Growth Of Second Phase Research Articles

Quantitative analysis of grain boundary diffusion, segregation and precipitation at a sub-nanometer scale

Grain boundaries are intrinsic and omnipresent microstructural imperfections in polycrystalline and nanocrystalline materials. They are short-circuit diffusion paths and preferential locations for alloying elements, dopants, and impurities segregation. They also facilitate heterogeneous nucleation and the growth of secondary phases. Therefore, grain boundaries strongly influence many materials' properties and their stabilities during application. Here, we propose an approach to measure diffusion, segregation, and segregation-induced precipitation at grain boundaries at a sub-nanometer scale by combining atom probe tomography and scanning transmission electron microscopy. Nanocrystalline multilayer thin films with columnar grain structure were used as a model system as they offer a large area of random high-angle grain boundaries and inherent short diffusion distance. Our results show that the fast diffusion flux proceeds primarily through the core region of the grain boundary, which is around 1 nm. While the spatial range that the segregated solute atoms occupied is larger: below the saturation level, it is 1,2 nm; as the segregation saturates, it is 2–3.4 nm in most grain boundary areas. Above 3.4 nm, secondary phase nuclei seem to form. The observed distributions of the solutes at the matrix grain boundaries evidence that even at a single grain boundary, different regions accommodate different amounts of solute atoms and promote secondary phase nuclei with different compositions, which is caused by its complex three-dimensional topology.

Machine learning-based microstructure prediction during laser sintering of alumina

Predicting material’s microstructure under new processing conditions is essential in advanced manufacturing and materials science. This is because the material’s microstructure hugely influences the material’s properties. We demonstrate an elegant machine learning algorithm that faithfully predicts the microstructure under new conditions, without the need of knowing the governing laws. We name this algorithm, RCWGAN-GP, which is regression-based conditional generative adversarial networks with Wasserstein loss function and gradient penalty. This algorithm was trained with experimental SEM micrographs from laser-sintered alumina under various laser powers. The RCWGAN-GP realistically regenerates the SEM micrographs under the trained laser powers. Impressively, it also faithfully predicts the alumina’s microstructure under unexplored laser powers. The predicted microstructure features, including the morphology of the sintered particles and the pores, match the experimental SEM micrographs very well. We further quantitatively examined the prediction accuracy of the RCWGAN-GP. We trained the algorithm with computer-created micrograph datasets of secondary-phase growth governed by the well-known Johnson–Mehl–Avrami (JMA) equation. The RCWGAN-GP accurately regenerates the micrographs at the trained time series, in terms of the grains’ shapes, sizes, and spatial distributions. More importantly, the predicted secondary phase fraction accurately follows the JMA curve.

Nanoscale constraints on a fluid-induced transformation of monazite during postmagmatic alteration – A case of the Jawornik granitoid (NE Orlica-Śnieżnik Dome, Sudetes, SW Poland)

Two monazite grains altered during postmagmatic, hydrothermal processes in the Variscan Jawornik granitoid (NE Orlica-Śnieżnik Dome, Sudetes, SW Poland) were investigated. Different assemblages of secondary phases replacing and forming close to altered monazite indicate two different courses of the mineral-fluid reactions. The first monazite is surrounded by two zones comprising rhabdophane group minerals (rhabdophane henceforth) and hematite (inner zone) and allanite (outer zone) that form a concentric corona texture. The growth of secondary phases is interpreted as being related to decreasing temperatures below ca. 500 °C. The inner zone formed due to alteration of the monazite that was driven by fluid-induced coupled dissolution-reprecipitation reactions. This alteration resulted in the progression of partial replacement by submicron layers of rhabdophane along the monazite surface and pseudomorphic replacement of a substantial part of the monazite by rhabdophane and hematite. The presence of the hematite suggests oxygen fugacity above the hematite-magnetite buffer. Alteration of the second investigated monazite is related to dissolution induced by a K-bearing alkali fluid, partial replacement by thorianite (ThO2), and overgrowth by K-feldspar with minor titanite formed during the chloritization of biotite and the alteration of feldspars. These results demonstrate that the local character of mineral-fluid reactions can take an entirely different course at the microscale, within a distance of several hundreds of microns. The assemblages of secondary phases replacing two investigated monazite grains indicate that the alteration processes are related to moderate temperature (<500 °C) hydrothermal processes assigned to upper greenschist facies conditions. The absence of secondary apatite (close to the first monazite) and secondary REE- and P-bearing phases (close to the second monazite) indicates the high mobility of the elements removed by the fluid, in contrast to the low mobility of the Th taken up by secondary rhabdophane or thorianite crystallizing at the monazite surface. The presence of rhabdophane group minerals and thorianite in hydrothermal secondary phases assemblages replacing monazite can be used as indicators of the local character of the environment at the microscale.

