Nucleation Behavior Research Articles

Nanopore facilitated monohydrocalcitic amorphous calcium carbonate precipitation.

Predicting the precipitation of solids is important in both natural systems and subsurface energy applications. The factors controlling reaction mechanisms, phase selection and conversion between phases are particularly important. In this contribution the precipitation and growth of an amorphous calcium carbonate species from flowing aqueous solution in a nanoporous controlled pore glass is followed in situ with differential X-ray pair distribution function analysis. It is discovered that the local atomic structure of this phase indicates monohydrocalcite-like pair-pair correlations, yet is functionally amorphous because it lacks long-range structure. The unexpected occurrence of synthetic proto-monohydrocalcite amorphous calcium carbonate, precipitated from a solution undersaturated with respect to published solubilities, suggests that nanopore confinement facilitates formation of an amorphous phase at the expense of more favorable crystalline ones. This result illustrates that confinement and interface effects are physical factors exerting control on mineral nucleation behavior in natural and geological systems.

Influence of a polymeric additive on the crystallisability and nucleation mechanism for the model fuel system of eicosane crystallising from supersaturated toluene solutions

The impact of a commercially available cold flow improver additive (IA) on crystallisation within a model fuel system of eicosane in toluene solutions is investigated. Polythermal crystallisation method utilising turbidometric detection is used to assess the influence of IA addition upon eicosane solubility and nucleation inhibition at different solution IA treat rates, ranging from 0.1 to 10 wt%. Whilst IA is found to have negligible effect upon eicosane solubility, its nucleation is found to be disrupted, resulting in a reduction in crystallisation temperatures as a function of higher IA treat rate, with IA and eicosane phase separation also taking place at treat rates of 2 wt% IA and above. A change from an instantaneous to a progressive nucleation mechanism of eicosane in the absence and presence of IA within solution, respectively, is found. This behaviour is rationalised through the proposition that IA is incorporated within developing eicosane prenucleation clusters, hence disrupting their ability to readily grow to a critical size for nucleation. Nucleation kinetic analysis confirms that at given solution undercoolings, higher IA treat rate reduces the rate of nucleation due to an increase in the effective interfacial tension to nucleation, in line with a higher degree of thermodynamic control of the nucleation process, as might be expected for the progressive nucleation behaviour.

Analysis of Heterogeneous Nucleation and Erosion Behavior Considering the Injection of Impurities into Wet Steam Flow Using Poly-dispersed Method

Nowadays, an enormous amount of electricity is gained from steam turbines, and solid salt injection into the steam turbines is used to control pH. Therefore, the heterogeneous nucleation will occur because of the presence of impurity (solid salt) in the steam turbine's low-pressure stage; and some changes take place in the operation of the turbine stage and power of the steam turbine due to the presence of impurities. As a result, there will be two groups of liquid droplets: the first group includes liquid droplets arising from spontaneous homogeneous condensation, and the next group includes liquid droplets arising from heterogeneous condensation. In this study, the poly-dispersed method has been used in the Eulerian-Eulerian approach for modeling the liquid droplets of each group. In more researches, the effects of heterogeneous nucleation on the supersonic stream among blades of steam turbines of the steam cycle have been neglected. The purpose of this study is to calculate and consider the effects of heterogeneous nucleation by injecting NaCl into the parameters of wet-steam flow, erosion rate, and turbine power. The numerical results gained due to the presence of solid particles (compared to their absence) show a reduction in condensation shock, a decrease in erosion rate, and a 0.022 MW increase in the power of the steam turbine. Overall, the erosion rate of blades and power generation can be considered two important parameters in designing and repairing steam turbine blades.

