Steep Compositional Gradients Research Articles

Microstructure transition gradients in titanium dissimilar alloy (Ti-5Al-5V-5Mo-3Cr/Ti-6Al-4V) tailored wire-arc additively manufactured components

The nature of the chemical mixing and microstructure gradients that occur across the interface transition, when manufacturing tailored components with the two high-performance dissimilar titanium alloys (Ti-6Al-4V (Ti-64) and Ti-5Al-5V-5Mo-3Cr (Ti-5553)) by the wire-arc additive manufacturing (WAAM) process, are reported. It has been shown that a relatively long-range chemical gradient occurs during the transition between layers produced with the two different titanium alloys, due to convective mixing in the melt pool between the substrate layers and new alloy wire. This resulted in a stepwise exponential decay composition profile normal to the layers, the width of which can be described by a simple dilution law, with steep local composition gradients seen within the boundary layers at the fusion boundary of each individual layer. The alloy-alloy composition gradients had little effect on the β-grain structure. However, they strongly influenced the transformation microstructure, due to their effect on the parent β-phase stability and the β → α transformation kinetics and reaction sequence. The microstructure gradient seen on transitioning from Ti-64 → Ti-5553 was significantly more abrupt, compared to when depositing the two alloys in the reverse order. Under WAAM thermal conditions, Ti-64 appears to be more sensitive to the effect of adding β-stabilising elements than when Ti-5553 is diluted by Ti-64, because at high cooling rates, stabilisation of the β phase readily suppresses α nucleation when cooling through the β transus, and the normal Ti-64 lamellar transformation microstructure is abruptly replaced by finer scale α laths generated by precipitation during subsequent reheating cycles.

Thermal evolution of Uranus and Neptune

Thermal evolution models suggest that the luminosities of both Uranus and Neptune are inconsistent with the classical assumption of an adiabatic interior. Such models commonly predict Uranus to be brighter and, recently, Neptune to be fainter than observed. In this work, we investigate the influence of a thermally conductive boundary layer on the evolution of Uranus- and Neptune-like planets. This thermal boundary layer (TBL) is assumed to be located deep in the planet and be caused by a steep compositional gradient between a H–He-dominated outer envelope and an ice-rich inner envelope. We investigate the effect of TBL thickness, thermal conductivity, and the time of TBL formation on the planet’s cooling behaviour. The calculations were performed with our recently developed tool based on the Henyey method for stellar evolution. We make use of state-of-the-art equations of state for hydrogen, helium, and water, as well as of thermal conductivity data for water calculated via ab initio methods. We find that even a thin conductive layer of a few kilometres has a significant influence on the planetary cooling. In our models, Uranus’ measured luminosity can only be reproduced if the planet has been near equilibrium with the solar incident flux for an extended period of time. For Neptune, we find a range of solutions with a near constant effective temperature at layer thicknesses of 15 km or larger, similar to Uranus. In addition, we find solutions for thin TBLs of a few km and strongly enhanced thermal conductivity. A ~ 1 Gyr later onset of the TBL reduces the present ΔTby an order of magnitude to only several 100 K. Our models suggest that a TBL can significantly influence the present planetary luminosity in both directions, making it appear either brighter or fainter than the adiabatic case.

Predicting the Rate of Homogeneous Intermetallic Nucleation within Steep Composition Gradients

Simulations of isothermal homogeneous nucleation from deeply undercooled amorphous melts exhibit systematic variations in nucleation behavior depending upon the strength of an imposed composition g...

Characterization of internal and intergranular oxidation in Alloy 690 exposed to simulated PWR primary water and its implications with regard to stress corrosion cracking

Oxidation testing of Ni-base Alloy 690 was conducted in simulated PWR primary water, and the internal and intergranular (IG) oxidation layers were systematically characterized by analytical transmission electron microscopy to obtain insight into its IG stress corrosion cracking (IGSCC) resistance and behavior. The internal oxidation layer consisted of upper NixFe1-xCr2O4 and innermost Cr2O3. Each internal oxidation layer had a crystallographic orientation relationship with the Alloy 690 matrix, indicating that the layers formed through solid-state reactions between inward diffusing oxygen and the alloying elements. Strong evidence that the Cr2O3 layer formed through the outward grain boundary diffusion of Cr was found, leaving severe Cr depletion along the grain boundary with a steep composition gradient toward the surface. The innermost Cr2O3 layer had a continuous and compact band-shaped morphology around the surface grain boundary. This Cr2O3 layer was revealed to be protective of the inward grain boundary diffusion of oxygen from the surface and therefore responsible for the prevention of grain boundaries from IG oxidation. IG Cr carbides suppressed the internal and IG oxidation further. The key findings obtained in this work may present the main reason for the high resistance of Alloy 690 to IGSCC under PWR normal operating conditions.

