Al Enrichment Research Articles

Early-Stage Oxidation Behavior of γ'-Ni<sub>3</sub>Al-Based Alloys with and without Pt Addition

The early-stage oxidation behavior of γ '-Ni3Al-based alloys of composition (in at.%) Ni-22Al and Ni-22Al with 10, 20, and 30Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150°C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the γ '-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al2O3 scale during heating the γ '-based alloys through to about 700°C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in γ '. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L12 crystal structure of γ ', which has the effect of increasing the Al:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ ' surface would kinetically favor the formation of Al2O3 relative to NiO. A further contributing factor is that the Pt-containing γ '-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment would reduce Ni availability and increase the Al supply to the evolving scale, thus kinetically favoring Al2O3 formation.

Influence of Ca on Corrosion Resistance of AZ91D

By adding 3% Ca in AZ91D, the reticular phase on grain boundaries formed and the corrosion rate decreased to 14.1% of the AZ91D corrosion rate in NaCl solution. The corrosion morphologies showed that the reticular phase effectively confined the development of corrosion from grain to grain and hence significantly reduced the corrosion rate. The Auger depth profiles indicated the corrosion films on Ca had a three-layered structure: a thick Al enrichment outer layer, an intermediate layer, and a thin protective inner layer. The thin inner layer should consist of an phase, which acted as an effective corrosion barrier.

Interface formation in infiltrated Al(Si)/diamond composites

This study pertains to the investigation of interface formation in infiltrated Al-based composites with high-volume fractions of monocrystalline synthetic diamond particles. The interface characteristics are discussed with respect to process conditions and Al matrix chemistry. To this end, two infiltration techniques, i.e., squeeze-casting and gas pressure infiltration are compared and the effect of Si-addition to the Al matrix is addressed. Eventually, thermal properties of the composite materials are presented and are in turn related to the interface characteristics. Electron microscopy investigations reveal a distinct 50–200-nm-thick layer at the interface between the metal matrix and the diamond particle, regardless of process history and matrix chemistry. This layer is amorphous and consists of carbon, aluminium and oxygen. Additionally, nanocrystallites of Al 2OC and enrichment of Si are observed within this interface layer in the Si-free, squeeze-casted material and in the gas pressure-infiltrated material with 7 wt.% of Si, respectively. Despite the fact that no evidence of SiC is found in the Si-containing composites, the process conditions experienced in gas pressure infiltration are clearly more favourable than those experienced in squeeze casting with respect to interfacial bonding and thermal properties. Actually, between 25 and 50 °C, the gas pressure-infiltrated AlSi/diamond composite yields a thermal conductivity of 375 W/m K along with a coefficient of thermal expansion of 7 × 10 − 6 /K.

A High-Resolution Photoemission Study of Nanoscale Aluminum Oxide Films on NiAl(110)

Using high-resolution soft X-ray photoemission, Al 2p, we have been able to quantify the relative populations of tetrahedrally (Al(tet)) and octahedrally (Al(oct)) coordinated Al(3+) in three distinct phases of nanoscale aluminum oxide films on NiAl(110). We have hence determined the bulk alumina phases that the nanoscale films most resemble. Adsorption of oxygen at room temperature produces a layer which predominately (90%) contains Al(tet) and is analogous to the amorphous bulk phase of alumina. Annealing this layer results in an Al enrichment of the oxide layer, through the diffusion of metal from the substrate, and an increase in the relative amount of Al(oct), producing a gamma-alumina-like layer with a relative Al(oct)/Al(tet) occupancy of 28 +/- 3%/72 +/- 3%. Oxygen adsorption at 823 K also produces a gamma-like phase, with a relative Al(oct)/Al(tet) occupancy of 27 +/- 3%/73 +/- 3%, although this layer is thicker than that formed at room temperature. Both oxidation methods produce gamma-alumina layers that display poor translational order. However, these poorly ordered layers have a relative Al(oct)/Al(tet) occupation similar to that of well-ordered oxide films produced using different oxidation conditions in previous studies. Both gamma layers undergo partial decomposition upon annealing to 1273 K, producing an alpha-alumina-like oxide, which contains only Al(oct), and is highly deficient in Al. There are significant oxide-free areas within the alpha-alumina oxide layer, which is characteristic of crystallite formation. Repeated cycles of oxidation and annealing to 1273 K do not produce a homogeneous film, but they do make the alpha-like oxide more Al rich.

