Elastic constant softening and martensite nucleation in a CuZnAl single crystal

The ultrasonic attenuation A and the elastic constants C₄₄, C_L and C′ have been measured during thermal cycles carried out above the martensite start-temperature M_s (M_s=267 K) in a single crystal of the Cu_67.93Zn_19.03Al_13.04 alloy. The ultrasonic attenuation displays a sharp maximum at a temperature which is about 6 degrees higher than M_s. This temperature difference can be accounted for either in terms of the Clausius–Clapeyron law for the martensite induced by the applied ultrasonic stress-field or in terms of premartensitic effects. Thermal hysteresis occurs for all the elastic constants C₄₄, C_L and C′ and the data are consistent with those previously obtained for the Young’s modulus in a polycrystalline alloy of the same system. The thermal hysteresis is attributed to the formation of stable nuclei of martensite at strong defects.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

On the peritectoid formation of Ni5Al3

The peritectoid formation of Ni₅Al₃ from the two phases NiAl and Ni₃Al was studied in a Ni---Al alloy containing 66 at% Ni by means of transmission electron microscopy. The product phase does not form as a uniform layer between the two initial phases as expected and already observed in a few systems. In the system studied here there are only very few nucleation sites located at the NiAl/Ni₃Al interface. The further growth of Ni₅Al₃ takes place only into one of the initial phases which is NiAl. A strict orientation relationship between NiAl and Ni₅Al₃ was observed; the growth direction was [221]. The transformation is presumably diffusion controlled; it is very sluggish and it can be described by a nucleation and growth process. From the study presented here we conclude that the formation of Ni₅Al₃ proceeds by a micromechanism which differs from that normally assumed for peritectoid reactions.     

Synthese und kristallstruktur von KCr0,8Al0,2Mo2O8

Single crystals of KCr_0.8Al_0.2Mo₂O₈ were prepared and investigated by the X-ray diffractometer technique. It shows a structure type related to trigonal KAIMo₂O₈, monoclinic NaCrMo₂O₈ or orthorhombic KInMo₂O₈, space group C_2h⁶—C2/c; a=17.445 Å, b=5.649 Å, c=8.997 Å, β=119.37°; Z=4. KCr_0.8Al_0.2Mo₂O₈ is characterized by isolated MoO₄ tetrahedra, isolated (Cr/Al)O₆ octahedra and a distorted square antiprism around K⁺. The crystal structure is discussed with respect to those of related compounds.

Zusammenfassung

Einkristalle von KCr_0.8Al_0.2Mo₂O₈ wurden synthetisiert und mit Vierkreisdiffraktometertechnik röntgenographisch untersucht. Sie zeigen einen mit trigonal-KA1Mo₂O₈, monoklin-NaCrMo₂O₈ oder orthorhombisch-KlnMo₂O₈ verwandten Strukturtyp, Raumgruppe C_2h⁶—C2/c; a=17,445 Å, b=5,649 Å, c=8,997 Å, β=119,37°; Z=4. KCr_0.8Al_0.2Mo₂O₈ zeichnet sich durch isolierte MoO₄-Tetraeder, isolierte (Cr/Al)O₆-Oktaeder und ein verzerrtes quadratisches Antiprisma um K⁺ aus. Die Kristallstruktur wird mit solchen verwandter Verbindungen diskutiert.     

Characterization of crystallization kinetics of a Ni- (Cr, Fe, Si, B, C, P) based amorphous brazing alloy by non-isothermal differential scanning calorimetry

The thermal stability and crystallization kinetics of a Ni- (Cr, Si, Fe, B, C, P) based amorphous brazing foil have been investigated by non-isothermal differential scanning calorimetry. The glass transition temperature T_g, is found to be 720 ± 2 K. The amorphous alloy showed three distinct, yet considerably overlapping crystallization transformations with peak crystallization temperatures centered around 739, 778 and 853 ± 2 K, respectively. The solidus and liquidus temperatures are estimated to be 1250 and 1300 ± 2 K, respectively. The apparent activation energies for the three crystallization reactions have been determined using model free isoconversional methods. The typical values for the three crystallization reactions are: 334, 433 and 468 kJ mol⁻¹, respectively. The X-ray diffraction of the crystallized foil revealed the presence of following compounds Ni₃B (Ni₄B₃), CrB, B₂Fe₁₅Si₃, CrSi₂, and Ni_4.5Si₂B.     