Solidification behaviour and microstructure of welding transition zone using low-transformation-temperature welding consumables

The solidification behaviour and microstructure of welding transition zone between low-transformation-temperature deposited metals and high-strength low-alloy steels were investigated. It was found that the steep composition gradient provided driving forces for the diffusion of carbon from base metal to weld metal, leading to the hardened and softened regions near fusion boundary. In weld metal near fusion boundary, there were retained δ-ferrites when the base metal dilution rate below 35% and Creq/Nieq value larger than 2.62. Compared with martensite, the mixed microstructures of martensite + δ-ferrite obtained less strain localisation, dislocation density and more percentages of large misorientation, which were more liable to resist microcrack initiation and propagation during deformation.

Low heat input gas metal arc welding for dissimilar metal weld overlays part II: the transition zone

Dissimilar metal weld overlays (DMWOL) of nickel base alloys on low alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding. This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of dissimilar weld overlays. In order to evaluate the quality of these overlays regarding resistance against hydrogen-assisted cracking, their transition zone region is investigated in this part of the publication series. Metallurgical characterization, including energy-dispersive x-ray spectroscopy, is performed on Alloy 625/grade 22 steel overlays. The transition zone is characterized by a narrow planar growth zone and steep compositional gradients from the fusion boundary towards the weld metal. Evidence of low carbon contents in the planar growth zone, as well as for carbide precipitation in the cellular growth zone was found. The microstructure in the transition zone region of the fusion zone shows characteristics known to be suitable for good resistance against hydrogen embrittlement.

Ankerite grains with dolomite cores: A diffusion chronometer for low- to medium-grade regionally metamorphosed clastic sediments

Ankerite grains with dolomite cores occur in marls, pelites, and psammites from a Buchan terrain in Maine and a Barrovian terrain in Vermont (U.S.A.). Dolomite cores are typically ≤20 μm in diameter, have sharp but irregular contacts with ankerite, and have the same crystallographic orientation as ankerite rims. Ankerite grains with dolomite cores are common in the chlorite zone, less abundant in the biotite and garnet zones, and rare (Vermont) or absent (Maine) at higher grades. The texture and crystallographic orientation of dolomite and ankerite and the sharpness of the dolomite-ankerite contact are consistent with partial replacement of detrital dolomite by ankerite by solution-reprecipitation. Metamorphic biotite is in Fe-Mg exchange equilibrium with ankerite rims but not with dolomite cores, implying that ankerite did not form long after biotite (biotite has no phlogopite cores). Possible sources of iron for the formation of ankerite are reduction of ferric iron hydroxide or the smectite-to-illite reaction during diagenesis. The sharpness of the dolomite-ankerite contact is a diffusion chronometer that constrains timescales of metamorphic process. Relatively low spatial resolution analyses of Fe/Mg across the contact with a NanoSIMS instrument and a FEG TEM give upper bounds on the thickness of the transition from ankerite to dolomite of ~2 and ~0.5 μm, respectively. Higher resolution analysis of BSE grayscale contrast with a FEG SEM gives a thickness ~100 nm. Fit of the grayscale profile to a model of one-dimensional diffusion across an infinite plane gives Dt = 10^(−15) m^2 (± a factor of 5), where D is the effective Fe-Mg interdiffusion coefficient and t is the duration of diffusion. Using the published experimental determination of D, upper bounds on the residence time of ankerite grains with dolomite cores at peak T = 400–500 °C, on the duration of linear cooling from peak T to 100 °C, and on the duration of linear heating from 100 °C to peak T followed by linear cooling to 100 °C are all 100 °C. The question is what explains the occurrence of ultrasteep composition gradients between dolomite and ankerite. Regional metamorphism on a timescale of a year or less is unrealistic. No barrier to diffusion at the dolomite-ankerite contact was observed in TEM images. Post-metamorphic formation of ankerite at very low temperature is ruled out by Fe-Mg exchange equilibrium between biotite and ankerite but not dolomite. It is unlikely that the steep composition gradients were preserved by intracrystalline pressure gradients. Alternatively, the steep composition gradients would be consistent with timescales of metamorphic process ~10^6 years or longer if D values during metamorphism were approximately six orders of magnitude or more smaller than those measured in the laboratory. The error of measurement is much less, approximately ± a factor of 2. A correction to D for the difference in P between measurements (0.1 MPa) and metamorphism (350–800 MPa) is likely an order of magnitude or less. Oxygen activity (a_(O2)), however, was 17–20 orders of magnitude larger during the laboratory measurements than during metamorphism. A correction to measured D for the difference in a_(O2_ between experiment and metamorphism appears to be the likeliest way to reconcile the steep composition gradients with realistic timescales of metamorphism. Before ankerite grains with dolomite cores are fully realized as a useful diffusion chronometer for low- and medium-grade metamorphic rocks, the rates of Fe-Mg interdiffusion in ankerite and dolomite need to be calibrated as a function of a_(O2).

