Pre-existing Nucleus Research Articles

A three-dimensional phase-field model for studying the orientation-dependent interface evolution in stress-induced martensitic phase transformation

In this study, we address the challenging task of predicting the motion of interfaces in stress-driven martensitic phase transformations within shape memory alloys. The novelty of our approach lies in the introduction of a monolithically solved thermodynamic-based phase-field method, specifically tailored for large strain conditions at the nanoscale for three-dimensional problems. To achieve this, we have developed a custom finite element software seamlessly integrated into the FEniCS open-source framework, providing a sophisticated and efficient tool for the examination of nanostructure evolution. Our investigation delves into five distinct and complex scenarios under diverse loading conditions for two– and three-dimensional problems, each presenting unique challenges: (i) a straightforward uniaxial tension scenario featuring a square domain with a pre-existing martensitic nucleus; (ii) the evolution of interfaces in a square sample incorporating a circular central nanovoid under biaxial tensile stress; (iii) the analysis of a rectangular beam subjected to horizontal and vertical compressive loads; (iv) the assessment of an initially voided rectangular beam experiencing mixed loading conditions; and (v) the three-dimensional simulations of cubic-to-tetragonal phase transformation using various orientation of the habit plane. In contrast to prior studies, our analysis not only explores the standard factors of habit plane reorientation and strain transformation values but also delves into the impact of nanovoids on austenite–martensite interface evolution.

New towns: the dilemma of newness and Genius Loci. The case of Bouinan, Algeria

ABSTRACT Newness, or building from scratch, has long been considered as the mantra of the New Towns. Most early New Towns were built on Ex-nihilo sites. That provides the state of mental and physical blankness - best known as tabula rasa - that modern planning favours. It is only after generations that relying on pre-existing nucleus is realised to be an alternative that helps to achieve place-making and promotes Genius Loci. The paper first presents a contribution to this alternative approach, encapsulates its philosophy and turns it into a working tool. It then discusses the process of its deployment. Bouinan New Town, (Algeria), provides a raw example of a New Town on which the approach was applied. It discusses the potentials and limits of the approach through its three parameters that are Man, Space, and Time, and resolving the dilemma between Genius Loci and Newness. Results and findings aim at fine-tuning the next New Towns generation that is envisaged in the 2030 Vision Plan and enrich the New Towns world literature in countries having similar conditions.

Thermodynamically consistent nonlocal kernel with boundary effect compensation and its application to the coupled phase field-nonlocal integral elasticity equations for modeling of martensitic transformations

In this article, the thermodynamically consistent compensated two-phase (CTP) kernel is employed for nonlocal integral elasticity based phase field modeling of stress-induced martensitic transformations. Utilizing a proper thermodynamic framework, the stress-strain relation with the CTP kernel is shown to be thermodynamically consistent. The coupled Ginzburg–Landau and local/nonlocal elasticity equations are solved using the finite element method. The advantages of the CTP kernel over previous kernels are shown through stress-induced martensitic growths in a simply connected region, in presence of a hole, in presence of a crack, and in a sample with a preexisting nucleus. In contrast to other widely used nonlocal kernels, for the CTP kernel, no ill-posedness is observed, the normalization and locality recovery conditions are satisfied and the boundary effects are entirely compensated. The numerical convergence of a phase field-nonlocal integral elasticity problem is studied, which indicates that the CTP kernel does not suffer from the numerical convergence issues of previous kernels. The present study provides a better insight into the CTP kernel and its application to the modeling of various phenomena at the nanoscale.

Elemental and radiogenic isotope perspective on formation and transformation of cratonic lower crust: Central Slave craton (Canada)