Influence of Al- and Cu-doping on the thermodynamic properties of the LaNiIn–H system

The Pressure–Composition–Temperature (P–C–T) relations for the LaNiIn, LaNi_0.95Cu_0.05In and LaNiIn_0.98Al_0.02–H systems were measured by a volumetric Sieverts’ method at 398–423 K. All isotherms show plateau pressure regions indicating equilibria between two hydride phases. The replacements of Ni by Cu and In by Al affect the P–C–T diagrams, stability of the hydrides, homogeneity regions of the hydrides formed, slope of the isotherms and critical temperatures of the β–γ transition. In addition, the Cu-doping induces a significant hysteresis between the hydrogen absorption and desorption processes. The relative partial molar thermodynamic properties for the studied systems are: ΔH_H = −34.6 ± 2.1 kJ (mol_H)⁻¹, ΔS_H = −70.7 ± 3.6 J (K·mol_H)⁻¹ for LaNiIn–H; ΔH_H = −34.1 ± 0.5 kJ (mol_H)⁻¹, ΔS_H = −74.9 ± 1.0 J (K·mol_H)⁻¹ for LaNi_0.95Cu_0.05In–H; ΔH_H = −33.2 ± 0.8 kJ (mol_H)⁻¹, ΔS_H = −68.3 ± 1.2 J(K·mol_H)⁻¹ for LaNiIn_0.98Al_0.02–H.     

Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

1Introduction NiTi alloys are the most successful shape memory alloys as a result of their combination of good functional properties and excellent mechanical strength[1,2].The thermal and mechanical shape memory behavior in these alloys is dependent upon …     

Dislocation dissociations and fault energies in Ni3Al alloys doped with palladium

Dislocation structures in polycrystalline Ni₃A1 alloy doped with palladium deformed at room temperature have been investigated by transmission electron microscopy. The structure consists mainly of dislocations dissociated into a/2<0 1 1> super partials bounding an anti-phase boundary (APB). Dislocations dissociated into a/3<112> super Shockley partials bounding a superlattice intrinsic stacking fault (SISF) were often observed. The majority of the SISFs are truncated loops, i.e. the partials bounding the SISF are of similar Burgers vector. These faulted loops are generated from APB-coupled dislocations, according to a mechanism for formation of SISFs proposed by Suzuki et al. (Suzuki K, Ichihara M, Takeuchi S. Acta Metall 1978;26:183) and recently modified by Chiba et al. (Chiba A, Hanada S. Philos Mag 1994;69:751). The APB energies for {111} and {100} slip planes are measured to be 144±20 and 102±11 mJ/m², respectively, and the SISF energy has been estimated to be 12 mJ/m² in this alloy. It is concluded that the dislocation structure in Ni_74.5Pd₂Al_23.5 alloy deformed at room temperature is similar to that in binary Ni₃Al and the difference in fault energies between these two alloys is small. Thus, it seems unlikely that the enhancement of ductility of Ni_74.5Pd₂Al_23.5 results from only such a small decrease of the ordering energy of the alloy. SISF bounding dislocations also have no apparent influence on the ductilization of Ni_74.5Pd₂Al_23.5 alloy.     

Electrochemical investigation of the iron-containing and no iron-containing AB5-type negative electrodes

The electrochemical behaviour of LaNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x (x = 0, 0.15, 0.55, 0.75) intermetallic compounds has been studied and presented [C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Guégan, Int. J. Hydrogen Energy 29 (2004) 307–311; C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Guégan, J. Alloys Compd. 360 (2003) 266–271; C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Guégan, J. Alloys Compd. 384 (2004) 249–253]. It has been deduced that the LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 compound has interesting electrochemical properties. In this paper we present the electrochemical study of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 compound properties compared with the parent LaNi_3.55Mn_0.4Al_0.3Co_0.75 compound. Several techniques, such as, the chronopotentiometry, the constant potential discharge (CPD), the cyclic voltammetry (CV) and the linear polarization (LP) were applied to characterize these electrochemical properties. The electrochemical discharge capacity of the LaNi_3.55Mn_0.4Al_0.3Co_0.75 alloy increases to reach 294 mAh g⁻¹ after few cycles only (five cycles). However, the activation of the LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloy takes more than 20 cycles to be achieved and the obtained maximum discharge capacity is 194 mAh g⁻¹. The hydrogen diffusion coefficient D_H was determined by constant potential discharge and cyclic voltammetry techniques. The obtained values of the LaNi_3.55Mn_0.4Al_0.3Co_0.75 and LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 compounds are 6.29 × 10⁻¹¹ and 7.62 × 10⁻¹¹, and 2 × 10⁻⁸ and 7.5 × 10⁻⁸ cm² s⁻¹ by CPD and CV techniques, respectively. The exchange current density values, determined by a linear polarization technique, are 44 and 27 mA g⁻¹, respectively, for LaNi_3.55Mn_0.4Al_0.3Co_0.75 and LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloys.     