Atomistic Study on the Energetic and Mechanical Behaviors of Helium Bubble Nucleation and Growth in RAFM Steel

Helium produced by neutron irradiation is a crucial inducement to bring about the property of deterioration of structural materials served in a fusion reactor. To investigate the nucleation and growth behavior of helium bubbles in reduced activation ferritic/martensitic steels, which comprise one of the most promising candidate structural materials, the Molecular Statics method and the Metropolis Monte Carlo algorithm are combined to investigate the energetic and mechanical behaviors of HenVm clusters in α-Fe. The simulation results show that the vacancy and helium atom binding energy are inclined to reach a saturation state, i.e., 4.0 eV for the vacancy and 2.4 eV for the helium atom; however, the binding energy of self-interstitial atoms decreases to minus values at high helium-to-vacancy (He/V) ratios. The crossover of the binding energy curve of the helium and vacancy indicates that the equilibrium He/V ratio is 1.68 during the nucleation of helium bubbles. Meanwhile, the dissociation energy analysis indicates that the stable He/V ratio of the clusters is 1.3 at high temperatures. Moreover, the pressure analysis of the HenVm clusters indicates that the He/V ratio corresponding to their mechanical equilibrium state varies from 0.50 to 0.65 at 0 K. Furthermore, the analysis combined with the relevant experimental data of helium density in helium bubbles indicates that the actual He/V ratio of helium bubbles in the served materials is closely relevant to the irradiation condition, such as helium production rate, temperature, etc. The investigation results in this paper contribute to elucidate the microscopic process of helium bubble nucleation and growth and provides the energetic and mechanical parameters of small-sized helium bubbles with different sizes for large-scale simulation studies.

Potassium Sulfate: A New Candidate to Explore Non-Photochemical Laser-Induced Nucleation Mechanisms

In this paper, we report a study on the nucleation behavior of potassium sulfate (K2SO4) from aqueous solutions under the influence of unfocused nanosecond laser pulses. The objective is to contribute to the general understanding of the Non-Photochemical Laser-Induced Nucleation (NPLIN) mechanism. First, the influence of several parameters such as supersaturation as well as laser parameters (pulse energy, number of pulses, and laser polarization) on induction time, probability of nucleation and mean number of crystals in comparison with spontaneous nucleation was investigated. Then, we examined the influence of gas composition (i.e., degassing and gas bubbling (CO2 and N2)) of the supersaturated solutions on the NPLIN kinetics, showing no correlation between gas content (or nature) on the crystallization behavior. Our study questions the role of impurities within the solution regarding the mechanism of laser-induced nucleation.

Lithiophilic anchor points enabling endogenous symbiotic Li3N interface for homogeneous and stable lithium electrodeposition

The safety hazards and short lifecycles caused by uncontrollable dendrite growth and infinite volume expansion hamper the widespread deployment of Li metal anodes. To address these issues, it is critical to regulate the nucleation and electrodeposition behaviors of Li. Herein, tetraaminophthalocyanine (TAPC) as lithiophilic anchor points modified on three-dimensional (3D) carbon skeleton (denoted as TAPC@CF) as both current collector and host material, is rationally designed for stabilizing Li metal anodes. Li3N spontaneously generated from the reaction between TAPC and Li enables upgraded electrochemical kinetics. Moreover, density functional theory (DFT) results reveal that the evenly distributed TAPC nanoparticles can effectively guide the homogeneous smooth deposition of metallic Li via strong affinity between Li and conjugation groups. Symmetrical cells containing TAPC@CF-Li anodes afford a stable cycling for 3500 h at 10 mA cm−2. Further, coupled with a high-loading LiFePO4 cathode (15.0 mg cm−2), the stability and feasibility of the modified electrode in practical cells are proved.

Dependence of Crystallization Behavior of Interacting Telechelic Poly(butylene succinate) Oligomer on Molecular Weight

A large spherulite structure deteriorates the mechanical properties of crystalline polymers, and therefore various methods have been explored to increase primary nucleation density. Recently, chain-end modification has been proposed as an effective approach for regulating polymer crystal nucleation. However, the relevant nucleation mechanism still requires investigation. Therefore, in this work, 2-ureido-4[1H]-pyrimidinone (UPy) units, which can form stacks via quadruple hydrogen bonds with each other, are introduced as end groups for the preparation of interacting telechelic poly(butylene succinate) (PBS-UPy) oligomers with different molecular weights (Mns). The crystallization, especially the nucleation behavior of PBS-UPy, is studied in detail by comparing with the corresponding pre-polymer, the hydroxyl-terminal PBS (PBS-OH). The thermal properties of PBS-UPy exhibit similar Mn-dependent tendency to those of PBS-OH, but with weaker total crystallization rate. The spherulite growth rate is significantly reduced, whereas the primary nucleation density is highly promoted, after introducing UPy groups. Further investigation reveals that the mechanism of UPy stacks’ influence on nucleation ability changes from inhibition to promotion with respect to Mn. Even under an inhibition of nucleation ability, the final nucleation density is obviously increased because of a significant decline of the growth rate. In addition, the change in the impact of UPy stacks on nucleation ability is speculated to originate from the memory expression feasibility of ordered conformation in the melt during crystallization.

