Al-rich Glasses Research Articles

Phenomenological description of the effect of glass dissolution on cement paste evolution in an integrated glass dissolution experiment

The interaction between nuclear glass surrogate and ordinary Portland cement (OPC) was investigated through integrated static leach tests conducted at ambient temperature. Tests were performed in stainless-steel cells in which OPC hydrated cement pastes were separated from glass powders by a stainless-steel filter. This article gives a phenomenological description of the processes affecting the hydrated cement paste arising from glass dissolution. The results show that the ingress of Si from glass dissolution leads to the degradation of the cement paste over a distance of 100–200 μm within 900 days of interaction. The Hardened Cement Paste (HCP) is affected by two main processes, which are the pozzolanic reaction and the alkali-silica reaction (ASR). These processes lead to porosity changes of HCP in contact with the stainless-steel filter and significant changes in the diffusive properties of the HCP. Especially, the effect of such processes on boron and lithium retention was studied and showed that precipitation of ASR-like gel and low Ca/Si C–S–H (Calcium Silicate Hydrate) highly retain these two elements. In the presence of an Al-rich glass (SM539), ASR is unlikely due to the preferential formation of C-A-S-H (Calcium Aluminium Silicate Hydrate).

Determination of the maximum dissolution rates of the Belgian reference glasses at very alkaline pH and 30 °C

To assess the chemical durability of the Belgian reference glasses SON68, SM513, and SM539 under hyper-alkaline conditions, maximum glass dissolution rates were measured at 30 °C and pH 13.5 by performing Single-Pass Flow-Through tests with a KOH solution and a synthetic young cementitious water containing Ca (YCWCa). In both leachants, the highest dissolution rate was determined for the Al-rich glass SM539, while similar rates were found for SM513 and SON68 glasses, whose compositions are comparable. For all glasses, the dissolution rates in YCWCa were lower than in KOH due to the presence of Ca, which leads to the formation of a slightly protective calcium silicate layer. The dissolution rates in YCWCa were similar to those determined from glass monolith mass losses in static tests carried out at 30 °C in YCWCa in the presence of hardened Ordinary Portland Cement paste powder with a cement-to-glass ratio of 1.Graphical abstract

Enhancement of ductile deformation in polycrystalline anorthite due to the addition of water

We performed shear deformation experiments on synthetic anorthite (An) aggregates with various amounts of water added externally. The samples were pure An aggregates and aggregates of An with 5% Si–Al-rich glass (An + glass hereafter), each with up to 0.5 wt% added water. The experiments were conducted using a Griggs-type deformation apparatus at 900 °C and 1.0 GPa, with shear strain rates ranging from 10−3.5 to 10−5.0 s−1. We observed a clear reduction of up to two orders of magnitude in effective viscosity with an increase in the amount of added water. Microstructural observations indicate that the deformation behavior evolves from brittle to ductile with an increasing amount of added water. The pure An sample with 0.5 wt% added water is more than an order of magnitude stronger than the equivalent An + glass sample. Infrared spectroscopy measurements reveal that the water contents trapped in the An + glass and pure An samples increase up to 1710 and 840 ppm H2O, respectively, during the 0.5 wt% added-water experiments. The difference in the water contents of the two samples indicates that approximately half of the water content is trapped in glass.

Strain localization and fabric development in polycrystalline anorthite\xa0+\xa0melt by water diffusion in an axial deformation experiment

