TEM study of superstructure in a perovskite lead lanthanum zirconate stannate titanate ceramic

The superstructure of an antiferroelectric Pb_0.97La_0.02(Zr_0.66Sn_0.25Ti_0.09)O₃ phase, whose composition is near the morphotropic-phase boundary, was characterized. Systematic selected-area diffraction revealed that there were two kinds of superlattice reflections in the pseudocubic reciprocal lattice, i.e. superlattice reflections (h, k, l all odd), and one-dimensional incommensurate superlattice reflections, where g denotes the vectors of fundamental or superlattice reflections. Convergent-beam electron diffraction disclosed that the average structure of the phase was rhombohedral with space group of . Based on the rhombohedral reciprocal lattice, the reflections were no longer superlattice but fundamental reflections, and the reciprocal vector of the one-dimensional incommensurate reflections was re-expressed as

Full-size image

, where h, k, l are integers and (−h + k + l) = 3n. In the light of the average structure and the reflection condition, the superspace Bravais class of the phase with one-dimensional incommensurate structure was determined to be in a (3 + 1)-dimensional space. In addition, the origins of the superlattice reflections were also examined and discussed.     

Quantitative representation of the anisotropy and tension/compression asymmetry of the critical resolved shear stress of an L12-ordered intermetallic

The anomalous anisotropy and tension (t)/compression (c)-asymmetry of the critical resolved shear stress τ_o of the L1₂-long-range ordered intermetallic γ′_NIM are analysed in the light of a published (Miner RV, Gabb TP, Gayda J, Hember, KJ. Met Trans A 1986; 17A: 507) analytical expression

Full-size image

. Particles of γ′_NIM strengthen the commercial nickel-base superalloy NIMONIC 105. characterises the orientation of the specimen, and T_o≈300 K and T_peak≈ 1000 K mark the temperature (T) range in which τ_o is anomalous. Except for γ′_NIM-single crystals with the [011]-orientation, the formula describes the anisotropy and (t/c)-asymmetry of the critical resolved shear stress satisfactorily.     

Microstructure and mechanical properties of Cr–Si–N coatings prepared by pulsed reactive dual magnetron sputtering

Cr–Si–N thin films were deposited by pulsed DC reactive dual-magnetron sputtering using Cr and Si targets, while various currents applied to the Si target allowed one to vary the Si content (C_Si) in the films. Microstructure, composition and mechanical properties were studied as a function of C_Si using XRD, ERD-TOF and depth-sensing indentation. Three regions of C_Si were distinguished: (i) C_Si < 2.3 at.%, where the grain size (D) does not significantly change with increasing C_Si; (ii) 2.3 < C_Si < 6.7 at.%, where D decreases as C_Si increases; and (iii) 6.7 ≤ C_Si ≤ 11.6 at.%, where a relatively rapid decrease of D is observed with increasing C_Si. We found that the hardness (H) and the reduced Young's modulus (E_r) of the films reached maximum values of H ~ 24 GPa and E_r ~ 240 GPa for C_Si ~ 2.3 at.%. Based on the evolution of the microstructural and mechanical properties of the Cr–Si–N films, we propose to explain the hardening observed for C_Si < 2.3 at.% in terms of the solid solution mechanism rather than the nanocomposite formation.     

Structures and superparamagnetic properties of overaged Fe–Al–Mn–C alloys

Microstructures and superparamagnetic properties in aged-hardened Fe–9%Al–30%Mn– (x)C,Si alloys, resulting from overaging at a temperature of 823 K for 48 h to 313 days, have been investigated by transmission electron microscopy (TEM), X-ray diffraction patterns, and vibrating sample magnetometry (VSM). The results reveal that the precipitate κ-phase [(Fe,Mn)₃AlC] decomposition in this alloy, overaged at 823 K for one week, resulted from two separate mechanisms: (1) wetting of the antiphase boundary segment (APBs) of D0₃ [(Fe/Mn)₃Al] domains by the B2 [(Fe/Mn)Al] phase; and (2) precipitation of the B2 [(Fe/Mn)Al] phase within the domain. A superparamagnetic behaviour was discovered when the alloy was overaged at 823 K for ≈120–313 days. The super-soft magnetic property was mainly attributable to the ferromagnetic spinel-ordered (B2 [(Fe/Mn)Al]+D0₃ [(Fe/Mn)₃Al]) phases and ordered B2 with monoclinic ′Mn structures.     

Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: Part I – Chemical compositions of Al3(Sc1−xZrx) precipitates

Atom-probe tomography (APT) and high-resolution transmission electron microscopy are used to study the chemical composition and nanostructural temporal evolution of Al₃(Sc_1−xZr_x) precipitates in an Al–0.09 Sc–0.047 Zr at.% alloy aged at 300 °C. Concentration profiles, via APT, reveal that Sc and Zr partition to Al₃(Sc_1−xZr_x) precipitates and Zr segregates concomitantly to the -Al/Al₃(Sc_1−xZr_x) interface. The Zr concentration in the precipitates increases with increasing aging time, reaching a maximum value of 1.5 at.% at 576 h. The relative Gibbsian interfacial excess of Zr, with respect to Al and Sc, reaches a maximum value of 1.24 ± 0.62 atoms nm⁻² after 2412 h. The temporal evolution of Al₃(Sc_1−xZr_x) precipitates is determined by measuring the time dependence of the depletion of the matrix supersaturation of Sc and Zr. The time dependency of the supersaturation of Zr does not follow the asymptotic t^−1/3 law while that of Sc does, indicating that a quasi-stationary state is not achieved for both Sc and Zr.     

High-pressure ultrasonic properties of spin-density-wave Cr–Re alloy single crystals

The pressure dependence of the elastic constants (c_ij) of Cr–Re alloy single crystals, containing 0.3 and 0.5 at.% Re, are reported. For 0.3 at.% Re the concentration (c) is below the triple point concentration (c_t) on the (c–T) magnetic phase diagram. This crystal therefore remains in the incommensurate (I) spin-density-wave (SDW) antiferromagnetic phase at all temperatures below the ISDW–paramagnetic (P) Néel transition temperature (T_N). The pressure derivatives of the elastic constants, dc_ij/dp, were measured for the Cr+0.3 at.% Re crystal as a function of temperature through T_N. The Cr+0.5 at.% Re crystal has c>c_t and exhibits an incommensurate–commensurate (I–C) SDW phase transition at a temperature T_IC<T_N on heating. In the case of this crystal dc_ij/dp was studied at temperatures close to and above T_IC. The acoustic mode Grüneisen parameters (γ_n), which quantify the lattice vibrational anharmonicity, were calculated as a function of temperature from the dc_ij/dp data for each crystal. These calculations indicate much larger coupling of the SDW to the long-wavelength longitudinal phonons than to the shear modes for both Cr–Re crystals. Longitudinal mode softening under applied pressure, due to strong magnetoelastic interactions between the SDW and the longitudinal acoustic phonons, in Cr+0.3 at.% Re at T<T_N, gives nearly discontinuously way to extremely large mode stiffening as the crystal is heated through T_N. γ_n in the CSDW and pressure-induced ISDW phases of the Cr+0.5 at.% Re crystal shows unusual behaviour with temperature. This is particularly so for the shear mode Grüneisen parameters which are relatively large and negative just above T_IC, implying significant shear mode softening under applied pressure. γ_n (shear) then increases on further increasing the temperature of this crystal. In comparison, γ_n (shear) for both the CSDW and pressure-induced ISDW phases of other Cr alloys, with c>c_t, for instance Cr–Ru and Cr–Ir alloys, is near zero, slightly positive, and remains constant with temperature. The measurements on the Cr+0.5 at.% Re crystal suggest the existence of a new phase line, separating the ISDW phase present at atmospheric pressure from that induced from the CSDW phase by applying high pressure, on the pressure–temperature magnetic phase diagrams of dilute Cr alloys with c>c_t. More experimentation, particularly high pressure neutron diffraction studies, are needed to verify this point.     