Selected growth of second phase on BiOBr facets via spatial charge separation towards enhanced photocatalysis activity

The recombination rate of electrons and holes determine the photocatalytic effect. Au nanoparticle (NPs) were loaded on the (110) plane of BiOBr via a ultrasonic method while PbO2 NPs were loaded on the (001) plane. The ultrasound not only promoted the formation of the NPs but also resulted indirectionally transfer of electrons and holes in the different crystal planes, where electronsmainly exist in (110) planes while holes accumulate on (001) planes. Namely, ultrasonic waves caused BiOBr layer spacing to increase and destroy the balance of its built-in electric field. The loading of Au NPs enhanced the photocatalytic performance.

Impact melting of the largest known enstatite meteorite: Al Haggounia 001, a fossil EL chondrite

AbstractAl Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low‐Ca pyroxene; (2) dispersion of metal‐sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S2 bubbles that infused the melt; (5) formation of immiscible silicate and metal‐sulfide melts; (6) shock‐induced transportation of the metal‐sulfide melt to distances &gt;10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite's bulk chemical composition: e.g., impact melting and loss of a metal‐sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.

Structural Characterization of Aluminum (Oxy)hydroxide Films at the Muscovite (001)–Water Interface

The formation of Al (oxy)hydroxide on the basal surface of muscovite mica was investigated to understand how the structure of the substrate controls the nucleation and growth of secondary phases. Atomic force microscopy images showed that solid phases nucleated on the surface initially as two-dimensional islands that were ≤10 Å in height and ≤200 Å in diameter after 16-50 h of reaction in a 100 μM AlCl3 solution at pH 4.2 at room temperature. High-resolution X-ray reflectivity data indicated that these islands were gibbsite layers whose basic unit is composed of a plane of Al ions octahedrally coordinated to oxygen or hydroxyl groups. The formation of gibbsite layers is likely favored because of the structural similarity between its basal plane and the underlying mica surface. After 700-2000 h of reaction, a thicker and continuous film had formed on top of the initial gibbsite layers. X-ray diffraction data showed that this film was composed of diaspore that grew predominantly with its [040] and [140] crystallographic directions oriented along the muscovite [001] direction. These results show the structural characteristics of the muscovite (001) and Al (oxy)hydroxide film interface where presumed epitaxy had facilitated nucleation of metastable gibbsite layers which acted as a structural anchor for the subsequent growth of thermodynamically stable diaspore grown from a mildly acidic and Al-rich solution.

Slag Resistance Mechanism of Lightweight Microporous Corundum Aggregate

The microstructures of the reaction interfaces between slag and corundum aggregates, microporous corundum produced in the laboratory and tabular corundum were observed after slag resistance experiments, and their associated slag resistance mechanisms were investigated. A continuous isolation layer was observed around the microporous corundum, which showed a significantly better slag resistance than tabular corundum. The formation of columnar crystals of CaAl12O19 (CA6) and CaAl4O7 (CA2) in the isolation layer was the main reason for the difference in slag resistance. With respect to the nucleation and growth of second phase, the slag resistance mechanism of lightweight microporous corundum was explored by thermodynamic and kinetic analysis. Due to its smaller pore size, the second phase is more likely to achieve supersaturation, and large quantities of crystal nuclei are generated for microporous corundum. The critical dissolved depths of the microporous and tabular corundum in saturated slag were calculated to be 0.14 and 0.27 μm, respectively. Additionally, the small pore sizes lead to an increase in the Ostwald ripening rate of the second phase, and the Ostwald ripening rate of microporous corundum was 12 times that of the tabular corundum based on Ardell's theory.

Strontium, nickel, cadmium, and lead substitution into calcite, studied by density functional theory.