基于第一性原理的CrCuFeNiTi高熵合金涂层的相及稳定性分析

采用等离子弧在45钢基体上熔覆了CrCuFeNiTi高熵合金涂层,经X射线衍射分析涂层中的物相有BCC、FCC和Fe₂Ti相,并对BCC、FCC和Fe₂Ti相进行了成分测试,采用CASTEP中的虚拟晶体近似方法建立了BCC、FCC和Fe₂Ti相的晶体结构模型,基于第一性原理计算了涂层中BCC、FCC和Fe₂Ti相的晶格常数,并与涂层中BCC、FCC和Fe₂Ti相的X射线测试结果进行了比较,另外计算了BCC和FCC相的弹性常数C_ij、体积模量B、剪切模量G、杨氏模量E和泊松比ν。结果表明,涂层中BCC和FCC相以及Fe₂Ti的晶格常数计算值与试验值的误差为0.43%~3.05%,BCC、FCC和Fe₂Ti相的生成热均为负值且BCC和FCC相的弹性常数C₁₁、C₁₂和C₄₄满足立方结构高熵合金的力学稳定性限制条件,可知BCC、FCC和Fe₂Ti相是稳定的。另外由C₁₂-C₄₄>0,G/B<0.57,ν>0.26可知BCC和FCC相以金属键结合且呈现韧性的特征。     

Electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.75−xFex (x = 0.55 and 0.75) compounds

The hydrogen storage alloys MmNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x (x = 0.55 and 0.75) were used as negative electrodes in the Ni-MH accumulators. The chronopotentiommetry and the cyclic voltammetry were applied to characterize the electrochemical properties of these alloys. The obtained results showed that the substitution of the cobalt atoms by iron atoms has a good effect on the life cycle of the electrode. For the MmNi_3.55Mn_0.4Al_0.3Co_0.2Fe_0.55 compound, the discharge capacity reaches its maximum of 210 mAh/g after 12 cycles and then decreases to 190 mAh/g after 30 charge–discharge cycles. However, for the MmNi_3.55Mn_0.4Al_0.3Fe_0.75 compound, the discharge capacity reaches its maximum of 200 mAh/g after 10 cycles and then decreases to 160 mAh/g after 30 cycles.

The diffusion behavior of hydrogen in the negative electrodes made from these alloys was characterized by cyclic voltammetry after few activation cycles. The values of the hydrogen coefficient in MmNi_3.55Mn_0.4Al_0.3Co_0.2Fe_0.55 and MmNi_3.55Mn_0.4Al_0.3Fe_0.75 are, respectively, equal to 2.96 × 10⁻⁹ and 4.98 × 10⁻¹⁰ cm² s⁻¹. However, the values of the charge transfer coefficients are, respectively, equal to 0.33 and 0.3. These results showed that the substitution of cobalt by iron decreases the reversibility and the kinetic of the electrochemical reaction in these alloys.     

Effect of alloy composition on enthalpy and entropy changes of hydride formation for stoichiometric and nonstoichiometric hydrogen storage alloys

Both enthalpy change ΔH° and entropy change ΔS° of hydride formation were evaluated from the measurement of pressure-composition isotherms (P-C-T curves) for hydrogen desorption in stoichiometric and nostoichiometric Mm (Ni₁₆Mn_11.4Al_0.3Co_0.7)₃ (Mm = misch metal, 0.88 x 1.12 values decreased with increasing x value, supporting that the low stability of the hydride causes the improved high-rate dischargeability of the Mm-poor Mm(Ni₁₆Mn_0.4Al_0.3Co_0.7)₁₁₂ alloy electrode.     