Effects of thermal aging on the microstructure of Type-II boundaries in dissimilar metal weld joints

In order to investigate the effects of long-term thermal aging on the microstructural evolution of Type-II boundary regions in the weld metal of Alloy 152, a representative dissimilar metal weld was fabricated from Alloy 690, Alloy 152, and A533 Gr.B. This mock-up was thermally aged at 450°C to accelerate the effects of thermal aging in a nuclear power plant operation condition (320°C). The microstructure of the Type-II boundary region of the weld root, which is parallel to and within 100μm of the fusion boundary and known to be more susceptible to material degradation, was then characterized after different aging times using a scanning electron microscope equipped with an energy dispersive X-ray spectroscope for micro-compositional analysis, electron backscattered diffraction detector for grain and grain boundary orientation analysis, and a nanoindenter for measurement of mechanical properties. Through this, it was found that a steep compositional gradient and high grain average misorientation is created in the narrow zone between the Type-II and fusion boundaries, while the concentration of chromium and number of low-angle grain boundaries increases with aging time. A high average hardness was also observed in the same region of the dissimilar metal welds, with hardness peaking with thermal aging simulating an operational time of 15years.

Modeling microbial reaction rates in a submarine hydrothermal vent chimney wall

The fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and, through mixing with seawater, are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities. Although several models have been developed to better understand the dynamic interplay of seawater, hydrothermal fluid, minerals and microorganisms inside chimney walls, none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems. In an effort to remedy this, a fully coupled biogeochemical reaction-transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow, solute transport and oxidation–reduction reactions associated with fluid mixing as a function of temperature. The metabolisms included in the reaction network are methanogenesis, aerobic oxidation of hydrogen, sulfide and methane and sulfate reduction by hydrogen and methane. Model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney (77–102°C), and methane and sulfide oxidation peak near the seawater-side of the chimney. The fastest metabolisms are aerobic oxidation of H2 and sulfide and reduction of sulfate by H2 with maximum rates of 140, 900 and 800pmolcm−3d−1, respectively. The maximum rate of hydrogenotrophic methanogenesis is just under 0.03pmolcm−3d−1, the slowest of the metabolisms considered. Due to thermodynamic inhibition, there is no anaerobic oxidation of methane by sulfate (AOM). These simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature. The model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the role of microbial activity in the deep subsurface.

Nucleation and Crystallization Kinetics of a Multicomponent Lithium Disilicate Glass by in Situ and Real-Time Synchrotron X-ray Diffraction

In this study, the high-temperature phase transformation of a multicomponent lithium disilicate glass was investigated by in situ and real-time synchrotron X-ray diffraction in the SiO2–Li2O–P2O5–Al2O3–ZrO2 glass system. Quantitative phase analysis via the Rietveld method was performed on the high-resolution data aiming to reveal the crystallization sequence, crystallization kinetics, and the role of P2O5 on nucleation. It is found that the nucleation of lithium metasilicate (LS) and lithium disilicate (LS2) in this complex glass is triggered by the steep compositional gradients associated with the disordered lithium phosphate (LP) precursors in the glass matrix. The LS2 crystals grow at the expense of the LS, cristobalite, and quartz phases in the glass during the isothermal crystallization process at 770 °C. The nucleation kinetics is temperature dependent, and the induction period of nucleation is longer at a lower temperature.

