A note on the peculiarities of producing of high-temperature super-conducting polymer–ceramic composites

High-temperature super-conducting polymer–ceramic composites were obtained based on Y₁Ba₂Cu₃O_7−x and a range of binders. Independent of the matrix type used, superconducting properties of the samples are missing (Maisner effect is absent) for the polymer–ceramic composites, formed by the hot pressing method at 200 °C. Restoration of superconducting processes occurs only after treatment of samples in the stream of dry oxygen at -transition temperature of the polymeric binder. Articles formed at 130 °C possess stable superconducting properties. And polyolefin binder, preserve the same critical superconducting (SC) transition parameters as the initial ceramic has, while acryl and vinyl homo- and co-polymers enhance the critical SC initiation transition temperature by 1–2 °C. Broadening of SC transition temperature interval of the composites at higher temperatures of formation (≥160 °C) is associated with thermo-oxidation destruction of the polymeric binder. Factors facilitating to the decline of destruction of the matrix reducing pressing duration, addition of anti-oxidants) allows to uphold the critical SC transition parameters of the initial ceramic. Polymerization methods of obtainment of polymer–ceramic composites permit to enhance SC characteristics of the materials versus the initial ceramic.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.8Ti0.2O3−δ solid solutions

Series of perovskite-type compounds La_1−aCa_aCr_0.8Ti_0.2O_3−δ (a=0–1.0) were synthesized by the ceramic technique in air (final heating 1350 °C). The crystal structure of the compounds after cooling in air to room temperature was characterized as orthorhombic in space group Pbnm. Analysis of the lattice constants shows a noticeable decrease with increasing Ca content. All compounds prepared were stable in air and in a stream of Ar/1 Pa O₂ at 20–1400 °C, as also in Ar/5% H₂ (pH₂O/pH₂=0.01) at 850–1000 °C. Oxygen stoichiometry and electrical conductivity of the solid solutions with a=0.0–1.0 are investigated. Increasing Ca contents decrease the stability of the oxides in respect to the thermal dissociation of oxygen. All compounds are p-type semiconductors in the temperature range 20–1000 °C at oxygen partial pressures of 10⁻¹⁵ to 0.21×10⁵ Pa. A maximum conductivity of about 30 S/cm in air at 1000 °C is observed for the composition with a=0.6 corresponding to a ratio of Cr³⁺/Cr⁴⁺=1 at an oxygen stoichiometry near 3.0, and oxidation states of La, Ca, Ti, and O ions of 3+, 2+, 4+, and 2−, respectively.     

Structural changes of Co70Fe5Si10B15 amorphous alloy induced during heating

In this study we present the results on complex structural changes of the Co₇₀Fe₅Si₁₀B₁₅ amorphous alloy induced during heating in the temperature range between 20 and 1000 °C. The structural and phase transformation changes were correlated with DTA, XRD and SEM properties. It is shown that initial Co₇₀Fe₅Si₁₀B₁₅ alloy during heating undergoes complex crystallochemical changes. In the range between ambient temperature and near 400 °C, investigated alloy retains the solid-state amorphous properties. Prolonged heating induces complete transformation to crystalline solid state. The solid–solid amorphous to crystalline state transformation process is completed at 500 °C, when two nanocrystalline phase alloy systems are formed. Prolonged thermal treatment between 600 and 1000 °C, influenced further elemental segregation and phase transition. At 1000 °C, the composite material consisting of two FCC cobalt-rich alloys and a hexagonal unidentified alloy are formed.     

Mechanical properties and oxidation behaviour of two-phase iron aluminium alloys with Zr(Fe,Al)2 Laves phase or Zr(Fe,Al)12 τ1 phase

Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al)₂ or τ₁ phase Zr(Fe,Al)₁₂. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ₁ phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0₃ Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0₃ Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%.     