Kimberlite-borne granulite xenoliths provide rare insights into the age, chemical composition and tectonothermal evolution of the otherwise largely inaccessible deep cratonic crust. The formation and transformation of the lower continental crust (LCC) beneath the central Slave craton (Canada) is here illuminated using whole-rock trace-element and Sr-Nd isotope compositions of nine metabasaltic (MBG), one gabbroic (MGG) and two metasedimentary/hybrid (MSG) granulite xenoliths. On the one hand, published sulphide Re-Os and a few zircon U-Pb data indicate that at least a portion of the LCC beneath the central Slave craton has a Palaeoarchaean origin (∼3.3 Ga), which apparently coincides with a period of juvenile crust and deep lithospheric mantle formation during plume impingement beneath the pre-existing cratonic nucleus. On the other hand, enrichment in Li, Sr, LREE, Pb and Th, but relative depletion in Ti, Hf and HREE, suggest formation of (picro)basaltic protoliths by partial melting of a subduction-modified garnet-bearing source, Crystallisation in the crust after fractionation of plagioclase is inidicated by their Sr and Eu negative anomalies, which are complementary to the positive anomalies in the MGG. Samarium-Nd isotopes in MBG and MGG show large scatter, but fall on Neo- or Mesoarchaean age arrays. These elemental systematics are suggested to fingerprint deserpentinisation fluids plus small amounts of sedimentary melt as the main contaminants of the mantle source, supporting the operation of at least regional and transient subduction at 3.3 Ga. Evidence for quasi-coeval plume impingement and subduction beneath the central Slave craton in the Mesoarchaean is reconcilable in a dynamic regime where vertical tectonics, though waning, was still active and plate interactions became increasingly important. Unradiogenic 87Sr/86Sr (down to 0.7017) is consistent with significant loss of Rb and probably other heat-producing elements (K, Th, U) plus H2O during Neoarchaean metamorphism, which helped to enhance LCC viscosity and stabilise the cratonic lithosphere.

Effect of pre-existing nuclei on crystallization during laser welding of Zr-based metallic glass

In this study, experiments are carried out to demonstrate that in laser welding of a Zr-based (Zr52.5Ti5Al10Ni14Cu17.9) bulk metallic glass (BMG), pre-existing nuclei nucleus density has significant influence on its crystallization behavior. Based on the classical nucleation/growth theory, it is concluded that a small amount of pre-existing nuclei in a BMG can shift the time-temperature-transformation (TTT) curve from a well-known ‘C-shape’ to a ‘ε-shape.’ This result provides fundamental understanding on why the shape of the TTT curve for a heating process is different from that for a cooling process for the same BMG. Two quality factors were defined as a measure of the effect of pre-existing nucleus density. By integrating the classical nucleation/growth theory with the heat transfer model, the evolution of crystalline phase during laser welding for a BMG with pre-existing nuclei was studied, and the modeling predictions compared favorably with the experimental results.

Correlation between disorder-order transformations in a Ni-based alloy and its mechanical properties

It is demonstrated that the disorder-order transformations in a commercial Ni-Mo alloy with minor concentrations of Fe and Cr and the corresponding effect on mechanical strength are sensitive functions of the exact chemical composition. A slight change in composition within the specified limits of the alloy is found to change the nature of the ordered phases as well as the kinetics of the transformation and the mechanical strength. The results show that while the increase in tensile yield strength due to atomic order is accompanied by moderate decrease in tensile ductility, additional loss of ductility can occur at constant yield strength due to changes in morphology of the ordered microstructure during isothermal aging. It is also shown that the state of short-range order present in an otherwise disordered solid-solution acts as a pre-existing nucleus for the transformation, which accelerates the respective kinetics. However, elimination of the short-range order by cold work is found to decelerate the kinetics of the transformation. It is concluded that although the disorder-order transformations in the alloy investigated have detrimental effects on mechanical strength, some beneficial effects of technological importance can be realized.

Paleoproterozoic juvenile crust formation and stabilisation in the south-eastern West African Craton (Ghana); New insights from U-Pb-Hf zircon data and geochemistry

