Phase transitions in wetting films at the surface of Ga–Pb alloys

Surface phase transitions at Ga-rich liquid surfaces have been investigated in Ga–Pb alloys with low lead content. In the region of the liquid–liquid miscibility gap, the Pb-rich liquid phase completely wets the surface of the Ga-rich phase at coexistence. Observations have been made of demixing and solidification of the Pb-rich liquid film. Ga-rich alloys, which are single-phase below the monotectic temperature, can be undercooled below the liquidus, as far as the metastable binodal line where the Pb-rich wetting liquid film forms and solidifies into thin {111} Pb crystals. These films completely redissolve upon reheating to the liquidus temperature. Freezing occurs at surfaces because of complete wetting of the liquid rich in the high melting point component and the hysteretic character of the solidification transformation. Such “surface” experiments allow assessment of the stable and metastable liquidus lines of the Ga–Pb phase diagram in the vicinity of the monotectic temperature.     

Melting of thin γFe–C films having (100), (110) and (111) surfaces in terms of molecular dynamics simulation

Structural changes associated with melting of thin γFe–C films having (100), (110) and (111) surfaces have been investigated by molecular dynamics simulation. Structures of thin film and bulk models of γFe containing about 0–4 at.% C were calculated at constant temperatures between 1000 and 1800 K. The liquidus temperature for each thin film model decreased with increasing the C concentration. Comparison between the atomic number density distributions of Fe and C showed: (i) The atomic number density of C near the surface increases before the formation of liquid near the surface. (ii) This increase becomes more prominent as temperature rises. (iii) Melting of γFe–C alloy would be rate-controlled by diffusion of C from the solid phase to the solid–liquid interface. These findings suggest that the increase in the C concentration enhances atomic vibrations of Fe near the surface and promotes melting of Fe at lower temperatures. Furthermore, it has been concluded from Lindemann's law of melting that surface melting occurs in γFe–C alloy having (110) surface more easily.     

Wetting and premelting of triple junctions and grain boundaries in the Al–Zn alloys

Phase transitions in grain boundaries (GBs) and GB triple junctions (TJs) can change drastically the properties of polycrystals. The GB and TJ wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid and solid phases are in equilibrium. The GB and TJ wetting tie-lines can continue in one-phase area of the bulk phase diagram as a GB or TJ solidus line. This line represents the GB or TJ premelting phase transition. The structure and composition of grain boundaries and GB triple junctions were studied by high-resolution electron microscopy and analytical transmission electron microscopy in the Al–5 at.% Zn polycrystals and by differential scanning calorimetry (DSC) in the Al–7.5 at.% Zn polycrystals. Between bulk solidus and GB or TJ solidus the metastable Zn-rich β_m-phase was observed in the GB triple junctions of quenched samples. This phase appears neither in the samples annealed above the bulk solidus nor in those annealed below the GB solidus. Zn-content in this β_m-phase corresponds to that of bulk liquidus. This is a structural indication that if the melt wets the GBs or TJs, the GB (or TJ) solidus line appears in the bulk phase diagram, and the liquid-like phase exists in GBs and TJs between bulk solidus and GB (or TJ) solidus lines. The structural observation of this phase is also supported by our data obtained by means of DSC.     

Theoretical temperature-dependence surface tension of pure liquid gold

A calculation, based on statistical thermodynamics, of the surface tension of pure liquid gold, in the temperature range 1337-1653 K, is described. The calculated surface tension of liquid gold was found to be 1100 mJ/m² that agrees well with the reported experimental value (1149 mJ/m²). Results reveal that below 1480 K, the surface excess entropy changes by one order of magnitude (0.0151 mJ/m² K) as compared to the value above this temperature (0.1773 mJ/m² K), and thus predicting a structural changes at the surface of pure molten gold at high temperature.     

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated. Paper presented at the poster session of MRSI AGM VI. Kharagpur. 1995     

Two-dimensional Mn–Co surface alloy on Co(0 0 1)

Surface alloy formation of Mn on Co(0 0 1) was detected by low-energy electron diffraction and in situ magneto-optic Kerr effect experiments. Here we discuss the electronic and magnetic properties of an equivalent Mn monolayer (ML) at the surface of Co(0 0 1) using the tight-binding linear muffin-tin orbitals method. Magnetic configurations and stability of ordered 2D Mn–Co alloy 2 ML thick vs Mn/Co(0 0 1) and Co/Mn/Co(0 0 1) have been considered. Multiple magnetic configurations have been always obtained but a comparison between the ground states display an instability of the perfect Mn ML on Co(0 0 1). Total energy differences are considerably reduced in the case of spin-polarization calculations as compared with the non-magnetic one.     

Phase transition and mechanical properties of constraint-aged Ni–Mn–Ga–Ti magnetic shape memory alloy

Ni–Mn–Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni–Mn–Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni–Mn–Ga–Ti alloy and about 900 MPa in Ni–Mn–Ga–Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni–Mn–Ga–Ti alloy.     