The influence of imperfect accretion and radial mixing on ice:rock ratios in the Galilean satellites

We investigate the origin of the steep compositional gradient inferred for the Galilean satellites. We analyze N-body simulations of satellite accretion (Ogihara, M., Ida, S. [2012]. Astrophys. J. 753, 60) to: (1) determine the extent to which individual satellites accrete material from different semi-major axes (‘radial mixing’); and (2) calculate the change in rock:ice ratios due to vapor production or physical erosion during collisions. Because of inwards migration, satellites experience enough radial mixing that any initial compositional gradient is efficiently smoothed out. Mean-motion resonances generally prevent large proto-satellites from colliding with each other; as a result, neither vapor production nor physical erosion are capable of explaining the steep compositional gradient. According to the models presented here, even combining an initial compositional gradient with impact-related mass loss cannot reproduce the observed gradient. Some other physical process must have been responsible, perhaps tidally-driven volatile loss at Io and Europa. Impact-related mass loss was probably more important in the saturnian system, and may explain some of the observed satellite diversity there.

Erratum to: Geometrical Properties and Turbulent Flame Speed Measurements in Stationary Premixed V-flames Using Direct Numerical Simulation

Three dimensional, fully compressible direct numerical simulations (DNS) of premixed turbulent flames are carried out in a V-flame configuration. The governing equations and the numerical implementation are described in detail, including modifications made to the Navier–Stokes Characteristic Boundary Conditions (NSCBC) to accommodate the steep transverse velocity and composition gradients generated when the flame crosses the boundary. Three cases, at turbulence intensities, u′/sL, of 1, 2, and 6 are considered. The influence of the flame holder on downstream flame properties is assessed through the distributions of the surface-conditioned displacement speed, curvature and tangential strain rates, and compared to data from similarly processed planar flames. The distributions are found to be indistinguishable from planar flames for distances greater than about 17δth downstream of the flame holder, where δth is the laminar flame thermal thickness. Favre mean fields are constructed, and the growth of the mean flame brush is found to be well described by simple Taylor type diffusion. The turbulent flame speed, sT is evaluated from an expression describing the propagation speed of an isosurface of the mean reaction progress variable \(\tilde{c}\) in terms of the imbalance between the mean reactive, diffusive, and turbulent fluxes within the flame brush. The results are compared to the consumption speed, sC, calculated from the integral of the mean reaction rate, and to the predictions of a recently developed flame speed model (Kolla et al., Combust Sci Technol 181(3):518–535, 2009). The model predictions are improved in all cases by including the effects of mean molecular diffusion, and the overall agreement is good for the higher turbulence intensity cases once the tangential convective flux of \(\tilde{c}\) is taken into account.

Numerical Modeling of Time-dependent Fluid Dynamics and Differentiation of a Shallow Basaltic Magma Chamber

This contribution presents a numerical model of time-dependent physical processes and differentiation in a cooling magma chamber to obtain the magma and liquid composition and model the distribution of minerals and H2O gas bubbles at any moment and place. The numerical experiments were mainly carried out simulating 10 kyr of differentiation of a 50 km cylindrical stock-like reservoir as one of the end-members of a wide spectrum of magma body geometries. A detailed space^time distribution of compositional varieties and exsolved phases in a chamber of initially superheated basaltic composition (corresponding to a lava of the HudsonVolcano, Chile) was obtained through numerical simulation using the finite-element method.The roof of the chamber is located at 2 km depth in a crust with a geothermal gradient of 308C/km. The model considers two spatial directions (z and r) and assumes the magma to be an incompressible non-Newtonian liquid, following the Navier^Stokes formulation, where the density and viscosity depend on the temperature and exsolved solid and volatile phase content, but viscosity also depends on the shear rate. The temperature transfer equation used includes conduction, convection and latent heat. The setting of the model considers that: (1) an extension of 1km into the country rocks around the chamber imposed on the heat flow across the margins is appropriate; (2) the reaction rates for phase (solid and gas) exsolution correspond to a Gaussian probability function; (3) the involved phases, calibrated from MELTS, are olivine, clinopyroxene, magnetite, plagioclase, orthopyroxene and H2O gas; (4) the velocity of the exsolved phase movements with respect to the hosting liquid is given by the Stokes’ velocity; (5) the sizes of crystals and bubbles in the melt vary in space and time up to a maximum given by the modal crystal size observed in the Hudson Volcano. Our results indicate that the convection dynamics of the reservoir are characterized by three distinct flows of decreasing velocity with time: convective cells, plumes and layer flows along the walls. The along-wall magma flow, which persists during most of the 10 kyr of magma cooling, contributes to the thermal insulation of the chamber interior, giving rise to a nearly permanent coexistence of liquids of contrasting composition. A strong compositional stratification is generated in the upper half of the chamber, with a downward-increasing thickness of layers. Such stratification is mainly the result of continuous upward flow and storage on top of the sidewall residual melts. The lower half of the chamber exhibits an independent convective pattern dominated by ascending plumes. A less pronounced stratification is generated here as a consequence of mixing between the residual liquid in the ascending plumes and the surrounding melt. Crystal accumulations of olivine, clinopyroxene and magnetite at the bottom generate a significant volume of ultrabasic magma at the end of 10 kyr. A plagioclase-rich solidification front along the walls is formed during the last stages of crystallization, when the along-wall magma flow diminishes in velocity. An application of the numerical modeling to a cylindrical sill-like chamber is presented to emphasize the role of the aspect ratio in the magma fluid dynamics, compositional gradients and exsolved phase distribution. The stock-like chamber favors the formation of steep compositional gradients and H2O gas concentration at the roof. Concentration of the exsolved phases along the margins is favored in a sill-like chamber. From the simulation results, it is possible to infer that stock-like chambers would be more eruptible and would exhibit a wider compositional spectrum of eruptive materials than sill-like chambers. Because solid and liquid dispersion follow different patterns in a convective chamber, the record of crystal^liquid equilibrium would be an exception.

