Interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures studied by aberration-corrected ultrahigh-resolution electron microscopy

The microstructure of interfacial defects in YBa₂Cu₃O_7-δ/SrTiO₃(0?0?1) heterostructures has been investigated by aberration-corrected ultrahigh-resolution electron microscopy. We determine that c-axis-oriented YBa₂Cu₃O_7-δ thin films epitaxially grow on SrTiO₃(0?0?1) with two types of interface structure. The coalescence of nucleation sites with different types of interface structure leads to the formation of antiphase domain boundaries in YBa₂Cu₃O_7-δ thin films, which terminate at planar faults with different configurations near the interface. Stand-off misfit dislocations are observed and the dislocation core structure is explored. Based on the interface structure and interfacial defects, the initial growth mode of YBa₂Cu₃O_7-δ thin films on SrTiO₃(0?0?1) is discussed.     

Enhancement of hardening and thermal resistance of Mg–Sn-based alloys by addition of Cu and Al

Mg–Sn-based alloys are considered as a promising precipitation-hardening system for applications at elevated temperatures, but the hardening effect is not satisfactory owing to sluggish nucleation and rapid coarsening of the major Mg₂Sn lath precipitates. In this study, Cu and Al are added to a Mg–6Sn–1Mn base alloy. The age-hardening response and the microstructures of these modified alloys have been investigated and are compared to that of the base alloy. The additional elements are found to bring several beneficial effects to the alloys for applications at elevated temperatures. Firstly, a eutectic structure consisting of strong intermetallic phases, i.e. Mg₂Cu in the Mg–6Sn–1Mn–2Cu alloy and Al_0.93Cu_1.07Mg in the Mg–6Sn–1Mn–2Cu–2Al alloy, remains stable along the grain boundaries after solution and ageing heat treatments. Secondly, the precipitate density has been increased significantly and the precipitate size has been refined remarkably during ageing at 200?°C. Moreover, the growth of the precipitates is inhibited remarkably during the over-ageing period. Therefore, the age-hardening response and over-ageing resistance are notably improved.     

Observation of phase separation in magnetron sputter-deposited Al–Cu thin films

The microstructures of Al–1.8 to 92.5?at.% Cu thin films prepared by radiofrequency (13.56?MHz) cathodic magnetron sputtering have been investigated by X-ray diffraction (XRD) and transmission electronic microscopy (TEM). A phase separation occurs in films of nominal Al–66.64?at.% Cu composition, consisting of a fcc Al solid solution phase, a fcc Cu solid solution phase and an unexpected sc Cu₃Al ordered phase with a Cu₃Au structure and a lattice parameter of about 0.36?nm.     

Microstructure and mechanical behavior of nanostructured composite Cu60Fe40 alloy

In this study, bulk nanostructured composite Cu₆₀Fe₄₀ alloy is prepared by a combustion synthesis technique. The prepared Cu₆₀Fe₄₀ alloy consists of Cu(Fe) solid solution and Fe(Cu) solid solution phases. The large-scaled compositional segregation in the Cu-rich and Fe-rich phases is not observed, respectively. A few micron-sized dendrite (Fe(Cu) solid solution) is embedded into the nanostructured matrix (Cu(Fe) solid solution). The grain size of the matrix is in the range 50–300?nm. The yield and fracture strength of the Cu₆₀Fe₄₀ alloy are 540 and 1050?MPa, respectively, and the fracture strain obtained from the compression test is about 20.9%. The Cu₆₀Fe₄₀ alloy displays notable work hardening in the compressive deformation.     

Synthesis,transformation and superconductivity of dual phase In–Sn alloy nanoparticles embedded in an Al matrix

We report the synthesis of nanoembedded biphasic alloy particles of In–Sn near eutectic alloy compositions embedded in an aluminium matrix by rapid solidification processing. Detailed transmission electron microscopy indicates that the two phases present at room temperature in as-synthesized samples are β and γ phases with tetragonal and hexagonal crystal structures, respectively. These co-exist with a small amount of single phase In or Sn particles with sizes less than 10?nm. Low temperature magnetization measurements indicate a superconducting transition temperature of 5?K, suggesting complete decomposition of the β-phase at small sizes and at low temperature. The small particles show type II behavior with a critical field Hc₁≈44?G and two values for Hc₂ of 250 and 1000?G, respectively. These values are considerably lower than those observed in bulk In–Sn alloys.     

