Thermal and grinding induced residual stresses in a silicon carbide particle-reinforced aluminium metal matrix composite

The residual stresses in a silicon carbide particle-reinforced aluminium (SiC_p/Al) metal matrix composite (MMC) were measured using the X-ray diffraction method. The thermal residual stresses induced by annealing were found to be hydrostatic tension for the Al matrix and hydrostatic compression for the SiC reinforcement. After grinding treatment, the force equilibrium between these hydrostatic stresses was disturbed and compressive stresses were measured in both constituents. The effect of grinding extended into the bulk, and depth profiles of the residual stresses in both constituents were obtained by layer removal. The behaviour exhibited in these depth profiles is explained and their usefulness is indicated.     

Microstructure and properties of ceramic coatings produced on 2024 aluminum alloy by microarc oxidation

The microstructures of the microarc oxidation coatings and 2024 aluminum alloy substrate were observed using the scanning electron microscope (SEM) and the phase composition of the coatings was analyzed by X-ray diffraction (XRD). Furthermore, the profiles of the nanohardness, H, and elastic modulus, E, along the coating depth were first determined using the mechanical properties microprobe. The microarc oxidation coatings consist of two layers—a loose layer and a compact layer. The H and E in the compact layer are about 18–32 GPa, 280–390 GPa, respectively. The H and E profiles are similar, and both of them exhibit a maximum value at a same depth of the coatings. The distribution of -Al₂O₃ phase content determines the H and E profiles in the coatings. The changes of -Al₂O₃ and -Al₂O₃ contents result from the different cooling rates of the molten alumina in the microarc discharge channel at the different depths of the coatings. After the microarc oxidation treatment, the microstructure of the alloy substrate, even near the Al/Al₂O₃ interface, has not been changed.     

Microstructure and properties of MgO-ZnO2-SiO2 alkali-free glass-ceramics

The glass-forming region of the MgO-ZnO-SiO₂ system with small additions of Al₂O₃ has been explored. Based on it, two alkali-free glass-ceramics nucleated by TiO₂ were produced, possessing high thermal expansion coefficient, microhardness and electrical resistivity. Investigation of the properties and microstructure of these glass-ceramics by means of scanning electron microscopy and X-ray diffraction indicates that the proportions of willemite solid solution, enstatite solid solution, and-cristobalite which were encountered mainly in the glass-ceramics dominates the variation in properties. The mismatch of thermal expansion coefficient between the crystalline phases and the residual glass phase plays an important role in changing the mechanical properties.The death of Professor Peter McMillan is sadly recorded.     

Measurement of residual stresses in Al2O3/Ni laminated composites using an X-ray diffraction technique

Using an X-ray diffraction technique, macro-residual stresses were measured in laminated composites consisting of alternating layers of -Al₂O₃ and nickel. The in-plane thermal mismatch stresses which develop during fabrication were found to be compressive and tensile in the -Al₂O₃ and nickel layers, respectively. The magnitude of the in-plane stresses was found to be 110 MPa. Models of laminate structures predict the stress state to be biaxial in the plane of the layers. However, substantial stresses were observed perpendicular to the plane of the laminate; this stress might be due to the hot-pressing procedure used to fabricate the samples. The stress on the side surface of a laminate was measured using the indentation method and the results were consistent with those obtained by the X-ray method. Three samples were heated to 700, 900 and 1000 °C, respectively, and then cooled to test the effect of stress relaxation of the residual stresses due to the thermal expansion. The heat treatments (700–1000 °C) had no effect on the measured stress states of the laminates.     

Synthesis and photocatalytic behaviors of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–CNT/TiO<Subscript>2</Subscript> composite materials under visible light

Cr₂O₃–CNT/TiO₂ composites derived from chromium acetylacetonate, multi-walled carbon nanotubes (MWCNT) and titanium n-butoxide (TNB) were prepared, and the photocatalytic activity of the Cr₂O₃–CNT and CNT/TiO₂ composites was examined. The Cr₂O₃–CNT/TiO₂ composites were characterized by BET surface area measurement, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray analysis. The photocatalytic activity was determined from the decomposition of methylene blue (MB) under visible light irradiation. Methylene blue was photodegraded successfully in the presence of the Cr₂O₃–CNT/TiO₂ composite under visible light irradiation.     

Double-step lamination of grain-oriented Y-Ba-Cu-O superconducting ceramics

The uniformity of grain alignment and anisotropic superconducting properties were investigated in laminated tape-cast YBa₂Cu₃O_7–x ceramics. Large, platey YBa₂Cu₃O_7–x grains were prepared by gentle grinding of pellets prepared by liquid-phase sintering. A mixture of pre-reacted BaCuO₂ and CuO was added as a molten flux. The grain-oriented specimens were then prepared by tape-casting followed by lamination and sintering. A high degree of grain orientation was observed near the surface of the specimen. Successive polishing and X-ray diffraction studies revealed that the effective depth of the oriented layer was approximately 200 m. The small anisotropy observed in the critical current density calculated from the magnetization provided additional evidence for the non-uniformity of the grain alignment across the sample. By reducing the thickness of the laminated sample to the effective depth for obtaining a uniform structure, polycrystalline ceramic samples with a homogeneous and high degree of grain orientation were obtained by a double lamination method.     

