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.     

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si₃N₄ material which was produced by the nitridation pressureless sintering (NPS) method using Y₂O₃ and Al₂O₃ as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.     

Electron-microscopic study of glassy As2Te3

Electron-microscopic investigations of glassy As₂Te₃ reveal that this material is a homogeneous glass and does not contain any crystalline phase. In order to verify the detection limit of the employed method with respect to crystalline phase the presence of the latter was provoked by heating the glass in DTA. The crystallization procceeds in two stages: (1) in T_g region the growth of crystallization centres originating probably from the initial melt starts; (2) closely below the recrystallization temperature T_r spontaneous crystallization in the whole glassy bulk is initiated.     

Nano structure of rapidly quenched Cu–(Zr or Hf) – Ti alloys and their devitrification process

The structure and primary devitrification process of the melt-spun Cu₆₀(Zr or Hf)₃₀Ti₁₀ alloys were investigated. It was confirmed that the compositional segregation in the diameter range of 5–10 nm exists in the as-quenched state. The nanocrystalline particles with cubic structure are observed in the glassy matrix in thehigh-resolution transmission electron microscopy images, of which size is corresponding to the scale of compositional segregation. Small-angle X-ray scattering measurement also indicates the development of nanoscale inhomogeneity with the same size as that of nanocrystalline particles. The nanocrystalline region has high Cu content. In contrast, Zr or Hf and Ti elements are enriched in the glassy region. These results are recognized as the formation of novel structure consisting of the glassy and nanocrystalline phases. It is suggested that the precipitation of bcc CuZr phase as a primary crystallization phase proceeds in the glassy phase remaining the nanocrystalline phase in the Cu–Zr–Ti alloy. Meanwhile, the glassy and nanocrystalline phases are transformed to an orthorhombic Cu₈Hf₃ phase at the initial crystallization stage in the Cu–Hf–Ti alloy. These differences of crystallization process are consistent with the results of thermodynamic and kinetic analyses of the transformation mode.     

An X-ray diffraction and electron spectroscopic study of metallic glass In20Te80

Results of X-ray diffraction and X-ray photoelectron spectroscopy investigations on the crystallization behaviour of an amorphous In₂₀Te₈₀ system and the effects of crystallization on the electronic core levels of In and Te atoms are presented. During controlled heat treatments three crystalline phases, Te, -In₂Te₃, and In₂Te₅-I, were observed in this system. In addition, a few splat-cooled samples were found to exhibit a new metastable crystalline phase. Photoelectron measurements revealed that the Te 3d and 4d core levels of amorphous In₂₀Te₈₀ were shifted downwards in energy from their characteristic values of pure Te metal. The In 3d and 4d levels experienced large energy shifts due to alloying, but remained unaffected by heating at temperatures below 520 K.     

Some aspects of splat-quenching in an inert atmosphere and of the formation of non-crystalline phases in Al-17.3 at. % Cu,germanium and tellurium

An apparatus for splat-quenching by the gun technique in a sealed, inert atmosphere is described. The importance of a low-oxygen quenching atmosphere in promoting efficient spreading of liquid particles and good thermal contact with the quenching surface is shown. A cooling rate of ～10¹⁰ K sec^?1 was estimated from the interlamellar spacing in a quenched Al-17.3 at. % Cu alloy. The process mechanisms of the gun technique are discussed with particular reference to the atomized droplet size and the effective specimen thickness for heat transfer. New non-crystalline phases are reported in electron-transparent areas of splat-quenched foils of Al-17.3 at. % Cu (eutectic composition) pure Ge and pure Te. The glassy Al-Cu phase was also observed in specimens which were chemically thinned from the thicker regions of foils; lattice image studies by high-resolution electron microscopy tentatively suggest that this phase has an amorphous, liquid-like atomic configuration. The peak positions in the electron diffraction patterns of the Ge and Te phases were compared, where possible, with those for the corresponding liquid and vapour-deposited phases. The results for Ge suggest that significant structural rearrangement took place during cooling and freezing from the liquid to give a paracrystalline, tetrahedral short-range order whereas, for Te, the liquid structure was probably largely preserved on freezing.     