Thermoelectric performance of p-type Bi–Sb–Te materials prepared by spark plasma sintering

In the present study, p-type (Bi,Sb)₂Te₃ thermoelectric materials have been fabricated through the spark plasma sintering (SPS) method by using powders with various particle sizes. Electrical conductivity (σ), Seebeck coefficient (), and thermal conductivity (κ) were evaluated in the temperature range of 300–500 K. The effect of annealing of the sintered samples on thermoelectric properties was also investigated. The maximum figure of merit ZT (ZT = ²σT/κ) of the sintered samples in the direction perpendicular to the pressing direction (with c-axis preferred orientation) was 1.15 at about 350 K, corresponding to be about 115% of that of the zone-melted ingot with the same composition in the same crystallographic orientation; while the bending strength was highly improved from about 10 to 80 MPa.     

Structure at abrupt copper–alumina interfaces: An ab initio study

The atomistic structure and the energetics of Cu(1 1 1)/-Al₂O₃(0 0 0 1) interfaces for two experimentally observed interface orientation relationships were investigated from first-principles by means of the mixed-basis pseudopotential method. In the first orientation relationship, the direction of Cu is parallel to the direction of -Al₂O₃, and in the second the Cu is rotated with respect to the -Al₂O₃ by 90° around the interface normal [0 0 0 1]. Numerous candidate systems were considered for each case, covering all high-symmetry interface configurations, and with either Al or O termination of the -Al₂O₃(0 0 0 1) surfaces. For each of the most stable interfaces, full relaxations of the positions of all atoms were performed. It is found that despite the different atomistic structures of the two interface types, their interface stabilities, in terms of the work of separation, and their local atomistic structures are similar.     

An atomistic analysis of the interface mobility in a massive transformation

A new multi-lattice kinetic Monte Carlo method has been used for an atomistic study on the interpretation of the interface mobility parameter for a massive face-centred cubic (fcc) to body-centred cubic (bcc) transformation in a single element system. For lateral growth of bcc in a system with an fcc(1 1 1)//bcc(1 1 0) and fcc[1 1 ]//bcc[0 0 ] interface orientation the overall activation energy for the interface mobility parameter is governed by energetically unfavourable atomic jumps. The atoms on the fcc lattice often cannot jump directly to bcc lattice sites because neighbouring atoms block the empty bcc sites. By single unfavourable jumps and by groups of unfavourable jumps a path from fcc to bcc is created. The necessity of these unfavourable jumps leads to an overall activation energy considerably larger than the activation energy barrier for a single atomic jump.     

Characterization of NH3 formation in desorption of Li–Mg–N–H storage system

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is “on board” storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this. Recently the hydrogen storage system, (Li₃N + 2H₂  LiNH₂ + 2LiH), was introduced by Chen et al. [P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302–304. [1]]. This type of material has attracted a great attention of the researchers from the metal hydride research community due to its high reversible storage capacity, up to 11.5 wt%. Currently the Li–Mg–N–H system has been shown to be able to deliver 5.2 wt% reversibly at a H₂ pressure of 30 bar and temperature of 200 °C. The accessibility of the capacity beyond 5.2 wt% is being actively explored. One of the issues related to the application of the metal–N–H storage systems is NH₃ formation that takes place simultaneously with H₂ release. NH₃ formation will not only damage the catalyst in a fuel cell, but also accelerate the cyclic instability of the H-storage material since the metal–N–H system turns into a metal–H system after loosing nitrogen and, therefore, it would not function at the temperature and pressure range designed for the metal–N–H system. The accurate determination of the amounts of NH₃ in the H₂ is, therefore, very important and has not been previously reported. Here a novel method to quantify NH₃ in the desorbed H₂, the Draeger Tube, is reported as being suitable for this purpose. The results indicate that the concentration of NH₃ in desorbed H₂ increases with the desorption temperature. For the (2LiNH₂ + MgH₂) system the NH₃ concentration was found to be 180 ppm at 180 °C and 720 ppm at 240 °C.     