We performed two axial deformation experiments on synthetic polycrystalline anorthite samples with a grain size of ~ 3 μm and 5 vol% Si–Al-rich glass at 900 °C, a confining pressure of 1.0 GPa, and a strain rate of 10−4.8 s−1. One sample was deformed as-is (dry); in the other sample, two half-cut samples (two cores) with 0.15 wt% water at the boundary were put together in the apparatus. The mechanical data for both samples were essentially identical with a yield strength of ~ 700 MPa and strain weakening of ~ 500 MPa by 20% strain. The dry sample appears to have been deformed by distributed fracturing. Meanwhile, the water-added sample shows plastic strain localization in addition to fracturing and reaction products composed of zoisite grains and SiO2 materials along the boundary between the two sample cores. Infrared spectra of the water-added sample showed dominant water bands of zoisite. The maximum water content was 1500 wt ppm H2O at the two-core boundary, which is the same as the added amount. The water contents gradually decreased from the boundaries to the sample interior, and the gradient fitted well with the solution of the one-dimensional diffusion equation. The determined diffusion coefficient was 7.4 × 10−13 m2/s, which agrees with previous data for the grain boundary diffusion of water. The anorthite grains in the water-added sample showed no crystallographic preferred orientation. Textural observations and water diffusion indicate that water promotes the plastic deformation of polycrystalline anorthite by grain-size-sensitive creep as well as simultaneous reactions. We calculated the strain rate evolution controlled by water diffusion in feldspar aggregates surrounded by a water source. We assumed water diffusion in a dry rock mass with variable sizes. Diffused water weakens a rock mass with time under compressive stress. The calculated strain rate decreased from 10−10 to 10−15 s−1 with an increase in the rock mass size to which water is supplied from < 1 m to 1 km and an increase in the time of water diffusion from < 1 to ~ 10,000 years. This indicates a decrease in the strain rate in a rock mass with increasing deformation via water diffusion.

In situ structural analysis of calcium aluminosilicate glasses under high pressure

In situ micro-Raman spectroscopy was used to investigate the structural evolution of OH−-free calcium aluminosilicate glasses, under high pressure and at room temperature. Evaluation was made of the role of the SiO2 concentration in percalcic join systems, for Al/(Al + Si) in the approximate range from 0.9 to 0.2. Under high pressure, the intensity of the main band related to the bending mode of bridging oxygen ([T-O-T], where T = Si or Al) decreased gradually, suggesting that the bonds were severely altered or even destroyed. In Si-rich glasses, compression induced a transformation of Qn species to Qn−1. In the case of Al-rich glass, the Al in the smallest Qn units evolved from tetrahedral to higher-coordinated Al ([5]Al and [6]Al). Permanent structural changes were observed in samples recovered from the highest pressure of around 15 GPa and, particularly for Si-rich samples, the recovered structure showed an increase of three-membered rings in the Si/Al tetrahedral network.

Petrology, mineralogy, and oxygen isotope compositions of aluminum-rich chondrules from CV3 chondrites

Bulk major element composition, petrography, mineralogy, and oxygen isotope compositions of twenty Al-rich chondrules (ARCs) from five CV3 chondrites (Northwest Africa [NWA] 989, NWA 2086, NWA 2140, NWA 2697, NWA 3118) and the Ningqiang carbonaceous chondrite were studied and compared with those of ferromagnesian chondrules and refractory inclusions. Most ARCs are marginally Al-richer than ferromagnesian chondrules with bulk Al2O3 of 10–15 wt%. ARCs are texturally similar to ferromagnesian chondrules, composed primarily of olivine, pyroxene, plagioclase, spinel, Al-rich glass, and metallic phases. Minerals in ARCs have intermediate compositions. Low-Ca pyroxene (Fs0.6–8.8Wo0.7–9.3) has much higher Al2O3 and TiO2 contents (up to 12.5 and 2.3 wt%, respectively) than that in ferromagnesian chondrules. High-Ca pyroxene (Fs0.3–2.0Wo33–54) contains less Al2O3 and TiO2 than that in Ca,Al-rich inclusions (CAIs). Plagioclase (An77–99Ab1–23) is much more sodic than that in CAIs. Spinel is enriched in moderately volatile element Cr (up to 6.7 wt%) compared to that in CAIs. Al-rich enstatite coexists with anorthite and spinel in a glass-free chondrule, implying that the formation of Al-enstatite was not due to kinetic reasons but is likely due to the high Al2O3/CaO ratio (7.4) of the bulk chondrule. Three ARCs contain relict CAIs. Oxygen isotope compositions of ARCs are also intermediate between those of ferromagnesian chondrules and CAIs. They vary from −39.4‰ to 13.9‰ in δ18O and yield a best fit line (slope = 0.88) close to the carbonaceous chondrite anhydrous mineral (CCAM) line. Chondrules with 5–10 wt% bulk Al2O3 have a slightly more narrow range in δ18O (−32.5 to 5.9‰) along the CCAM line. Except for the ARCs with relict phases, however, most ARCs have oxygen isotope compositions (>−20‰ in δ18O) similar to those of typical ferromagnesian chondrules. ARCs are genetically related to both ferromagnesian chondrules and CAIs, but the relationship between ARCs and ferromagnesian chondrules is closer. Most ARCs were formed during flash heating and rapid cooling processes like normal chondrules, only from chemically evolved precursors. ARCs extremely enriched in Al and those with relict phases could have had a hybrid origin (Krot et al. 2002) which incorporated refractory inclusions as part of the precursors in addition to ferromagnesian materials. The occurrence of melilite in ARCs indicates that melilite-rich CAIs might be present in the precursor materials of ARCs. The absence of melilite in most ARCs is possibly due to high-temperature interactions between a chondrule melt and the solar nebula.