A new metastable phase near 60 at% Zr from amorphous Ni-Zr

Metallic glass alloys of Ni-Zr were prepared by vapour and liquid quenching. For glass compositions at and near 60 at% Zr, crystallization proceeded by the sequence: amorphous metastable crystalline phase NiZr₂ + NiZr. The metastable Ni₄₀Zr₆₀ phase exhibited a very distinctive X-ray diffraction pattern and was present in both liquid- and vapour-quenched samples. Long-term anneals of samples with the metastable structure produced the equilibrium phases NiZr₂ and NiZr. The crystallization of the amorphous structure directly to a single metastable phase shows a correspondence between the compositions of the amorphous and crystalline phases. These results thus suggest a connection between the short-range structure of the glassy phase and the crystalline phases to which they transform. An observation of peaks and valleys in a plot of the X-ray scattering vector,Q _p, against glass composition is noted.     

Crystallization kinetics of Zr60Al15Ni25 bulk glassy alloy

The crystallization kinetics of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isochronal and isothermal conditions has been investigated by differential scanning calorimetry (DSC). The microstructure of as-cast Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy is observed by high-resolution electron microscopy (HREM). It is found that there exist nanocrystals with a size of about 7 nm in the glassy matrix, which are not observed in the XRD image. The results of Kissinger analysis show that the effective activation energies for glass transition (457 kJ/mol) and crystallization (345 kJ/mol) are high, indicating that it has large thermal stability against crystallization. The crystallization of Zr₆₀Al₁₅Ni₂₅ bulk glassy alloy under isothermal annealing can be modeled by the Johnson-Mehl-Avami equation. The crystallization kinetics parameters show that the isothermal crystallization starts from the growth of the pre-existing nanocrystals and the crystallization process is diffusion-controlled.     

Crystallization of Li-Si-O-N oxynitride glasses

Effects of nitrogen incorporation on the crystallization of a 3OLi₂O · 70SiO₂ glass have been studied in terms of the glass transition and crystallization temperatures, nucleation mechanism and precipitated crystalline phases. Glass transition and crystallization temperatures rise with increasing nitrogen content. Bulk crystallization is dominant in these oxinitride glasses. Precipitated crystallization phases and their relative amounts vary with nitrogen content. The experimental results are discussed assuming the formation of an [((-O-)₃Si-)₃N] unit and its evolution to an [SiON₃] unit in the residual glassy phase during the crystal growth.On leave from the Government Industrial Development Laboratory, Hokkaido, 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 004, Japan.     

The kinetics and mechanism of crystallization in enstatite-type glass-ceramic materials

The general crystallization behaviour of a model glass-forming melt having a composition (wt%) 7.5 SiO₂ · 1.0 Al₂O₃ · 1.0 MgO · 1.5 CaO + 10 TiO₂ is investigated. The composition and the morphology of the crystalline phases formed in the system and the structure of the resulting glass-ceramic material are examined by X-ray diffraction, electron microscopy and DTA. Detailed kinetic measurements indicate that the process of nucleation of the initially formed anosovite-type phase may be described from the viewpoint of the non-steady state theory of nucleation. It is also demonstrated that the crystal growth of the spherulites in this phase is diffusion-limited. The role played by TiO₂ in the system under investigation and in similar glass-forming melts is also discussed.     

High formability of glass plus fcc-Al phases in rapidly solidified Al-based multicomponent alloy

