Autocatalytic Nucleation Research Articles

Phenomenological kinetic model of the nano-bainitic steels to characterize the dynamics of the autocatalytic nucleation process

A kinetic model has been proposed in the present work, which formulates the dynamic nature of autocatalysis. The autocatalytic factor is quantified in the present work which involves interaction energy between bainite nucleus and stress field generated by existing bainite. This interaction energy provides the stored energy for autocatalytic nucleation and eventually reduces the nucleation barrier of bainite transformation. The change in Gibbs free energy due to carbon partitioning will also alter the autocatalytic nucleation with the bainite transformation. The model also evaluates the activation energy due to dislocation barrier, which is arising by solid solution strengthening of austenite. The slow kinetics at low transformation temperature is due to the larger activation energy of the dislocation barrier, which provides greater resistance to atomic displacement during transformation. The interaction energy between bainite nucleus and stress field generated by existing bainite was validated by determining the transformation shear strain exhibited in austenite which was found to be in the range as reported in the literature for displacive transformations.

Bainite Transformation-Kinetics-Microstructure Characterization of Austempered 4140 Steel

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.

Investigation of microstructural evolution and bainite transformation kinetics of multi-phase steel

In this study, multi-phase steel with a mixture of ferrite, bainite, retained austenite and martensite is obtained after specific heat treatments, which include intercritical annealing, isothermal holding and final quenching. The microstructural constituents and individual phase fractions are precisely quantified based on multi-scale characterizations. The focus is on bainite transformation kinetics and its impact on the microstructural evolution, which is systematically analyzed according to the assumption of displacive modeling theory. Bainite transformation begins with austenite grain-boundary nucleation and continues through autocatalytic nucleation based on nucleation kinetics of bainitic sub-units. A small discrepancy between the modeling and experimental results is indicative of carbon partitioning into austenite, which reduces the driving force and/or the rate parameter κ for bainite transformation. In addition, the continuous carbon enrichment in austenite decelerates the bainite transformation and prolongs the duration time. Moreover, the maximum carbon concentration in austenite is limited by the T0 line. Austenite with a lower carbon content but higher volume fraction will not transform to bainite at higher isothermal bainite transformation temperatures, and such mechanically unstable austenite is more likely to transform to martensite during final quenching. As a result, with increasing isothermal holding temperature, the bainite and retained austenite fractions decrease while the martensite fraction increases in the sepcimens after final quenching.

Influence of the parent phase on the bainitic transformation under large stress of manganese-boron steel 22MnB5

22MnB5 steel grade is widely employed for manufacturing tailored tempered components in the automotive industry, where locally heated tools lead to bainitic microstructures in the whole or some regions of the formed parts. Because heat treatment and forming occur simultaneously, phase transformations evolve under the influence of high stresses and in pre-deformed austenite. Isothermal bainitic transformations at 450 °C under the influence of applied stresses in the range of 0–200 MPa are evaluated in this work. Material response is experimentally studied by means of a thermomechanical simulator using sheet specimens and an enhanced gas cooling device. Bainite morphology is studied with electron backscatter diffraction (EBSD) analysis. Austenite grains are reconstructed from the EBSD determinations, showing a good consistency between the calculations and the experimental results. These reconstructed parent phase distributions are further employed to analyze individual and general misorientation angles in both austenitic and bainitic states. Special attention is paid to the influence of the parent phase on the bainitic transformation kinetics. A displacive-based model, recently proposed by the authors, accounting for the variants selection, autocatalytic nucleation, and deformation related defects density is used in this investigation to account for the transformation kinetics, incorporating into consideration the reconstructed austenite.

Microstructure Transformation on Pre-Quenched and Ultrafast-Tempered High-Strength Multiphase Steels.