Effect of Sn on microstructure and mechanical properties of (Ti–Cu)-based bulk metallic glasses

A Ti₄₃Zr₇Cu₄₃Ni₇ bulk metallic glass (BMG) exhibits a plastic strain of less than 0.2% under room-temperature compression. However, a Ti₄₅Zr₅Cu₄₀Ni_7.5Sn_2.5 BMG developed by addition of Sn to Ti₄₃Zr₇Cu₄₃Ni₇ shows a significant improvement of ～14.8% to the plastic strain with work hardening. Microstructural comparison of these BMGs reveals that the addition of Sn leads to formation of nano-scale chemical heterogeneities throughout the material. These nano-scale chemical fluctuations play an important role in enhancing the plastic strain of (Ti–Cu)-based BMGs.     

Surface tension measurement of undercooled liquid Ni-based multicomponent alloys

The surface tensions of liquid ternary Ni–5%Cu–5%Fe, quaternary Ni–5%Cu–5%Fe–5%Sn and quinary Ni–5%Cu–5%Fe–5%Sn–5%Ge alloys were determined as a function of temperature by the electromagnetic levitation oscillating drop method. The maximum undercoolings obtained in the experiments are 272 (0.15T _L), 349 (0.21T _L) and 363?K (0.22T _L), respectively. For all the three alloys, the surface tension decreases linearly with the rise of temperature. The surface tension values are 1.799, 1.546 and 1.357?N/m at their liquidus temperatures of 1719, 1644 and 1641?K. Their temperature coefficients are ?4.972?×?10^–4, ?5.057?×?10^?4 and ?5.385?×?10^?4?N/m/K. It is revealed that Sn and Ge are much more efficient than Cu and Fe in reducing the surface tension of Ni-based alloys. The addition of Sn can significantly enlarge the maximum undercooling at the same experimental condition. The viscosity of the three undercooled liquid alloys was also derived from the surface tension data.     

Partitioning and segregation of trace element Sn in a low-alloy steel

The partitioning of the trace element Sn into Cu-rich precipitates in a low-alloy pressure-vessel steel has been characterized using the three-dimensional atom probe (3DAP) technique. This investigation has revealed for the first time that the trace element Sn, present at only 0.007?at.% in the steel, partitions strongly to both small spherical precipitates (<4?nm in diameter) and to large non-spherical precipitates (largest dimension 10–50?nm) during thermal ageing. Sn was also seen to segregate strongly to the precipitate/matrix interface of a large Cu precipitate and particularly in the region where a dislocation appears to intersect the precipitate. The strong attraction of large solute atoms to special sites probably drives the interfacial segregation of Sn. This is consistent with the observation of stronger segregation of Sn to the interface of large precipitates than to the coherent interface of smaller precipitates.     

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al₇Cu₂Fe phase during laser cladding of quasicrystal-forming Al₆₅Cu_23.3Fe_11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.     

HIGH-Tc superconductivity in perovskite oxides of the Y─Ba─Cu─O system

Abstract

Several biphasic compositions of the type Y_3?xBa_3+xCu₆O₁₄ show an onset of superconductivity in the 90-115K range, attaining zero resistance in the 70-85K range. The phase responsible for superconductivity in these compositions is a perovskite oxide of composition YBa₂Cu₃ O₇. This oxide annealed in oxygen shows the onset of superconductivity at 120K and zero resistance at 87K. YBa₂,Cu₃O₇ shows the highest Meissner effect of all oxide superconductors. The superconducting behaviour of the two perovskite oxides, Y_0.95Ba_1.95,Cu₃O₇ and Y_1.05Ba_1.95Cu₃O₇ show interesting features; a marked decrease in resistivity is observed from room temperature itself in the former oxide with zero resistance at 89K. Electron microscopy and infrared spectra of these oxides are briefly discussed.     