The effect of H2O on F and Cl solubility and solution mechanisms of in aluminosilicate melts at high pressure and high temperature

Effects of H 2 O on the solution behavior of fluorine and chlorine in peralkaline sodium aluminosilicate glasses quenched from melts at high temperature (1400 °C) and pressure (1.5 GPa) were studied by combining solubility measurements and Raman spectroscopy. With increasing H 2 O content from 0 to ~10 wt%, the fluorine solubility increases from 3.3 to 4.4 mol% in Al-free glasses and from 6.3 to 9.3 mol% in Al-rich glasses (10 mol% Al 2 O 3 ). In contrast, in the same H 2 O concentration range the chlorine solubility decreases from 5.7 to 3.4 mol% in Al-free glasses and from 3.6 to 1.7 mol% in Al-rich glasses. In Al-free glasses, interaction between H 2 O and the silicate to depolymerize the network is Q 4 + H 2 O ↔ Q 2 (H) and Q 3 + H 2 O ↔ Q 2 (H). The effect of water on silicate melt structure is different in halogen-bearing melts because in hydrous melt systems both F and Cl can act to depolymerize the melt further. For fluorine, this is accomplished via formation of Si-F, Al-F, and Na-F bonding in addition to Si-OH, whereas in chlorine-bearing hydrous melts, there is no interaction between Si 4+ and Cl − . The halogen solubility in the magmatic liquid influences mineral/melt partition coefficients of chlorine and fluorine and implies partition coefficients different from unity. Moreover because of the contrasting effects of H 2 O on fluorine and chlorine solubility, the Cl/F ratio in magmas formed in water-rich environments such as subduction zones can be a sensitive indicator of H 2 O content during arc magmas genesis. Transport properties of melts, such as diffusion and viscosity, also vary differently in halogen-bearing hydrous melts compared with halogen-free systems. Moreover, those effects on melt properties are the strongest in F-bearing systems.

Murchisite, Cr5S6, a new mineral from the Murchison meteorite

Murchisite (IMA 2010-003), Cr_5S_6, is a new chromium sulfide mineral, discovered in the Murchison CM2 meteorite. The type material occurs as one subhedral crystal (1.3 × 4 μm in size) in contact with low-Ni iron (“kamacite”), martensitic iron, schreibersite, and a Ca-,Al-rich glass, all of which are included in an isolated forsteritic olivine grain in the meteorite’s matrix. The mean chemical composition determined by electron microprobe analysis of the type material is (wt%) Cr 53.32, S 42.87, V 1.44, Fe 1.14, P 0.10, Ni 0.10, sum 98.97. The empirical formula calculated on the basis of 6 S atoms is (Cr_(4.60)V_(0.13)Fe_(0.09)Ni_(0.01))_(∑4.83)(S_(6.00)P_(0.01))_(∑6.01). Murchisite was also identified in another isolated olivine grain from the same meteorite. These crystals are subhedral to round in shape, 300 nm to 1 μm in size, and occur in association with tochilinite and serpentine, within which it is included, chromite, and eskolaite. Its electron backscatter diffraction patterns are an excellent match to that of synthetic Cr_5S_6 with the P31c structure, showing a = 5.982, c = 11.509 A, V = 356.67 A^3, and Z = 2, based on previously published data from synthetic material. Murchisite is named for the locality (the Murchison meteorite). It is a low-temperature phase (~327 °C in the Cr-S system), probably formed from higher temperature Cr_(1−x)S exsolved or expelled from a Cr-,S-bearing, metal-rich spherule included in forsteritic olivine grains that were probably derived from chondrule fragments. The formation of this phase reflects sulfur fugacities intermediate between those that would have led to troilite and those that would have led to no sulfide at all. The high-temperature Cr_(1−x)S phase equilibrated with coexisting alloys to ~600 °C. Murchisite formed from this precursor at low temperatures (<~300 °C) through ordering of S vacancies.