A Hybrid Model to Predict Formulation Dependent Granule Growth in a Bi-Component Wet Granulation Process.

In this study, a hybrid modeling framework was developed for predicting size distribution and content uniformity of granules in a bi-component wet granulation system with components of differing hydrophobicities. Two bi-component formulations, (1) ibuprofen-USP and micro-crystalline cellulose and (2) micronized acetaminophen and micro-crystalline cellulose, were used in this study. First, a random forest method was used for predicting the probability of nucleation mechanism (immersion and solid spread), depending upon the formulation hydrophobicity. The predicted nucleation mechanism probability is used to determine the aggregation rate as well as the initial particle distribution in the population balance model. The aggregation process was modeled as Type-I: Sticking aggregation and Type-II: Deformation driven aggregation. In Type-I, the capillary force dominant aggregation mechanism is represented by the particles sticking together without deformation. In the case of Type-II, the particle deformation causes an increase in the contact area, representing a viscous force dominant aggregation mechanism. The choice between Type-I and II aggregation is determined based on the difference in nucleation mechanism that is predicted using the random forest method. The model was optimized and validated using the granule content uniformity data and size distribution data obtained from the experimental studies. The proposed framework predicted content non-uniform behavior for formulations that favored immersion nucleation and uniform behavior for formulations that favored solid-spreading nucleation.

Ice nucleation forced by transient electric fields.

Icing affects many technical systems, like aircraft or high-voltage power transmission and distribution in cold regions. Ice accretion is often initiated by ice nucleation in sessile supercooled water droplets and is influenced by several influencing factors, of which the impact of electric fields on ice nucleation is still not completely understood. Especially the influence of transient electric fields is rarely or not at all investigated, even though it is of great interest, e.g., for high-voltage transmission lines or for the food industry. In the present study the impact of transient electric fields on ice nucleation in supercooled sessile water droplets is experimentally investigated under well-defined conditions. A set of droplets is cooled down to a certain temperature and is subsequently exposed to electric fields generated from standard lightning or standard switching impulse voltages, which are commonly used for testing of high-voltage equipment. The nucleation behavior of individual droplets is captured using a high-speed camera and the effect of the transient electric field on ice nucleation is analyzed by considering both the singular and the stochastic nature of nucleation. While the singular nature of nucleation is referred to during analysis of the relative number of droplets remaining liquid long times after the impulse voltage, its stochastic nature is accounted for in the analysis of the temporal evolution of the relative number of frozen droplets. It is shown that low electric field strengths (E[over ̂]≤6.52kV/cm) only have a negligible impact on ice nucleation, independent of the supercooling. In contrast, high electric field strengths (E[over ̂]≥9.78kV/cm) promote significantly ice nucleation. It is also shown that depending on the supercooling, the freezing delay of the different droplets in the ensemble may vary over several magnitudes for the same conditions. It is demonstrated that the electric field appears to indirectly affect the nucleation rate by generating droplet oscillations, finally promoting ice nucleation. The experiments clearly demonstrate the possibility to actively force ice nucleation by applying transient electric fields. These results improve the understanding of ice accretion on high-voltage insulators and may also lend insight into freezing processes in food industry. We expect that these results will be a valuable contribution in formulating and/or validating new nucleation models.

Semiflexible oligomers crystallize via a cooperative phase transition.

