Origin of tweed in Au–Cu–Al alloys

The premartensitic tweed in Au–Cu–Al alloys, contrary to previous thought that resort to defects, is confirmed to be associated with the coherent embryos of an intermediate phase (I phase) embedded in parent phase. The parent?→?I phase transformation temperature was measured by differential scanning calorimeter and dynamic mechanical analysers, which shifts from 82.3 to 557.6?°C depending on the alloy composition. X-ray diffraction and transmission electron microscopes (TEM) results show that the parent?→?I phase transformation is a charge density wave transition that cannot be suppressed even by melt-spun method, which shows obvious compositional inhomogeneity between I phase and parent. The results imply that the parent?→?I phase transition is a fast displacive transformation coupled with diffusion. Moreover, accompanying the parent?→?I phase transformation, alloys demonstrate diversified microstructure revealed by TEM observation, from tweed to chessboard nanowires or twins. These findings provide the experimental evidence for that parent?→?I phase transformation in Au–Cu–Al alloys is originated from pseudospinodal decomposition as theoretically predicted.     

Methanol-driven structuring of Au–Pt bimetallic nanoclusters on a thin film of Al2O3/NiAl(100)

The adsorption of methanol altered structures of Au–Pt bimetallic nanoclusters on a thin film of Al₂O₃/NiAl(100). Methanol adsorbed on the Au–Pt intermixed bimetallic clusters, of which the surfaces consist of both Au and Pt, induced a segregation of Au from Pt. This segregation state was unstable, as the clusters returned to the initial Au–Pt intermixed state upon desorption or decomposition of adsorbed methanol. Ethanol and cyclohexene were adsorbed on Au–Pt bimetallic clusters for comparisons, indicating that the interaction of the hydroxyl group of methanol with the clusters accounts for the structural modifications.     

Spectral features of Co–Ge–Te amorphous thin film

Optical spectral features of Co_xGe_yTe_100?x?y amorphous thin films where 10≤x≤35 and 41≤y≤47 were studied for the first time. The transmittance and reflectance at normal incidence have been measured at room temperature in the spectral range 190–2500 nm. Refractive index and extinction coefficient have been evaluated in the above spectral range. Band tail width and energy gap were strongly affected by cobalt concentration in the “as prepared” amorphous thin film. Absorption band spectrum on the basis of the imaginary parts of the dielectric constant, ε₂, ε_1, refractive index, and extinction coefficient are also affected by cobalt content. On the other hand, the band edge parameter β remains almost constant.     

High-resolution transmission electron microscopy study on reversion of Al2CuMg precipitates in Al–Cu–Mg alloys under irradiation

Image deconvolution analyses showed that reversion of S-Al₂CuMg precipitates occurred in an Al–Cu–Mg alloy during high-resolution transmission electron microscopy observations. A fraction of Mg and Cu atoms in the precipitates diffused into Al matrix due to electron beam irradiation at 300 kV, resulting in structural/chemical reversion of the precipitates. The structural reversion of the S-Al₂CuMg precipitates is closely related with irradiation-induced displacement of atoms. The strong attraction between Cu and Mg atoms might assist the sub-threshold displacement of Cu atoms. One transitional structure is determined to be S′′-Al₁₀Cu₃Mg₃, a precursor of S-Al₂CuMg. Two other transitional structures, Al₃CuMg and Al₁₈Cu₅Mg₅ which have the same lattice parameters of a = c = 0.405 nm as that of S′′-Al₁₀Cu₃Mg₃, but different b values, are suggested.     

Theoretical study of optoelectronic properties of GaAs1−xBix alloys using valence band anticrossing model

The (12 × 12) and (14 × 14) valence band anticrossing (V-BAC) models were applied to calculate the electronic band structure of GaAs_1−xBi_x dilute alloys along Δ-, Λ- and Σ-directions at room temperature. A comparative study based on these models was performed in terms of energy levels, optical transitions, spin–orbit splitting and effective mass. We found a significant reduction of the band-gap energy E_g by roughly 81 meV/%Bi accompanied by an increase in the spin–orbit splitting Δ_so+ by about 56 meV/%Bi. Furthermore, Δ_so+ does come into resonance with E_g at ∼12%Bi for resonance energy equal to 0.73 eV. An excellent agreement has occurred between the (14 × 14) V-BAC model predictions and experimental results reported in the literature. In addition, we have investigated the Bi composition and k-directions dependence of the effective mass at Γ point. A slight increase of the holes effective mass with x can affect the holes transport properties of GaAsBi. The intrinsic carrier density increases with both x and the temperature T, but it remains below 10¹⁰ cm⁻³ for x ⩽ 5% and T ⩽ 300 K.     

Electrical behavior of amorphous indium–gallium–zinc oxide thin film transistors by embedding Au nanoparticles in the channel layer

We reported the effects on the electrical behavior of amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistors (TFTs) after introducing various positions and sizes of Au nanoparticles (NPs) in the channel layer. These TFTs showed an off-current increase and threshold voltage (V_th) shift compared to conventional a-IGZO TFTs. The effects of Au NPs are explained to form the carrier conduction path which causes the current leakage in the channel layer, and act as either electron injection sites or trap sites. Therefore, this study demonstrates that the optimized control of size and position of Au NPs in the channel layer is crucial for its application in the electrical stability improvement and V_th control of a-IGZO TFTs.     