Electrophoretically deposited alumina as protective overlay for thermal barrier coatings against CMAS degradation

TBCs are increasingly susceptible to degradation by airborne CMAS deposits. In order to mitigate the CMAS attack, we fabricated a dense, crack-free alumina overlay for TBCs by electrophoretic deposition (EPD) technique. Overlay coatings of controlled thickness were successfully fabricated for YSZ TBCs, by EPD followed by controlled sintering at 1200 °C. YSZ with alumina overlay coatings were tested for CMAS attack at 1300 °C. Detailed examination of microstructural changes and phase evolution in CMAS tested specimens was performed by X-ray diffraction and electron microscopy. Dense alumina overlay produced by EPD was found to physically suppress the infiltration of CMAS. Furthermore, CMAS was found to crystallize into anorthite (CaAl 2Si 2O 8) and MgAl 2O 4 spinel by chemically interacting with EPD α-Al 2O 3. A shift in CMAS glass composition to a crystallizable Al-rich glass composition promoted the formation of anorthite platelets (CaAl 2Si 2O 8) and localized enrichment of Mg promoted the formation of MgAl 2O 4 spinel. EPD alumina overlay on commercial-production TBCs retained its adhesion and structural integrity after 20 cycles of 1-hour furnace thermal cycle test at 1100 °C.

Structure−Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 − x)P2O5:70SiO2

New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.

Chemical composition of glass and crystalline phases in coarse coal gasification ash

A procedure has been developed for determining the chemical composition and relative abundance of the amorphous or glassy material, as well as crystalline phases, present in coarse coal gasification ash, in order to assist in predicting the behaviour of the material in cement/brick/concrete applications. The procedure is based on a combination of quantitative X-ray diffraction (XRD), chemical analysis and electron microprobe studies. XRD analysis indicates that the clinker samples contain a number of crystalline high temperature phases, including anorthite, mullite, cristobalite, quartz and diopside. Quantitative evaluation using Rietveld-based techniques has been used to determine the percentages of both the individual crystalline phases and the glass component. These data were then combined with the chemistry of the crystalline phases and the overall chemical composition of the ash to estimate the chemical composition of the glass phase, which is typically the most abundant component present in the different materials. Although there is some degree of scatter, comparison between the inferred glass composition from XRD and bulk chemistry and actual data on the glass composition using electron microprobe techniques suggest that the two approaches are broadly consistent. The microprobe further indicates that a range of compositions are present in the glassy and crystalline components of the ashes, including Si–Al-rich glass, metakaolin and Fe–Ca–Mg–Ti phases, as well as quartz, anorthite and an aluminophosphate material. Electron microprobe and XRD studies also show that pyrrhotite (FeS), representing a high temperature transformation product of pyrite, is present in some clinker and partially burnt carbonaceous shale samples.

The metasomatic record in the shallow peridotite mantle beneath Grenada (Lesser Antilles arc)

