Front Nucleation Research Articles

Pattern Formation under Deep Supercooling by Classical Density Functional-Based Approach.

Solidification patterns during nonequilibrium crystallization are among the most important microstructures in the natural and technical realms. In this work, we investigate the crystal growth in deeply supercooled liquid using the classical density functional-based approaches. Our result shows that the complex amplitude expanded phase-field crystal (APFC) model containing the vacancy nonequilibrium effects proposed by us could naturally reproduce the growth front nucleation (GFN) and various nonequilibrium patterns, including the faceted growth, spherulite, symmetric and nonsymmetric dendrites among others, at the atom level. Moreover, an extraordinary microscopic columnar-to-equiaxed transition is uncovered, which is found to depend on the seed spacing and distribution. Such a phenomenon could be attributed to the combined effects of the long-wave and short-wave elastic interactions. Particularly, the columnar growth could also be predicted by an APFC model containing inertia effects, but the lattice defect type in the growing crystal is different due to the different types of short-wave interactions. Two stages are identified during the crystal growth under different undercooling, corresponding to diffusion-controlled growth and GFN-dominated growth, respectively. However, compared with the second stage, the first stage becomes too short to be noticed under the high undercooling. The distinct feature of the second stage is the dramatic increments of lattice defects, which explains the amorphous nucleation precursor in the supercooled liquid. The transition time between the two stages at different undercooling is investigated. Crystal growth of BCC structure further confirms our conclusions.

Diffusion driven barite front nucleation and crystallisation in sedimentary rocks

The need to investigate mineral precipitation in heterogeneous sedimentary rock with complex pore network and mineral composition arises with problems like pore clogging by barite precipitation during sulfate-rich water injection in geological reservoirs, the durability of long term storage of nuclear waste or the damage induced by crystallisation. At the LFCR (Laboratoire des Fluides Complexes et leurs Réservoirs, Anglet, France), we aim to reproduce geological objects in laboratory conditions. In this frame, we built a contra diffusive set up to precipitate a barite front in two sedimentary rock samples, a Lavoux oolitic limestone and a Vosges Sandstone with a fraction of clays. Two reservoirs filled respectively with BaCl2 and Na2SO4 dissolved in milli-Q water are in contact with a porous sample so that the ions diffuse through it. X-ray tomography shows that a barite front is precipitated in both samples at different positions depending on the diffusion of the different ions. SEM/EDS microanalysis on polish sections highlight both the barite front location and its connectivity. In the Lavoux limestone, a connected barite front is present. Fine barite aggregates preferentially precipitate in the smallest pores of the Lavoux sample, whilst the crystals precipitating in large macropores (>20 μm) show a preferential orientation. We propose that the Gibbs free energy barrier for barite heterogeneous nucleation in the limestone is lower in contact with micrite in small pores than in contact with euhedral calcite in large pores. Finally, the Gibbs energy barrier for barite homogeneous nucleation in large pores is the highest. In the Vosges sandstone, the barite front is scattered with well-crystallized barite precipitating in large pores, and a more striking thin layer of barite is precipitated in the interfoliar space of chlorite-smectite complex. Consequently, we propose that smectite can concentrate barium by adsorption. Then because sulfate and sodium concentrations increase, ultimately barium is desorbed and barite precipitate.

Origins of plastic shock waves in single-crystal Cu

We investigate shock wave propagation in single-crystal Cu with large-scale molecular dynamics simulations. Plastic shock waves propagate via dislocation nucleation or growth. With decreasing particle velocity, a remarkable drop in plastic shock wave velocity relative to the linear shock velocity–particle velocity relation is observed in the elastic–plastic two-wave regime for different loading directions. This reduction can be attributed to the changes in the mechanisms of plastic shock wave generation/propagation, from the dislocation nucleation-dominant mode, to the alternating nucleation and growth mode, and to the growth-dominant mode. For weak shocks, the plastic shock advances at the speed of the growth of existing dislocations (below the maximum elastic shock wave speed), considerably slower than the dislocation nucleation front for strong shocks (above the maximum elastic shock wave speed).

Nucleation fronts ignite the interface rupture that initiates frictional motion

Nucleation fronts ignite the interface rupture that initiates frictional motion

Crystal nucleation and growth of spherulites demonstrated by coral skeletons and phase-field simulations