A first-principles study of the formation of θ′′ phase in Al–Cu alloys

The θ′′-Al₃Cu phase plays an important role in the precipitation process of Al–Cu alloys. This phase has a sandwich structure—every two {200}_Cu layers are separated by three {200}_Al layers. To analyse the formation mechanism of this structure, the elastic strain energy of the {200}_Cu and {200}_Al layers, and the chemical bonding energy that reflects the interaction between the electrons in Cu and neighbouring Al atoms are calculated and analysed by first-principles calculations, projected density of states and Bader analysis. Our computation results reveal that this sandwich structure is energetically preferred in the competition of elastic strain and chemical bonding energies. To minimise the elastic strain energy of {200}_Al and {200}_Cu layers, the {200}_Cu layers prefer being apart from each other, whereas the chemical bonding energy favours the opposite arrangement because the intermetallic bond between Al and Cu atoms may form through p-d hybridization.     

Local order description of an icosahedral Al–Cu–Fe quasicrystal

Abstract

A rapidly solidified Al₆₅Cu₂₀Fe₁₅ icosahedral alloy has been studied by extended X-ray absorption he structure above the Cu and Fe K absorption edges. The local order has been determined around Cu and Fe atoms and compared with results obtained previously in other icosahedral alloys     

Morphological and magnetic response of copper-substituted nickel ferrite nanoparticles

Ferrite nanoparticles are interesting materials owing to their unique physical and chemical properties. The metal-doped ferrites have well-defined structures and magnetic response, such as high permeability for a specific frequency range. In this study, copper-substituted nickel ferrite (Ni_1?xCu_xFe₂O₄) nanoparticles with a compositional range of 0?≤?x?≤?0.3 were synthesised through a co-precipitation technique. Metal chlorides were used as precursors and NaOH as a precipitating agent for the growth of ferrite nanoparticles. To minimise the internal stresses and maximise the magnetic response, ferrite nanoparticles were annealed in a furnace at 700°C for 6 h. The structural and magnetic response of Ni_1?xCu_xFe₂O₄ ferrite with different values of x were investigated using Scanning Electron Microscopy (SEM), Fourier Transform Infrared spectroscopy (FT-IR), Vibrating Sample Magnetometer (VSM) and X-ray Diffraction (XRD) techniques. XRD analysis confirmed the formation of cubic spinel structure of single phase for all the compositions. The lattice constant decreased with increase in the value of x. FT-IR study showed two main metal oxygen bonds in the range 500–700 cm^?1 confirming the formation of a single-phase cubic inverse structure of Cu-substituted Ni ferrite. VSM results revealed the formation of ferrimagnetic nanoparticles. The optical and magnetic response of the ferrite nanoparticles changed with Cu content.     

Enhanced age-hardening response and microstructure study of an Ag-modified Mg–Sn–Zn based alloy

A new Mg–Sn–Zn based alloy modified with a small amount of Ag exhibits a significantly higher aging peak than that of the base alloy and at a considerably shorter aging time. The enhanced aging response of the Ag-modified alloy is ascribed to the precipitation of densely distributed MgZn₂ needles and Mg₂Sn plates stimulated by the Ag. A wide and low plateau behind the hardness peak could be associated with rod-shaped precipitates of the orthorhombic Mg₅₄Ag₁₇ phase, aligned with the hexagonal axis of the Mg matrix.     

Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass

Cast Cu₅₀Zr₅₀ alloy rods with a diameter of 1?mm have been found to consist of a glassy phase containing fine crystalline particles with a size of about 5?nm. They have a glass transition temperature T _g of 675?K, and a large supercooled-liquid region extending 57?K above T _g. The rods exhibit a high yield strength of 1860?MPa and a Young's modulus of 104?GPa. Because they contain a dispersion of embedded nanocrystals, the as-cast bulk metallic glass rods can sustain a compressive plastic strain at room temperature of more than 50%, an exceptional value which is explicable by compensation of any shear softening by nanocrystal coalescence and pinning of shear bands.     