Semicrystalline polymers are ubiquitous, yet despite their fundamental and industrial importance, the theory of homogeneous nucleation from a melt remains a subject of debate. A key component of the controversy is that polymer crystallization is a non-equilibrium process, making it difficult to distinguish between effects that are purely kinetic and those that arise from the underlying thermodynamics. Due to computational cost constraints, simulations of polymer crystallization typically employ non-equilibrium molecular dynamics techniques with large degrees of undercooling that further exacerbate the coupling between thermodynamics and kinetics. In a departure from this approach, in this study, we isolate the near-equilibrium nucleation behavior of a simple model of a melt of short, semiflexible oligomers. We employ several Monte Carlo methods and compute a phase diagram in the temperature-density plane along with two-dimensional free energy landscapes (FELs) that characterize the nucleation behavior. The phase diagram shows the existence of ordered nematic and crystalline phases in addition to the disordered melt phase. The minimum free energy path in the FEL for the melt-crystal transition shows a cooperative transition, where nematic order and monomer positional order move in tandem as the system crystallizes. This near-equilibrium phase transition mechanism broadly agrees with recent evidence that polymer stiffness plays an important role in crystallization but differs in the specifics of the mechanism from several recent theories. We conclude that the computation of multidimensional FELs for models that are larger and more fine-grained will be important for evaluating and refining theories of homogeneous nucleation for polymer crystallization.

Insights into the Role of Dipentaerythritol in the Thermodynamics and Nucleation Behavior of a Pentaerythritol–Water System

The experimental metastable zone widths of PE–DPE–H2O systems were measured by a polythermal method, and the nucleation behavior of pentaerythritol (PE) and the influence of dipentaerythritol (DPE)...

Designing spacial skeleton for lithium metal anode with Li+ concentration regulation and interfacial modification

Lithium metal has been considered as one of the most promising anodes for the endeavoring pursuit of advanced batteries due to its ultra-high capacity of ~3860 mAh g−1. To solve the current problems of lithium metal anode (LMA), such as unrollable dendritic growth, dead Li accumulation, and the resulted pulverization, three-dimensional frameworks have been adopted to maintain the structural integrity of LMA, but the issues including interfacial stability and affinity, undesired Li-ion deletion at the electrolyte/electrode interface are still challenging. Herein, by adopting hierarchical porous graphitic carbon (HPGC) as LMA skeleton and the atomic-level MgO for interface modification, the ideally uniform interface with strong Li affinity can effectively regulate the nucleation and deposition behavior of Li metal; while with the large quantity of Li+ storage in the micro-porous of HPGC, both the spatial confinement of Li+ flux and the increase in Li+ concentration at electrode/electrolyte interface can be achieved to facilitate the planer Li metal electrodeposition. Furthermore, the delithiation of HPGC with slightly higher potential can effectively reduce the formation of dead-Li via preventing the depletion of Li with inexhaustible Li+ storage during stripping process. The superiority of MgO@HPGC hosted LMA can be demonstrated both as coating layer and interior skeleton for different kinds of LMA applications, with enhancement in both long term cycling stability (~650 h) and high coulombic efficiency (~97%) over 390 cycles.

Quantification and Characterization of Microdamage Resistance in Metals for Designing High-Strength Ductile Microstructures