A multicomponent Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ alloy was found to keep a mixed glassy + Al phases in the relatively large ribbon thickness range up to about 200 μm for the melt-spun ribbon and in the diameter range up to about 1100 μm for the wedge-shaped cone rod prepared by injection copper mold casting. The glassy phase in the Al-based alloy has a unique crystallization process of glass transition, followed by supercooled liquid region, fcc-Al + glass, and then Al + Al₃Y + Al₉ (Co, Fe)₂ + unknown phase. It is also noticed that the primary precipitation phase from supercooled liquid is composed of an Al phase instead of coexistent Al + compound phases, being different from the crystallization mode from supercooled liquid for ordinary Al-based glassy alloys. In addition, it is noticed that the mixed Al and glassy phases are extended in a wide heating temperature range of 588–703 K, which is favorable for the development of high-strength nanostructure Al-based bulk alloys obtained by warm extrusion of mixed Al + amorphous phases. The Vickers hardness is about 415 for the glassy phase and increases significantly to about 580 for the mixed Al and glassy phases. The knowledge of forming Al + glassy phases with high hardness in the wide solidification and annealing conditions through high stability up to complete crystallization for the multicomponent alloy is promising for future development of a high-strength Al-based bulk alloy.     

Glass formation and immiscibility in the TeO2-B2O3-Fe2O3-MnO system

The glass formation in the quaternary TeO₂-B₂O₃-MnO-Fe₂O₃ system and in its ternary systems was investigated. A range of liquid immiscible phases, located near to the binary TeO₂-B₂O₃ and B₂O₃-MnO systems was established. Using transmission electron microscopy, a trend to metastable liquid-phase separation in the single-phase glasses, located near to the boundary of immiscibility was observed. With an increase in the Fe₂O₃ and MnO content still in the process of cooling of the melts, it was possible for a fine glassy crystalline structure to be formed in them. It was shown that by changing the upper limit of the melting temperature and the cooling rate, the glassy crystalline structure and the Fe₃O₄ content could be modified.     

Characterization of crystal phases,morphology and crystallization processes in lithium aluminosilicate glass-ceramic

The nucleation and crystallization processes of Li₂O-Al₂O₃-SiO₂ glass-ceramics were investigated by differential thermal analysis. The crystalline phases produced during thermal treatment at different temperatures and the residual glassy phase were characterized by X-ray diffraction, SEM and image analysis techniques. The activation energy of the crystallization process was calculated as E=380±20 kJ mol^–1. The influence of nucleating agents (TiO₂, ZrO₂) was evaluated to obtain glass-ceramics transparent to visible light. The stability of the glassy phase in cooling was determined by means of temperature-time-transformation curves.     

Simulated body fluid electrochemical response of Zr-based metallic glasses with different degrees of crystallization

The bio-electrochemical response in simulated body fluid of the Zr₅₃Cu₃₀Ni₉Al₈ metallic glasses with different degrees of partial crystallization was systematically examined and discussed. Through thermal annealing, the volume fractions of the crystalline phases are determined to be 0, 34, 63, and near 100%. Based on the bio-corrosion voltage and current, as well as the polarization resistance, it is concluded that the fully amorphous alloy exhibits the highest bio-electrochemical resistance. With an increasing degree of partial crystallization, the corrosion resistance becomes progressively degraded. The passive current reveals that the fully amorphous metallic glasses can form a more protective and denser passive film on the metallic glass surface. The formation of reactive nanocrystalline phases in the amorphous matrix would reduce the bio-corrosion resistance.     

Effect of thermo-mechanical histories on the microstructure and properties of Zr<Subscript>65</Subscript>Al<Subscript>10</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>15</Subscript> metallic glassy plates

Effect of thermo-mechanical histories during hot rolling in the supercooled liquid region on the microstructure and properties of Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glassy plates was investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC), microhardness and electrical resistivity measurements. It was found that some nano-scale clusters and a few crystalline phases were dispersed in the amorphous matrix, which may depress the crystallization onset temperature (T_x). The microhardness increased while the electrical resistivity first increased and then decreased with hot rolling times. So, it is important for the working and forming of bulk metallic glasses in the supercooled liquid region to take the thermo-mechanical histories into account.     

A study on the crystallization of amorphous arsenic

The results of DTA analysis of amorphous arsenic in combination with electron microscopical study of the samples through the whole temperature range reveal that no changes of the initial state of the sample occur on heating up to crystallization temperature. Close above this temperature the spontaneous crystallization starts. It differs markedly from the case of glassy materials, like As₂Te₃, in which case at temperatures as low as the transition region T_g the crystallization nuclei of geometrically defined shapes appear and grow into crystalline grains on further heating.     