Spherulites are radial distributions of acicular crystals, common in biogenic, geologic, and synthetic systems, yet exactly how spherulitic crystals nucleate and grow is still poorly understood. To investigate these processes in more detail, we chose scleractinian corals as a model system, because they are well known to form their skeletons from aragonite (CaCO3) spherulites, and because a comparative study of crystal structures across coral species has not been performed previously. We observed that all 12 diverse coral species analyzed here exhibit plumose spherulites in their skeletons, with well-defined centers of calcification (CoCs), and crystalline fibers radiating from them. In 7 of the 12 species, we observed a skeletal structural motif not observed previously: randomly oriented, equant crystals, which we termed “sprinkles”. In Acropora pharaonis, these sprinkles are localized at the CoCs, while in 6 other species, sprinkles are either layered at the growth front (GF) of the spherulites, or randomly distributed. At the nano- and micro-scale, coral skeletons fill space as much as single crystals of aragonite. Based on these observations, we tentatively propose a spherulite formation mechanism in which growth front nucleation (GFN) of randomly oriented sprinkles, competition for space, and coarsening produce spherulites, rather than the previously assumed slightly misoriented nucleations termed “non-crystallographic branching”. Phase-field simulations support this mechanism, and, using a minimal set of thermodynamic parameters, are able to reproduce all of the microstructural variation observed experimentally in all of the investigated coral skeletons. Beyond coral skeletons, other spherulitic systems, from aspirin to semicrystalline polymers and chocolate, may also form according to the mechanism for spherulite formation proposed here. Statement of SignificanceUnderstanding the fundamental mechanisms of spherulite nucleation and growth has broad ranging applications in the fields of metallurgy, polymers, food science, and pharmaceutical production. Using the skeletons of reef-building corals as a model system for investigating these processes, we propose a new spherulite growth mechanism that can not only explain the micro-structural diversity observed in distantly related coral species, but may point to a universal growth mechanism in a wide range of biologically and technologically relevant spherulitic materials systems.

Breakdown of Barkhausen Criticality in an Ultrathin Ferromagnetic Film

Analysis of Barkhausen avalanches in thin films of magnetic material has been an active area of research due to the proximity of the dimensionality crossover. However, the measurement of avalanches in ultrathin films (thickness and . The observations could not be explained within the framework of the established front propagation or nucleation models under adiabatic evolution. We attempt to explain our findings by performing simulations of the 2D random field Ising model under non-adiabatic evolution with a constant field step, and obtain good qualitative agreement between experimental and simulated probability distributions for high disorder levels. We conclude that the observed peculiarities are a consequence of strong pinning and the departure from the adiabatic condition during magnetic field sweep.

Revealing the heterogeneous nucleation behavior of equiaxed grains of inoculated Al alloys during directional solidification

An in-situ study on the directional solidification of an inoculated Al-20 wt%Cu alloy under well-controlled constant cooling rates and temperature gradients has been carried out using a microfocus X-radiography set-up. The influences of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively. It is shown that under the same cooling rate, the nucleation rate of grains decreases with increasing temperature gradient. A high temperature gradient also promotes preferential growth of dendrite arms along the temperature gradient direction, and therefore the formation of elongated grains. However, the temperature gradient effects on nucleation and grain growth decrease with increasing cooling rate. It is revealed that the propagation velocity of the nucleation front in directional solidification castings is approximately equal to the ratio between cooling rate Ṫ and temperature gradient G. Based on the experimental observations, a novel numerical grain size prediction model has been proposed, in which the temperature gradient effect on the nucleation kinetics was rigorously treated by introducing two new concepts termed as ‘inhibited nucleation zone’ (INZ) and ‘active nucleation zone’ (ANZ). The model has been applied to simulate the present in-situ solidification experiments. A good agreement was achieved between the predicted grain number density and the experimental measurements, showing the importance of including the temperature gradient effect on heterogeneous nucleation. Furthermore, the present model also has the capability to predict the temperature gradient necessary for the transition from equiaxed to columnar grain growth.

A non-local implicit gradient-enhanced model for thermomechanical behavior of shape memory alloys

A three-dimensional, implicit gradient-enhanced, fully coupled thermomechanical constitutive model is developed within the framework of thermodynamic principles for NiTi shape memory alloys. This work focuses on unstable behaviors of NiTi samples under different thermomechanical loading conditions. Temperature variation and its coupling effect on non-local behavior of a shape memory alloy during a loading–unloading cycle at different strain rates are considered. The proposed constitutive equations are implemented into the finite element software ABAQUS, and the numerical investigations indicate that the used procedure is an effective computational tool for simulation of several behaviors of NiTi samples including phase front nucleation and propagation, stress–strain–temperature responses, and transformation-induced stress relaxation. The obtained results are shown to be in a good agreement with available experimental and numerical findings in the literature. The effectiveness of the model in removing mesh sensitivity is evaluated by investigating the mesh-dependence issue for the low strain rate problems through numerical examples.

Nucleation front instability in two-dimensional (2D) nanosheet gadolinium-doped cerium oxide (CGO) formation

Low temperature (<40 °C) transition from nanoparticles to 3D layered morphology is observed for gadolinium-doped cerium oxide.