We have used density functional theory to predict the ion exchange energies for divalent cations Ni(2+), Sr(2+), Cd(2+), and Pb(2+) into a calcite {10.4} surface in equilibrium with water. Exchange energies were calculated for substitution into the topmost surface layer, at the mineral-fluid interface, and into the second layer of the solid. This information can be used as an indicator for cation substitution in the bulk phase, such as for the uptake of toxic metals from the environment and the growth of secondary phases. In both the surface and in the second layer, Ni(2+), Cd(2+), and Pb(2+) substitute exothermically and Sr(2+) substitutes endothermically. Our results agree with published experimental data that demonstrate trace metal coprecipitation with calcite as a sink for Ni(2+), Cd(2+), and Pb(2+), whereas Sr(2+) has a distribution constant significantly smaller than 1. Ni(2+) substitution is favored at the mineral-fluid interface compared with bulk substitution, which also agrees with experimental data. Our results predict that Ni(2+), Cd(2+), and Pb(2+) form a stable solid solution with calcite. Successful prediction of the experimental results gives us confidence in our ability to predict the divalent cation preference for surfaces rather than for sites within the bulk crystal structure, which cannot be directly derived from experiment.

Flash-sintering of MnCo2O4 and its relation to phase stability

Abstract The flash-sintering behavior of manganese cobaltite spinel (MnCo 2 O 4 ) is analyzed in the present work. It is shown that the MnCo 2 O 4 is flash-sintered at 120–150 °C under 15.0–17.5 V cm −1 , which is substantially lower than the conventional-sintering temperatures of 1080 °C and more. We have also demonstrated that the flash-sintering is a transient phenomenon, where the power dissipation rises quickly at first and then results to Joule heating. The extent of sintering is confirmed through SEM, where a dense and pore - free morphology is observed for the well-sintered samples. The growth of secondary phase as a function of sintering temperature in both conventional and flash processes is monitored by XRD. Consistent changes in I – V curves observed at 200–700 °C, suggest that the rapid increase of the conductivity during flash-effect follows the hopping mechanism of usual conductivity phenomenon. On the basis of correlation between the conductivity, phase-stability and microstructure, a mechanism for flash-sintering has been proposed.

Formation mechanism of secondary phase in (La,Nb) codoped TiO2 ceramics varistor

Formation mechanism of secondary phase in (La,Nb) Codoped TiO2 ceramics varistor were investigated. The (La,Nb) Codoped TiO2 ceramics samples were prepared from anatase TiO2, Nb2O5 and La2O3 oxide powders by a traditional solid-state sintering method. Microstructure, chemistry composition, crystal structure, thermal etched groove and micrograph of in (La,Nb) Codoped TiO2 ceramics were by SEM, EDS, AFM and TEM. Formation mechanism of secondary phase were discussed by point defect thermodynamical analysis, grain boundary energy and materials structure measurement.The results show that secondary phase are originated from segregation of point defects NbTi andTiLa at grain boundaries in (La,Nb) Codoped TiO2 ceramics. The driving force of segregation is the elastic strain energy. The segregated ion nucleate at grain surface or grain boundaries interface with higher energy. The nucleation gradually grow to form secondary phase at during high-temperature sintering. The growth of secondary phase is mainly on crystal planes with higher energy to minimize the materials system energy.

Effect of precipitates on microstructures and properties of forged Mg–10Gd–2Y–0.5Zn–0.3Zr alloy during ageing process

The precipitate behavior during forging and ageing process of Mg–10Gd–2Y–0.5Zn–0.3Zr alloy has been investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The mechanical properties of the alloy after forging and ageing process have been evaluated using Vickers hardness and room-temperature tensile tests. The results show that precipitation of 14H-type long period stacking order (LPSO) phase is the main strengthening phase in the as-forged alloy. The LPSO phase and refinement of grains contribute to the strength improvement of the alloy after forging process. The optimal mechanical properties of the alloy are obtained when it is aged at 200 °C for 60 h, which mainly owes to the precipitation of large amounts of β′ and 14H-type LPSO phases on the α-Mg matrix. The growth of secondary phases, widening of soft precipitate free zones and coarsening of grains during subsequent ageing process at higher temperature lead to the decrease of mechanical properties of the alloy.