Effect of the ZrO<Subscript>2</Subscript> concentration on the crystallization behavior and the mechanical properties of high-strength MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> glass–ceramics

High-strength, colorless glass–ceramics in the MgO/Al₂O₃/SiO₂ system with high concentrations of ZrO₂ and a great potential for technical application, e.g., as high-performance hard disc substrates, are investigated. ZrO₂ concentrations from 6 to 9 mol% are added to a stoichiometric cordierite glass to investigate the influence of the concentration of the nucleating agent on the crystallization behavior and the mechanical properties. The phase formation and the microstructure of the glass–ceramics are studied using X-ray diffraction and scanning electron microscopy including electron backscatter diffraction. It is shown that the volume crystallization of ZrO₂, a low-/high-quartz solid solution (low-/high-QSS), and spinel is accompanied by the surface crystallization of indialite. This phase offers a much smaller coefficient of thermal expansion than the other crystal phases, which may induce high compressive stresses in the surface layer of the glass–ceramics after cooling and seems to result in excellent mechanical properties of the material. Biaxial flexural strengths of up to 1 GPa were measured. Higher ZrO₂ concentrations reduce the surface crystallization of indialite and decrease the mean size of the crystals resulting in a higher translucency. The volume-crystallizing phases and the mechanical properties of the glass–ceramics do not seem to be significantly affected by the analyzed ZrO₂ concentrations.     

Electrical properties of a partially crystallized aluminosilicate glass containing TiO2 and Nb2O5

A two-stage heat treatment of an aluminosilicate glass containing both TiO₂ and Nb₂O₅ may be used to precipitate a fine dispersion of doped TiO₂ crystals within a glassy matrix. The resulting partially crystalline product exhibits the properties of a semiconducting material with activation energies for conduction ranging from 0.2 to 0.35 eV. The d.c. conductivity of the specimens has been shown to be related to the size of the TiO₂ crystals. With prolonged crystallization treatment a second non-conducting crystalline phase, anorthite, reduces the conductivity of the glass-ceramics.     

Preparation and characterization of selenium incorporated anodic conversion coatings on titanium surfaces for biomedical applications

An anodic spark deposition process was used for preparation of inorganic, glass-ceramic like conversion coatings. The microstructure of the layers was characterized by surface and solid state techniques such as scanning electron microscopy, electron probe microanalysis and Raman spectroscopy. The porous coatings, typically up to 8 μm thick, consist mainly of titanium oxides and amounts of incorporated electrolyte constituents like Se, Ca or P. Beside nano crystalline anatase phases, a mostly amorphous structure is proposed in which network-forming [PO₄] tetrahedras and [TiO₆] octahedras in various degrees of condensation are connected. A drastic modification of the film structure was observed when selenium was incorporated into the glassy oxide structure of the coating. In these cases no nano crystalline phases of titanium oxides or other chemical compounds were detected. First cell culture investigations show a significant improvement of the biological properties. Cell proliferation and TGF-beta-expression of these coatings in comparison with commercial pure titanium (CPT) with native titanium oxide films were examined.     

Phases formed during rapid quenching of liquid carbon

Pulsed laser action upon a sample of highly oriented pyrolytic graphite (HOPG) in a gasostat filled with helium at a pressure above that corresponding to the triple point of carbon, followed by rapid quenching of the liquid phase at a rate of about 10⁶ K/s leads to the formation of a crater with a periodic spatial structure at the surface. The composition and structure of nongraphite carbon phases in the near-surface region of the crater have been studied using the Raman scattering spectroscopy, electron microdiffraction, and energy-dispersive X-ray analysis. It is established that rapidly quenched carbon possesses predominantly a hybrid structure of glassy carbon formed as a result of the high-temperature treatment, with inclusions of crystalline carbyne, chaoite, and a hybrid cubic phase of ultradense carbon (C₈). The hybrid phases of glassy carbon and C₈ had not been reported until now as possible products of solidification of liquid carbon.