Hydrodynamic theory of freezing: Nucleation and polycrystalline growth

Structural aspects of crystal nucleation in undercooled liquids are explored using a nonlinear hydrodynamic theory of crystallization proposed recently [G. I. Tóth et al., J. Phys.: Condens. Matter 26, 055001 (2014)JCOMEL0953-898410.1088/0953-8984/26/5/055001], which is based on combining fluctuating hydrodynamics with the phase-field crystal theory. We show that in this hydrodynamic approach not only homogeneous and heterogeneous nucleation processes are accessible, but also growth front nucleation, which leads to the formation of new (differently oriented) grains at the solid-liquid front in highly undercooled systems. Formation of dislocations at the solid-liquid interface and interference of density waves ahead of the crystallization front are responsible for the appearance of the new orientations at the growth front that lead to spherulite-like nanostructures.

Physical Modeling of the Mechanism of Modification with Wear-Resistant Surfacing

A modeling procedure is proposed based on established features of the effect of adding modifiers and process parameters simulating surfacing on the solidification front structure and intensity of primary phase equiaxed crystallite front nucleation and growth kinetics within the volume of melt.

Phase-field crystal modeling of heteroepitaxy and exotic modes of crystal nucleation

We review recent advances made in modeling heteroepitaxy, two-step nucleation, and nucleation at the growth front within the framework of a simple dynamical density functional theory, the Phase-Field Crystal (PFC) model. The crystalline substrate is represented by spatially confined periodic potentials. We investigate the misfit dependence of the critical thickness in the StranskiKrastanov growth mode in isothermal studies. Apparently, the simulation results for stress release via the misfit dislocations fit better to the PeopleBean model than to the one by Matthews and Blakeslee. Next, we investigate structural aspects of two-step crystal nucleation at high undercoolings, where an amorphous precursor forms in the first stage. Finally, we present results for the formation of new grains at the solid–liquid interface at high supersaturations/supercoolings, a phenomenon termed Growth Front Nucleation (GFN). Results obtained with diffusive dynamics (applicable to colloids) and with a hydrodynamic extension of the PFC theory (HPFC, developed for simple liquids) will be compared. The HPFC simulations indicate two possible mechanisms for GFN.

Extension of the compressible PISO algorithm to single-species aerosol formation and transport

In this paper, an Eulerian model for single-species aerosol production and transport is introduced, and solved using the Pressure Implicit with Splitting of Operators (PISO) algorithm. The aerosol droplets are described in terms of two moments of the droplet size distribution, i.e., the droplet number concentration and the liquid mass fraction. The compressible PISO algorithm for reacting flows is extended to incorporate the transport equations of these two moments. The scheme is applied to the simulation of vapor-to-droplet conversion in a Laminar Flow Diffusion Chamber (LFDC). In that setting, we show the numerical properties of the method for, first, carrier gas flow without the presence of vapor or droplets, and second, the production and evolution of aerosol droplets through nucleation and condensation. The method is shown to be second order in time and space. We adopt a TVD scheme the handle unphysical oscillations that may arise near sharp nucleation fronts. Good agreement is found with experimental data, in terms of the predicted temperature centerline profile (within 1%) and LFDC outlet droplet number concentration. The detailed validation and analysis of the model in combination with PISO may be used for more advanced aerosol modeling.

A route for the direct crystallization of dolomite

The direct crystallization of dolomite from an aqueous solution at temperatures between 60–220 °C was followed in situ through time-resolved synchrotron-based energy-dispersive X-ray diffraction combined with offline high-resolution imaging, X-ray diffraction, and infrared spectroscopy. Crystalline CaMg(CO3)2 phases form through a three-stage process. In the first stage, a nanoparticulate magnesium-deficient, amorphous calcium carbonate (Mg-ACC) with a nominal formula of Ca0.606Mg0.394CO3·1.37H2O forms. After a temperature-dependent induction time, during stage 2 the Mg-ACC partially dehydrates and orders prior to its rapid (<5 min) crystallization to non-stoichiometric proto-dolomite. This occurs via the dissolution of Mg-ACC, followed by the secondary nucleation of proto-dolomite from solution. The proto-dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de-nuovo and continuous formation of nanocrystalline proto-dolomite subunits that form spherical aggregates. In stage three of the reaction, the proto-dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale (hours to days), via an Ostwald-ripening mechanism. Such a three-stage crystallization can explain microbially induced proto-dolomites observed in modern hypersaline settings and may also be the route by which the Cryogenian cap dolomite deposits of the Neoproterozoic formed.

Direct observation of graphene growth and associated copper substrate dynamics by in situ scanning electron microscopy.

This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermore, the role of trace amounts of oxygen during growth is discussed and related to graphene-induced surface reconstructions during cooling. Above all, this work demonstrates the potential of the method for in situ studies of surface dynamics on active metal catalysts.