The composition and geochemical signatures of the mantle wedge beneath the Lesser Antilles arc are documented by the ultramafic xenoliths included in alkali basalts (M-series) on Grenada. Xenoliths consist of harzburgites, lherzolites, dunites and subordinate wehrlites and pyroxenites. Primary minerals phases are olivine, low-Al and high-Al orthopyroxene, clinopyroxene and Cr-Spinel. In addition to the primary assemblage, Grenada xenoliths contain metasomatic phases such as Al-rich clinopyroxene, plagioclase, Al-rich spinel, pargasitic amphibole and Si- and Al-rich glasses. The trace-element signatures of pyroxenes and glasses have been determined on selected samples by LA-ICP-MS. Pyroxenes from both lherzolite and harzburgite xenoliths have U-shaped rare earth element (REE) profiles, unusually high Th, U and Sr concentrations and large negative Nb, Ta and Zr, and Hf anomalies. The geochemical signatures of metasomatic clinopyroxene are different from those reported for clinopyroxene from fluid-metasomatised mantle wedge, and are clearly distinct from those of clinopyroxene in equilibrium with host lavas. Si-rich glasses show a narrow compositional range, with trace-element characteristics closely similar to those of reacted pyroxenes. This, along with the general lack of chemical gradients of LILE and LREE over more compatible elements suggests dacitic glasses represent the products of in-situ melting caused by temperature increase before and during the uptake of xenoliths by host lavas. Dacitic melts are believed to represent local re-melts of regions metasomatically enriched by earlier arc magmas that had stalled, fractionated, and solidified in the upper mantle. These local re-melts thus reflect the metasomatic component formed by earlier arc-related metasomatic agents and liable to be re-mobilised. This also appears to be the easiest way to explain the compositional similarities between erupted arc lavas and the metasomatised peridotites. The results of this study suggest that the mantle wedge beneath the Lesser Antilles underwent complex peridotite–melt reaction processes operated by sub-arc melts and, later on, by magmas similar in compositions to the host alkali basalts. The majority of the compositional range of erupted Grenada magmas, but adakites found at surface, can be obtained by the interaction of basalts, possibly formed by hydrous melting of MORB-source mantle, with the overlying mantle wedge.

Stability of Al-rich glasses in the Al–La–Ni system

The stability of Al-rich glasses in the Al–La–Ni ternary system has been measured. Glasses with critical thicknesses ranging from 270 to 780 μm have been achieved. An in-situ two-phase amorphous region containing as much as ∼10 vol.% of nanocrystals was also observed with critical thicknesses ranging from 420 to 950 μm. Measurements of T g, T x and T ℓ, as well as other empirical measures of thermal stability based on these quantities, confirm the unusual thermal stability of these glasses. These results suggest that bulk Al-based glasses with a maximum critical thickness exceeding 1 mm may be possible in alloys based on this system.

Extension and Representation of the Glass Forming Region in the Nd-Si-Al-O-N System

Recent work has indicated that the glass-forming region in the Y-Si-Al-O-N system could be more extensive than originally reported, particularly in the Al-rich part of the system. With knowledge gained from the Y-Si-Al-O-N system and bringing together published behaviour diagrams for the Nd-Si-Al-O-N system, a further study of the latter system was undertaken. The present work outlines the extension of the glass-forming region to Al-rich glasses compositions in the Nd-Si-Al-O-N system.

Application of high precision SIMS [formula omitted] analyses to the early solar system chronology

High precision SIMS 26 Al– 26 Mg isotopic analyses were performed in order to detect the excesses of 26 Mg that derived from in situ decay of 26 Al (half-life of 0.73 million years) in primitive meteorites. The electron multiplier is used to analyze Mg isotopes in Al-rich glass using 5 μm O 2 − primary ion beam. The EM dead time correction and Al/Mg sensitivity calibration are applied to obtain accurate data. The precisions of 26 Mg/ 24 Mg isotopic ratios and 27 Al/ 24 Mg atomic ratios were 2–5‰ and 10%, respectively. The relative formation time of less than 1 million years among chondrules is able to be determined. Further attempt was made to improve the precision of Mg isotopic ratios using the multi-collector Faraday cups. The preliminary tests showed both internal and external precisions of better than 0.1‰.

Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182, and El Djouf 001

Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites are chemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties of the glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) and its composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz). All glasses have chondritic CaO/Al 2O 3 ratios, unfractionated CI-normalized abundances of refractory trace elements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of a primitive condensate origin whose chemical composition has been established before chondrule formation and accretion, rather then the product of either crystal fractionation from chondrule melts or part melting of chondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase. Because they have Al 2O 3 contents and trace element abundances very similar to those of the Al-rich glasses, they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchange reactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe 2+, Mn 2+, and Cr 3+). Glasses formed contemporaneously with the host olivine. As the most likely process for growing nonskeletal olivines from a vapor we consider the VLS (vapor-liquid-solid) growth process, or liquid-phase epitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Such liquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition.