Al enrichment of carbon steels through weld overlay process for improved oxidation resistance

A simple method is described for enriching the surface aluminum concentration of carbon steel. The process of weld overlaying steel substrate with pure aluminum wire results in an aluminum concentration of ∼15–17 at.% (8–9 wt.%) for depths of ∼2 mm. The aluminum-enriched surface shows an improved oxidation and sulfidation resistance. The paper gives details of the process along with microstructural analysis and oxidation resistance improvement results.

Exchange capacity of soils, metal mobilization and soil pollution by heavy metals in West Bengal

Exchangeable and non-exchangeable (obtainable under stress conditions) capacities of K-ion (and of other ions like Na + , Ca 2 + , Mg 2 + ) and metal mobilization (Na, K, Ca, Mg, Ba, Mn, Fe, Cr, Ni, Cu, Zn) in soils from different regions of West Bengal were examined. The results show that the exchange capacities of selected soil samples are good. The exchangeable and non-exchangeable (obtainable under stress conditions) K-contents are fairly good [1.03-8.5% of the total K (exchangeable) and 4.3-18% of the total K (non-exchangeable)]. Hg is conspicuously absent. There is appreciable metal mobilization and the results are interpreted in terms of concentration factor F M and the enrichment factor EF. However, in view of very low Al contents (compared to natural abundance), EF values become abnormally high. The results in terms of F M values suggest depletion of Al (abnormal), Fe (fairly high), K and Mg (fairly high) and enrichment of Ba, Ni and Zn in most cases. There is no appreciable depletion or enrichment for Cu, Cr (except in Patharpratima) and Mn. Na concentrations are high. Bi content is also fairly high indicating the possibility of presence of As and Sb. Metal requirements for plants and microbes must be different from their macroscopic concentrations in soils and in absence of true pollutional limits of metals in soils, comparison of the results are difficult. However, metal mobilization cannot be regarded to be alarming in soils though the concentrations of some metals are on the increase.

Effects of Pt on the short-term oxidation behavior of γ-Ni+γ′-Ni3Al alloys

The aim of this study was to elucidate the beneficial role played by platinum addition in promoting the formation of a protective Al2O3 scale on γ′-Ni3Al+γ-Ni alloys during oxidation at high temperatures. To do this, the early-stage oxidation behavior of γ′-Ni3Al-based alloys of composition (in at.%) Ni–22Al and Ni–22Al with 10, 20, and 30 Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150°C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the γ′-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al2O3 scale during heating above about 600°C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in γ′. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L12 crystal structure of γ′, which has the effect of amplifying the increase in the Al:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ′ surface kinetically favors the formation of Al2O3 relative to NiO. A further contributing factor is that the Pt-containing γ′-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment reduces Ni availability and can increase the Al supply to the evolving scale, thus kinetically favoring Al2O3 formation.

Growth mechanism of incongruently melting terbium aluminum garnet (Tb 3Al 5O 12; TAG) single crystals by laser FZ method

Of all paramagnetic transparent materials, terbium aluminum garnet (Tb 3Al 5O 12; TAG) has the largest Verdet constant in visible and near infrared lights and can be applied to Faraday rotation devices for lasers in these wavelengths. Recently we successfully grew incongruently melting TAG single crystals using a hybrid laser floating zone method. Atomic concentration ratio [Tb]/[Al] of the grown crystals was measured by the electron probe microanalysis using the wavelength dispersive spectrometer to study the growth mechanism. The initially grown part of the growth was found to be Tb enriched compared to the TAG crystal composition. Contrary, Al enrichment was detected at the part where the growth was terminated instantaneously. This suggested that terbium aluminum perovskite (TbAlO 3; TAP) precipitated at the beginning of the growth stage, then a TAG single crystal was grown through the residual self-adjusted liquid phase, just as seen in the yttrium iron garnet (Y 3Fe 5O 12; YIG) growth.

