Rheological behaviour of lamellar eutectic Al?Al2?Cu elaborated by induction heating

This paper presents solutions to the experimental problems encountered in the elaboration of lamellar Al Al₂Cu or fibrous Al Al₃Ni eutectic samples using the Bridgman technique. The objective is to prepare samples for use in mechanical tests. The investigation of the elaborated structures of Al Al₂Cu (orientation, grain selection, shape of the solid-liquid (S.L.) interface, interlamellar spacing λ…) can then be used to confirm the predicted behavior. Creep tests under constant applied stress σ (from 2 to 5 · 10⁷ N/m²) at temperatures from 300 to 450 °C show:

• – that a stationary state exists (with constant strain rate )
• – the influence of an imperfect orientation
• – the existence of a Bailey-Norton creep law = kσⁿ. This law leads to the possiblity of finding a constitutive equation for high temperatures tests.

     

Laser produced amorphous phases in NbSi and VSi thin films

The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb₄Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu₃-type configuration of Nb₃Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.     

Si enhances the growth of B4C nanowires

The B₄C nanowires can be easily fabricated by employing an activated carbon, boron and Ni(NO₃)₂ whereas almost no B₄C nanowires can be obtained if activated carbon is replaced by pure carbon. The analyses show that there are Si and Al impurities in the activated carbon, and both Si and Ni are critical to the growth of the B₄C nanowires. Additionally, the yield of the B₄C nanowires is greatly increased if the B₄C nanowires are grown by the carbon nanotubes-confined method when Si is introduced. The mechanism of the Si enhanced growth of B₄C nanowires is discussed.     

Structural characterization of lanthanum aluminosilicate glasses by 29Si solid-state NMR

We investigate the network structures of LaSiAlO glasses by ²⁹Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR). Their compositions span most of the glass-forming region of the ternary La₂O₃Al₂O₃SiO₂ system at 1600 °C. The ²⁹Si NMR resonances narrow and become progressively deshielded when Al substitutes for Si in the network, as well as for increasing La-content of the glass, which leads to network depolymerization. We compare experimental and calculated center of gravities of the ²⁹Si NMR peaks, the latter generated from different simplified models for the distributions of Al and Si as well as bridging oxygen (BO) and non-bridging oxygen (NBO) atoms over the networks. The data do not permit accurate quantifications and may only be interpreted in limiting scenarios. However, they indicate that both distributions are essentially randomized, implying a clear deviation of the Al/Si ordering from that according to a Loewenstein Al-avoidance, coupled with a nearly uniform partitioning of the NBO atoms between Al and Si tetrahedra.     

Phase lead determined growth of 〈100〉 — textured germanium crystallites in eutectic Ag?Ge alloys

Al Si type eutectics (Al Si, Ag Si, Ag Ge, Zn Ge) form 〈100〉 — textured eutectic dendrites of the semiconductor phase when a certain growth rate R and liquid temperature gradient G are held. The spacings of these eutectic dendrites λ follow the relationship: λ = (r_λ ≈︁ 0.2; g ≈︁ 0.5). The exponents of this relationship can be explained by the model of phase lead determined growth. This model is based on the consideration of the analogy to dendrite growth. The Al Si type eutectics have geometrically similar morphologies for determined G^x/R-relations (x = 1.25 in the case of Ag Ge) and the relation λ²R = const. is valid. In this paper the Ag Ge eutectic will be discussed in close detail.     

Untersuchung der Korngrenzendiffusion von Zn-65 in α-Aluminium-Zink-Legierungen

The diffusion of Zn-65 in large angle grain boundaries in Al and in 8-Al Zn alloys was investigated. p_K = γδD_K rises exponentially up to a Zn content of 7 at-%, then at a lower rate with concentration. Above 40 at-% Zn p_K is almost independent of concentration. At the solidus temperature, D_K is of the order of 10⁻⁵ cm²/sec. A simple vacancy model for the diffusion in large angle grain boundaries is being discussed. From the model, the following parameters for the grain boundary self diffusion in Al and Cu that have not been investigated so far can be estimated: The activation energies for the electromigration in thin polycrystalline Al films and for the superplasticity at the eutectoid composition are compatible with the results of this investigation. The kinetics of the discontinuous precipitation and the eutectoid transformation in Al Zn alloys is governed by the phase boundary diffusion.     