Hydrogen storage behavior of ZrNi 70/30 and ZrNi 30/70 composites

The Zr–Ni compositional alloys, namely (i) ZrNi 70/30 and (ii) ZrNi 30/70 (both by weight) have been investigated for the reversible hydrogenation behavior. These composites show Zr–Ni intermetallic multi-phase formation as explored by X-ray diffraction studies. The sorption kinetics of ZrNi 70/30 seems much faster (3–4 times) than that of ZrNi 30/70 alloys. The initial desorption rate increasing with an increase in temperature. A well-defined plateau region was obtained for the ZrNi 70/30 with an equilibrium pressure range from <1 bar (300 °C) to 10 bars (390 °C). For ZrNi 30/70, the sloppy plateau region extends to higher equilibrium pressures. It is estimated that the total effective hydrogen concentration for ZrNi 70/30 (1.0 wt.%) is at least 2 times that of ZrNi 30/70 (0.5 wt.%) composites. From the PCT isotherms, the enthalpy of reaction (ΔH) has been calculated to be 39 kJ/mol H₂ for the ZrNi 70/30. The surface morphologies of the hydrogenated materials exhibit the presence of cracks and particle size pulverization in comparison to the pristine alloys.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Microstructure evolution in CoNiGa shape memory alloys

Magnetic shape memory CoNiGa alloys hold great promise as new smart materials due to the good ductility and a wide range of martensitic transformation (MT) temperatures as well as magnetic transition points. This paper reports the results of investigations on the equilibrium phase constitution and microstructure evolution in quenched or aged CoNiGa alloys using the optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The dendritic γ phase decreases as lowering of Ga content in studied two series of samples (Co₅₀Ni_50 − xGa_x, x = 0–50 and Co_100 − 2yNi_yGa_y, y = 15–35). Some γ′ precipitates with different morphologies were found in given alloys conducted with water quenching (WQ) at 800 °C or long-time ageing at 300 °C. After 800 °C quenching, the γ′ phase has a rod-like shape for the Co₅₀Ni₃₀Ga₂₀ alloy but shows a Widmanstätten morphology as Ga increases to 25 at%, and trends to be block structure in further high Ga content alloy. In the case of 300 °C aged alloys, the γ′ particles prefer to nucleate in interior of γ phase or at the interface of β–γ. We also presented an illustrative vertical section phase diagram keeping 50 at% Co, and isothermal section phase diagram at 1150 and 800 °C of the CoNiGa system. Based on the schematic ternary phase diagram, the composition scope which potentially holds over the magnetic pure martensite phase structure at room temperature (RT) was pointed out. It is believed that this optimized range alloys would play an important role in the functional materials design for application.     

Microstructures and properties of Mg–7Gd alloy containing Y

Mg–7 mass%Gd–x mass%Y (x = 0, 1, 3 and 5) alloys were prepared by casting method, and the microstructures, age hardening behavior and mechanical properties have been investigated. The results show that the addition of Y to the binary Mg–7Gd alloy could reduce the grain size of the as-cast alloys, and enhance the age hardening response and improve mechanical properties during the investigated temperature range. The Mg–7Gd–5Y alloy exhibits maximum ultimate tensile strength and yield strength at peak hardness, and the values are 258 and 167 MPa at room temperature, and 212 and 140 MPa at 250 °C, respectively, which is about 1.8 times as high as the Mg–7Gd binary alloy. When x is more than 3, the amount of Mg₅(Gd,Y) phase is observed at the peak hardness of aged alloys. The significant improvement of the tensile strength at peak hardness is mainly attributed to the fine dispersion of the β-Mg₅(Gd,Y) precipitate.     

Hydrogen disproportionation by reactive milling and recombination of Nd2(Fe1−xCox)14B alloys

Stoichiometric Nd₂(Fe_1−xCo_x)₁₄B alloys (x=0, 0.25, 0.5, 0.75 and 1) have been disproportionated into NdH_2+δ and bcc–(Fe,Co) (0≤x≤0.75) or fcc–Co (x=1), respectively, by milling in hydrogen at enhanced temperatures. Reactive milling leads to the disproportionation of the thermodynamically very stable Nd₂Co₁₄B alloy. This reaction is not possible via the conventional hydrogenation disproportionation desorption and recombination (HDDR) process. Grain sizes of disproportionated and recombined Nd₂(Fe,Co)₁₄B materials were found to be <10 nm and 40–50 nm, respectively — approximately an order of magnitude smaller than those of conventional-HDDR processed alloys. The recombined Nd₂Co₁₄B alloy shows on average slightly smaller grain sizes than the Nd₂Fe₁₄B compound. A more effective exchange coupling leading to enhanced remanences, possibly due to the slightly smaller grain size, has been observed for Nd₂Co₁₄B powders recombined at 600–700°C.     

