Liquid–liquid structure transition in Sn–3.5Ag–3.5Bi melts

The liquid–liquid structure transition (LLST) as the function of temperature and time in Sn–3.5Ag–3.5Bi melts was investigated with the help of direct current four-probe method. The LLST which occurs during first cycle heating of two cycles heating/cooling experiments can divide into two different structure changes: irreversible LLST of 650°C isothermal and step reversible LLST at 770°C–806°C on subsequent heating process. Obvious kinetic phenomena are observed during isothermal experiments. Irreversible and reversible LLST are analysed from the viewpoint of short-range order. These results will help to understand the law and mechanism of liquid field, and provide some scientific reference for the innovation of lead-free solder manufacturing.     

Electrical resistivity of liquid Bi–Sb alloys

The electrical resistivities of liquid Bi–Sb alloys have been measured by DC four-probe method within Bi-rich composition through a wide temperature range. The distinct anomaly of a hump shape was observed on resistivity–temperature (ρndash;T) curves for liquid Bi–Sb alloys on heating at the relatively high temperatures. These anomalies have revealed the temperature-induced liquid–liquid phase transition in Bi–Sb melts. The DSC results for BiSb20wt% alloy further prove the existence of liquid–liquid transition. Measuring the ρ–T curves first on heating and then on cooling we have found that on cooling the ρ–T curve remained linear. It means that the postulated liquid–liquid transition may be irreversible.     

Surface tension and density of liquid Bi–Pb,Bi–Sn and Bi–Pb–Sn eutectic alloys

Surface tension and density measurement of liquid Bi₅₆Pb₄₄, Bi₄₃Sn₅₇ and Bi₄₆Pb₂₉Sn₂₅ eutectic alloys was carried out by using the large drop method over the temperature range of 380–750 K. The regular solution model has been used in conjunction with Butler's equation to calculate the surface tension of binary and ternary alloys of the Bi–Pb–Sn system, while the surface tension of ternary alloys has also been predicted by using geometric models. The new experimental results were compared with the calculated values of the surface tension as well as with the data available in the literature.     

Electrical resistivity of liquid Sb–Zn alloys

Using the DC four-probe method, temperature dependence of the electrical resistivity (ρ???T) of Sb_{100? x} Zn _x (x?=?25,?40,?50,?57,?61,?80 at%) alloys was investigated in the temperature range of 500–860°C. The results showed that resistivity of each liquid alloy decreased non-linearly with temperature increasing above their liquidus (T _L) until reaching critical temperature, at which the resistivity–temperature coefficients dρ/dT–T converts from negative into positive. The phenomena of liquid phase transformation might relate with the dissociation of covalent bonds, chemical orders and associations in Sb–Zn melts.     

Experimental evidence of the ferroelectric nature of the λ-point transition in liquid water

We studied the dielectric properties of nano-sized liquid water samples confined in polymerized silicates MCM-41 characterized by pore sizes 3–10 nm. Freezing temperature suppression in nanopores helps keep the water samples in liquid form at temperatures well below 0°C and thus effectively study the properties of supercooled liquid water. We report the first direct measurements of the dielectric constant by the dielectric spectroscopy method and demonstrate very clear signatures of the second-order phase transition of ferroelectric nature at temperatures next to the λ-point in the supercooled bulk water in full agreement with the recently developed model of the polar liquid.     

Spatial heterogeneity in liquid–liquid phase transition

Molecular dynamics simulations are performed to investigate the liquid–liquid phase transition(LLPT) and the spatial heterogeneity in Al–Pb monotectic alloys. The results reveal that homogeneous liquid Al–Pb alloy undergoes an LLPT,separating into Al-rich and Pb-rich domains, which is quite different from the isocompositional liquid water with a transition between low-density liquid(LDL) and high-density liquid(HDL). With spatial heterogeneity becoming large, LLPT takes place correspondingly. The relationship between the cooling rate, relaxation temperature and percentage of Al and the spatial heterogeneity is also reported. This study may throw light on the relationship between the structure heterogeneity and LLPT, which provides novel strategies to control the microstructures in the fabrication of the material with high performance.     

Abnormal resistivity behavior of Cu–Ni and Cu–Co alloys in undercooled liquid state

The resistivity behavior of undercooled liquid Cu–Ni and Cu–Co alloys had been studied in the contactless method, to probe the structure transition in undercooled melts during the cooling process. Over the entire concentration range, linear behavior of resistivity with temperature was obtained in liquid and undercooled liquid Cu–Ni system. It implied that the formation of icosahedral order might not influence the electron scattering in undercooled liquid Cu–Ni alloys. Similar results were obtained in Cu–Co system in the vicinity of liquidus temperature. A turning point was obvious in temperature coefficient of resistivity for undercooled liquid Cu–Co alloys around the bimodal line, which was interpreted to be responsible for metastable liquid–liquid phase separation. During liquid phase separation process, resistivity decreased and the temperature coefficient of resistivity was larger than that of homogeneous melts. In combination with transmission electron microscopy and scanning electron microscope studies on the as-solidified microstructure, this was interpreted as the formation of egg-type structure and concentration change in Cu-rich and Co-rich phases. The mechanism controlling the separation and droplets motion was also discussed in undercooled liquid Cu–Co system.     