Energy and lifetime of surface plasmon from first-principles calculations

The dependence of energy and line width of Al(1 1 1) and Mg(0 0 0 1) surface plasmons on two-dimensional momenta is evaluated within an ab initio pseudopotential approach. Quantitative agreement with experimental dispersion and damping of the surface plasmon on a metal surface is obtained without any adjustable parameters taking explicitly into account both the bulk and surface electronic structures on the same basis. The study of the impact of the band structure and dynamical exchange-correlations on the surface plasmon energy and line width dispersion shows that the main effect comes from the three dimensional band structure, although to achieve quantitative agreement with experiment the inclusion of exchange-correlation effects is needed.     

The surface tension of copper-nickel alloys

The surface tension of copper-nickel alloys was measured, both as functions of concentration and temperature, using electromagnetic levitation and the oscillating drop technique. The measurements covered the entire concentration range from c _Cu = 0 to 100%, and a temperature range that included the molten as well as the undercooled regime. The results agree well with a model based on Butler's formula for the surface tension and chemical potentials measured in the bulk liquid phase. This allows one to calculate the surface segregation of this system.     

Thermo-induced plasmon excitations in the near surface region of ternary Co-Cr-Mo alloy

Electron Energy Loss Spectroscopy has been employed for investigation of the surface and bulk plasmon excitations versus heating in the Co-Cr-Mo alloy surface for the primary electron beam energies Е₀ ranging from 150 to 800 eV. For the annealed alloy the experimental values of the plasmon energy are localized at more energies as compared to the non-annealed alloy. It was established that heating of the alloy promotes to insignificant deviations of the plasmon excitations. Damping of the intensity line of surface plasmon with a increase of heating was established. Physical processes which can influence on the energy displacements of long wavelength plasmon oscillations and damping of surface plasmon in the characteristic loss spectra at heating are considered.     

Complex magnetic behavior at the surface of B2 ordered FeCr alloy

Bouzar et al. (1997) have recently investigated the surface of B2 FeCr alloy. In all cases of crystal growth 0 0 1, 1 1 0 and 1 1 1 they found the local polarization at the surface to be antiferromagnetically coupled with the subsurface layer in contrast to parallel coupling between Fe and Cr in bulk FeCr. In order to assert these results, we have investigated the local polarization of Fe at the (0 0 1) surface of this alloy with a tight-binding linear muffin tin Orbitals (TB-LMTO) model. Using general gradient approximation with Langreth-Mehl-Hu functional for p(1 × 1) and c(2 × 2) configurations we found the local polarization at the Fe surface layer to be antiferromagnetically coupled with the subsurface Cr layer and high magnetic moments compared to the bulk values.     

The dependence of the phase transition temperature of Ga–Zn eutectic alloy on its morphology

The effect of the crystallization conditions and the properties of the constituent components on the type of structures formed and the phase-transition temperature of Ga–Zn eutectic alloy is investigated. The structure of the Ga–Zn alloy is compared with the corresponding structures of the Ga–In and Ga–Sn alloys. Translated from Izmeritel’naya Tekhnika, No. 1, pp. 34–37, January, 2009.     

Sintering of Bi–Tl, Bi–In, Bi–Pb, and Hg–In Contacts

Experimental evidence is presented that the sintering of Bi–Tl, Bi–In, Bi–Pb, and Hg–In contacts is associated with two concurrent processes opposite in thermal effect: the formation of a nonequilibrium liquid phase and nucleation of intermetallic phases.     

Solidification of phase change material on vertical cylindrical surface in holdup air bubbles

Solidification of phase change material around a vertical cylindrical surface was studied to investigate the performance of ice storage system and stored thermal energy. Air bubbles were generated in the phase change material at various air flow rate as a gas holdup to enhance the heat transfer rate and accelerate the ice layer growth at the solid–liquid interface. The test tube surface was cooled by ethylene glycol–water solution at a flow rate of 40% concentration by weight. The ice layer growth and solidification front velocity at solid–liquid interface were estimated from the temperature–time recorded data of a set of thermocouples fixed in a radial position perpendicular to cooled surface. The ice layer growth at the first instants of solidification process is much higher. Thereafter it decreased gradually according to the increasing of thermal resistance of ice layer. The increasing of ethylene glycol–water solution mass flow rate seems to accelerate the solidification process with small rate. The effect of air bubbles agitation was found to increase the ice layer growth rate and solidification front velocity by about of 20–45%. As a consequence the stored thermal energy was increased by about 55–115% with increasing air bubbles flow according to the attribute of generates turbulence at the solid–liquid interface. The measured data showed that with stirring the bulk water in energy storage tank, the storage time can be reduced by 10–35% of that without stirring.     