Integrated intensity changes of X-ray diffraction lines for crystalline powders by grinding and compression. Relations between effective Debye parameter and lattice strain

The relations between the effective Debye parameter and lattice strain are investigated on some non-metallic crystalline powders, CoO, Co_0.5 Mg_0.5O, CoAl₂O₄, TiO₂, CaF₂, BaTiO₃ and graphite. The effective Debye parameter B _eff was determined from the integrated intensities of X-ray diffraction lines with different sin /-values and the lattice strain, , was determined from the half-widths of the diffraction lines. Both B _eff and increased with increase in grinding time and pressure. B _eff showed a relatively sharp increase, but tended to saturation. However, increased slowly even after prolonged grinding, and B _eff increased with increase in . When >0.2×10^–2, however, B _eff tended to a limit. The limiting values of B _eff were varied from sample to sample. An attempt was made to explain the changes of B _eff and with grinding and compression to large shear by the introduction of various defects into the structure. B _eff was found to give some information on the deformed structure of crystalline powders.     

Investigation of the phase composition in sintered lanthana-doped (Zr,Sn)TiO4 ceramics

Ceramic materials of the ZrO₂-SnO₂-TiO₂ system, modified by La₂O₃ and ZnO additions, were investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry. All the samples sintered at 1330, 1360 and 1400 °C contain a (Zr_xSn_z)TiO₄ (x + z= 1, 0 < z 0.4) solid solution coexisting with ZrTiO₄, the last gradually disappearing with rising temperature. At 1400 °C, the (Zr_0.8Sn_0.2)TiO₄ solid solution matrix was not quite homogeneous, containing small amounts of a grain-boundary phase assumed to be La₂(Zr,Sn,Ti)₂O₇ and the expected TiO₂-basis solid solution rich in Zn and La. The evolution of the chemical composition of (Zr_xSn_z)TiO₄ solid solution to (Zr_0.8Sn_0.2)TiO₄ and the simultaneous disappearance of ZrTiO₄ are thought to explain the variation trends of lattice parameters with increasing sintering temperature.     

Preparation of CVD diamond coatings on gamma titanium aluminide using MPECVD with various interlayers

CVD diamond coatings were deposited on to γ-TiAl surfaces using a microwave plasma enhanced CVD to improve wear properties and the performance of γ-TiAl. Diamond coatings were directly deposited on to γ-TiAl substrates and deposited on to TiC, Ti₅Si₃, Al₂O₃ + TiO₂, and Si interlayers prepared on γ-TiAl substrates. The diamond coatings deposited directly on γ-TiAl suffered severe delamination and cracked. Those deposited on TiC and Ti₅Si₃ interlayers partially delaminated, whereas those deposited on Al₂O₃ + TiO₂ and Si interlayers adhered well to the underlying surfaces. The diamond films obtained were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that polycrystalline and nanocrystalline diamond films grew on γ-TiAl. Residual internal stresses of the diamond coatings deposited on interlayered-γ-TiAl were estimated experimentally from Raman spectra. The coatings prepared on Al₂O₃ + TiO₂/γ-TiAl and Si/γ-TiAl showed lower residual stresses.     

Modeling of the Phase Transformation Induced by Ball Milling in Anatase TiO2

A high-pressure and high-temperature phase of TiO₂ : TiO₂ II is formed transiently during room-temperature high-energy ball milling of anatase TiO₂ : TiO₂ anatase TiO₂ II TiO₂ rutile. Rutile is the only phase present after prolonged ball milling. The present paper focuses on the influences of physical and chemical processing conditions on the transformation kinetics. The effects of two milling parameters on the kinetics of phase transformation of anatase TiO₂ were investigated: the nature of milling tools and the powder-to-ball weight ratio R. Granulometric characterizations and TEM observations have demonstrated that the transformation of TiO₂ anatase into TiO₂ II occurs without fracturing of particles and that TiO₂ II nanograins form at the surface of anatase particles. The parameter R affects only the transformation rate. For a given R, the transformation rate is the largest with alumina grinding tools, intermediate with zirconia tools, and the smallest with steel tools. The parameters involved in current models of the mechanical alloying process do not suffice to explain the differences in transformation rates observed here. A parameter, which takes into account the influence of the mechanical properties of grinding materials, is considered.     

