The crystallographic orientation relationship between Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> in the composite material Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Al–Mg–Si alloy

The formation mechanism of spinels on Al₂O₃ particles in the Al₂O₃/Al–1.0 mass% Mg₂Si alloy composite material has been investigated by transmission electron microscopy (TEM) in order to determine the crystallographic orientation relationship. A thin sample of the Al₂O₃/Al–Mg–Si alloy composite material was obtained by the FIB method, and the orientation relationship between Al₂O₃ and MgAl₂O₄, which was formed on the surface of Al₂O₃ particles, was discovered by the TEM technique as follows:

Dissociation reaction of the 1/3$$ \left\langle {\bar{1}101} \right\rangle $$ edge dislocation in α-Al2O3

It has been reported that dislocations with 1/3\( \left\langle {\bar{1}101} \right\rangle \) edge component of the Burgers vector are formed in {1\( \bar{1} \)04}/\( \left\langle {11\bar{2}0} \right\rangle \) low-angle grain boundaries of alumina (α-Al₂O₃). These dislocations dissociate into two partial dislocations with a stacking fault on the (0001) plane (Tochigi et al. in J Mater Sci 46:4428–4433, 2011). However, the dissociation reaction of these dislocations has not been determined so far. In this study, the structures of the dissociated dislocations and the (0001) stacking fault were investigated by transmission electron microscopy and theoretical calculations. It was revealed that the dissociated dislocations were generated from the 1/3\( \left\langle {\bar{1}101} \right\rangle \) perfect edge dislocation by the reaction of 1/3\( \left\langle {\bar{1}101} \right\rangle \) → 1/18\( \left\langle {\bar{4}223} \right\rangle \) + 1/18\( \left\langle {\bar{2}4\bar{2}3} \right\rangle \). Furthermore, electron energy loss spectroscopy analysis was performed to examine the atomic/electronic structure of the (0001) stacking fault. In the observed spectra, a chemical shift and intensity decrease were found at the oxygen K-edge. Theoretical spectrum analysis using first-principles calculations revealed that the characteristic features of the spectra are originated from the local atomic configurations of the (0001) stacking fault.     

Dielectric function for the Anderson model

In recent years the study of alloys and compounds containing rare-earth and actinide elements is receiving increasing attention. The Anderson model is most popularly used for studying the theory of these systems. As it displays a large number of anomalous characters in magnetic and electrical properties, it was felt worthwhile to study the dielectric properties of this model. Using the linear response theory of Kubo, the energy and wave vector-dependent dielectric function \(\varepsilon \left( {\bar q,E} \right)\) is related to the retarded Green’s function of Fourier components of electron density fluctuations \(\left\langle {\left\langle {\rho _{\bar q} ;\rho _{ - \bar q} } \right\rangle } \right\rangle \) . Thus a many-body calculation of \(\varepsilon \left( {\bar q,E} \right)\) requires the calculation of \(\left\langle {\left\langle {\rho _{\bar q} ;\rho _{ - \bar q} } \right\rangle } \right\rangle \) . The Greens function is calculated using the equation-of-motion method with RPA decoupling. Further, since certain ensemble averages are required as inputs to the calculation, the relevant single-particle Green’s functions are also evaluated.     

Crystallization of amorphous silicon thin films: comparison between experimental and computer simulation results

Polycrystalline silicon obtained by the crystallization of thin amorphous silicon films has been an important material for microelectronics technology during the last decades. Many properties are improved in crystallized amorphous silicon compared to the as-deposited polysilicon such as larger grain size, smoother surface, and higher-carrier mobility. In this work, the crystallization of amorphous silicon is investigated by combining transmission electron microscopy (TEM) observations and molecular dynamics calculations. TEM observations on a series of specimens have shown that the majority of the silicon grains are oriented with a zone axis normal to the surface. In order to understand the crystallization mechanism molecular dynamic simulations were performed. It is found that the c/amorphous interface exhibits the lowest reduced interfacial energy density while the c/amorphous has the lowest reduced energy differences per unit interfacial area. The most energetically unfavorable interface is c/amorphous.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