Characterization of quenched-in vacancies in Fe–Al alloys

Physical and mechanical properties of Fe–Al alloys are strongly influenced by atomic ordering and point defects. In the present work positron lifetime (LT) measurements combined with slow positron implantation spectroscopy (SPIS) were employed for an investigation of quenched-in vacancies in Fe–Al alloys with the Al content ranging from 18 to 49 at.%. The interpretation of positron annihilation data was performed using ab-initio   theoretical calculations of positron parameters. Quenched-in defects were identified as Fe-vacancies. It was found that the lifetime of positrons trapped at quenched-in defects increases with increasing Al content due to an increasing number of Al atoms surrounding the Fe vacancies. The concentration of quenched-in vacancies strongly increases with increasing Al content from ≈10⁻⁵$\approx {10}^{-5}$ in Fe₈₂Al₁₈${\mathrm{Fe}}_{82}{\mathrm{Al}}_{18}$ (i.e. the alloy with the lowest Al content studied) up to ≈10⁻¹$\approx {10}^{-1}$ in Fe₅₁Al₄₉${\mathrm{Fe}}_{51}{\mathrm{Al}}_{49}$ (i.e. the alloy with the highest Al content studied in this work).     

High-resolution transmission electron microscopy and tomographic atom probe studies of the hardening precipitation in an Al–Cu–Mg alloy

The hardening precipitation of an Al–Cu–Mg aluminium alloy designed for aeronautics was investigated using high-resolution transmission electron microscopy (HREM) and tomographic atom probe techniques. The observed precipitates clearly belong either to the Guinier–Preston–Bagaryatskii (GPB) zones type or to the so-called S-Al₂CuMg precipitation. We analysed a large number of precipitates in order to obtain statistical information on the precipitation. We focused on the structural and/or chemical composition of the different precipitates. It was found, in particular, that the very numerous GPB zones do not present a single chemical composition. Evidence is also given for the presence of two different kinds of S-precipitate/matrix orientation relationships, strongly linked to the morphology of the precipitate. The structure of the S precipitates was confirmed by direct comparison with simulated HREM images. Particular attention was paid to the nature of the S-precipitate/matrix interfaces.     

Precipitation and fracture behaviour of Fe–Mn–Ni–Al alloys

The effects of Al addition on the precipitation and fracture behaviour of Fe–Mn–Ni alloys were investigated. With the increasing of Al concentration, the matrix and grain boundary precipitates changed from L1₀ θ-MnNi to B2 Ni₂MnAl phase, which is coherent and in cube-to-cube orientation relationship with the α′-matrix. Due to the suppression of the θ-MnNi precipitates at prior austenite grain boundaries (PAGBs), the fracture mode changed from intergranular to transgranular cleavage fracture. Further addition of Al resulted in the discontinuous growth of Ni₂MnAl precipitates in the alloy containing 4.2?wt.% Al and fracture occurred by void growth and coalescence, i.e. by ductile dimple rupture. The transition of the fracture behaviour of the Fe–Mn–Ni–Al alloys is discussed in relation to the conversion of the precipitates and their discontinuous precipitation behaviour at PAGBs.     

Substrate-induced stress and transformation characteristics of a deposited Ti–Ni–Cu thin film

This article presents the investigation results on the transformation characteristics of a sputter-deposited Ti–Ni–Cu shape memory alloy thin film and its relation to the substrate-induced stress. Experimental results show that, with the substrate attachment, the transformation interval increases while the transformation hysteresis decreases in comparison with those of the same thin film in the free-standing condition. By assuming a stress distribution through the film thickness, a layer-by-layer transformation sequence in the substrate-attached film is proposed and the transformation interval and hysteresis are analysed. The analysis results show qualitative agreement with the experimental observations, suggesting that the approach taken is plausible. This approach may also be used to examine the transformation characteristics of other thin films having thermally induced phase transformations.     

Thermodynamics and surface properties of liquid Al–Ga and Al–Ge alloys

The surface properties of Al–Ga and Al–Ge liquid alloys have been theoretically investigated at a temperature of 1100 K and 1220 K respectively. For the Al–Ga system, the quasi chemical model for regular alloy and a model for phase segregating alloy systems were applied, while for the Al–Ge system the quasi chemical model for regular and compound forming binary alloys were applied. In the case of Al–Ga, the models for the regular alloys and that for the phase segregating alloys produced the same value of order energy and same values of thermodynamic and surface properties, while for the Al–Ge system, the model for the regular alloy reproduced better the thermodynamic properties of the alloy. The model for the compound forming systems showed a qualitative trend with the measured values of the thermodynamic properties of the Al–Ge alloy and suggests the presence of a weak complex of the form Al₂Ge₃. The surface concentrations for the alloys show that Ga manifests some level of surface segregation in Al–Ga liquid alloy while the surface concentration of Ge in Al–Ge liquid alloy showed a near Roultian behavior below 0.8 atomic fraction of Ge.     