Microstructure Evolution in a 9%Cr Heat Resistant Steel during Creep Tests

Dynamic structural changes during creep tests for about 103 hours at 600 and 650oC were investigated in a P92-type 9%Cr martensitic heat resistant steel. The structural changes are characterised by the development of relatively large equiaxed subgrains with relatively low dislocation densities in place of initial martensite laths. The coarsening of substructure was accompanied by a growth of second phase precipitates. The final grain/subgrain sizes and dislocation densities evolved after the creep tests were in rough correlation with applied stresses, i.e. larger (sub)grains developed under lower stresses. The structural mechanism responsible for microstructure evolution was considered as a kind of continuous dynamic recrystallization.

Mechanical properties and non-destructive evaluation of chromium–molybdenum ferritic steels for steam generator application

The paper presents high temperature mechanical properties and non-destructive evaluation (NDE) of chromium–molybdenum (Cr–Mo) ferritic steels widely used as structural materials for steam generator (SG) applications. Creep and low cycle fatigue deformation and damage (the important design considerations for SGs) of ferritic steels are presented. Recent trend and advances in ferritic steels for SG applications are discussed. The paper also highlights the recent results obtained on creep and fatigue properties of indigenously developed steels. Weld and weld joints of ferritic steels are known to be the weak links affecting the life of the components. The creep properties of weld joints are presented. Apart from mechanical behaviour of ferritic steels, the paper deals with some of the recent results on the characterisation of microstructural features such as grain size, nucleation and growth of secondary phases, degradation of microstructure due to thermal ageing and creep, and assessment of creep and fatigue damage of ferritic steels using advanced NDE techniques.

Observation of the formation and growth of secondary phases in Bi-2223/Ag tapes

Formation and growth of secondary phases in Bi-2223/Ag tapes have been investigated. Samples were heat treated at several temperatures for different periods under reduced oxygen partial pressure. The composition and microstructural evolution of secondary phases during the sintering process were characterized by SEM, XRD and EDS. Using high resolution SEM, the (Ca,Sr) 2CuO 3 (2:1), CuO and (Ca,Sr) 14Cu 24O 41 (14:24) phases could be clearly identified, and details of the Bi-2212, Bi-2223 structure, as well as the quenched liquid region could be also observed. Experiments showed the liquid region of quenched samples contained small Cu-rich precipitates, and the majority of the liquid was rich in Pb, Bi and Ca. The increase of the liquid region area fraction and the disappearance of the 2212 phase both take place quickly, in contrast to the slower development of the total secondary phase area fraction. The amount of the large secondary phase particles was lowest for samples sintered at 830 °C in 8.5% O 2. At higher temperatures, the amount of the 2:1 phase increased and the particle size of the 14:24 became larger, while at lower temperatures, the CuO particles appeared at very beginning and tended to grow rather fast.

The effect of high-temperature exposure on the microstructural stability and toughness property in a 2205 duplex stainless steel

A series of aging treatments at the temperature range of 650–975 °C for different time intervals in a solution-treated 2205 duplex stainless steel were carried out. The corresponding microstructure of aged specimens was observed and the impact toughness was measured. More attention was paid to the secondary phase precipitation and the transition of ductile-to-brittle fracture. The results indicated that the impact toughness of duplex stainless steel was sensitive to the precipitation of σ phase even at the initial stage of aging. Two kinds of Cr-, Mo-enriched intermetallic phases, σ and χ, were found to precipitate preferentially at δ/ γ interface boundary and within δ-ferrite grain after 5 min of aging at the temperature range of 875–900 °C. The volume fraction of σ phase was continuously increased with the time of aging and σ phase developed into a coarse particle due to the high diffusibility of solute atoms at high temperatures. The precipitation of Mo-enriched χ phase at the initial stage of aging is presumably enhanced due to the low interfacial energy of highly coherent χ/ δ interface with a characteristic cubic-to-cubic orientation relationship. However, this pre-formed χ phase was re-dissolved eventually into the σ phase when the specimen aged above 750 °C. Accompanied with the growth of secondary phases, the γ phase nearby the interface of σ or χ phase was induced to grow into the δ-ferrite region which was depleted in Cr and Mo. The δ-ferrite in original duplex structure would be completely decomposed into the σ phase and secondary γ after long-term aging.