High-strength, multiphase steels consisting of pearlite surrounded by tempered martensite were prepared by pre-quenching and ultrafast tempering heat treatment of high-carbon pearlitic steels (0.81% C). The microstructures were analyzed by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. With an increasing quenching temperature from 120 °C to 190 °C, the quenched martensite variants nucleated via autocatalytic nucleation along the interface. Furthermore, the tempered nodules exhibited a distinct symmetrical structure, and the tempered martensite and pearlitic colonies in the group also showed a symmetrical morphology. In addition, a reasonable model was formulated to explain the transformation process from quenching martensite to the multiphase microstructure. When the quenching temperature was set to 120 °C, followed by ultrafast heating at 200 °C/s to 600 °C and subsequent isothermal treatment for 60 s, the multiphase structure showed highest strength, and the pearlite volume fraction after tempering was the lowest. The microhardness softening mechanism for the tempered structures consisted of two stages. The first stage is related to martensitic sheets undergoing reverse transformation and the nucleation of cementite on dislocations. The second stage involves the transformation of austenite into pearlite and continued carbide coarsening in the martensitic matrix.

Preparation, optimization and evolution of the kinetic mechanism of an Fe-MIL-88A metal–organic framework

Investigating the kinetics of crystallization, growth behavior and morphological changes through statistical studies of Fe-MIL-88A suggested an autocatalytic nucleation mechanism.

Formation and autocatalytic nucleation of co-zone [formula omitted] deformation twins in polycrystalline Mg: A phase field simulation study

A phase-field model is developed to study the formation and autocatalytic nucleation of {101¯2} twins in polycrystalline Mg. The twins are found to nucleate most favourably in grains with the most negative interaction energy. Within such grains, the energetically most favoured nucleation site is determined by stresses concentrated near the grain boundaries that are related to the elastic anisotropy of the material. Furthermore, in a structure consisting of three lamellar grains with an incoming twin in the central grain, the simulation results show that before autocatalytic nucleation, the incoming twin often has a lenticular shape. The stress field around the tip of the incoming twin plays the major role in the autocatalytic nucleation. After a twin has nucleated in the neighbouring grain, the incoming and the outgoing twins evolve simultaneously, and the shape of the incoming twin gradually changes from lenticular to parallel-sided plate. Under the condition that the crystallographic orientation of the central grain and the applied strain remains unchanged, the driving force for twin nucleation decreases with increasing misorientation (up to 90°) across the grain boundary. It is further derived that the interaction energy values between the pre-existing stress field of the polycrystalline structure and the eigenstrain of the to-be-nucleated twin is mathematically related to the resolved shear stress of twins.

Multiphase-field modeling of martensitic phase transformation in a dual-phase microstructure

Ferritic-martensitic dual-phase steels (DP) offer beneficial material properties for industrial applications. Understanding the microstructural evolution within the production chain and its consequences for the mechanical characteristics allows to improve the material properties by changing specific process parameters. In addition to experiments, numerical methods are a helpful tool to explore these correlations. For this purpose, we present an elastic multiphase-field model to study the martensitic phase transformation in a DP microstructure. An EBSD scan of an as-received commercial DP600 finished strip is used to generate a virtual, initial ferritic-austenitic microstructure. Based on the initial polycrystalline microstructure, the applied multiphase-field model is able to predict the evolution of the martensitic variants in austenitic grains, including grain boundary and autocatalytic nucleation. The approach identifies grain boundaries lying parallel or perpendicular to the habit plane of martensitic variants as preferred nucleation sites. We discuss the resulting stress distribution and show its correlations to retained austenite.

Bainite formation kinetics in steels and the dynamic nature of the autocatalytic nucleation process

Over the years, a quantitative theory to explain bainite formation kinetics has been proposed based on the nucleation kinetics of bainitic sub-units. Although the theory shows acceptable correlation with experimental results, it is observed that the kinetic models show a certain degree of discrepancy with actual kinetics. It is identified that these mainly arise due to the inadequate estimation of autocatalytic nucleation, especially as a function of progress of bainite formation. With the help of this observation, the kinetic model is modified and a better insight into the process of autocatalytic nucleation, essential in bainite formation, is obtained.

Influence of crystalization temperature on morphological properties of dibutylphtalate alumosilicate adsorbents

Alumosilicates have advantages over other adosrbents because they have high adsorption capacity and high selectivity. Due to this fact, the paper investigates the synthesis and characterization of the aluminosilicate materialused as a potential adsorbent of dibutyl phthalate (DBP) The following analyses were conducted on the samples synthesized at various crystallization temperatures (70, 80 and 90° C): the adsorption of DBP, crystallinity (XRD), medium particle diameter (Ds50%) and scanning electron microscopy (SEM) It has been found that the aluminosilicate species obtained at temperatures of 80 and 90 C have good adsorbent properties for DBP The medium particle diameter varied depending on the crystallization temperature according to the principles of autocatalytic nucleation and the ‘memory effect’ of the gel. At the temperature of 90°C, crystallinity decreases significantly (37.87%) whereas dibutylphthalate adsorption increases due to the increase in the content of other nonzeolite species.