ConspectusFractures in metallic materials such as ductile austenitic, ferritic, and dual-phase steels often occur after significant plastic deformation. The dislocation-driven plasticity enhances the microscopic stress concentration and induces vacancies (e.g., by the jogged screw dislocation motion), resulting in microstructure-scale cracks and voids at specific crystallographic planes or microstructure boundaries. (Hereafter, cracks and voids are referred to as damage.) In addition, plasticity plays important roles in microscopic damage growth in terms of damage tip blunting and coalescence. In fact, plasticity-related damage evolution in various fracture phenomena such as metal fatigue and hydrogen embrittlement still contains many uncertainties, particularly in high-strength materials containing fine and complex microstructures. Therefore, microscopy-based damage quantification and characterization are required for designing damage-resistant microstructures. In this context, because damage evolution is a multiscale phenomenon from atomistic to over millimeter scales, the target size of damage in the measurements is important to realize reliable analyses. The intrinsic origin of damage evolution is an atomistic/nanometer scale phenomenon, which requires transmission electron microscopy for its analysis. However, from mechanical viewpoints, revealing damage nucleation behavior is not necessary, because general mechanical analyses are performed for oversubmicrometer-sized damages that can be observed by optical microscopy and scanning electron microscopy. Hence, submicrometer/micrometer-sized damage is the major target in this damage quantification. As a post-mortem analysis method, the damage area fraction, the number of damages, the damage size, and the damage shape at various plastic strains can be quantified by observing micrometer-sized damages. In the case of monotonic tensile deformation, the number density of damages plotted against strain indicate the damage initiation probability. In addition, when damages stop growing, a strain range where the average damage size remains nearly constant appears. The strain range quantitatively indicates damage arrestability. For instance, dual-phase steel consisting of soft and hard body-centered cubic phases clearly shows three strain-dependent damage evolution regimes: (1) damage incubation regime (no damage appears), (2) damage arrest regime (damage initiates, but stops growing immediately), and (3) damage growth regime. Hydrogen uptake increases damage initiation probability and decreases damage arrestability. Specifically, a comparison of the quantified damage parameters between specimens with and without hydrogen revealed that large amounts of hydrogen in the dual-phase steel degrades the resistance to microscopic damage growth, which critically decreases the macroscopic ductility. In this case, the damage arrestability (damage growth resistance) is more important than crack initiation resistance, which is dependent on the dislocation-driven stress accommodation capability of soft microstructure components surrounding the crack initiation site. Thus, damage quantification enables the evaluation and classification of microscopic problems in plasticity-related fractures. After the classification of the problems, the underlying mechanisms for damage initiation and growth must be clarified separately. To reveal the mechanisms, full-field micrometer-scale strain mapping and site-specific dislocation characterization are required for damage initiation and growth. To fulfill these demands, new techniques were developed, such as micrometer-scale replica digital image correlation and dislocation-resolved in situ electron channeling contrast imaging. Based on the deformation/dislocation characterization results, metallurgical models of damage evolution can be developed, enabling to propose a mechanism-based microstructure-design strategy for creating damage-resistant high-strength materials.

Atomistic interpretation of extra temperature and strain-rate sensitivity of heterogeneous dislocation nucleation in a multi-principal-element alloy

Incipient plasticity of crystals is usually initiated from versatile pre-existing crystalline defects of different geometries and length scales that formed during materials fabrication and processing, which is closely associated with the mechanical properties. Here we study the heterogeneous dislocation nucleation behaviors from an existing nanoscale void embedded in a prototypical multi-principal-element (MPE) NiCoCr alloy via atomistical exploration of possible minimum energy pathways and analyzes of the dislocation kinetics by a continuum-level mechanistic model, in comparison with the same plastic mechanism in an elemental face-centered cubic copper. It is found that the rough nucleation pathway of dislocation brings about extra thermal softening and strain-rate sensitivity for the critical nucleation stress of dislocation in the MPE alloy, which reproduces the experimental observations. The extra temperature softening is revealed to be associated with the high total entropy of the MPE alloy, in which the configurational entropy plays a dominating role rather than the vibrational counterpart. The extra strain rate sensitivity is caused by the relatively small activation volume in MPE alloy. Our strategy offers physical understanding of the experimental phenomena with atomistic details, which also emphasizes the critical role of configurational disorder in the dislocation accommodated plasticity in the generic complex concentrated alloys.

Li migration, nucleation and growth behavior regulated by a lithiophilic cobalt phosphide-doped carbon nanofibers derived ion/electron conductive framework