Theory of band gap bowing of disordered substitutional II–VI and III–V semiconductor alloys

For a wide class of technologically relevant compound III?CV and II?CVI semiconductor materials AC and BC mixed crystals (alloys) of the type A _x B_1?x C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A _x B_1?x C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight-binding models for the pure AC and BC systems. The electronic properties of the A _x B_1?x C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1 ? x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA) We apply this treatment to the II?CVI system Cd _x Zn_1?x Se, to the III?CV system In _x Ga_1?x As and to the III-nitride system Ga _x Al_1?x N.     

Simulation of displacement cascades in Ni–Al ordered alloys

Abstract

Molecular dynamics was used to investigate defect production induced by displacement cascades in ordered intermetallic alloys NiAl and Ni₃Al. The composite potentials obtained from the embedded atom potentials (EAM) and the universal function of Biersack and Ziegler were used. The number of point defects and their final structure produced by displacement cascades were investigated and compared with the standard NRT prediction. Crystalline structure, atomic mixing and chemical disordering were also studied during the evolution of the cascades, by measuring their characteristic parameters in the cells of the subdivided crystal.     

Ultraviolet photovoltaic characteristic of MgB2 thin film

Fast photoelectric effects have been observed in MgB₂ thin film fabricated by chemical vapour deposition. The rise time was $\sim $10 ns and the full width at half-maximum was \sim185\,ns for the photovoltaic pulse when the film was irradiated by a 308\,nm laser pulse of 25\,ns in duration. X-ray diffraction and the scanning electron microscope revealed that the film was polycrystalline with preferred c-axis orientation. We propose that nonequilibrium electron--hole pairs are excited in the grains and grain boundary regions for MgB₂ film under ultraviolet laser and then the built-in electric field near the grain boundaries separates carriers, which lead to the appearance of an instant photovoltage.     

Readout of super-resolution marks with Ti thin film

Using Ti as the super-resolution reflective film to replace the Al reflective layer in conventional read-only optical disk, the recording marks with a diameter of 380 nm and a depth of 50 nm are read out in a dynamic testing device whose laser wavelength is 632.8 nm and numerical aperture of the lens is 0.40. The optimum Ti thin film thickness is 18 nm and the corresponding signal-noise-ratio is 32 dB.     

Interface mixing behaviour of Lennard–Jones FCC (100) thin film

A modified Monte Carlo method combined with quenched molecular dynamics simulation is used to determine mixing energetics and concentration profiles at interface for systems containing mono-and bilayers of adatoms adsorbed on FCC (100) crystal surface. The systems under consideration are constructed via Lennard–Jones potential at temperatures near 0 K. For systems with monolayer of adatoms, intermixing at the interface becomes preferable with increasing magnitude of the potential well-depth ratio of adatom to substrate atom. The increasing tendency of intermixing is linearly enhanced when the adatom becomes smaller than the substrate atom, otherwise, the intermixing trend is non-linear and weaker. For systems with bilayers of adatoms, complex development of concentration profile is observed along with increasing magnitude of the potential well-depth ratio and atomic size difference between adatom and substrate atom. This behaviour is related to the interplay between contributions of asymmetric bond interaction and relaxation to minimise the total energy of the system.     

High-resolution inverse Raman spectroscopy of the ν1 band of water vapor

The Raman spectrum of the ν₁ band of water vapor at 7 Torr and 296 K, and at 84 Torr and 404 K, has been measured using high-resolution inverse Raman spectroscopy. The frequencies and relative intensities of the observed transitions are compared to a spectral model based on infrared data. Self-broadening coefficients have been determined for 22 lines from J = 0 to J = 8.     

Compositional dependence of the deformation behaviour of ultrahigh-purity Ti–Al alloys

The present study is concerned with the effect of the O and Al concentrations on the deformation behaviour of ultrahigh-purity (UHP) Ti–(48,?50,?52)?at.%?Al alloys using UHP Ti with 30?wt?ppm?O. It has been shown that yield strength increases with increasing O content. Stoichiometric Ti–50?at.%?Al alloys had the lowest yield strength and the highest ductility when the O content was sufficiently low. It is suggested that the deformation mechanism of UHP binary Ti–Al is strongly related to the Al concentration. The deformation substructure of UHP Ti–48?at.%?Al is shown to be dominated by ordinary dislocation as well as deformation twinning and a small portion of superdislocations. The deformation substructure of UHP Ti–50?at.%?Al alloy was similar to that of Ti–48?at.%?Al, but deformation twinning was not observed. Most of dislocation structures of UHP Ti–52?at.%?Al alloy consisted of faulted dipoles. The major deformation mode of UHP Ti–48?at.%?Al and UHP Ti–50?at.%?Al alloys was ordinary dislocation in deformation orientation, which takes advantage of ordinary dislocation slip. However, the major deformation mode in this orientation for UHP Ti–52?at.%?Al alloys was superdislocation slip.