Feasibility assessment of self-grading metallic bond coat alloys for EBCs/TBCs to protect Si-Based ceramics

A sputtered γ (TiAl) + Laves TiAlCr coating formed an adherent Al 2 O 3 scale on SN 282 Si 3 N 4 when oxidized. A Ti–N rich layer was formed at the TiAlCr–Si 3 N 4 interface, which effectively resulted in beneficial local Al enrichment underneath the growing Al 2 O 3 scale, rather than detrimental Al loss to the substrate.

Geochemical characteristics of the slip zones of a landslide in granitic saprolite, Hong Kong: implications for their development and microenvironments

Geochemical investigations of the slip zones of a landslide in granitic saprolite revealed that they have signatures distinct from their host materials. These distinctions include stronger Si depletion, higher Al enrichment, greater LOI, significant fixations of Mn, Ba and Ce, stronger negative Eu anomalies, and greater accumulations of other rare earth elements (REE). Altogether, these geochemical characteristics indicate that: (a) the slip zones have greater abundance of clays, consistent with field and microscopic observations; (b) concentration of clay size particles within the slip zones may have been from downward leaching and deposition, and lateral transportation of Al-Si solutions and colloids through pores and fractures within the saprolite; and (c) there were prevailing oxidation and poor drainage, and occasional reduction conditions within the slip zones. It was concluded that geochemical analyses could be effective in gathering clues for understanding the development and nature of slip zones in landslide investigations.

Acicular iron nanoparticles protected against sintering with aluminium oxide

Acicular iron nanoparticles have been obtained by thermal reduction with hydrogen of a goethite precursor protected against sintering with Al cations, either by doping during the synthesis or by a further coating and the relation between microstructure and magnetic properties of the final Fe particles has been studied in order to evaluate the efficiency of both protecting methods. Uniform goethite and Al-doped goethite precursors were prepared by oxidation with air of FeSO 4 solutions, containing Al(NO 3 ) 3 when required, previously precipitated with Na 2 CO 3 , while the Al oxide coating on the undoped goethite precursor was carried out by heterocoagulation. In both protecting methods, Al cations were mainly concentrated in the particle outer layers of the goethite precursors and the final iron. Due to this Al enrichment, the growth of iron crystals during the reduction process is minimised resulting Fe particles with improved coercivity and squareness. Nevertheless, the coating procedure seems to be more effective giving rise to the iron particles with the largest coercivity, probably due to a better preservation of the acicular morphology in this case.

Compositional shift in AlxGa1−xN beneath annealed metal contacts

Interfacial reactions between Ni, Pd, or Au contacts and AlxGa1−xN with compositions x=0.15 and 0.47 were studied in this investigation. X-ray photoelectron spectroscopy and scanning transmission electron microscopy coupled with energy dispersive spectroscopy showed that reaction between these metals and AlxGa1−xN results in a pronounced shift in the AlxGa1−xN beneath the contact to an Al-enriched composition. This Al enrichment results from the preferential outdiffusion of Ga into the metal films, and this reaction is consistent with thermodynamic driving forces in these systems. Possible consequences of the observed compositional shift on the electrical properties of contacts to AlxGa1−xN are also discussed.