Existence, structure and valence properties of the skutterudites CeyFe4−xCoxSb12

We report on sample preparation, annealing effects, electron microprobe analysis in the series Ce_yFe_4−xCo_xSb₁₂ which shows that a phase separation occurs for substituted samples (0<x<4) annealed at 650 and 550 °C. Single phase samples are obtained for either Ce_yFe₄Sb₁₂ or Ce_yCo₄Sb₁₂ samples annealed at 650 °C and for all compositions when annealed at 700 °C. The valence state of Ce in homogeneous samples has been studied using X-ray absorption spectroscopy (XAS). Ce ions are trivalent throughout the series and the XAS spectra does not show effect of the crystal field on the 5d-final state.     

The influence of titanium on the electrochemical properties of the AB5-type metal hydrides

AB₅-type intermetallic compounds were prepared by arc-melting in argon atmosphere. The composition of a stoichiometric compound LaNi_3.6Al_0.4Co_0.7Mn_0.3 with a hexagonal CaCu₅ structure was varied by stoichiometric and nonstoichiometric addition of Ti. With the increase of the Ti y0.05 content in LaNi_3.6Al_0.4Co_0.7Mn_0.3Ti_y, the hydrogen storage capacity is enhanced, whereas when y=0.1–0.3, it is decreased. The discharge capacity and cyclability are increased considerably by addition of titanium in the range of 0.02–0.1 with a maximum value at about 0.1%. The highest maximum capacity is achieved for a nonstoichiometric addition of 0.05% Ti. The kinetic properties are also additionally improved by the formation of a titanium-rich second phase. This can explain the improvement of the capacity for alloys with low Ti content. The decrease in capacity for high Ti content was also correlated with the amount of the Ti-rich phase. Therefore, the improvement of kinetics are due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This study has been aimed to improve the electrode properties of a series of multicomponent LaNi_3.6Al_0.4Co_0.7Mn_0.3Ti_y (y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3) alloys which have mutual complementary properties. All the prepared alloys have been subjected to analyses by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (P–C–T curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.     

Effects on direct synthesis of large scale mono-disperse Ni0.5Zn0.5Fe2O4 nanosized particles

Mono-disperse Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite particles have been synthesized directly via the hydrothermal method using sodium dodecyl sulfate (SDS) as surfactant. Particle size could be varied from 6 to 19 nm by changing the experiment parameters. X-ray diffraction, high resolution TEM images confirmed the high crystallinity of ferrite nanocrystals. The effects of precursor suspension pH value, reaction temperature and surfactant (SDS) concentration on phase purity, particle size and dispersed property were discussed. The results indicated that mono-disperse Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite nanoparticle had been obtained at pH value range (8–9), reaction temperature (90 °C) and moderate SDS concentration (>0.2 mM). The magnetic measurement shows that as prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possesses good super-paramagnetic behavior. We also put forward a primary experimental model to shed light on the controllability of the monodispersity of the nanosized particles.     

Grain boundary self-diffusion of Ni in pure and boron-doped Ni3Al

Grain boundary (gb) self-diffusion in pure Ni-rich Ni₃Al was measured between 882 and 1374 K using the radiotracer ⁶³Ni, a serial sectioning technique and sensitive liquid scintillation counting. The results of the gb diffusivity P = δD_gb (δ : gb width, D_gb : gb diffusion coefficient) can be represented by the Arrhenius parameters P₀ = 3.27 · 10¹³and Q_gb = 168 kJ/mol. Additionally gb diffusion was investigated in boron-doped (0.24 at%) Ni-rich Ni₃Al in the range from 882 to 1352 K yielding P₀ = 1.24 · 10⁻¹² m³/s and Q_gb = 187 kJ/mol. The increase in the activation enthalpy Q_gb and the decrease of P upon boron-doping is explained by the segregation of B in Ni₃Al gbs, which may lead to an increase in the vacancy formation enthalpy and to a blocking of energetically favourable diffusion paths in the gbs. For comparison gb self-diffusion in pure Ni was remeasured yielding Q_gb = 112 kJ/mol. Ordering of the lattice and the preservation of ordering up to the gb planes, as predicted in Ni₃Al, therefore has a pronounced decelerating influence on gb diffusion, stronger than on bulk diffusion. Applying the semi-empirical relation of Borisov et al. (Phys. Met. Metallogr., 17 (1964) 80) gb energies γ_gb were determined for arbitrary high angle gbs in pure and B-doped Ni₃Al, resulting in 915 and 870 mJ/m², respectively, at 1100 K.     