The growth of oriented ceramic eutectics

Controlled microstructures of the two eutectics in the alumina-titania system have been grown using a special electron beam heating technique. In the aluminium titanate-titania system, the eutectic interlamallar spacing varies with the freezing rate R as =AR ^–n where n=0.5 and the value of the constant A is 8.5×10^–6 cm^3/2sec^–1/2. Primary plate-like dendrites of aluminium titanate in a matrix of discontinuous aluminium titanate-titania eutectic are formed on solidifying a composition TiO₂-20 wt % Al₂O₃. These dendrites appear to deflect cracks in this ceramic. In the alumina-aluminium titanate system, primary rod-like dendrites of alumina were grown in a ribbon-like eutectic of alumina and aluminium titanate on solidifying a composition Al₂O₃–38.5% TiO₂.     

Crystallization and structure of a MgO-Al2O3-SiO2-TiO2 glass-ceramic

The structure and changes occurring during the volume crystallization of a MgO-Al₂O₃-SiO₂-TiO₂ glass induced by heat-treatment at temperatures in the range 740 to 1300° C were investigated by means of DTA, scanning electron microscopy, X-ray diffraction, electron microprobe analysis, density and other measurements. Crystallization proceeds by liquid phase separation and coalescence of small particles forming larger microcrystalline regions dispersed in the glassy phase, these regions fill the bulk of the glass during the heat-treatment. Such a mechanism of crystallization leads to the formation of a fine-grained homogeneous structure whose chemical composition is identical to that of the glassy matrix (as is shown by the electron microprobe). The scanning electron microscope revealed two crystalline phases in the initial stage (850 to 890° C) of crystallization while X-ray diffraction identified only a -cordierite type structure, which at temperatures above 1100° C is transformed into the -form. The X-ray diffraction patterns showed further that a cordierite 2MgO.2Al₂O₃.nSiO₂ solid solution may be formed dependent on the heattreatment temperature. Stereoscan micrographs of various stages of crystallization show the development of the glass-ceramic. In addition, the influence of the technique employed to prepare the electron microscope specimens, on their surface topography is also shown.     

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solutions from Al₂O₃-TiO₂-MgAl₂O₄ powder mixtures of 1 m particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (pure Al₂TiO₅), 10^–3, 10^–2 and 10^–1. Analysis of the kinetic data and microstructural observation indicates that MgAl₂O₄ affects the mechanism of Al₂TiO₅ formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg_0.5AlTi_1.5O₅, i.e. the equimolar Al₂TiO₅-MgTi₂O₅ solid solution, and are formed by reaction between MgAl₂O₄ and TiO₂ at temperatures above 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x10^–2, the prevailing mechanism is the nucleation and growth of Al₂TiO₅ nodules for fractional conversion up to 0.8. Further conversion of the residual Al₂O₃ and TiO₂ particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al₂TiO₅. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al₂TiO₅ becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al₂TiO₅ formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.     

Formation of niobium-tantalum pentoxide orthorhombic solid solutions under chlorine-bearing atmospheres

This paper studies, by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), the structural changes suffered by amorphous hydrated Nb oxide and mixtures of amorphous and crystalline Nb and Ta oxides subjected to thermal treatments in air and chlorine atmospheres. The air heating of amorphous Nb₂O₅·nH₂O leads to different crystalline phases depending on the working temperature. The hexagonal phase of Nb₂O₅ is obtained at 773 K and the monoclinic phase -Nb₂O₅ is obtained at 1173 K. The thermal treatment of amorphous Nb oxide in chlorine atmosphere decreases the temperature at which phase -Nb₂O₅ appears and at 1173 K the stable monoclinic phase -Nb₂O₅ is obtained. Air calcination for 5 hours at temperatures between 973 and 1273 K of different amorphous Nb₂O₅·nH₂O - Ta₂O₅·nH₂O mixtures does not lead to solid solution of these oxides. Thermal treatment in chlorine atmosphere of amorphous Nb and Ta oxides leads to the formation of Nb₂O₅ and Ta₂O₅ orthorhombic solution in one hour at 973 K and in 24 hours at 1223 K, when starting from crystalline oxides. The effect of chlorine is due to the dissolution-recrystallization of the metallic chlorides and the oxygen formed, when the system evolves to chemical equilibrium between solid phases and gaseous chlorine.     

Synthesis and alteration of α-LiFeO2 by mechanochemical processes

The effects of grinding on a stoichiometric mixture of LiOH · H₂O and -FeOOH were studied. It was found that, in the course of grinding, losses of structural water occurred and a phase structurally related to disordered -LiFeO₂ was formed. X-ray diffraction data suggest the occurrence of an ordered phase as intermediate and both -Fe₂O₃ and -Fe₂O₃ were undetected during the comminution process. A prolonged mechanical treatment of this mixture originated an elimination of Li⁺ from the -LiFeO₂ structure and the appearance of the spinel phase, -LiFe₅O₈. Additionally, the mechanical activation of a sample of -LiFeO₂ prepared at high temperatures also leads to a similar rearrangement of cations. The structural transformation is explained with the help of a model in which the vacancies of Li⁺ created during grinding promote the migration of the Fe³⁺ ions from octahedral to tetrahedral sites.     