The crust of the Paleoproterozoic West African Craton in northern Ghana is dominantly made of Tonalite-Trondhjemite-Granodiorite (TTG) suites, low-K to high-K calc-alkaline granites, with minor LILE-enriched diorites, and chemically related lavas. Results of zircon U-Pb dating indicate that all these different rock types were formed together over a prolonged period of ca. 100Ma, between 2.21 and 2.11Ga. Zircon Hf isotope compositions of five samples are supra-chondritic (εHft from +1 to +6) indicating that the granitoids derive from the reworking of juvenile crustal components as old as 2.60Ga. Geochemical data show that the TTGs were derived from the reworking of a low-K mafic crust, while biotite granites formed by the reworking of older felsic crustal rocks, and diorites derived from the melting of LILE-enriched mantle sources.The combined datasets support a three-step model for the formation of the crust of the West African craton in northern Ghana: (1) a juvenile, mafic proto-crust was extracted from the depleted mantle between 2.60 and 2.30Ga, in an oceanic environment remote from any pre-existing continental nucleus; (2) the mafic proto-crust was reworked in several primitive magmatic arcs or in accreted oceanic plateaux. Crustal reworking initially produced minor felsic magmatism between 2.35 and 2.21Ga, followed by the 2.21–2.15Ga emplacement of low- to medium-P TTG and K-rich calc-alkaline granitoid suites, and was coeval with the formation of an enriched mantle source. (3) Various proto-continental terranes collided during the Eburnean orogeny between 2.14 and 2.11Ga, triggering intense reworking of the composite crust and mantle reservoirs formed in the previous two steps, i.e. remnants of the juvenile mafic crust, older TTGs and biotite granites, and previously enriched mantle sources. The emplacement of high-P TTGs, granites and LILE-enriched diorites resulted from lower crustal heating in response to the delamination of the lower crust and lithospheric mantle. The Eburnean collision therefore marks the final stages of a 200–500Ma long period of juvenile continental crust formation and stabilisation. The datasets further reveal that the Paleoproterozoic West African Craton comprises a minimum of two distinct crustal blocks, with north-western Ghana lying at their interface.

Heterogeneous bubble nucleation on heated surface from insoluble gas

Based on thermodynamic analysis, a model of heterogeneous bubble nucleation on a heated surface from an insoluble gas bubble is developed. Line tension is included in the model, which not only contributes to the change of thermodynamic potential (Gibbs free energy) during bubble formation but accounts for the variation of contact angle during nucleation process. The dependence of surface tension on temperature is also considered. The change of Gibbs free energy during bubble formation, which can be used to evaluate the stability of bubbles, is calculated. In the process of bubble growth, two stable or metastable bubble radii are found before and after the critical radius of bubble formation is reached. In addition, a parametric study is conducted to investigate the influence of the initial bubble size, line tension, wall temperature, wall temperature gradient and surface tension on the nucleation process. The present study provides a basis for heterogeneous nucleation from a pre-existing nucleus.

Micro-/macro-responses of a ferroelectric single crystal with domain pinning and depinning by dislocations

Phase field simulations are conducted to investigate the micro-structural signature and the macro-response of a ferroelectric single crystal with domain pinning and depinning phenomena by dislocation arrays. It is shown that due to the presence of the dislocation arrays, a domain with polarizations antiparallel to an applied field can survive under the small amplitude of applied field. The residual domain serves as a pre-existing nucleus during the following macroscopic switching via only domain wall motion. The pinned domain will be depinned when the external electric field amplitude exceeds a critical value, which highly depends on the dislocation spacing in the dislocation array. Due to the pinning and depinning effect, an asymmetric hysteresis loop of polarization versus electric field might appear when a bias field is applied.

Phase field simulation of heterogeneous cubic → tetragonal martensite nucleation

The mechanisms for heterogeneous cubic→tetragonal martensite nucleation due to different types of microscopic defects (voids, stress-concentration site, inertial inclusion and pre-existing nucleus) and the temporal evolution of martensite morphology are monitored with finite element simulation of phase filed model. The results demonstrate that the nucleation prefers to occur around void and stress-concentration site initially; high residual stress exists around inertial inclusion; pre-existing nucleus promotes nearby martensite phase to develop on it. The effects of various defects on heterogeneous nucleation are different, and stress relaxation behavior is the dominant factor which characterizes the whole microstructure evolution process.

Molecular Dynamics Simulation of Heterogeneous Nucleation of Argon Vapor onto a Spherical Solid Particle

Heterogeneous nucleation often occurs in the atmosphere, but its microscopic mechanism is mostly unknown. In our work, molecular dynamics simulations were performed to explore the dynamic characteristics of the heterogeneous nucleation of supersaturated argon vapor onto a spherical solid nanoparticle. We discuss the effect of the cooling rate on the evolution of the system temperature, the cluster distribution, the cluster size, and the nucleation rate during the nucleation process. Our results show that the pre-existing nucleus plays an important role in the cluster formation stage. Furthermore, in the system with a pre-existing heterogeneous nucleus, a critical cooling rate (1.8×10 J·s-1) exists at which homogeneous nucleation emerges and coexists with heterogeneous nucleation but heterogeneous nucleation still dominates the entire nucleation process.