Physical basis of large microtubule aster growth.

Microtubule asters - radial arrays of microtubules organized by centrosomes - play a fundamental role in the spatial coordination of animal cells. The standard model of aster growth assumes a fixed number of microtubules originating from the centrosomes. However, aster morphology in this model does not scale with cell size, and we recently found evidence for non-centrosomal microtubule nucleation. Here, we combine autocatalytic nucleation and polymerization dynamics to develop a biophysical model of aster growth. Our model predicts that asters expand as traveling waves and recapitulates all major aspects of aster growth. With increasing nucleation rate, the model predicts an explosive transition from stationary to growing asters with a discontinuous jump of the aster velocity to a nonzero value. Experiments in frog egg extract confirm the main theoretical predictions. Our results suggest that asters observed in large fish and amphibian eggs are a meshwork of short, unstable microtubules maintained by autocatalytic nucleation and provide a paradigm for the assembly of robust and evolvable polymer networks.

Self-twinning in solid-state decomposition

Twinning in face-centered cubic materials has received increasing attention owing to its favorable contributions to materials properties. While the models designed to account for twin nucleation have been mainly concerned with deformation twinning under external stress, addressing autocatalytic nucleation of twin (self-twinning) remains a challenge. Here, we report experimental evidences and a dislocation-based model demonstrating that a new source for twinning (self-twinning) operatives during solid-state decomposition. The initial stage of self-twinning is a stress relaxation process to relieve the local stress concentration caused by pile-up of interfacial dislocations at the precipitate-matrix interface. Dynamical two-beam analyses on dislocations configurations showed that three types of partial dislocations were involved in self-twinning: two Shockley partials, 16[112] on primary (111¯) plane and 16[112¯] on conjugate (111) plane; a stair-rod dislocation of 16[110] connecting primary and conjugate planes. We further show that the thermally-activated cross-slip of Shockley partials with aid of stair-rod dislocation plays a crucial role in self-twinning process.

Sol–Gel Synthesis and Crystal Nucleation of Photosensitive Au/Ag- Containing Glasses of Lithium Silicate System

A sol–gel method has been proposed for obtaining the sintering mixture used in the preparing of a photosensitive Au/Ag-containing glasses of the Li 2 O - SiO 2 system. It has been established that the use of sol–gel method for the production of the batch for synthesizing photosensitive Au/Ag-containing glasses gives more homogeneous distribution of Au/Ag metals in the bulk of the glass. Crystal-nucleation main characteristics have been defined in a large area of compounds of photo-structured compositions of a lithium–silicate system (23.4–46.0 Li 2 O mol % by analysis) both for spontaneous and catalyzed nucleations. The spontaneous (homogeneous) nucleation happened in the samples without any photosensitive metals and with photosensitive gold–silver additives (0.05 wt % above 100%) without irradiation; the catalyzed (heterogeneous) nucleation happened in the samples with photosensitive silver ore gold additives under x-ray CuK I± irradiation. The ranges of the compositions of the glasses in which the effect of catalyzing irradiation is reversed (suppressing nucleation in accordance with the autocatalytic mechanism) under certain conditions. The spontaneous and catalyzed nucleation of crystals in the glasses of Li 2 O – SiO 2 system caused by the addition of photosensitive impurities and X-ray irradiation was analyzed in glasses with a very wide range of compositions in Li 2 O – SiO 2 system. Composition ranges with positive (+) and negative (-) differences in the rates of catalyzed and spontaneous steady-state nucleation were established. This finding point is favorable for obtaining photostructured (photosensitive) materials. The concentration limit was determined to be 37.98 mol % Li 2 O. About this value, we observe a modification of the mechanism of nucleation of crystals. The behavior of crystal nucleation in photostructured glasses under X-ray irradiation changes to the reverse: autocatalytic nucleation is suppressed rather than induced by x-ray CuK I± irradiation. These peculiarities of crystallization mechanism in glasses under study give one the possibility to choose the correct composition range depending on the problems facing researchers.