In Situ Reaction Kinetics and Microstructure Evolution in Preparing Particle-Containing Copper Alloys by Mechanical Alloying

To improve alloy properties and the distribution level of TiB2 particles in the alloy matrix, mechanical alloying was used to prepare the Cu-TiB2 alloy. But even after long-time ball milling, the in situ reaction between Cu-B and Cu-Ti alloy powders still cannot be observed in the whole process. The kinetics of the in situ reaction during high-temperature heat treatment was first investigated and led to the following results. With the increase of heat-treatment time t or diffusion depth X, both the moving speed Δx/Δt of reaction front edge and nucleation rate Z(x) of TiB2 particles decrease at a fast rate and finally stabilize at a very low level. The falling rate of moving speed Δx/Δt increases with the decrease of temperature. Although the CTi′ value of the Ti element gives a poor effect on the moving speed Δx/Δt, it significantly affects the nucleation rate of TiB2 particles, and its falling rate increases with a decreasing CTi′ value. And for the same diffusion depth X and CTi′, the nucleation rate of TiB2 decreases with increasing the heat-treatment temperature. These results were verified by the microstructure change after the heat treatments at temperatures of 773 K, 973 K, and 1173 K (500 °C, 700 °C, and 900 °C), respectively. Finally, the best combination of ball-milling and heat-treatment conditions was obtained.

Additive manufacturing of nickel-based superalloy Inconel 718 by selective electron beam melting: Processing window and microstructure

Abstract

Phase-Field Modeling of Polycrystalline Solidification: From Needle Crystals to Spherulites—A Review

Advances in the orientation-field-based phase-field (PF) models made in the past are reviewed. The models applied incorporate homogeneous and heterogeneous nucleation of growth centers and several mechanisms to form new grains at the perimeter of growing crystals, a phenomenon termed growth front nucleation. Examples for PF modeling of such complex polycrystalline structures are shown as impinging symmetric dendrites, polycrystalline growth forms (ranging from disordered dendrites to spherulitic patterns), and various eutectic structures, including spiraling two-phase dendrites. Simulations exploring possible control of solidification patterns in thin films via external fields, confined geometry, particle additives, scratching/piercing the films, etc. are also displayed. Advantages, problems, and possible solutions associated with quantitative PF simulations are discussed briefly.

A free-boundary problem for concrete carbonation: Front nucleation and rigorous justification of the $\sqrt{{t}}$-law of propagation

We study a one-dimensional free-boundary problem describing the penetration of carbonation fronts (free reaction-triggered interfaces) in concrete. Using suitable integral estimates for the free boundary and involved concentrations, we reach a twofold aim: (1) We fill a fundamental gap by justifying rigorously the experimentally guessed \sqrt{t} asymptotic behavior. Previously we obtained the upper bound s(t)\leq C'\sqrt{t} for some constant C' ; now we show the optimality of the rate by proving the right nontrivial lower estimate, i.e. there exists C''&gt;0 such that s(t)\geq C''\sqrt{t} . (2) We obtain weak solutions to the free-boundary problem for the case when the measure of the initial domain vanishes. In this way, we allow for the nucleation of the moving carbonation front – a scenario that until now was open from the mathematical analysis point of view.

The importance of amino acid interactions in the crystallization of hydroxyapatite

Non-collagenous proteins (NCPs) inhibit hydroxyapatite (HA; Ca(5)(PO(4))(3)OH) formation in living organisms by binding to nascent nuclei of HA and preventing their further growth. Polar and charged amino acids (AAs) are highly expressed in NCPs, and the negatively charged ones, such as glutamic acid (Glu) and phosphoserine (P-Ser) seem to be mainly responsible for the inhibitory effect of NCPs. Despite the recognized importance of these AAs on the behaviour of NCPs, their specific effect on HA crystallization is still unclear, and controversial results have been reported concerning the efficacy of HA inhibition of positively versus negatively charged AAs. We focused on a positively charged (arginine, Arg) and a negatively charged (Glu) AA, and their combination in the same solution. We studied their inhibitory effect on HA nucleation and growth at physiological temperature and pH and we determined the mechanism by which they can affect HA crystallization. Our results showed a strong inhibitory effect of Arg on HA nucleation; however, Glu was more effective in inhibiting HA crystal growth during the growth stage. The combination of Glu and Arg was less effective in controlling HA nucleation, but it inhibited HA crystal growth. We attributed these differences to the stability of complexes formed between AAs and calcium and phosphate ions at the nucleation stage, and in bonding strength of AAs to HA crystal faces during the growth stage. The AAs also influenced the morphology of synthesized HA. Presence of either Arg or Glu resulted in the formation of spherulites consisting of preferentially oriented nanoplatelets orientation. This was attributed to kinetic factors favoring growth front nucleation (GFN) mechanism.