Experimental and computer simulation determination of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (K _p), heights (K _h) and full-widths at half maximum (K _FWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ～0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr_{1? x} Cu _x (0.1 ≤ x ≤ 0.9), K _p, K _h and K _FWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu–Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu–Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, x _Cu ～ 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid.     

Surface tension modelling of liquid Cd–Sn–Zn alloys

The thermodynamic model in conjunction with Butler equation and the geometric models were used for the surface tension calculation of Cd–Sn–Zn liquid alloys. Good agreement was found between the experimental data for limiting binaries and model calculations performed with Butler model. In the case of ternary alloys, the surface tension variation with Cd content is better reproduced in the case of alloys lying on vertical sections defined by high Sn to Zn molar fraction ratio. The calculated surface tension is in relatively good agreement with the available experimental data. In addition, the surface segregation of liquid ternary Cd–Sn–Zn and constituent binaries has also been calculated.     

Disorder in liquid Cu–Pd alloys

The disorder in thermodynamic and microscopic structure of liquid Cu–Pd alloy at 1350?K has been studied using regular associated solution model. For this, we have calculated free energy of mixing (G_M ), activity (a), concentration fluctuation in long wavelength limit [S_CC (0)] and chemical short-range order parameter (α ₁) of liquid Cu–Pd alloy at 1350?K. The energetic and structural asymmetry of liquid Cu–Pd alloys has been successfully explained on the basis of regular associated solution model.     

Structural asymmetry in liquid Fe–Si alloys

The thermodynamic properties and microscopic structure of liquid Fe–Si alloys at 1873 K were studied by using the regular associated solution model. The model was utilized to determine the complex concentration in a regular associated solution of Fe, Si and Fe₂Si. The complex concentration was then used to calculate the integral excess free energy of mixing, activity, concentration fluctuations in the long-wavelength limit, S_CC (0), and the Warren–Cowley short-range parameter α ₁. The analysis suggests that heterocoordination leading to the formation of complex Fe₂Si is likely to exist in the liquid and is of a strongly interacting nature. The theoretical analysis reveals that the alloy is more ordered towards the Fe-rich region. The observed asymmetry in the properties of mixing of Fe–Si alloys in the molten state is successfully explained on the basis of the regular associated solution model.     

Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

Thermophysical properties of Cu–In–Sn liquid Pb-free alloys: viscosity and surface tension

The viscosity of a few Cu–In–Sn liquid alloys has been investigated by a number of geometric (Muggianu, Kohler, Toop) and physical thermodynamic models (Kozlov–Romanov–Petrov, Budai–Benko–Kaptay, Schick et al.) and GSM for the cross section (z/y = 1/3) in Pb-free liquid alloy Cu_x–In_y–Sn_z at 1073 K. Moreover, the surface tensions of the same liquid alloys have been investigated by a number of geometric models and the Butler model for the cross section Cu_x–In_y–Sn_z (z/(y + z) = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) at the same temperature. The best agreement of the surface tensions was obtained in the Kohler model for x_Cu = 10 at % and the Butler model for x_Cu = 20 at % and x_Cu = 30 at.%, respectively. The best agreement among chosen geometric and physical models and experiment for these selected sections Cu₈₀In₁₅Sn₅, Cu₇₅In₁₅Sn₁₀, Cu₅₅In₇Sn₃₈, Cu₃₃In₅₀Sn₁₇ and Cu₂₆In₅₅Sn₁₉ at 1073 K was obtained for the Budai–Benkö–Kaptay model.     

Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti–(50???x)Ni–xFe (x?=?2,?4,?6,?8 in at.%) alloys. We found that a Ti–44Ni–6Fe alloy exhibits a second-order-like incommensurate–commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3? 110? as the temperature decreases and lock into the commensurate position at 180?K. The commensurate phase is not expanded along one of the ? 111? directions, unlike the R-phase formed by a first-order transformation in Ti–48Ni–2Fe and Ti–46Ni–4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti–44Ni–6Fe alloy. In the case of a Ti–42Ni–8Fe alloy, the incommensurate state remains even at 19?K.     