Peculiarity and defect structure of the natural and synthetic zeolite mordenite: A single-crystal X-ray study

Single-crystal X-ray data were collected from a natural fibrous mordenite crystal of composition K2.99Ca1.85Na1.06Al7.89Si40.15O96⋅28H2O and from a platy synthetic mordenite crystal of composition Na6Al6.02Si42.02O96⋅19H2O. Diffraction data were measured with a point detector using a sealed X-ray tube and an image plate using synchrotron radiation, respectively. Both structures exhibit the same defect features visible in difference-Fourier maps. Domains of the entire Cmcm framework structure are reproduced by a non-crystallographic (001) mirror plane at z = 0 and z = 1/2. An identical description is a shift of framework domains 1/2 along the c axis. The concentration of this defect domain is 2.7(2) and 3.1(1)% for the natural and synthetic mordenite crystals, respectively. Reproductions of reciprocal layers from synchrotron image-plate data reveal diffuse scattering for hkl layers with l = 2n + 1. The diffuseness of these layers is not homogeneous but concentrates in the form of halos around selected reflections allowed for C-centering. Diffuse features in electron diffraction patterns of natural and synthetic mordenite have been described before and were interpreted either as evidence of c/2 faults or intergrowth with different mordenite-related structure-types. We have modeled a (100) defect layer that is modified from the mordenite characteristic puckered sheet of six-membered rings and allows coherent intergrowth of identical structural subunits shifted by c/2. These defect domains do not influence or obstruct the 12-membered ring-channels characteristic of this zeolite. The major difference in Si,Al distribution between the two samples is that the natural crystal has Al strongly enriched at T3, which is part of the four-membered rings. We suggest that a synergetic effect between extraframework cations and Si, Al ordering during crystal growth is responsible for Al enrichment in natural mordenite with ca. 2 Ca p.f.u. close to T3.

The effect of metasomatism on the Cr-PGE mineralization in the Finero Complex, Ivrea Zone, Southern Alps

The magmatic metasomatism that was responsible for producing chromitite–dunite bodies in the unusual phlogopite peridotite of the Finero Complex in Permian to Triassic times also influenced the Cr-platinum group elements (PGE) mineralization. At least the end stages of this metasomatism are recorded in compositional zoning of chromite grains in the podiform chromitite. Metasomatic melt, with or without vapor, reacted with chromite to produce core-to-rim Cr enrichment of extant chromite grains and was concurrent with pyroxene crystallization. Under conditions of lower melt/rock ratio, metasomatism resulted in core-to-rim Al enrichment in chromite and crystallization of amphibole between chromite and clinopyroxene. This early, high-temperature metasomatism is unrelated to the later and pervasive K-metasomatism that crystallized phlogopite and was associated with the intrusion of clinopyroxenite dikes that cut the peridotite. Much later, serpentinization of olivine locally depleted chromite in Al and enriched it in Fe and formed minor amounts of magnetite. The PGE, which are present mainly as laurite inclusions in chromite, were remobilized by the early metasomatism. This resulted in substantial variation in the PGE contents of chromitites and imposed a characteristic PGE pattern in which chondrite-normalized Os, Ir, Ru and Rh contents are high but Pt and Pd contents are low. The slopes of PGE chondrite-normalized concentration patterns are systematically related to absolute PGE abundance and to rock mode. Chromitites with low modal orthopyroxene, clinopyroxene, and amphibole exhibit negative PGE slopes and contain relatively high PGE concentrations, whereas chromitites rich in these silicate minerals have positive slopes and low PGE contents.

Micrometeorites from the northern ice cap of the Novaya Zemlya archipelago, Russia: The first occurrence

Abstract— Glacial deposits at the margins of the ice cap of the northern island of the Novaya Zemlya archipelago, Russia, contain numerous spherules and rare scoriaceous particles thought to be extraterrestrial. The 1 Kyr old glacier has decreased in volume and coverage during the last 40 years, leaving the spherules contained in the ice at the margins of the glacier where they can be easily collected.The spherules are similar in their appearance, texture, and mineralogy to cosmic spherules found in deep‐sea sediments in Greenland and Antarctica. Silicate spherules have typical bar‐like textures (75%) or porphyritic textures (15%), while other spherules are glassy (7%). The spherules from Novaya Zemlya are altered only slightly. There are spherules consisting of iron oxides, metal cores with iron oxide rims, a continuous network of iron oxide dendrites in a glass matrix, and particles rich in chromite (3%). Some spherules contain metal droplets and relict forsterite and low‐Ca pyroxene. Silicate spherule compositions match compositions of other cosmic spherules. Both Nova Zemlya and other cosmic spherules are close to carbonaceous chondrite matrices in patterns of variations for Ca, Mg, Si, and Al, which might suggest that their predecessor was similar to carbonaceous chondrite matrices. Unmelted micrometeorites are generally depleted in Ca and Mg and enriched in Al relative to cosmic spherules. The depletion of the micrometeorites in Ca and Mg can be connected with their terrestrial alteration (Kurat et al. 1994), while the Al enrichment seems to be primary.