Mathematical model of influence of rapid induction heating on nucleation and growth of precipitates

A mathematical model of the influence of rapid induction heating on the nucleation and growth of secondary phases in diffusion controlled processes has been developed. The total stress produced by the electromagnetic field and acting on the volume V of a specimen was derived and added to other external stresses applied to the system. Then, the total effective stress was introduced into mathematical equations of diffusion controlled nucleation and growth processes. In addition, the effect of rapid induction heating on the age hardening treatment of a selected cast nickel base superalloy, IN738LC, was investigated experimentally. For this purpose, two types of rapid aging with equal heating rates were applied: one treatment was induction aging and the other was salt bath aging. Microstructural characteristics of γ′ precipitates and hardening behaviour were studied by means of scanning electron microscopy (SEM), electron image analysis, transmission electron microscopy (TEM), and hardness testing. According to the results obtained, although the rates of heating in induction and salt bath aging were equal, the rates of nucleation and growth of γ′ precipitates in induction aging were much faster than those obtained in salt bath aging, especially in the first minutes of the aging process. Furthermore, the characteristics of γ′ precipitates with induction aging were more favorable than those with salt bath and normal aging. It was observed that the growth rate of γ′ in induction aging deviated considerably from the t1/3 growth law of the standard modified Lifshitz, Slyozov, and Wagner (MLSW) theory. The remarkable improvement of microstructural characteristics obtained with induction aging can be attributed to the existence of the external electromagnetic force produced by rapid induction heating.

Isothermal melt processed Bi-2212/Ag tapes containing MgO and Al 2 O 3 additions

The development of the superconducting phase in melt-processed Bi-2212 tapes depends on controlling the development of solid phases in the partial melt and the conversion of the partial melt into a highly-aligned superconducting phase. MgO and Al2O3 additions have been used with isothermal melt processing (IMP) for the grain refinement of phases in the partial melt and the prevention of secondary phase growth during solidification. Controlled solidification with reduced oxidation rates also significantly increased the transport properties of the tapes with or without the oxide additions. The optimal oxidation rate was found to depend upon the processing temperature. Critical current densities in excess of 100 kA cm−2 were obtained in tapes melt-processed below 800 °C.

Controlled decomposition and reformation of the 2223 phase in Ag-clad (Bi, Pb)2Sr2Ca2Cu3Ox tapes and its influence on the microstructure and critical current density

The decomposition of almost fully reacted (Bi, Pb)2Sr2Ca2Cu3Ox (BSCCO-2223) tapes caused by heating in 1 atm of pure O2 at 825 °C has been studied. It was found that partially decomposing 2223 tapes to a mixture of Bi2Sr2Ca1Cu2Oy, (Ca, Sr)2PbO4, and other secondary phases reduced the critical current density (77 K, 0 T) from ∼20 kA/cm2 to nearly zero. Reheating the tapes in 7.5% O2 restored the 2223 phase and, while there was some degradation of the 2223 grain alignment due to residual secondary phase growth, the critical current density was also restored to nearly its original value. We hypothesize that such a decomposition/reformation process can be useful in increasing the connectivity and relative density of polycrystalline 2223, by encouraging the formation of a liquid phase which heals residual cracks in the BSCCO core.

Growth and shrinkage of precipitates under irradiation

Abstract The growth of second phase precipitates from the supersaturated solid solution under irradiation is investigated taking into account a new mechanism of precipitate dissolution. This mechanism is of a purely diffusion origin, i.e. it is based on diffusion outfluxes of point defects produced by irradiation within the precipitates into the host matrix, provided that the interface boundary is transparent for the point defects. The point defect production rate within a precipitate is proportional to its volume while the total diffusion influx of substitutional impurity atoms is proportional to its radius meaning that there exists a maximum size at which the precipitate growth rate equals the rate of its radiation-induced dissolution. This size is shown to be a stable one implying that under irradiation a stationary state can be achieved far away from the thermodynamic equilibrium.

The degradation of YBa2Cu3O7 resulting from exposure to wet and dry steam

The severity of degradation of YBa2Cu3O7−x from exposure to an aqueous phase has the following dependence: wet steam (condensed thin film) &gt; bulk fluid phase &gt; dry steam, other variables being constant. Also, the surface reactivity increases with oxygen deficiency. It is found that the early stages of degradation are associated with surface hydroxylation and preferential extraction of Ba. Subsequently, dissolution takes place, followed by solution saturation and sequential precipitation, nucleation, and growth of secondary phases. These observations form the basis of a model which incorporates surface hydroxylation and OH− penetration of the bulk, formation of planar defects and extraction of Ba, lattice dissolution, and mobility and solubility constraints.