The interface microstructure in alumina (FP) fibre/magnesium alloy composite

The objective of this work was to characterize the interfacial reaction zone in the metal matrix composite system-Al₂O₃(FP)/Mg (ZE41A). The composite was fabricated by liquid infiltration method. The reaction zone, a result of the reaction between magnesium in the alloy and the alumina fibres, was analysed for its morphology, chemistry, and crystallographic orientation using transmission electron microscopy. The results of this study showed the reaction zone to be, on average, 100nm wide and composed of MgO. The grains of the reaction zone ranged from less than 10 nm at the fibre/reaction zone interface to greater than 100nm at the matrix/reaction zone interface. It is proposed that the growth of the reaction zone was controlled by a seepage mechanism involving infiltration of liquid magnesium between MgO crystalS. Finally, it was observed that the MgO grains have the following crystallographic orientation relationship with the alumina grains from which they grew:

Turbidity Data of Weightless SF<Subscript>6</Subscript> Near its Liquid–Gas Critical Point

Light transmission measurements performed in SF₆ close to its liquid–gas critical point are used to obtain turbidity data in the reduced temperature range (T is temperature, T _c is the critical temperature). Automatic experiments (ALICE 2 facility) were made at a near critical density, i.e., , in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( is the average density, ρ _c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal compressibility and the correlation length ξ. These latter formulations are also used to analyze very near T _c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57 436, 1998).     

Thermodynamic properties of Pt5La,Pt5Ce,Pt5Pr,Pt5Tb and Pt5 Tm intermetallics

The Gibbs’ energies of formation of Pt₅La, Pt₅Ce, Pt₅Pr, Pt₅Tb and Pt₅ Tm intermetallic compounds have been determined in the temperature range 870–1100 K using the solid state cell: $$Ta,M + MF_3 /CaF_2 /Pt_5 M + Pt + MF_3 ,Ta$$ . The reversible emf of the cell is directly related to the Gibbs’ energy of formation of the Pt₅M compound. The results can be summarized by the equations: $$\begin{gathered} \Delta G_f^ \circ \left\langle {Pt_5 La} \right\rangle = - 373,150 + 6 \cdot 60 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Ce} \right\rangle = - 367,070 + 5 \cdot 79 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Pr} \right\rangle = - 370,540 + 4 \cdot 69 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Tb} \right\rangle = - 372,280 + 4 \cdot 11 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \Delta G_f^ \circ \left\langle {Pt_5 Tm} \right\rangle = - 368,230 + 4 \cdot 89 T\left( { \pm 300} \right)J mol^{ - 1} \hfill \\ \end{gathered} $$ relative to the low temperature allotropic form of the lanthanide element and solid platinum as standard states The enthalpies of formation of all the Pt₅M intermetallic compounds obtained in this study are in good agreement with Miedema’s model. The experimental values are more negative than those calculated using the model. The variation of the thermodynamic properties of Pt₅M compounds with atomic number of the lanthanide element is discussed in relation to valence state and molar volume.     

Phase relations in the system Cu-La-O and thermodynamic properties of CuLaO2 and CuLa2O4