The Effect of Sulfur Segregation on the Adherence of the Thermally-Grown Oxide on NiAl—I: Sulfur Segregation on the Metallic Surface of NiAl(001) Single-Crystals and at NiAl(001)/Al2O3 Interfaces

The aim of this study was to improve the understanding of the deleterious effect of sulfur impurities on the adherence of the thermally-grown oxide on the boundary layer in thermal-barrier-coating systems. In Part I, the sulfur segregation on the free surface of NiAl(001) and at different interfaces between metal and transient alumina scales has been characterized by AES, XPS and LEED. The sulfur diffusion coefficient in the alloy has been determined (D = 0.15 exp(−218,000/RT) cm2/s). It is by three orders of magnitude larger than the nickel and aluminum self-diffusion coefficients. It has also been observed that the sulfur de-segregates upon Al enrichment of the metallic surface. The saturation of the metallic surface with an amorphous alumina layer formed at room temperature blocks the segregation of S. However, in the initial stages of oxidation where the transient θ-alumina grows by cationic transport and inject vacancies at the interface, S segregates at the interface between the alumina thin films and the metallic substrate.

MINERALOGY AND ORIGIN OF SPOTS IN SPOTTED SLATE FROM THE MALAGUIDE COMPLEX, BETIC CORDILLERAS, SPAIN: AN XRD, EMPA AND TEM-AEM STUDY

We have studied the nature of spots occurring in spotted slates from the Malaguide Complex, in the Betic Cordilleras of southern Spain, by optical microscopy, X-ray diffraction, electron-microprobe analysis and transmission electron microscopy. These spots developed during an episode of contact metamorphism after the Alpine regional event. They are optically and mineralogically zoned, the extent of the zones being dependent upon both the composition of the slates (or of the different bands in these) and the degree of spot development. The mineral zonation is clearly correlated with the chemical variations, as revealed by electron-microprobe analyses obtained across the spots. In the best-developed spots, an inner isotropic zone, characterized by Al enrichment relative to the slate, is followed by an intermediate quartz- or mica-rich zone showing a relative enrichment in either Si or in Si, K, and Na; an outer yellow to reddish zone shows a notable increase in Fe. TEM–AEM studies indicate that the innermost isotropic zone mainly consists of “amorphous” Si–Al or Si–Al–Fe phases that contain numerous phyllosilicate inclusions. Two main phyllosilicate associations have been identified in the spots: muscovite + berthierine ± chlorite and muscovite + biotite + chlorite. The first assemblage is characterized by the development of Al-poor oxidized berthierine, which preferentially developed in Fe-rich spots, whereas the second association is better developed in micaceous spots. Even where both assemblages coexist in a single spot, they are restricted to different microdomains. Textural features indicate that berthierine formed as a product of the reaction of both chlorite and muscovite. The first transformation occurred through disordered polytypes of chlorite and involved a notable Fe enrichment relative to the original chlorite. In contrast, formation of berthierine from muscovite occurred through a complex set of muscovite–berthierine interstratifications and intergrowths. AEM data for biotite and berthierine suggest that the availability of Fe may be an important factor controlling the formation of berthierine in spotted slate in the Malaguide complex.