Polymer Brushes with Nanometer‐Scale Gradients

Surfaces with steep height or composition gradients at the nanometer scale are synthesized from self-asssembled photoinitiator monolayers. The monolayers are prestructured by interference lithography followed by brush growth. Through step-and-repeat processes, surfaces with nano meter-scale composition gradients are obtained.

Development of high-efficiency flexible Cu(In,Ga)Se 2 solar cells: A study of alkali doping effects on CIS, CIGS, and CGS using alkali-silicate glass thin layers

CuInSe 2 (CIS), Cu(In,Ga)Se 2 (CIGS), and CuGaSe 2 (CGS) solar cells were fabricated on flexible 50-μm thick zirconia sheet substrates. Alkali doping into CIS, CIGS, and CGS absorber layers was demonstrated using alkali-silicate glass thin layers (ASTL) deposited on substrates prior to the sputtering of the Mo back contact layer. Enhanced cell efficiencies with the use of ASTL were demonstrated regardless of the In/Ga composition ratio in CIGS. The external quantum efficiency (EQE) curves of CIGS solar cells fabricated with ASTL showed an enhanced absorption in the long wavelength region, whereas the EQE curves of CIS and CGS cells showed no such variation. This result implies that the presence of alkali elements in CIGS lead to a reduction in elemental inter-diffusion of In and Ga during growth, resulting in a decrease of the nominal band-gap energy of the CIGS layer due to the steep Ga composition gradient present in the CIGS layer.

Inter-comparison of the g-, f- and p-modes calculated using different oscillation codes for a given stellar model

In order to make astroseismology a powerful tool to explore stellar interiors, different numerical codes should give the same oscillation frequencies for the same input physics. This work is devoted to test, compare and, if needed, optimize the seismic codes used to calculate the eigenfrequencies to be finally compared with observations. The oscillation codes of nine research groups in the field have been used in this study. The same physics has been imposed for all the codes in order to isolate the non-physical dependence of any possible difference. Two equilibrium models with different grids, 2172 and 4042 mesh points, have been used, and the latter model includes an explicit modelling of semiconvection just outside the convective core. Comparing the results for these two models illustrates the effect of the number of mesh points and their distribution in particularly critical parts of the model, such as the steep composition gradient outside the convective core. A comprehensive study of the frequency differences found for the different codes is given as well. These differences are mainly due to the use of different numerical integration schemes. The use of a second-order integration scheme plus a Richardson extrapolation provides similar results to a fourth-order integration scheme. The proper numerical description of the Brunt-Vaisala frequency in the equilibrium model is also critical for some modes. An unexpected result of this study is the high sensitivity of the frequency differences to the inconsistent use of values of the gravitational constant (G) in the oscillation codes, within the range of the experimentally determined ones, which differ from the value used to compute the equilibrium model.