Phase relations in the system Cu-La-O at 1200 K have been determined by equilibrating samples of different average composition at 1200 K, and phase analysis of quenched samples using optical microscopy, XRD, SEM and EDX. The equilibration experiments were conducted in evacuated ampoules, and under flowing inert gas and pure oxygen. There is only one stable binary oxide La₂O₃ along the binary La-O, and two oxides Cu₂O and CuO along the binary Cu-O. The Cu-La alloys were found to be in equilibrium with La₂O₃. Two ternary oxides CuLaO₂ and CuLa₂O₄₊ were found to be stable. The value of varies from close to zero at the dissociation partial pressure of oxygen to 0.12 at 0.1 MPa. The ternary oxide CuLaO₂, with copper in monovalent state, coexisted with Cu, Cu₂O, La₂O₃, and/or CuLa₂O₄₊ in different phase fields. The compound CuLa₂O₄₊, with copper in divalent state, equilibrated with Cu₂O, CuO, CuLaO₂, La₂O₃, and/or O₂ gas under different conditions at 1200 K. Thermodynamic properties of the ternary oxides were determined using three solid-state cells based on yttria-stabilized zirconia as the electrolyte in the temperature range from 875 K to 1250 K. The cells essentially measure the oxygen chemical potential in the three-phase fields, Cu + La₂O₃ + CuLaO₂, Cu₂O + CuLaO₂ + CuLa₂O₄ and La₂O₃ + CuLaO₂ + CuLa₂O₄. Although measurements on two cells were sufficient for deriving thermodynamic properties of the two ternary oxides, the third cell was used for independent verification of the derived data. The Gibbs energy of formation of the ternary oxides from their component binary oxides can be represented as a function of temperature by the equations:

Dislocation structure at a \{ {\overline{1} 2\overline{1} 0} \}/ {\langle} 10\overline{1} 0 {\rangle} low-angle tilt grain boundary in LiNbO3

LiNbO₃ is a ferroelectric material with a rhombohedral R3c structure at room temperature. A LiNbO₃ bicrystal with a $ \{ {\overline{1} 2\overline{1} 0} \}/ {\langle}10\overline{1} 0{\rangle}$ 1° low-angle tilt grain boundary was successfully fabricated by diffusion bonding. The resultant boundary was then investigated using high-resolution TEM. The boundary composed a periodic array of dislocations with $ b = { 1}/ 3{\langle} \overline{1} 2\overline{1} 0{\rangle} $ . They dissociated into two partial dislocations by climb. A crystallographic consideration suggests that the Burgers vectors of the partial dislocations should be $ 1/ 3{\langle}01\overline{1} 0{\rangle} $ and $ 1/ 3{\langle}\overline{1} 100{\rangle} $ , and a stacking fault on $ \{ {\overline{1} 2\overline{1} 0} \} $ is formed between the two partial dislocations. From the separation distance of a partial dislocation pair, a stacking fault energy on $ \{ {\overline{1} 2\overline{1} 0} \} $ was estimated to be 0.25?J/m² on the basis of isotropic elasticity theory.     

Oxidation Behavior of C/SiC Composite with CVD SiC-B4C Coating in a Wet Oxygen Environment

A two-layered self healing coating with a B₄C internal layer and a SiC external layer is prepared on C/SiC composite by chemical vapor deposition (CVD). Microstructure and component of the coating was analyzed by SEM, EDS, and XRD. Oxidation behavior of SiC-B₄C coated C/SiC composite was compared with SiC-SiC coated C/SiC in an environment of at 700°C, 1,000°C and 1,200°C for 100 h, respectively. It is demonstrated that the SiC-B₄C coating is more efficient to protect the composite from oxidation than SiC-SiC coating below 1,000°C due to the self healing behavior. After oxidized at 700°C for 100 h, the residual flexural strength of SiC-B₄C coated C/SiC is about 86%, and that of SiC-SiC coated is about 64%. While after oxidized at 1,200°C, the former is about 86% and the later is about 89%. This is due to the enhanced evaporation of B₂O₃ at higher temperature.     

Determination of standard Gibbs energies of formation of ternary oxides in the system Co-Sb-O by solid electrolyte emf method

An isothermal section of the phase diagram of the system Co-Sb-O at 873 K was established by isothermal equilibration and XRD analyses of quenched samples. The following galvanic cells were designed to measure the Gibbs energies of formation of the three ternary oxides namely CoSb₂O₄, Co₇Sb₂O₁₂ and CoSb₂O₆ present in the system.