Comparison of thermally induced and deformation induced martensite in Fe–29% Ni–2% Mn alloy

The morphology and crystallography of martensite either formed by cooling to subzero temperatures (thermal effect) or by compression deformation were compared for different austenite grain sizes of Fe–29% Ni–2% Mn alloy by transmission electron microscope (TEM). TEM observations revealed both and ′ martensite formation within large grained austenite phases by thermal effect whereas only ′ martensite formation was observed in small grained austenite phases. On the other hand, compression deformation effect caused only ′ martensite formation in both small and large grained austenite phases of Fe–29% Ni–2% Mn alloy. Thermally induced ′ martensite exhibited a lenticular morphology with partial twinnings that are peculiar to this kind of morphology. The crystallographic orientation relationship between austenite and thermally induced ′ lenticular martensites was found to be as Kurdjumov–Sachs (K–S) type relationship.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

The ternary system Na2O–ZnO–WO3: Compounds and phase relationships

The subsolidus phase relationships of ternary system Na₂O–ZnO–WO₃ have been investigated by X-ray diffraction (XRD) and differential thermal analyzer (DTA). All the samples were synthesized in the temperature range from 530 to 850 °C in air. There are one ternary compound and five binary compounds in the Na₂O–ZnO–WO₃ system, which can be divided into eight three-phase regions. The crystal structure of the ternary compound Na_3.6Zn_1.2(WO₄)₃ is determined by single-crystal structure analysis method. It belongs to triclinic system with space group and lattice constants a = 7.237 (5) Å, b = 9.172 (6) Å, c = 9.339 (6) Å and  = 94.920 (4)°, β = 105.772 (9)°, γ = 103.531 (8)°, Z = 2. DTA analyses indicate that the compound Na₂WO₄ is not suitable to be the flux for ZnO crystal growth below 1250 °C, since no liquidus was observed in the system before 1250 °C.     

Growing ledge structures of AlN crystals in a Fe–Mn–Al–C alloy

The Fe–30.4Mn–8.7Al–1.0C (wt.%) alloy was nitridized at 1000 °C. The AlN formed into a Widmanstätten side-plate shape. The side-plate of the AlN is parallel to the basal plane of the HCP crystal. There are three possible growing riser planes; namely ()_AlN, ()_AlN, and ()_AlN planes. The angle for growing riser planes met on the (0 0 0 1) terraces is 120°.     

An investigation of the Pd–Ag–Ru–Gd quaternary system phase diagram

On the basis of the Ag–Pd–Gd, Ag–Ru–Gd and Pd–Ru–Gd ternary systems, the partial phase diagram of Pd–Ag–Ru–Gd (Gd < 25 at.%) quaternary system has been studied by means of X-ray diffraction analysis, differential thermal analysis, electron probe microanalysis and optical microscopy. The 700 °C isothermal sections of the Ag–Pd–5Ru–Gd, Ag–Pd–20Ru–Gd and Ag–Pd–50Ru–Gd (Gd ≤ 25 at.%) phase diagrams were determined, respectively. And the 700 °C isothermal section of the Pd–Ag–Ru–Gd (Gd ≤ 25 at.%) quaternary system phase diagram was finally inferred. The section consists of four single-phase regions: solid solution Pd(Ag), (Ru), Pd₃Gd and Ag₅₁Gd₁₄; five two-phase regions: Pd(Ag) + (Ru), Pd(Ag) + Ag₅₁Gd₁₄, (Ru) + Ag₅₁Gd₁₄, Pd(Ag) + Pd₃Gd and (Ru) + Pd₃Gd; three three-phase regions: Pd(Ag) + Pd₃Gd + (Ru), Pd(Ag) + Ag₅₁Gd₁₄ + (Ru) and (Ru) + Ag₅₁Gd₁₄ + Pd₃Gd; one four-phase region Pd(Ag) + (Ru) + Ag₅₁Gd₁₄ + Pd₃Gd. No new quaternary intermetallic phase has been found.     