Gradient blending of poly(dimethylsiloxane) with polystyrene and polyethylene in supercritical carbon dioxide

Direct impregnation and blending of poly(dimethysiloxane) (PDMS) with polystyrene and polyethylene has been investigated. Blends of polystyrene were prepared by first swelling and impregnation of polystyrene pellets in 0.1, 3 and 5 wt% solutions of PDMS in CO 2 at 80 °C and 45.7 MPa and then depressurization either directly at the impregnation temperature or after cooling to 50 or 27 °C. Blends of polyethylene were formed by impregnation at 130 °C and 57 MPa with 0.1 wt% PDMS solutions in CO 2 and depressurization at either 130 or 50 °C. The microstructural features of the blends were investigated by scanning electron microscopy (SEM). PDMS distribution across the pellet cross sections was investigated in terms of silicon atom distribution using energy dispersive X-ray (EDX) analysis. It is shown that the blends display pore-wise and compositionally gradient features. They typically show a steep compositional gradient, with a high PDMS concentration near the edge of the pellets which decrease and reach a plateau value a short distance away from the outer surface. In blends with polystyrene, when decompressions were carried out at temperatures lower than the impregnation temperature, the PDMS distributions were found to be significantly different, displaying a characteristic local maximum in the 20–100 μm range from the outer surface. In these blends, the SEM results show the formation of a dense non-porous layer with a thickness that corresponds closely with the location of the observed PDMS maximum, pointing to a competition between the vitrification of the host matrix and the escaping ability of PDMS dissolved in carbon dioxide before its entrapment during depressurization. At decompressions at the impregnation temperature, or with impregnations with PDMS solutions of higher concentrations, polystyrene pellets were found to expand more, and display a pore-gradient feature with layers of progressively increasing pore sizes from the outer edges towards the central region of the pellets. In blends of polyethylene, in contrast to polystyrene, the local maximum in PDMS concentration was not observed. The overall PDMS levels in the central region away from the outer surface were lower. The central region of the pellet showed a nanoporous morphology.

Feedback‐Controlled Firing of Reaction‐Bonded Aluminum Oxide

The reaction‐bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina‐based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1°C/min to 1100°C. Heat‐treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30°C/mm) and compositional gradients (>5000 mol·(m3·mm)−1), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback‐controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback‐controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat‐treatment cycle, a 1 mm sample was fired in 18 h. With feedback‐controlled firing, the same sized sample was fired in only 5 h.

Thermodynamic modelling of diffusion-controlled garnet growth

Numerical thermodynamic modelling of mineral composition and modes for specified pressure-temperature paths reveals the strong influence of fractional garnet crystallisation, as well as water fractionation, on garnet growth histories in high pressure rocks. Disequilibrium element incorporation in garnet due to the development of chemical inhomogeneities around porphyroblasts leads to pronounced episodic growth and may even cause growth interruptions. Discontinuous growth, together with pressure- and temperature-dependent changes in garnet chemistry, cause zonation patterns that are indicative of different degrees of disequilibrium element incorporation. Chemical inhomogeneities in the matrix surrounding garnet porphyroblasts strongly affect garnet growth and lead to compositional discontinuities and steep compositional gradients in the garnet zonation pattern. Further, intergranular diffusion-controlled calcium incorporation can lead to a characteristic rise in grossular and spessartine contents at lower metamorphic conditions. The observation that garnet zonation patterns diagnostic of large and small fractionation effects coexist within the same sample suggests that garnet growth is often controlled by small-scale variations in the bulk rock chemistry. Therefore, the spatial distribution of garnet grains and their zonation patterns, together with numerical growth models of garnet zonation patterns, yield information about the processes limiting garnet growth. These processes include intercrystalline element transport and dissolution of pre-existing grains. Discontinuities in garnet growth induced by limited element supply can mask traces of the thermobarometric history of the rock. Therefore, thermodynamic modelling that considers fractional disequilibrium crystallisation is required to interpret compositional garnet zonation in terms of a quantitative pressure and temperature path of the host rock.

Computational and experimental study of the effect of the composition on the morphology of the ordered domains and APBs structures

The formation and growth of ordered domains in Fe–Al alloys is explored computationally in two-dimensions (2D) using the modified Khachaturyan diffusion equation by utilizing the thermodynamic parameters related to the equilibrium phase diagram and experimentally by using a single sample containing a steep composition gradient. The effect of alloy composition on the morphology of the ordered domains as well as the APBs in a single phase is presented. Experiment and computer simulations show that the time to form an ordered structure increases rapidly as the composition approaches the order–disorder transition line. Moreover, diffuse APBs are found to be formed near the transition line. The early stage of ordering is also described.