The uncontrollable Li dendritic plating and infinite volumetric expansion significantly plague the practical applications of lithium (Li) metal, which is regarded as the most promising high-energy-density anode material for rechargeable batteries. The well regulated Li-ion (Li+) flux distribution and Li deposition is the prerequisite for feasible Li-metal batteries. In this work, the Li migration, nucleation and growth behavior are investigated with a lithiophilic CoP-doped carbon nanofibers (CoP@CNF)-guided self-standing substrate. The high adsorption effect and low migration barrier of CoP to Li+ lead to the reversible conversion reaction between Li and CoP, enabling an even charge transportation and a homogeneous Li+ concentration gradient. The uniform nucleation and dense electrodeposition of Li over through the CoP@CNF is demonstrated via combined in-situ/ex-situ morphologic and electrochemical characterizations. Benefiting from the small nucleation and growth overpotential of Li, the symmetrical cell of CoP@CNF@Li can steadily operate under 0.5 mA cm−2 over 2000 h with a low voltage hysteresis of ca. 12 mV. When paired with a LiFePO4 cathode, a specific capacity exceeding 90 mAh g−1 up to 1000 cycles is achieved at 5 C (1 C = 170 mA g−1), demonstrating the dendritic Li suppression capability of the CoP@CNF@Li composite anode.

Role of martensitic transformation sequences on deformation-induced martensitic transformation at high strain rates: A quasi in-situ study

For the mechanical behavior of steels with metastable austenite, it was widely accepted that high strain rates could weaken deformation-induced martensitic transformation (DIMT) and reduce work-hardening behavior. This limits the application of these steels for energy-absorption applications, e.g., impact conditions. However, the mechanism of strain-rate dependent DIMT is still not well understood and it greatly limited the further alloy design. Thus, with the aim of revealing the intrinsic relations between strain rate and martensitic transformation, quasi-in-situ EBSD (Electron Backscatter Diffraction) tests were performed under different strain rates in this study. The influence of the martensitic transformation sequences on the DIMT volume fraction was investigated. With increasing strain rate, the γ→ε→α’ transformation was inhibited, and the γ→α’ transformation sequences dominated. The martensitic transformation sequences firstly affect the nucleation behavior. The α′-martensite nucleation sites changed from ε or twin to twin only, and thus, the α′-martensite nucleation barrier increased. In addition to nucleation, the variant selection phenomenon caused by the absence of ε-martensite under high strain rates increased the elastic strain energy, which inhibited the growth of α′-martensite. By affecting both nucleation and growth, high strain rates suppress the γ→ε→α’ transformation, and inhibit DIMT kinetics. Thus, by tailoring the martensitic transformation sequences, two methods are proposed to enhance DIMT behavior via suitable control of the stacking fault energy. This work provides a comprehensive explanation for the strain-rate dependence of DIMT, which can be used to tailor the mechanical properties of steels with metastable austenite.

Stochastic shelf-scale modeling framework for the freezing stage in freeze-drying processes

Freezing and freeze-drying processes are commonly used to improve the stability and thus shelf life of pharmaceutical formulations. Despite strict product quality requirements, batch heterogeneity is widely observed in frozen products, thus potentially causing process failure. Such heterogeneity is the result of the stochasticity of ice nucleation and the variability in heat transfer among vials, which lead to unique freezing histories of individual vials.We present for the first time a modeling framework for large-scale freezing processes of vials on a shelf and publish an open source implementation in the form of a python package on pypi. The model is based on first principles and couples heat transfer with ice nucleation kinetics, thus enabling studies on batch heterogeneity. Ice nucleation is assumed to be an inhomogeneous Poisson process and it is simulated using a Monte Carlo approach. We applied the model to understand the individual pathways leading to batch heterogeneity. Our simulations revealed a novel mechanism how ice nucleation leads to heterogeneity based on thermal interaction among vials. We investigated the effect of various cooling protocols, namely shelf-ramped cooling, holding steps and controlled nucleation, on the nucleation and solidification behavior across the shelf. We found that under rather general conditions holding schemes lead to similar solidification times, as in the case of controlled nucleation, thus identifying a potential pathway for freezing process optimization.