Sol–Gel Synthesis and Crystal Nucleation of Photosensitive Au/Ag- Containing Glasses of Lithium Silicate System

A sol–gel method has been proposed for obtaining the sintering mixture used in the preparing of a photosensitive Au/Ag-containing glasses of the Li2O - SiO2 system. It has been established that the use of sol–gel method for the production of the batch for synthesizing photosensitive Au/Ag-containing glasses gives more homogeneous distribution of Au/Ag metals in the bulk of the glass. Crystal-nucleation main characteristics have been defined in a large area of compounds of photo-structured compositions of a lithium-silicate system (23.4–46.0 Li2O mol % by analysis) both for spontaneous and catalyzed nucleations. The spontaneous (homogeneous) nucleation happened in the samples without any photosensitive metals and with photosensitive gold–silver additives (0.05 wt % above 100%) without irradiation; the catalyzed (heterogeneous) nucleation happened in the samples with photosensitive silver ore gold additives under x-ray CuKa irradiation. The ranges of the compositions of the glasses in which the effect of catalyzing irradiation is reversed (suppressing nucleation in accordance with the autocatalytic mechanism) under certain conditions. The spontaneous and catalyzed nucleation of crystals in the glasses of Li2O–SiO2 system caused by the addition of photosensitive impurities and X-ray irradiation was analyzed in glasses with a very wide range of compositions in Li2O–SiO2 system. Composition ranges with positive (+) and negative (-) differences in the rates of catalyzed and spontaneous steady-state nucleation were established. This finding point is favorable for obtaining photostructured (photosensitive) materials. The concentration limit was determined to be 37.98 mol % Li2O. About this value, we observe a modification of the mechanism of nucleation of crystals. The behavior of crystal nucleation in photostructured glasses under X-ray irradiation changes to the reverse: autocatalytic nucleation is suppressed rather than induced by x-ray CuKa irradiation. These peculiarities of crystallization mechanism in glasses under study give one the possibility to choose the correct composition range depending on the problems facing researchers.

Acicular ferrite formation during isothermal holding in HSLA steel

Microstructural observation and high-resolution dilatometry were employed to investigate acicular ferrite formation during isothermal holding in the HSLA steel. A decrease in isothermal temperature suppresses formation of polygonal ferrite and promotes formation of acicular ferrite. Island-like martensite/austenite constituents are dispersed inside of the acicular ferrite grain. The displacive model assuming autocatalytic nucleation was developed to describe the incomplete transformation phenomenon well. Decrease of isothermal temperature lowers the activation energy, and thus enhances the formation of acicular ferrite. Increase of amount of polygonal ferrite and acicular ferrite both results in decrease of M s. Formation of polygonal ferrite causes diffusion of solute alloying atoms into the untransformed austenite, which lowers M s. Acicular ferrite transformation suppresses martensitic transformation by diffusion of solution atoms during isothermal holding, and introduction of internal stress resulting from volume expansion during FCC → BCC transformation.

Exploring bainite formation kinetics distinguishing grain-boundary and autocatalytic nucleation in high and low-Si steels

Bainite formation in steels begins with nucleation of bainitic ferrite at austenite grain boundaries (γ/γ interfaces). This leads to creation of bainitic ferrite/austenite interfaces (α/γ interfaces). Bainite formation continues through autocatalysis with nucleation of bainitic ferrite at these newly created α/γ interfaces. The displacive theory of bainite formation suggests that the formation of bainitic ferrite is accompanied by carbon enrichment of surrounding austenite. This carbon enrichment generally leads to carbide precipitation unless such a reaction is thermodynamically or kinetically unfavourable. Each bainitic ferrite nucleation event is governed by an activation energy. Depending upon the interface at which nucleation occurs, a specific activation energy would be related to a specific nucleation mechanism. On the basis of this concept, a model has been developed to understand the kinetics of bainite formation during isothermal treatments. This model is derived under the assumptions of displacive mechanism of bainite formation. The fitting parameters used in this model are physical entities related to nucleation and microstructural dimensions. The model is designed in such a way that the carbon redistribution during bainite formation is accounted for, leading to prediction of transformation kinetics both with and without of carbide precipitation during bainite formation. Furthermore, the model is validated using two different sets of kinetic data published in the literature.

Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

The austenite to martensite transformation in a semi-austenitic stainless steel containing 17 wt-%Cr, 7 wt-%Ni and 1 wt-%Al was investigated with vibrating sample magnetometry and electron backscatter diffraction. Magnetometry demonstrated that, within experimental accuracy, martensite formation can be suppressed on fast cooling to 77 K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron backscatter diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of a retardation of the transformation followed by a new acceleration during slow isochronal (i.e. at constant rate) cooling is interpreted in terms of the combined effect of the strain energy introduced in the system during martensite formation, which thermodynamically and/or mechanically stabilises austenite, and autocatalytic nucleation of martensite.

INFLUENCE OF CRYSTALLIZATION TEMPERATURE ON THE ABSORPTION OF DIBUTYL PHTHALATE IN ZEOLITE A

In order to determine the effect of crystallization temperature on the absorption and properties of zeolite A particles, we have monitored the modified dibutyl phthalate absorption ( DBP ), a degree of crystalline phase, specific surface area, mean diameter of particle ( D S 50 % ), and performed scanning electron microscopy ( SEM ) of the synthesized samples of zeolites. The synthesis of samples was carried out at crystallization temperatures of 70, 75, 80, 85 and 90°C ; the raw materials (sodium aluminate and sodium silicate) were heated at 90°C. The particle size of the synthesized samples is similar in most studied systems at all temperatures of crystallization, which is in accordance with the principles of the autocatalytic nucleation and „memory effect” of the gel. An increase in the specific surface area of the synthesized samples with the increase in the crystallization temperature was observed in all the analyzed systems (3.25 to 35.31 m 2 /g). It was found that the increase of crystallization temperature increases the absorption of dibutyl phthalate(0.90-1.20 cm 3 /g ); however, at the same time, the proportion of zeolite A in the same samples is reduced, as confirmed by SEM analysis.

Effect of yttrium on nucleation and growth of zirconium hydrides

Addition of yttrium in zirconium causes precipitates of yttrium, which form two types of particles and are oxidized upon heat treatment. One type of particles with sub-micrometer scale sizes has a low population, whereas the other with nano scale sizes has a high population and cluster distribution. Owing to strong affinity of yttrium to hydrogen, the nanoparticles, mostly within the grains of the Zr–Y alloy, attract nucleation of hydrides at the clusters of the nanoparticles and cause preferential distribution of intragranular hydrides. In comparison with that of Zr, additional nanoparticles in the Zr–Y alloy impede further growth of hydride precipitates during hydriding. It is deduced that the impediment of growing hydride precipitates by the nanoparticles is developed during an auto-catalytic nucleation process, which leads to formation of thin and intragranular hydrides, favorable to mitigation of hydride embrittlement.

INFLUENCE OF CRYSTALLIZATION TEMPERATURE ON THE ABSORPTION OF DIBUTYL PHTHALATE IN ZEOLITE A

In order to determine the effect of crystallization temperature on the absorption and properties of zeolite A particles, we have monitored the modified dibutyl phthalate absorption ( DBP ), a degree of crystalline phase, specific surface area, mean diameter of particle ( DS50 % ), and performed scanning electron microscopy ( SEM ) of the synthesized samples of zeolites. The synthesis of samples was carried out at crystallization temperatures of 70, 75, 80, 85 and 90°C ; the raw materials (sodium aluminate and sodium silicate) were heated at 90°C. The particle size of the synthesized samples is similar in most studied systems at all temperatures of crystallization, which is in accordance with the principles of the autocatalytic nucleation and „memory effect” of the gel. An increase in the specific surface area of the synthesized samples with the increase in the crystallization temperature was observed in all the analyzed systems (3.25 to 35.31 m2/g). It was found that the increase of crystallization temperature increases the absorption of dibutyl phthalate(0.90-1.20 cm3/g ); however, at the same time, the proportion of zeolite A in the same samples is reduced, as confirmed by SEM analysis.