Experimental evidence of the transformation from microheterogeneous to microhomogeneous states in Ga–Sn melts

The electrical resistivities of liquid Ga–Sn system have been carefully measured as a function of temperature for different compositions employing direct-current four-probe method. It was well known that the electrical resistivity varies linearly with temperature for typical liquid metals. However, an abnormal change on the resistivity-temperature curve in the intermediate temperature region Ts–Te$T_s\text{–}{T}_e$ (385–422 °C for Ga₂₀Sn₈₀ melt and 395–449 °C for liquid Ga₁₀Sn₉₀ melt, here Ts$T_s$ and Te$T_e$ are denoted as the start-temperature and the end-temperature of the abnormal change) is observed in the initial Ga₂₀Sn₈₀ and Ga₁₀Sn₉₀ melts during first heating, but this abnormal behavior disappears during subsequent cooling as well as reheating process. This result indicates that in the initial Ga₂₀Sn₈₀ and Ga₁₀Sn₉₀ melts there may exist the microheterogeneities, during heating in the temperature range from Ts$T_s$ to Te$T_e$ the irreversible structure transition possibly takes place from microheterogeneous melt to microhomogeneous melt. It suggests that the necessary overheating above liquidus up to Te$T_e$ is a guarantee of the molten alloy getting the microhomogeneous sate or the true solution and conserving it during cooling down to liquidus at any cooling rates.     

Interdiffusion and thermotransport in Ni–Al liquid alloys

In this paper, we present extensive self-consistent results of molecular dynamics (MD) simulations of diffusion and thermotransport properties of Ni–Al liquid alloys. We develop a new formalism that allows easy connection between results of the MD simulations and the real experiments. In addition, this formalism can be extended to the case of ternary and higher component liquid alloys. We focus on the temperature and composition dependence of the self-diffusion coefficients, interdiffusion coefficients, thermodynamic factor, Manning factor and the reduced heat of transport. The two latter quantities both represent measures of the off-diagonal Onsager phenomenological coefficients. The Manning factor and the reduced heat of transport can be related to experimentally obtainable quantities provided the thermodynamic factor is available. The simulation results for the reduced heat of transport show that for all compositions, in the presence of a temperature gradient, Ni tends to migrate to the cold end. This is in agreement with an available experimental study for a Ni_21.5Al_78.5 melt (only qualitative result is available so far).     

Study of electrical conductivity and phase transition in Bi2O3–V2O5 system

The solid solutions of bismuth–vanadate were prepared by the conventional solid-state reaction. The sample characterization and the study of phase transition were done by using FT-IR, X-ray diffraction (XRD) and DSC measurements. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contributions as two well-defined semicircles are clearly seen along with an inclined spike. The electrical conductivity of Bi₂O₃–V₂O₅ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with an increase in the concentration of V₂O₅ due to the vacancy migration phenomenon. It has been found that the conductivity of different compositions of Bi₂O₃–V₂O₅ increases and shows a jump in the temperature range 230–260°C due to the phase transition of BiVO₄ from monoclinic scheelite type to that of tetragonal scheelite type. The endothermic peak in DSC at around 260°C reveals the phase transition, which is also confirmed by the XRD and FT-IR analysis. The XRD patterns confirmed the monoclinic structure of BiVO₄.     

High-temperature superconductivity in transition metal oxypnictides: A rare-earth puzzle?

Extensive ab initio LDA and LSDA + U calculations of an electronic structure of newly discovered high-temperature superconducting series ReO_{1 − x} F _x FeAs (Re = La, Ce, Pr, Nd, and Sm and the hypothetical case of Re = Y) have been performed. In all cases, almost identical electronic spectrum (both energy dispersions and the densities of states) has been obtained in a rather wide energy interval (about 2 eV) around the Fermi level. This fact is unlikely to be changed by strong correlations. This leads inevitably to the same critical temperature T _c of a superconducting transition in any theoretical BCS-like mechanism of the Cooper pair formation. The experimentally observed variations of the T _c for different rare-earth substitutions are either due to the disorder effects or less probably because of possible changes in the spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers. The text was submitted by the authors in English.     

The reassessment of the structural transition regions along the liquidus of Fe–Si alloys and a possible liquid–liquid structural transition in FeSi2 alloy

We reassessed the structural transition regions along the liquidus of Fe–Si alloys by using ab initio molecular dynamics simulation. Except for 50 at.% Si, structural transition compositions are found at both 30 at.% Si and 67 at.% Si (FeSi₂) which are eutectic alloys. We demonstrated that the liquid structure in the sub-region of 0～30 at.% Si is close-packed, and in the sub-region of 67～100 at.% Si liquid alloys have very open structure. From 30 at.% Si to 67 at.% Si, the close-packed structure gradually change into open one. These structure transition sub-regions are also supported by the formation enthalpy of liquid alloys. Furthermore, the predicted enthalpy change between 1585 K and 1873 K is so large that there is probably liquid–liquid transition with temperature at FeSi₂ alloy which is an important thermoelectric material. Discussions have been made on the materials phenomenon of several Fe–Si alloys based on the structural information.