Mechanical loss of cubic zirconia

Cubic zirconia can be stabilized by doping with lower valent oxides such as Y₂O₃ or CaO. Oxygen vacancies are then created as charge compensating defects. Mechanical loss measurements were performed in the temperature range 300–1600 K using both free decaying (3 Hz, 3 kHz) and forced vibrations (10⁻²–10 Hz). The influence of Y₂O₃ (10–24 mol%) and CaO (10–16 mol%) on the loss spectra was studied. The low temperature spectra (<1000 K) show a composite loss maximum which can be decomposed into two peaks, I and I_A (I′, I_A′). Both peaks, with activation enthalpy values between 1 and 2 eV, rely on local jumps of oxygen vacancies which are trapped by dopant cations. Submaxima I (I′) are assigned to defect pairs of oxygen vacancies and dopant ions (Y or Ca) forming elastic (electric) dipoles. These are oriented parallel to 111 (trigonal symmetry) with the vacancies on nearest neighbor sites. Maxima IA (I_A′) are assigned to relaxation of vacancies within larger clusters and interaction effects (Y or Ca). The high temperature loss spectra (≥1000 K) show various relaxation phenomena. In Y₂O₃ stabilized ZrO₂ we observe around 1400 K a loss peak with frequency independent temperature position which is assigned to a structural phase transition. ZrO₂–CaO shows a high temperature peak of Debye type (H=4.0±0.5 eV) which is related to local diffusional jumps of cations via vacancies.     

Reduced Annealing Time and Enhanced Magnetocaloric Effect of La(Fe,Al)13 Alloy by La-nonstoichiometry and Si-doping

LaFe_13-xM_x (M = Si, Al) alloys are promising for use in magnetic refrigeration. However, they require long annealing time (30 days) in order to optimize the magnetocaloric properties. Research has shown that the addition of extra La in off-stoichiometric alloys can greatly shorten the annealing time. Therefore, the purpose of this study is to investigate the influence of the extra addition of La on the annealing properties of a new off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x (x = 0, 0.1, 0.4) alloys. It was demonstrated that after a 36h annealing time, a large volume fraction of 1:13 magnetocaloric phase was obtained for all alloys. Further microstructural analysis of the off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x alloys revealed a facet-like grain morphology. The La_1.7Fe_11.6Al_1.4 and La_1.7Fe_11.6Al₁Si_0.4 alloys were shown to contain large 1:13 phase precipitates separated in a La-rich matrix, while the La_1.7Fe_11.6Al_1.3Si_0.1 alloy had a continuous 1:13 phase matrix with a fine dispersion of La-rich precipitates throughout. When the magnetic field varied between 0 and 2 T, the corresponding magnetic entropy change and relative cooling capacity for the La_1.7Fe_11.6Al_1.3Si_0.1 specimen were determined as 4.58 J/kg K and 173.6 J/kg, respectively. More importantly, the La_1.7Fe_11.6Al_1.3Si_0.1 alloy displayed only a slight volume change when the meta-magnetic phase transition occurred, which is promising for cyclic use.     

Preparation, characterization and crystal structure of 4,4′-bipyridinium nitrate trinitratodioxo-uranium(VI) and 4,4′-dipyridinium diaqua hexanitrato-thorate(IV)

The compounds 4,4′-bipyridinium nitrate trinitratodioxo-uranium(VI) [UO₂(NO₃)₃][4,4′-bipyH]·HNO₃ (1) and 4,4′-dipyridinium diaqua hexanitrato-thorate(IV) [Th(NO₃)₆][4,4′-bipyH₂]·2H₂O (2) were prepared and characterized. Their crystal structures have been determined using X-ray diffraction techniques. Both complexes crystallize in the monoclinic system with space groups P2₁/c for 1 and C2/c for 2. The geometry of the anion in 1 is a distorted hexagonal bipyrimidial coordination, while that in 2 is a distorted icosahedron. The cation in 1 is formed from hydrogen-bonded monoprotonated bipyridyl with a neutral nitrate molecule, whereas the cation in 2 is formed from hydrogen-bonded diprotonated bipyridyl with a water molecule. The cations in both complexes are connected with the anions through hydrogen bonds via the nitrate molecule in 1 and the water molecule in 2.