Laser cladding of stainless steel with Ni–Cr3C2 and Ni–WC for improving erosive–corrosive wear performance

The microstructure and the erosive–corrosive wear (ECW) performance of laser-clad Ni–Cr₃C₂ and Ni–WC coatings with overlapping clad tracks (OCT) on a 0.2% C martensitic stainless steel were investigated by scanning electron microscopy (SEM), XRD, EDX techniques and ECW testing. The coating produced by completely dissolving Cr₃C₂ particles in laser melted pool is composed of austenite (γ) dendrites surrounded by a γ-M₇C₃ eutectic, whereas another one is of granular solidifying structure in which contains the incompletely dissolved WC particles. The microhardness of Ni–WC coating is higher than that of Ni–Cr₃C₂, about 300 HV average. The main reason of microhardness difference is that two coatings have different solidified structure. The comparison of ECW tests found that the reduction of ECW rate dose not occur with the increase of hardness. The Ni–Cr₃C₂ coating with lower hardness has a lower ECW rate with respect to the Ni–WC one. Both average ECW rate decreased by approximately 30% and 60% as compared to that of stainless steel substrate, and both coatings had different ECW mechanism. The increase of ECW resistance is closely related to structure state, kind and amount of carbides, microhardness and toughening ability of the clad layer.     

Phase equilibria on the ternary Mg–Mn–Ce system at the Mg-rich corner

Three isopleths at the Mg-rich corner of Mg–Mn–Ce ternary system were investigated via thermal analysis, SEM/EPMA and XRD. A ternary eutectic reaction was observed at 1 wt.% Mn and 23 wt.% Ce and 592 °C. A solid-solution type ternary intermetallic compound, (Mg,Mn)₁₂Ce, was observed with 0.5 at% solid solubility of Mn in the tetragonal Mg₁₂Ce. With the aid of thermodynamic modeling and experiments, a revised phase diagram for the binary Mg–Ce system and the isopleths of 0.6, 1.8 and 2.5 wt.% Mn were proposed up to 25 wt.% Ce.     

Phase transformation and morphology of the intermetallic compounds formed at the Sn–9Zn–3.5Ag/Cu interface in aging

The morphology and phase transformation of the intermetallic compounds (IMCs) formed at the Sn–9Zn–3.5Ag/Cu interface in a solid-state reaction have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The monoclinic η′-Cu₆Sn₅ transforms to the hexagonal η-Cu₆Sn₅ and the orthorhombic Cu₅Zn₈ transforms to the body-centered cubic (bcc) γ-Cu₅Zn₈ as aged at 180 °C. The scallop-shaped Cu₆Sn₅ layer is retained after aging at 180 °C for 1000 h. In the solid-state reaction, Ag is repelled from η′-Cu₆Sn₅ and reacts with Sn to form Ag₃Sn, and the Cu₅Zn₈ layer decomposes. Kirkendall voids are not observed at the Sn–9Zn–3.5Ag/Cu interface even after aging at 180 °C for 1000 h.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

The ternary system: silicon–titanium–uranium

Phase equilibria in the system Si–Ti–U were established at 1000 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed and were characterised by X-ray powder data refinement: (1) stoichiometric U₂Ti₃Si₄ (U₂Mo₃Si₄-type) with a small homogeneity region of about 3 at.% exchange U/Ti and (2) U_2−xTi_3+xSi₄ (Zr₅Si₄-type) extending at 1000 °C for 0.7<x<1.3. Mutual solubility of U-silicides and Ti-silicides was found to be below about 1 at.%. The Ti,U-rich part of the diagram was also investigated at 850 °C establishing the tie-lines to the low temperature compounds U₂Ti and U₃Si. U₂Ti₃Si₄ is weakly paramagnetic following a Curie–Weiss law above 50 K with μ_eff.=2.67 μ_B/U, Θ_P=−150 K and χ₀=1.45×10⁻³ emu/mol (18.2×10⁻⁹ m³/mol).