High energy impact techniques application for surface grain refinement in AZ91D magnesium alloy

A nanostructured surface layer was fabricated on magnesium alloy AZ91D by using the high-energy impact technique (HEIT). With the help of transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM), the microstructure features of the surface layer were systematically observed and characterized in different stages of microstructure evolution. The result revealed the mechanism of grain refinement and strain accommodation. The process of grain refinement, accompanied by an increase in strain in the surface layer, resulted from several processes. The onset of deformation twinning and the intersection with twins system are one of them. The operation of basal slip and pyramidal slip led to the formation of dislocation cells and low-angle dislocation boundaries. The successive subdivision of grains to a finer scale resulted in the formation of highly disoriented nanocrystalline grains. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization. The minimum size of such refined grains was about 40 nm.     

Structural features of Σ = 19, [110] GaAs tilt grain boundaries

= 19, [110] tilt grain boundaries have been observed to facet parallel to particular planes; the facets lie along _A/ _B, ( )_A/ )_B and ( )_A/( )_B. The structural unit of the = 19 ( )_A/( )_B [110] boundaries consists of 5- and 7-member rings, which are similar to the core structure of a/2[110] edge dislocations. The polarities in each grain on either side of the boundaries has been confirmed by CBED methods; a lower number of anti-site type cross-boundary bonds occur along the boundaries compared to when the polarity of one grain is reversed. The presence of 7-member rings and anti-site cross-boundary bonds results in a more open atomic structure at the boundary, shortening the distance between the first and the second {331} atomic planes from the boundary plane by 40%.     

On the meaning of the impingement parameter in kinetic equations for nucleation and growth reactions

If certain preconditions are met, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic equation is exactly accurate for nucleation and growth reactions with linear growth and is, at least, a good approximation for nucleation and growth reactions with parabolic growth. These preconditions include randomly distributed product phases, isotropic growth and constant equilibrium state. Mechanisms causing deviations from these preconditions include: capillarity effect, vacancy annihilation, blocking due to anisotropic growth. It is shown that deviations lead to a modification of the overall transformation, which can be approximated well by a single equation:

Hydrogen reduction of Cu-Mn-Fe-O spinel solid solutions

The crystal-chemical transformations accompanying the hydrogen reduction of Cu_xMn₁ - xFe₂O₄ (x = 0.2, 0.5, 0.8) solid solutions are investigated, and general equations describing these processes over the entire composition range (0 x 1) are derived. The results on the character of the processes involved differ radically from earlier findings. The equilibrium oxygen pressure is evaluated for the low-oxygen phase boundary of the spinel solid solutions throughout their stability region. For solid solutions, which are of practical interest, the data are well represented by the equation , which is of importance in controlled synthesis of single-phase materials in this system.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 332–338.Original Russian Text Copyright © 2005 by E. Zinovik, M. Zinovik.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Precipitation in non-stoichiometric spinel

Precipitation in non-stoichiometric spinel (MgO · 3.5Al₂O₃) single crystals has been investigated in the temperature range 900 to 1600° C. The formation of a coherent intermediate phase precedes precipitation of the equilibriumα-Al₂O₃, and the time-temperature-extent of transformation (T-T-T) curve for the formation of this metastable phase has been determined. Nucleation ofα-Al₂O₃ occurs only at free surfaces, presumably because of the large strain energy accompanying the transformation of the anion sublattice from cubic to hexagonal close-packing. The growth rate ofα-Al₂O₃ from the free surfaces follows a linear law and appears similar to the “autocatalytic reaction” found in some metallic alloys. The following orientation relationship between spinel andα-Al₂O₃ is observed: $$\langle 10\overline 1 0\rangle _{\alpha - {\rm A}l_2 O_3 } \parallel \langle 110\rangle _{spinel} $$ and $$(0001)_{\alpha - {\rm A}l_2 O_3 } \parallel \{ 111\} _{spinel.} $$ .     