Precipitation in a Cu–4Ti–2Be alloy

It is known that the binary Cu–2?wt.% Be alloy can be strengthened by ageing its supersaturated solution at 523?K. The increase in the strength arising from ageing is because of the formation of GP zones. The same level of strength is achieved by ageing the supersaturated solid solution of a dilute binary Cu–4?wt.% Ti alloy at 723?K. In contrast to Cu–2?wt.% Be alloy, the increase in strength in this alloy occurs on account of spinodal decomposition of the as-quenched microstructure through the mechanism of clustering and ordering. In the present study, decomposition of a supersaturated solid solution of the ternary Cu–4?wt.% Ti–2?wt.% Be alloy was studied. Whereas ageing of this alloy at 523?K resulted in the formation of GP zones like in a Cu–2?wt.% Be alloy, ageing at 723?K resulted in a spinodally decomposed microstructure as in a Cu–4?wt.% Ti alloy. The decomposition of the ternary alloy at the two temperatures by two different mechanisms indicates that Be and Ti in solution in Cu act, more or less, independently of each other during ageing.     

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon undergoes a B2 ? B19′ transformation upon cooling and heating. When the Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)₂Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)₂Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)₂Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.     

A comparative study of Sr-doped LaMnO3 synthesised via solid-state reaction and sol–gel methods

ABSTRACT

This work deals with the synthesis of hole-doped La_1-xSr_xMnO₃ (x?=?0.25 and 0.33) perovskite via two different ways, namely a solid-state reaction method and a sol–gel process. Various properties of the samples have been investigated and compared by means of different techniques, namely X-ray diffraction (XRD), temperature-dependent resistivity measurements in the temperature range 25–300?K using a closed cycle refrigerator and vibrating sample magnetometry (VSM). All samples have orthorhombic crystal symmetry and an irregular variation in unit-cell volume with increase in Sr content. The average crystallite size was determined from the XRD data and found to lie in the range 19–22?nm. Metal–insulator transitions (MIT) were observed in all the samples via low-temperature electrical characterisations. The magnetic measurements confirm that the magnetisation does not saturate for the 25% Sr doped samples up to the available field of 10?kOe. The magnetic response also supports the interpretation of XRD and resistivity data. The obtained results are explained on the basis of double-exchange and super-exchange mechanisms.     

Possibility of a shape phase transition for solidification of tin at scallop-like surfaces of Cu6Sn5

The possibility of solder-spreading transitions in solidification of solder at a rough rigid intermetallic surface is proven theoretically. Depending on the misorientation of scallops on the interface, one can observe a two-dimensional spreading transition over the scallops or a one-dimensional spreading transition along the triple-junction of two intermetallic compound scallops and liquid solder. The extent of undercooling can be determined not only by different interface energies, but by different angles between neighbouring scallops as well.     

Influence of Cd and Ge on the kinetics of Guinier–Preston zone formation in Cu–Be alloys

The influence of Cd and Ge on the kinetics of Guinier–Preston zone formation has been studied for Cu–2?wt%?Be–0.3?wt%?Cd and Cu–2?wt% Be–0.3wt%?Ge solid solutions. The relative volume fractions of these Guinier–Preston (GP) zones are determined for different times of ageing at 473?K by a method based on microhardness measurements. Cd atoms in Cu–Be–Cd accelerate GP zone formation. This is due to the supersaturation of Cd atoms in Cu and their strong binding energy with vacancies. In contrast, the Ge atoms do not influence GP zone formation in Cu–Be–Ge. The binding energies between the two solute atoms and a vacancy, and the concentration of the quenched-in vacancies bound to the solute atoms, are calculated using the Hasiguti formula and the Kimura equation.