1,499 research outputs found
Effects of solute content on grain refinement in an isothermal melt
This is the port-print version of the article. The official published version can be obtained from the link below - Copyright @ 2011 Acta Materialia Inc. Published by Elsevier LtdIt is well accepted in the literature that for effective grain refinement some solute is required in the melt to restrict the growth of the solid even if potent nucleating particles with a favourable physical nature are present. In this paper we investigate the effect of the solute on grain initiation in an isothermal melt, and an analytical model is developed to account for the effect of solute elements on grain size. This study revealed that the solute elements in the liquid ahead of the growing crystals reduce the growth velocity of the nucleated crystals and increase the maximum undercooling achievable before recalescence. This allows more particles to be active in nucleation and, consequently, increases the number density of active particles, giving rise to a finer grain size. The analytical model shows that the final grain size can be related to the maximum undercooling, average growth velocity and solid fraction at the moment of recalescence. Further analysis using the free growth model and experimental data in the literature revealed that for a given alloy system solidified under similar conditions the grain size can be empirically related to 1/Q (Q is the growth restriction factor) to a power of 1/3, which is considerably different from the empirical linear relationship in the literature. It is demonstrated that the 1/3 power law can describe the experimental data more accurately than a linear relationship.The EPSRC is gratefully acknowledged for providing financial support under Grant EP/H026177/1
Effects of lattice mismatch on interfacial structures of liquid and solidified Al in contact with hetero-phase substrates: MD simulations
Published under licence in IOP Conference Series: Material Science and Engineering by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.In this study, the effects of the misfit on in-plane structures of liquid Al and interfacial structure of solidified Al in contact with the heterophase substrates have been investigated, using molecular dynamics (MD) simulations. The MD simulations were conducted for Al/fcc (111) substrates with varied misfits. The order parameter and atomic arrangement indicated that the in-plane ordering of the liquid at the interface decreases significantly with an increase of the misfit, i.e., solid-like for small misfit and liquid-like for large misfit. Further, our MD simulation results revealed that a perfect orientation relationship forms at the interface between the substrate and the solidified Al for a misfit of less than -3% and the boundary is coherent. With an increase in the misfit, Shockley partial and extended dislocations form at the interface, and the boundary becomes a semi-coherent or low-angle twist boundary.EPSR
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A joint diffusion/collision model for crystal growth in pure liquid metals
Data availability:
The datasets generated in this study have been deposited in the Brunel University London database, Figshare [https://doi.org/10.17633/rd.brunel.26029045.v1]71. Additional raw data can be found in Source Data file. Source data are provided with this paper.Code availability:
The code used in the current study has been deposited in Code Ocean [https://doi.org/10.24433/CO.8127284.v1]72.Supplementary information is available online at: https://www.nature.com/articles/s41467-024-50182-7#Sec14 .Source data are available online at: https://www.nature.com/articles/s41467-024-50182-7#Sec15 .The kinetics of atomic attachments at the liquid/solid interface is one of the foundations of solidification theory, and to date one of the long-standing questions remains: whether or not the growth is thermal activated in pure liquid metals. Using molecular dynamics simulations and machine learning, I have demonstrated that a considerable fraction of liquid atoms at the interfaces of Al(111), (110) and (100) needs thermal activation for growth to take place while the others attach to the crystal without an energy barrier. My joint diffusion/collision model is proved to be robust in predicting the general growth behaviour of pure metals. Here, I show this model is able to quantitatively describe the temperature dependence of growth kinetics and to properly interpret some important experimental observations, and it significantly advances our understanding of solidification theory and also is useful for modelling solidification, phase change materials and lithium dendrite growth in lithium-ion battery.The EPSRC was gratefully acknowledged for providing financial support under Grant No. EP/N007638/1 and EP/S036296/1. We also were grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (Grant Nos. EP/P020194/1 and EP/T022213/1), and maintained with support from Brunel University London
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A molecular dynamics study on the boundary between homogeneous and heterogeneous nucleation
Data Availability: The data that support the findings of this study are available within the article.Supplementary material: The supplementary material is available online at:
https://pubs.aip.org/jcp/article-supplement/3267965/zip/094702_1_5.0192069.suppl_material .The large discrepancy among the nucleation kinetics extracted from experimental measurements and computer simulations and the prediction of the classical nucleation theory (CNT) has stimulated intense arguments about its origin in the past decades, which is crucially relevant to the validity of the CNT. In this paper, we investigate the atomistic mechanism of the nucleation in liquid Al in contact with amorphous substrates with atomic-level smooth/rough surfaces, using molecular dynamics (MD) simulations. This study reveals that the slightly distorted local fcc/hcp structures in amorphous substrates with smooth surfaces can promote heterogeneous nucleation through a structural templating mechanism, and on the other hand, homogeneous nucleation will occur at a larger undercooling through a fluctuation mechanism if the surface is rough. Thus, some impurities, previously thought to be impotent, could be activated in the homogeneous nucleation experiments. We further find that the initial growth of the nucleus on smooth surfaces of amorphous substrates is one order of magnitude faster than that in homogeneous nucleation. Both these factors could significantly contribute to the discrepancy in the nucleation kinetics. This study is also supported by a recent study of the synthesis of high-entropy alloy nanoparticles assisted with the liquid metal Ga [Cao et al., Nature 619, 73 (2023)]. In this study, we established that the boundary existed between homogeneous and heterogeneous nucleation, i.e., the structural templating is a general mechanism for heterogeneous nucleation, and in its absence, homogeneous nucleation will occur through the fluctuation mechanism. This study provides an in-depth understanding of the nucleation theory and experiments.EPSRC of the UKRI under Grant Nos. EP/N007638/1 and EP/S35296/1. We are grateful to the UK Materials and Molecular Modeling Hub for computational resources, which is partially funded by the EPSRC (Grant Nos. EP/P020194/1 and EP/T022213/1), and maintained with support from Brunel University London