Effect of void morphology on void facilitated plasticity in irradiated Cu/Nb metallic nanolayered composites

Voids are common radiation-induced defects in nuclear materials and produce detrimental effects on mechanical properties. The influence of a pre-existing void on mechanical behavior has been investigated in single crystals, but the effects of void located at bimetal interfaces on the dislocation nucleation mechanisms and associated mechanical performance have not been fully studied. Using Cu/Nb metallic nanolayered composites (MNCs) as a model system, with atomic-scale simulations, we report on the influence of a pre-existing void at the Cu/Nb interface on dislocation nucleation and deformation behavior. Compared with the void-free system, the size, location and shape of the voids can influence dislocation behavior significantly, such as the initial nucleation site, the critical nucleation stress, and the preferred slip system. As the size of the faceted void increases from 3 to 4 nm, the dominant mechanism of dislocation nucleation changes from a misfit dislocation-assisted localized shear to a void edge-assisted Frank-Read-like mechanism. Also, the location of the faceted void can change the coupled strain field between the void and the misfit dislocation, ultimately affecting the preferred slip system selection. In contrast, the spherical void cannot effectively activate the Frank-Read-like dislocation source due to the absence of edges. This work reveals the effects of void morphology on dislocation behavior, and provides an explanation for co-dominated plasticity of bimetal interface and void.

Relaxation effect on crystal nucleation in a glass unveiled by experimental, numerical, and analytical approaches

In this work, we significantly extend our new approach for analyzing nucleation kinetics below the glass transition, Tg, which considers the structural relaxation effect. Using a 5BaO·8SiO2 (B5S8) glass, we showed a significant stepwise change in the nucleation relaxation time, τsr, with temperature in the Tg region, in contrast with the expected monotonous variation with temperature: τsr is four times shorter for the supercooled liquid than for the glass. Then, using a combination of new experimental data and theoretical (analytical and numerical) analyses, we advance an interpretation within the Classical Nucleation Theory (CNT) framework that brings together the nucleation rates in the T > Tg and T < Tg ranges. We show once more that the so-called “breakdown” of the CNT is merely the result of underestimated nucleation rates due to incomplete glass relaxation during limited experimental nucleation times, confirming our recent findings for Li2O·2SiO2 and 2Na2O·CaO·3SiO2. Finally, for the first time, we also show that due to the almost complete crystallization of the B5S8 glass, and likely of other glasses that exhibit sufficiently high nucleation and growth rates, the relaxation process cannot be finished in the low-temperature range and the ultimate steady-state nucleation regime cannot be reached. These combined results shed light on the complex nucleation behavior below Tg.

Systematic study on orientation relationships between acicular ferrite and Ti-Mg oxide at different cooling rates in low-carbon steel

In this paper, the influence of cooling rate on acicular ferrite (AF) nucleation and growth on Ti-Mg oxide was studied using high-temperature confocal laser scanning microscopy. The compositions and elemental distributions of the inclusions were studied via electron probe microanalysis, and the crystallographic orientation relationships between AF and inclusions were investigated systematically via electron back scattered diffraction. The results indicated that the initial transformation temperature of AF decreased from ~656 to ~609 °C and the growth rate of AF raised from 6.74 to 55.52 μm/s as the cooling rate increased from 1 to 12 °C/s. The area fraction and the nucleation probability of AF reached a maximum at 2 °C/s, which was the most suitable cooling rate for AF formation. The Ti-Mg-Mn-O, Ti-Mg-Mn-O·TiN and Ti-Mg-Mn-O·MnS inclusions in Ti-Mg deoxidized steel were all effective for AF nucleation. For the four samples at different cooling rates, all AF grains had four kinds of orientation relationships with Ti-Mg-Mn-O oxides: Baker–Nutting (B-N), Nishiyama–Wasserman (N-W), {100} Ferrite ║{110} Oxide , and irrational orientation relationships. The {100} Ferrite ║{110} Oxide orientation was also a common parallel relationship between the B-N and N-W orientation relationships. The lattice mismatch between the AF and oxide was low, and the interface between the AF and oxide was close to a coherent interface when the first three kinds of orientation relationships were adopted. The formation of a Mn-depleted zone (MDZ) around Ti-Mg-Mn-O oxide could increase the driving force of ferrite formation, and it contributed more to the nucleation of those AF grains that had no orientation relationships with oxides. • The nucleation and growth behaviors of acicular ferrite on Ti-Mg oxides at different cooling rates were observed. • Orientation relationships between acicular ferrite and Ti-Mg oxides were examined via electron back scattered diffraction. • The nucleation mechanisms of acicular ferrite induced by Ti-Mg oxides were discussed in detail.