Indentation-induced deformation mechanisms in laser-processed directionally solidified WC–W<Subscript>2</Subscript>C eutectoids

Indentation-induced surface and subsurface deformation mechanisms in directionally solidified WC–W₂C eutectoids are investigated. The microstructures of the WC–W₂C composites consist of primary WC regions, W₂C-rich regions, and lamellar-type eutectoid regions, which form during the sequential phase transformations during laser processing. Indentation-induced surface crack profiles indicate little propensity for debonding at the WC–W₂C eutectoid interfaces, although some interactions with larger-scale microstructural features are observed. In the subsurface indentation regions, dislocation networks predominantly composed of partial dislocation pairs having Burgers vectors \( b = \frac{1}{6}\left\langle {2\bar{1}\bar{1}3} \right\rangle \) are observed in the primary WC regions, whereas shear banding and grain refinement are commonly observed in the micrometer-scale W₂C-rich regions. Deformation in the sub-micrometer eutectoid regions is distinct, with the dislocation activity primarily limited to the WC phase, with Burgers vectors of \( b = \frac{1}{3}\left\langle {1\bar{2}13} \right\rangle \) that terminate at the interfaces.     

The Ideal Strengths of Superconducting MgCNi<Subscript>3</Subscript> and CdCNi<Subscript>3</Subscript>

The stress-strain curves under tensile deformation in the 〈100〉, 〈110〉, and 〈111〉 directions and under shear deformation in the (001)〈110〉, \((110)\langle \overline {1}10\rangle \), \((111)\langle 1\overline {1}0\rangle \), and \((111)\langle 11\overline {2}\rangle \) slip systems have been systematically calculated by first-principles method to study the ideal strengths of superconducting MgCNi₃ and CdCNi₃. The ideal strengths in the three tensile directions are found to be reduced in the order of 〈100〉 → 〈110〉 → 〈111〉 and those for the four shear slip systems in the order of \((110)\langle \overline {1}10\rangle \rightarrow (111)\langle 11\overline {2}\rangle \rightarrow (111)\langle 1\overline {1}0\rangle \rightarrow (001)\langle 110\rangle \) for both superconductors. Their lowest ideal tensile strengths are found to be larger than the corresponding highest ideal shear strengths, which could explain why both superconductors have the ductility. The obtained lattice constants and elastic properties coincide well with the the available experimental and theoretical values.     

Study by electrical conductivity and infrared spectrometry of the thermal stability of defective iron spinels with aluminium and chromium

Precipitation in defective chromium- or aluminium-substituted magnetites, \(\gamma - \left( {Fe_{\left( {8/3} \right) - \left( {8/9} \right)x}^{3 + } M_{\left( {8/9} \right)x}^{3 + } \square _{1/3} } \right)O_4^{2 - } \left( {M^{3 + } = Al^{3 + } ,Cr^{3 + } ;0< x< 2} \right)\) , and defective iron aluminium chromium spinels, \(\gamma - \left( {Fe_{8/9}^{3 + } Al_{\left( {8/9} \right)\left( {2 - x} \right)}^{3 + } Cr_{\left( {8/9} \right)x}^{3 + } \square _{1/3} } \right)O_4^{2 - } \) has been investigated by electrical conductivity and infrared spectrometry in the temperature range 600 to 1200° C. For highly γ-AI-substituted magnetites and γ-iron aluminium chromium spinels the transformation of the spinel lattice into an α-rhombohedral lattice has been found to be preceded by the formation of an intermediate phase at about 900° C with a high alumina content, approximately identical to disordered γ-Al₂O₃. It is only at higher temperatures (> 1100° C) that the formation of an α-rhombohedral phase is observed. In the case of γ-Cr-substituted magnetites, temperatures of only about 700° C are required for the transformation γ → α.