Study of the aerodynamics of a circulating fluidized-bed reactor particularly for coal pyrolysis

Investigations into control methodologies for mitigating ash-related problems such as particle agglomeration and bed defluidisation during fluidised-bed combustion of low-rank coals are conducted using a laboratory scale spouted bed combustor. Two control methods are investigated viz., the use of alternative bed materials and pretreatment of coal. Bauxite and calcined sillimanite are used as alternative bed materials in the spouted bed combustor while burning a South Australian low-rank coal. Samples of the same coal subjected to Al pretreatment, water washing and acid washing are also tested. Experimental results indicated that both methods are effective to different extents in reducing particle agglomeration and bed defluidisation. Tests with calcined sillimanite and bauxite as bed materials showed no agglomeration for longer periods than with sand runs at the same bed temperatures. Al pretreatment and water-washing were also found to be effective and resulted in extended combustion operation. Al enrichment in ash coating on bed particles has been identified as the key mechanism for prevention of agglomeration and defluidisation by these control methodologies. For water-washing, the principal reason behind agglomeration and defluidisation control is the reduction in sodium levels.

Cumulatic Diorites Related to Post-Collisional, Brasiliano/Pan-African Mafic Magmatism in the Vila Nova Belt, Southern Brazil

The Capivaras Diorite, in the Vila Nova region, NW of the Sul-rio-grandense Shield, is composed of six NE- to NNE-oriented rock bodies of late-tectonic emplacement relative to the D3 deformation phase which forms subvertical high-strain zones in basement gneiss sequences. Within these intrusive bodies, a shape foliation is present, generally parallel to contacts and displaying a local solid-state deformational component. The internal structure of the Capivaras Diorite main intrusion is marked by a zone of intense flow and mingling, characterized by strong shape foliation and layers of variable texture and composition, which result from cumulative processes, heterogeneous flow and interaction of coeval, compositionally contrasting magmas. The central part of this intrusion is texturally homogeneous and slightly foliated, even though cumulative processes have remained important during its formation. Along the contact with basement gneisses, fine-grained diorites are found, which are considered to be compositionally close to the parental magma of the Capivaras Diorite. This magma has a mildly alkaline affinity and shows moderate to high contents of Zr, Ti and P. Highly-fractionated REE patterns, low Nb contents, as well as high contents of K, Sr, Ba, and Rb, are suggestive of its provenance from mantle sources which have been previously affected by subduction processes, such as those of mature magmatic arcs or post-collisional settings. Magmatic evolution was controlled by cumulative processes and gave origin to pyroxene orthocumulates, plagioclase-pyroxene orthocumulates, pyroxene adcumulates, and more rarely plagioclase adcumulates. The cumulative origin of these rocks is indicated by field, textural and geochemical features, which are distinct from those of crystallized liquids. The compositional diversity of cumulates has led to the generation of compositionally different melts. The early-formed pyroxene cumulates have caused Ca, Al, Na, Ba, and Sr enrichment in the magmatic liquid, leading to plagioclase crystallization and accumulation. Coarse-grained mafelsic cumulates were formed during the late stages of magmatic crystallization, due to volatile enrichment of the intercumulus liquid. Considering geological relations, as well as tectonic and compositional features of the Capivaras Diorite, it is interpreted as part of Neoproterozoic magmatism related to the post-collisional stage of Brasiliano/Pan-African Orogenic Cycle in southern Brazil.

Microstructural processes in creep of an AZ 91 magnesium-based composite and its matrix alloy

Constant stress tensile creep tests were conducted on an AZ 91–20 vol.% Al 2O 3 short fiber composite and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material is shown to be considerably improved compared with the matrix alloy. The creep strengthening arises mainly from the effective load transfer between plastic flow in the matrix and the fibers. Microstructural investigations by TEM revealed good fiber–matrix interface bonding during creep exposure. The microstructures of the AZ 91 alloy and its composite were similar with regard to two types of β-phase precipitates; the enhanced precipitation of the Mg 17 (Al, Zn) 12 phase on the fibers is promoted by heterogeneous nucleation due to the Al enrichment of the matrix near to the alumina fibers.