THE DISTRIBUTION OF STARS AND STELLAR REMNANTS AT THE GALACTIC CENTER

Motivated by recent observations that suggest a low density of old stars around the Milky Way supermassive black hole, models for the nuclear star cluster are considered that have not yet reached a steady state under the influence of gravitational encounters. A core of initial radius 1-1.5 pc evolves to a size of approximately 0.5 pc after 10 Gyr, roughly the size of the observed core. The absence of a Bahcall-Wolf cusp is naturally explained in these models, without the need for fine-tuning or implausible initial conditions. In the absence of a cusp, the time for a 10-solar-mass black hole to spiral in to the Galactic center from an initial distance of 5 pc can be much greater than 10 Gyr. Assuming that the stellar black holes had the same phase-space distribution initially as the stars, their density after 5-10 Gyr is predicted to rise very steeply going into the stellar core, but could remain substantially below the densities inferred from steady-state models that include a steep density cusp in the stars. Possible mechanisms for the creation of the parsec-scale initial core include destruction of stars on centrophilic orbits in a pre-existing triaxial nucleus, inhibited star formation near the supermassive black hole, or ejection of stars by a massive binary. The implications of these models are discussed for the rates of gravitational-wave inspiral events, as well as other physical processes that depend on a high density of stars or stellar mass black holes near Sagittarius A*.

Shock-induced collapse of a gas bubble in shockwave lithotripsy

The shock-induced collapse of a pre-existing nucleus near a solid surface in the focal region of a lithotripter is investigated. The entire flow field of the collapse of a single gas bubble subjected to a lithotripter pulse is simulated using a high-order accurate shock- and interface-capturing scheme, and the wall pressure is considered as an indication of potential damage. Results from the computations show the same qualitative behavior as that observed in experiments: a re-entrant jet forms in the direction of propagation of the pulse and penetrates the bubble during collapse, ultimately hitting the distal side and generating a water-hammer shock. As a result of the propagation of this wave, wall pressures on the order of 1 GPa may be achieved for bubbles collapsing close to the wall. The wall pressure decreases with initial stand-off distance and pulse width and increases with pulse amplitude. For the stand-off distances considered in the present work, the wall pressure due to bubble collapse is larger than that due to the incoming shockwave; the region over which this holds may extend to ten initial radii. The present results indicate that shock-induced collapse is a mechanism with high potential for damage in shockwave lithotripsy.

Cavitation in creeping shear flows

AbstractLiquid failure has been observed in low Reynolds number (Stokes) shear flows where reduction of the hydrodynamic pressure should not occur. Cavitation in Stokes flows has implications in lubrication, polymer processing, and rheological measurements. Such cavitation can be predicted by a principal normal stress cavitation criterion (PNSCC). We present results of a direct experimental test of the PNSCC. Imaging of the cavitation events suggests that the cavitation is gaseous and originates from preexisting nuclei. Crevice‐stabilized gas nuclei are assumed, and numerical simulations are used to investigate the cavitation event for a Newtonian liquid. The inception of cavitation from a preexisting nucleus, the persistence of suitable nuclei, and the growth and deformation of shed bubbles are considered. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Treatment of low level liquid waste by an in‐situ freezing‐melting process

This paper is to present the results of an initial study of applying a new purification process for the treatment of low level liquid waste (LLLW) produced by nuclear power plants, the so‐called in‐situ freezing‐melting process. In order to purify the liquid waste, the wastewater is first frozen to form clean ice. The clean ice is then melted to yield purified water that is allowable to be discharged. The in‐situ freezing‐melting process utilizes the ice formation of falling film on a vertical wall of ice‐former as a wastewater treatment procedure. A falling film of an aqueous lithium bromide solution (LiBraq) is formed on the inner wall surface as a coolant. The three tested experimental statuses on the outer wall surface include pre‐existing nucleus of crystallization, no pre‐existing nucleus of crystallization, and the affixed fiber grids. The effects of the three experimental statuses on the amount of ice formation, the degree of purification, and the overall heat‐transfer coefficient were investigated. The present study found that the maximum values of both the amount of ice formation and the degree of purification could be found by varying the Reynolds number of wastewater. A lower inlet temperature of wastewater or aqueous lithium bromide solution can increase the amount of ice formation, but causes a decrease in the degree of purification. In addition, the preexisting nucleus of crystallization has positive effects on both the amount of ice formation and the degree of purification. However, when the wastewater is able to wet the outer surface of the wall thoroughly, affixing fiber grids to the surface will, contrarily, cause unfavorable effects on the degree of purification.