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.     

An electrochemical method for the preparation of Mg–Li alloys at low temperature molten salt system

Mg–Li alloys have been prepared by electrolysis in a molten salt electrolyte of 50% LiCl–50% KCl (mass%) at low temperature of 420–510 °C. The effects of electrolytic temperature and cathodic current density on alloy formation rate and current efficiency were studied. For the deposition of metallic lithium on the cathode consisting of solid Mg and liquid Mg–Li, both electrolytic temperature and cathodic current density have no obvious influence on current efficiency; while for the deposition of metallic lithium on the solid magnesium cathode, both electrolytic temperature and cathodic current density greatly affect alloy formation rate and current efficiency. The optimum electrolysis condition is—molten salt mixture, LiCl:KCl = 1:1 (mass%), electrolytic temperature: 480 °C, cathode current density: 1.13 A cm⁻². Mg–Li alloys with low lithium content (about 25 wt% Li) were prepared via electrolysis at low temperature following by thermal treatment at higher temperature.     

The stress anomaly in FeAl–Fe3Al alloys

The anomalous stress peak observed near 500–600 °C in Fe–Al alloys has now been convincingly explained using a model of hardening by immobile thermal vacancies on the lower temperature side of the peak and the loss of hardening as these vacancies become mobile at higher temperatures. The large numbers of vacancies required for such hardening are associated with compositions close to stoichiometry, i.e. 40–50%Al, raising the question of whether such a vacancy hardening model can be adopted for Fe₃Al alloys, which show a similar stress peak anomaly. Examination of data on vacancy formation over the entire range of composition, Fe–Fe₃Al–FeAl, shows that, indeed, a vacancy hardening model appears capable of explaining the stress anomaly for both FeAl and Fe₃Al.     

A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

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).     

Austenite stability in Fe–Mn–Si-based shape memory alloys

The stability of austenite in a number of Fe–Mn–Si-based shape memory alloys has been investigated. It was found that a grain boundary precipitate of BCC structure is formed over a wide range of alloy compositions and heat treatment temperatures. This grain boundary phase has been identified as the chi (χ) phase. Although up to 3 vol.% of the grain boundary precipitate was generated by isothermal aging in the range 500–800 °C, it was found not to markedly affect the mechanical properties or the shape memory effect. Nano-indentation indicated that the hardness and strength of the parent and precipitate phase are very similar, as are their compositions.     

The microstructure, tensile properties, and creep behavior of as-cast Mg–(1–10)%Sn alloys

The microstructure, tensile properties, and creep behavior of Mg–(1–10)wt%Sn alloys were studied in this paper. The microstructure of as-cast Mg–Sn alloys consisted of dendrite -Mg and second Mg₂Sn phases and the secondary dendrite arm spacing (DAS) of the -Mg phase was decreased with the increase of tin content. The micro-hardness of the alloys increased when tin content rises, while the greatest tensile and elongation were exhibited by Mg–5 wt%Sn. The indentation creep experiments were conducted at 150 °C for applied loads of 30 kg, it suggested that the indentation creep resistance of Mg–Sn alloys could be obviously improved with the increase of tin content, and Mg–10%Sn alloy had better indentation creep resistance than that of AE42.     

Mechanical alloying and spark plasma sintering of the intermetallic compound Ti50Al50

This study investigates the microstructures and mechanical properties of Ti₅₀Al₅₀ alloys prepared via mechanical alloying (MA) starting from elemental powders. The process of the spark plasma sintering (SPS) has also been studied. It is found that the nanocrystallization process of the Ti–Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100–1200 °C with a compaction pressure of 30 MPa and a heating rate of 30 °C min⁻¹. Specimens with high densities and approaching the equilibrium state can be obtained in short time by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth.     

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.