Band alignment at the interfaces of Al2O3 and ZrO2-based insulators with metals and Si

Internal photoemission of electrons was used to determine the band alignment in metal (Mg, Al, Ni, Cu, Au)-oxide-silicon structures with Al₂O₃- and ZrO₂-based insulators. For Al₂O₃- and ZrO₂ layers grown on Si by atomic layer deposition the barrier height between the Si valence band and the oxide conduction band was found to be 3.25 and 3.1 eV, respectively. Thermal oxidation of the Si/oxide stacks results in a barrier height increase to ≈4 eV for both cases due to formation of a silicate interlayer. However, there is a significant sub-threshold electron emission both from silicon and metals, indicating a high density of states in the band gap of the insulators. These states largely determine the electron transport across metal oxides and may also account for a significant dipole component of the potential barrier at the metal/ZrO₂ and metal/Al₂O₃ interfaces.     

Theoretical Approach to the Deformation-induced Luminescence in Metals

The present paper reports the mechanoluminescence (ML) produced during fracture of metals. The mechanism of ML in metals is based on the generation of holes during the decay of mobile dislocations on the surface and subsequent electron-hole radiative recombination. The following equation is derived for the transient ML intensity of the fracto-induced ML in metals . The above equation shows that I should be maximum for a particular value of time t. For β 〉 α, . This equation shows that I should decrease exponentially with time. For the fracto-induced ML in metals, t_M and I_m may be expressed by the following equations . The spectroscopy of ML in metals is also discussed. A good correlation is found between the theoretical and experimentals results.     

A theoretical interpretation of the electrical behaviour of individual edge dislocations in Si as determined by combined EBIC/TEM studies

The EBIC data obtained from individual edge and Frank partial dislocations ⁱn Si, and described in other publications, are related to defect parameters, such as the capture cross-section for minority carriers, which determine electrical recombination properties of dislocations. A two stage model for the capture of minority carriers by dislocations is developed and shown to correctly account for the temperature-dependence of the EBIC contrast obtained from these dislocations.     

Structural investigation of an amorphous Si24Nb76 alloy produced by mechanical alloying using reverse Monte Carlo simulations

The atomic structure of an amorphous Si₂₄Nb₇₆ alloy produced by mechanical alloying was investigated by using one X-ray total structure factor S(K) as input data for reverse Monte Carlo (RMC) simulations. The partial S_Si–Si(K), S_Si–Nb(K), S_Nb–Nb(K) structure factors and G_Si–Si(r), G_Si–Nb(r), G_Nb–Nb(r) pair probability functions were obtained from the RMC simulations. The structural parameters (interatomic distances and co-ordination numbers) for the first neighbors were extracted and compared with those found in the Nb₃Si compound. It was observed some resemblance between these phases.     

RHEED Studies of MBE-grown Aluminium Layers on {111}-Oriented Silicon Substrates

Thin Al-layers were prepared by Knudsen-type evaporation of Al onto {111} Si in a custom built UHV-reactor. Epitaxial growth was found on clean substrate surfaces with (7 × 7) reconstruction. In situ RHEED provides the epitaxial relationship The epitaxial films show a twelve-fold symmetry axis according to the combination of the 4-fold and 3-fold symmetry of the {001} deposit plane and of the substrate surface, respectively.     

On the Formation of the Garnet Phase in Oxidic High-Temperature Solutions

The paper informs on the formation of the garnets Y₃Fe₅O₁₂, Y₃Al₅O₁₂ and Gd₃Ga₅O₁₂ as primary phase in the ternary solvent mixtures PbO PbF₂ B₂O₃ and PbO MoO₃ B₂O₃. There are correlations with formerly measured oxygen ion concentrations (OIC) in the solvents:

• i each garnet grows in a special region of the OIC, the phase boundaries are parallel to the lines of constant OIC and
• ii the garnet solubility increases distinctly with decreasing OIC. This emphasizes the important role of the oxygen ions for the garnet formation.

Solubility models are discussed taking into consideration both previous and present results. We conclude that the garnet formation is described best by a model reaction of the type RE rare earth elements, Y; M Al, Fe, Ga.     

The precipitation of barium chromate from aqueous solution (with rapid mixing): Kinetics of the crystal growth

The kinetics of precipitation of barium chromate from well-stirred aqueous solutions of initial solute concentrations C₀ = 0.0001 to 0.0010 M (supersaturations 8 to 80) was studied at 25 °C by conductivity measurements and chemical analysis. Nucleation occurred during induction periods and regular crystal growth then took place onto the crystallites formed during the induction periods. The crystal growth was rate-controlled, in this range, by the rate of deposition of metal salt ions onto the growing crystal surfaces. This rate, at any time, then depended on both the overall surface area (A_t) and on the residual excess solute concentration (ΔC_t) in solution according to the relation, the growth rate expressed in terms of degree of crystallisation was then The rate constant (K_α) for the crystallisation of barium chromate at 25 °C was 1.5 10⁶ sec⁻¹ M⁻².     