Galactic nuclei activity sustained by globular cluster mass accretion

The decay of globular clusters to the centre of their mother galaxy corresponds to carrying a quantity of mass sufficient to sustain the gravitational activity of a small pre-existing nucleus and to accrete it in a significant way. This is due to both dynamical friction of field stars and tidal disruption by the compact nucleus. The results of the simplified model presented here show that the active galactic nuclei luminosity and lifetime depend on the characteristics of the globular cluster system and are quite insensitive to the nucleus' initial mass.

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon, Sm-Nd and Pb-Pb whole-rock isotope study

The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and TDM model ages (2.65–2.62 Ga) and the positive ɛNdT values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ1 values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older TDM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material.

Leidenfrost temperature on an extremely smooth surface

Most existing heat transfer models on transition boiling employ the hypothesis on the mechanism limiting the so-called liquid-solid contact fraction that the activation of preexisting nuclei upon liquid-solid contact to be intermittent and localized and that conventional activation criteria can be used to predict the activation of preexisting nuclei in the wetted area upon contact. Supposing this hypothesis to be valid, the behavior of liquid-solid contact on an extremely smooth surface such as a single-crystal surface is expected to be very different from that on a metal surface of standard roughness. In this report, to examine the hypothesis, the Leidenfrost temperature is measured on single-crystal and metal plates. The maximum surface roughness of the former is 0.03 μm, and that of the latter is 1.25 μm. Results of the experiment show that the Leidenforst temperatures on these two surfaces do not differ from each other as long as the surfaces are the same in wettability and thermal conductivity (or thermal diffusivity). Visual observation of the transient and localized wetted area on the single-crystal surface shows that numerous tiny dry spots are formed in the wetted area even at initial surface superheats lower than the minimum superheat to activate the preexisting nucleus of 0.03 μm radius. These results imply that the hypothesis employed in most of the existing heat transfer models on transition boiling must be reexamined.

Leidenfrost temperature on an extremely smooth surface

Abstract Most existing heat transfer models on transition boiling employ the hypothesis on the mechanism limiting the so-called liquid-solid contact fraction that the activation of preexisting nuclei upon liquid-solid contact to be intermittent and localized and that conventional activation criteria can be used to predict the activation of preexisting nuclei in the wetted area upon contact. Supposing this hypothesis to be valid, the behavior of liquid-solid contact on an extremely smooth surface such as a single-crystal surface is expected to be very different from that on a metal surface of standard roughness. In this report, to examine the hypothesis, the Leidenfrost temperature is measured on single-crystal and metal plates. The maximum surface roughness of the former is 0.03 μm, and that of the latter is 1.25 μm. Results of the experiment show that the Leidenforst temperatures on these two surfaces do not differ from each other as long as the surfaces are the same in wettability and thermal conductivity (or thermal diffusivity). Visual observation of the transient and localized wetted area on the single-crystal surface shows that numerous tiny dry spots are formed in the wetted area even at initial surface superheats lower than the minimum superheat to activate the preexisting nucleus of 0.03 μm radius. These results imply that the hypothesis employed in most of the existing heat transfer models on transition boiling must be reexamined.

Stability of preexisting vapor nucleus in uniform temperature field.

As for the preexisting vapor-nuclei trapped in the cavities on a solid surface, the minimum superheat required to activate them is determined by the critical nucleus, where the radius of the curvature of the liquid-vapor interface reaches a minimum. In the present paper, the thermodynamic analysis is used to investigate the stability condition of preexisting vapor nuclei and to make clear the physical meaning of the critical nucleus. The thermodynamic consideration shows that preexisting nuclei can hold the thermodynamically-stable equilibrium state when the curvature of the liquid-vapor interface increases due to increased volume. Further, the stability of preexisting vapor nuclei trapped in a doubly reentrant cavity under large subcooling is examined concretely for freon R-113, and the temperature overshoots which occur during the inception of nucleation on enhanced boiling surfaces are estimated.