Conversion of Al 2 O 3 –SiO 2 powder mixtures to 3:2 mullite following the stable or metastable phase diagram

A model system, composed of powder blends of amorphous isomorphic silica spheres, being 500 nm in diameter, and also monosized crystalline α-Al 2 O 3 powder, was investigated. Two different particle sizes of the corresponding alumina powder were employed: 300 nm and 2 μm. This particular assembly enabled a distinction between amorphous silica and crystalline alumina merely by their difference in particle morphology. The powder blends were sintered at temperatures between 1400 and 1700 C and the microstructure evolution was characterized by scanning (SEM) and transmission electron microscopy (TEM). It is worth noting that upon annealing at 1700°C, both microstructures were indistinguishable. However, depending on the Al 2 O 3 particle size, different conversion mechanisms were monitored. When using the 300 nm Al 2 O 3 powder, fast dissolution of alumina into the coalesced silica glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 μm Al 2 O 3 particles, however, resulted in the formation of two Al-containing glasses (phase separation into a Si- and Al-rich glass). In this case, the transformation to mullite can be rationalized by the conversion of the metastable Al-rich transient glass into mullite, which forms an epitaxial, single crystalline coating on the host Al 2 O 3 particle. Therefore, depending on the initial Al 2 O 3 particle size, mullite formation follows either the stable or metastable phase diagram.

A neutron diffraction study of Al-rich glasses

Incoherent, multiple, inelastic and transmission corrections in Al 8.5Y x Ni 15− x glasses are negligible, making them very well-suited for quantitative structural studies by neutron scattering. Differential scanning calorimetry has shown that there is an abrupt change in the mode of crystallization when the Y/Ni ratio equals to one. For x⩽7.5 crystallization proceeds by a pure growth process, while for x>7.5, nucleation as well as growth are involved in the crystallization process. This suggests a distinct change in the configurational short-range order of these glasses. Neutron diffraction results indicate that Y-rich glasses show exceptionally long short-range order (12–15 Å for x = 10) and homogeneous glass nearest-neighbor distances. Y-poor glasses show a decreased short-range order (about 8 Å for x=5) with nearest-neighbor distances indicative of compositional inhomogeneity.

Strategies for the development of nanocrystalline materials through devitrification

The crystallization of metallic glass through devitrification reactions can yield ultrafine nanocrystalline product structures. The recent discovery of Al-rich glasses containing85 at.% Al and a combination of transition and rare earth element additions has yielded microstructures of Al nanocrystals in an amorphous matrix with nanocrystal volume fractions approaching 20% and excellent mechanical properties. Similar behavior is reported for Fe-based alloy glass systems such as Fe-Nb-B, and Fe-Si-B. This characteristic microstructure is synthesized by a primary crystallization reaction that yields a density of > 10 20 m −3 nanocrystals and limited growth. The systematic control of the nanocrystal development can be addressed by strategies based on thermodynamic and kinetic factors. The first issue is the initial glass formation which may be approached on the basis of the Egami size factor criteria. However, this must be applied to the amorphous phase that coexists with the nanocrystals following primary crystallization based on metastable phase boundaries. The growth control limitation has been identified to arise from diffusion field impingement. This control is enhanced for compositions that yield the highest initial particle densities for a given nanocrystal volume fraction. For example, the addition of transition elements to the Al-based glasses initially enhances the range of glass-forming conditions as the solute level increases, but excess levels diminish the driving force for fcc nanocrystal formation while increasing the driving force for intermetallic formation. The kinetic strategies also indicate processing directions to promote and retain a high density of nanocrystal dispersions including the possible utilization of sequential intermetallic crystallization reactions to modify the nanocrystal-amorphous matrix stability. The alloying strategies to promote and retain a high density of nanocrystal dispersions are quite general and may be applied to any system exhibiting primary nanocrystal formation through devitrification, including the Fe-based FINEMET alloys.

EXAFS study of the coordination environment of aluminum in a series of silica-rich glasses and selected minerals within the Na 2OAl 2O 3SiO 2 system

Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na 2OAl 2O 3SiO 2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61. Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.