Gas atomization of cobalt ferrite-phosphate melts

A rapidly solidified (Co, Fe)₃O₄ spinel was formed in a cobalt-iron-phosphate glass matrix by gas atomization of melts of composition 37.5% mol % CoO, 37.5%Fe₂O₃, 25% P₂O₅; and 40% CoO, 40% Fe₂O₃, 20% P₂O₅, and the material has been characterized using size analysis, X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy.In the 20 mol % P₂O₅ composition, atomized powder 50–100m in diameter contained 33 wt % (Co, Fe)₃O₄. The ferrite crystallized as randomly oriented, faceted dendrites parallel to 100, and the growth was apparently diffusion controlled. Unlike the ferrite, where iron is in both tetrahedral and octahedral coordination, the iron in the glassy matrix was predominantly in distorted octahedral coordination. Overall particle size distributions were broad (15–700m), with 50 wt % smaller than 200m. The maximum in the size distribution shifted from 150 to 80m on increasing the atomizing pressure from 7 to 20 bar (100 to 300 psi). Calculations have shown that the cooling rates obtained with oxide melts vary strongly with droplet size, and less strongly with melt temperature.     

Microwave dielectric properties of temperature-stable (Mg<Subscript>0.95</Subscript>Co<Subscript>0.05</Subscript>)<Subscript>2</Subscript>TiO<Subscript>4</Subscript>–Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite ceramics for LTCC applications

In this paper, the effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the phase formation, sintering characteristic, the microstructure and microwave dielectric properties of temperature-stable (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ ceramics were investigated. (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ powders were obtained by using the traditional solid-state process. A small amount of LBBS doping can effectively reduce sintering temperature and promote the densification of the ceramics. X-ray diffraction analysis revealed not only the primary phase (Mg·Co)₂TiO₄ associated with Li₂TiO₃ minor phase but also a third phase (Mg·Co)TiO₃. The dielectric constant and Qf values vary with the doping amount of LBBS and sintering temperatures. With the compensation of the positive temperature coefficient (τ _f ) of Li₂TiO₃ and the negative τ _f of (Mg_0.95Co_0.05)₂TiO₄, the τ _f of the specimens fluctuates around zero. The (Mg_0.95Co_0.05)₂TiO₄ ceramic with 2.5 wt% LBBS addition and sintering at 900?°C for 4 h exhibited excellent microwave dielectric properties: ? _r ?=?19.076, Qf?=?126100 GHz, and τ _f ?=?0.98 ppm/°C.     

Lead-strontium titanate glass ceramics: I – crystallization and microstructure

The crystallization behaviour of glasses in the system [(Pb Sr ) O·TiO₂]-[2SiO₂· B₂O₃]-[K₂O]-[BaO] (0.2 ≤ x ≤ 0.9) have been studied. Perovskite titanate was found to be the major phase in all the glass ceramic samples investigated. The actual composition of crystalline phases could not be confirmed on the basis of shift in the X-ray diffraction (XRD) peak positions because of similar effects due to solid solution formation and strain due to crystal clamping. Comparison of the observed intensities of various XRD peaks of the perovskite titanate phase with the calculated intensities for (Pb Sr )TiO₃ with same lead/strontium ratio confirmed the formation of lead strontium titanate solid solution. Microstructural characteristics of various glass ceramics are also discussed. The advantages of using K₂O and BaO as additives instead of only K₂O are also discussed.     

The mechanism of nucleated crystallisation of glasses in lithia-alumina-silica and cordierite systems

Two principal lines of investigation are discussed. In the first of these, in which glasses were crystallised under high pressure, it was shown that the low temperature -forms of spodumene and eucryptite could be obtained. In further experiments the effects of substituting GeO₂ for SiO₂, Ga₂O₃ for Al₂O₃, MgO for Li₂O and TiO₂ for Al₂O₃ in Li₂O-Al₂O₃-SiO₂ glasses were studied. It was found possible to crystallise both and -forms of germanium and gallium containing spodumene and eucryptite. Infra-red absorption spectra, X-ray diffraction data and thermal expansion data are presented for the various crystal types. Based on the results of the investigation, it is suggested that nucleated crystallisation in the glasses may be due to the initial precipitation of a crystal phase which is isomorphous with the predominant crystal phase that is to be developed in the glass-ceramic, and that during the heat-treatment the crystals of the latter phase grow on the crystals of the primary phase.