Solidification behavior and microstructural evolution of the Cr‐Nb eutectic alloy

A series of microstructures including fully coupled eutectic, both α‐Cr and β‐Cr₂Nb primary dendrites embedded in eutectic and only β‐Cr₂Nb primary dendrites plus eutectic were observed in the arc‐melted Cr‐Cr₂Nb eutectic alloy. By employing EPMA analysis performed at the eutectic regions, the eutectic composition of the Cr‐Cr₂Nb system was indicated to contain less than 18 at.%Nb. Based on the solidification phase selection involving phase competitive nucleation and growth, the α‐Cr phase was predicted to be the primary nucleating phase and the β‐Cr₂Nb the primary growing phase. Under large undercooling conditions, the solidification process was controlled by nucleation, which led to the formation of α‐Cr primary particles. With the decrease in undercooling, the solidification process developed into growth controlling, which caused the occurrence of β‐Cr₂Nb primary phase since the actual solidification path of the alloy lay within the hypereutectic region. The explanation was confirmed by the experimental composition analysis.     

Strukturuntersuchung Al-dotierter Ni/Cr-Schichten und Untersuchungen zur Legierungsbildung des Al-Zusatzes zur Dotierung mit der Legierung 80Ni-20Cr

The temperature coefficient of the electric resistance by sublimation deposition thin Ni/Cr-layers (>100 Ω/□ to nearly 2 k Ω/□) is reduced almost to zero by endow the layer with aluminium. The electron diffraction investigation of this layer shows three phases: the γ–γ′-phase [Ni₃(Cr, Al)], the α-phase (Cr) and NiCr₂O₄.— The Al-addition for endow the Ni/Cr layer give a different formation of alloy with the wire (2 mm ∅︁) of 80Ni/20Cr. Couse for the bad alloy formation is the existence of a continuous barrier layer of chromium. This barrier layer are exist in case that aluminium has a strong reaction to formation an intermetallic Ni/Al-phase.     

Effect of Buoyancy-driven Convection on Lamellar Spacing of an Al?Zn Eutectic

The influence of buoyancy-driven convection on the lamellar spacing of an Al–Zn eutectic was studied under conditions of oriented crystallization. The convection due to buoyancy was affected by the mold position respective to the direction of action of the force of gravity. It has been found that the buoyancy-driven convection does not exert any direct effect on the lamellar spacing. Rather, it only affects the rate of advance of the crystallization front. The lamellar spacing λ of the Al–Zn eutectic can be described as function of this rate of advance V by the relation .     

The precipitation of alkaline-earth metal and transition metal oxalates from aqueous solution. Induction periods and nucleation rates

The precipitation of magnesium, calcium, strontium and barium oxalates and of manganous, ferrous, cobalt, nickel and copper oxalates was studied from equivalent aqueous solutions at 22°C: the initial overall concentrations (C) generally varied from 0.001 to 0.2 M and the saturation ratios (S_mox) varied from <10 to >3000. The induction periods before the main growth surge were measured and nucleation rates were determined from final crystal numbers and induction periods. Precipitation occurred through homogenous nucleation: the critical nuclei in supersaturated alkaline-earth metal oxalate solutions were formed by aggregation of 6–8 M⁺⁺Ox⁻ ion-pairs while the critical nuclei in supersaturated transition metal oxalate solutions were formed by aggregation of 6–8 MOx complexes (to units of 3–4 M⁺⁺MOx₂⁻ ion-pairs). Over the range studied, the nucleation rates then varied with saturation ratios according to the relation, Nucleation rates at any saturation ratio decreased in the order Mg > Sr, Ba > Ca and Fe > Mn > Co, Cu > Ni; that is, generally in the order of increasing M⁺⁺–Ox⁻ and M⁺⁺–MOx₂⁻ bond strengths and increasing surface energies of the metal oxalate crystals. Induction periods decreased with increasing-concentration and saturation ratio; over The factors t _C1 and t _S1 depended in turn on the ‘rate constants’ for nucleation and growth during the induction periods and on metal oxalate solubilities.     

K2.6Nb11.6O30: a novel tetragonal bronze structure

K_2.6Nb_11.6O₃₀, Mr = 1659.33 g mol^–1, crystallizes in the tetragonal system, space group P4/mbm, Z = 1. The lattice parameters are a = 12.549(3) Å, c = 3.978(2) Å, V = 626.4(4) ų, D _x _{x} = 4.398 g cm^–3. The crystal structure was solved and refined from 413 independent reflections, obtained by X-ray diffraction on a single crystal. The final R index and weighted R _w index are 0.0500 and 0.1178, respectively. The three-dimensional network of K_2.6Nb_11.6O₃₀ is similar to that of tetragonal tungsten bronze with K and Nb partially occupying the interstitial pentagonal and trigonal sites. The most interest of this structure is the planar pentagonal coordination of Nb atoms.