Dynamics and microstructural evolution of bulk amorphous Cu12.5Ni10Zr41Ti14Be22.5 during phase separations: a small-angle neutron scattering approach

Small-angle neutron scattering has been applied to investigate in detail the structural relaxation of bulk amorphous Cu_12.5Ni₁₀Zr₄₁Ti₁₄Be_22.5 alloy in the supercooled liquid range (620–673 K) and its effect on subsequent crystallization around the crystallization point (673 K). The interference events from typical phase separation were recorded as the alloy was annealed in the supercooled liquid range. It was revealed that the crystallization in the alloy which was previously relaxed in the supercooled liquid range was significantly prohibited, and further phase separation was observed. Considerable temperature dependence of these phase separations was observed. It has been demonstrated that the phase separation developed via the spinodal mode and the achieved microstructure consisted of one droplet-like supercooled liquid phase embedded in the similarly disordered matrix. The droplets showed a bar-like pattern and distributed in a relatively regular form in the matrix. The phase separations exhibited sluggish coarsening kinetics with much smaller exponent than the Lifshitz–Slyozov–Wagner value. However, the dynamic scaling property of the phase separations at different temperatures has been approved by our scaling analysis. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructure Development and Mechanical Properties of Ni Matrix/Carbide Composites

Nickel alloy matrix/dispersed carbide (VC, NbC, WC) composites were prepared by either flame-spray, liquid phase sintering, or solid state sintering. A commercial Ni-B-Si alloy was mixed with 15% vol. of carbide particles and used to prepare composite coatings by flame-spray, bulk composite materials by solid state sintering (below 1045^°C) or liquid phase sintering (above 1050^°C). Phase characterization was performed by X-ray diffraction, optical microscopy, scanning electron microscopy and X-ray fluorescence in energy and wavelength dispersive spectroscopy modes. Similar microstructural features were produced by thermal spray and liquid phase sintering: a Ni-rich matrix and a boron-rich intergranular phase. Sintered samples showed higher wear resistance than the coatings obtained by flame spraying. In both cases the wear mechanism is dominated by the plastic flow of the Ni-rich ductile matrix and the fracture of a boron-rich intergranular phase, the latter serving as a retainer. Carbide removal was observed for solid state sintered samples.     

Melting of thin γFe–C films having (100), (110) and (111) surfaces in terms of molecular dynamics simulation

Structural changes associated with melting of thin γFe–C films having (100), (110) and (111) surfaces have been investigated by molecular dynamics simulation. Structures of thin film and bulk models of γFe containing about 0–4 at.% C were calculated at constant temperatures between 1000 and 1800 K. The liquidus temperature for each thin film model decreased with increasing the C concentration. Comparison between the atomic number density distributions of Fe and C showed: (i) The atomic number density of C near the surface increases before the formation of liquid near the surface. (ii) This increase becomes more prominent as temperature rises. (iii) Melting of γFe–C alloy would be rate-controlled by diffusion of C from the solid phase to the solid–liquid interface. These findings suggest that the increase in the C concentration enhances atomic vibrations of Fe near the surface and promotes melting of Fe at lower temperatures. Furthermore, it has been concluded from Lindemann’s law of melting that surface melting occurs in γFe–C alloy having (110) surface more easily.     

Microwave power dependence in Gd 123 and Tl 2212 thin films: Examining the evidence for limiting behavior

Using a 5-GHz parallel-plate resonator, several Gd 123 films and one Tl 2212 film have been studied as a function of microwave power. The variation of the surface resistance as a function of dc magnetic fields applied parallel to the crystallographicc axis reflects the gross microstructure of the films. Two broad categories of behavior are observed in dc field. The evidence for intrinsically limited power dependence is discussed in terms of impedance plane analysis.     

Wetting properties and interfacial microstructures of Sn–Zn–xGa solders on Cu substrate

The wetting properties and interfacial microstructures of Sn–9Zn–xGa lead-free solders with Cu substrate were investigated. The wetting property is improved remarkably with the increase of Ga content in the Sn–9Zn lead-free solder. The lower surface tension, which results from the decrease of the oxidation of the Zn atoms owing to the formation of the Ga-rich protective film covered on the liquid solder, is the key reason for the better wettability. During soldering, the Cu₅Zn₈ compounds layer form at the interface of Sn–9Zn/Cu and the IMCs formed at the solder/Cu surface become much thicker when the Ga content is from 0.1 wt.% to 3 wt.%. However, neither Cu–Sn compounds nor Ga-rich phases are observed at the solder/Cu surface.     

Residual strength of aluminum–lithium alloy center surface crack tension specimens at cryogenic temperatures

A three parameter fracture criterion which correlates the stress and the stress intensity factor at failure, is followed for the residual or fracture strength estimations of cracked configurations made of aluminum–lithium (Al–Li) alloys. The three fracture parameters are determined from the fracture data of Al–Li alloy center surface crack tension (SCT) specimens at cryogenic temperatures. It is found that the estimated fracture strength values compare well with the test results.