanced corrosion resistance allows us to expect future progress as a new type of soft magnetic materials.     

Formation,thermal stability and corrosion behavior of glassy Ti45Zr5Cu45Ni5 alloy

Ti-rich Ti₄₅Zr₅Cu₄₅Ni₅ bulk metallic glass with critical diameter reaching 3 mm and supercooled liquid region of 42 K was prepared by copper mold casting. The glass transition temperature and onset temperature of crystallization are 673 K and 715 K, respectively. The glassy Ti–Zr–Cu–Ni alloy is passive in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, although pitting corrosion occurred by anodic polarization at higher potential in the Cl⁻-containing solutions. Corrosion rates of the glassy Ti₄₅Zr₅Cu₄₅Ni₅ alloy are of the order of 10⁻³ mm/year in the NaCl and H₂SO₄ solutions and about 1 × 10⁻² mm/year in the HCl acid.