Accounting for the tongue-and-groove effect using a robust direct aperture optimization approach
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98733/1/MPH001266.pd
Transition of amorphous to crystalline oxide film in initial oxide overgrowth on liquid metals
It is important to understand the mechanism of oxidation in the initial stage on the free surface of liquid metals. Mittemeijer and co-workers recently developed a thermodynamic model to study the oxide overgrowth on a solid metal surface. Based on this model, we have developed a thermodynamic model to analyse the thermodynamic stability of oxide overgrowth on liquid metals. The thermodynamic model calculation revealed that the amorphous oxide phase is thermodynamically preferred up to 1.3 and 0.35 nm respectively, in the initial oxide overgrowth on liquid Al and Ga at the corresponding melting point. However, the amorphous phase is thermodynamically unstable in the initial oxide overgrowth on liquid Mg. The thermodynamic stability of amorphous phase in the Al and Ga oxide systems is attributed to lower sums of surface and interfacial energies for amorphous phases, compared to that of the corresponding crystalline phases.Financial support under grant EP/H026177/1 from the EPSRC was used
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A study on prenucleation and heterogeneous nucleation in liquid Pb on solid Al using molecular dynamics simulations
Data Availability: The data that support the findings of this study are available within the article.Copyright © 2023 Author(s). In this paper, we investigate prenucleation and heterogeneous nucleation in the liquid Pb/solid Al system as an example of systems with large lattice misfit using molecular dynamics simulation. Solid Pb and Al have a large positive lattice misfit (f) of 18.2% along the densely packed [110] direction. This study reveals that prenucleation occurs at 600 K (an undercooling of 15 K), and a 2-dimensional (2D) ordered structure forms at the interface with a coincidence site lattice (CSL) between the first Pb and first Al layers. The CSL accommodates the major part of the f, and only a small residual lattice misfit (fr) of 1.9% remains. The formation of the CSL transforms the original substrate into a considerably potent nucleant, where the first Pb layer becomes the new surface layer of the substrate. At an undercooling of about 22 K, nucleation proceeds by merging 2D ordered structure through structural templating: the second Pb layer is epitaxial to the CSL Pb layer, the third Pb layer largely accommodates the fr, and the fourth Pb layer is a nearly perfect crystalline plane. Further analysis indicates that the interface with the CSL has a lower interfacial energy than with a cube-to-cube orientation relationship. For the first time, we established that the CSL was an effective mechanism to accommodate the f for systems with a large positive misfits. Heterogeneous nucleation is governed not by a single mechanism (misfit dislocations in Turnbull’s model), but instead by various mechanisms depending on f. This study sheds new light on the atomistic mechanism of heterogeneous nucleation.The EPSRC was gratefully acknowledged for providing financial support under Grant No. EP/N007638/1. We were grateful to the UK Materials and Molecular Modeling Hub for computational resources, which is partially funded by EPSRC (Grant Nos. EP/P020194/1 and EP/T022213/1), and maintained with support from Brunel University London
Heterogeneous Nucleation and Grain Initiation on a Single Substrate
Data Availability Statement: All data are available in the main text.Copyright: © 2022 by the authors. Recently, we have proposed a new framework for early stages solidification, in which heterogeneous nucleation and grain initiation have been treated as separate processes. In this paper, we extend our atomic-level understanding of heterogeneous nucleation to spherical cap formation for grain initiation on a single substrate using molecular dynamics calculations. We first show that heterogeneous nucleation can be generally described as a three-layer mechanism to generate a two-dimensional (2D) nucleus under a variety of atomic arrangements at the solid/substrate interface. We then introduce the atomistic concept of spherical cap formation at different grain initiation undercoolings (ΔTgi) relative to nucleation undercooling (ΔTn). When ΔTn ΔTgi, spherical cap formation becomes barrierless and undergoes three distinctive stages: heterogeneous nucleation to produce a 2D nucleus with radius, rn; unconstrained growth to deliver a hemisphere of rN (substrate radius); and spherical growth beyond rN. This is followed by a theoretical analysis of the three-layer nucleation mechanism to bridge between three-layer nucleation, grain initiation and classical nucleation theory.EPSRC of the UKRI under the grant number EP/N007638/1
An Overview on Atomistic Mechanisms of Heterogeneous Nucleation
Our current understanding of heterogeneous nucleation has been dominated by the classical nucleation theory (CNT) with little progress of significance being made in past 100 years. In recent years under the financial support from EPSRC for the LiME Research Hub, we have made substantial progress on understanding heterogeneous nucleation at atomic level using a combination of molecular dynamics simulations and advanced high-resolution electron microscopy. We found that heterogeneous nucleation proceeds through a three-layer nucleation mechanism to produce a 2D nucleus. The atomistic mechanisms responsible for accommodating lattice misfit are dependent on misfit (f): (1) for systems with small negative misfit (−12.5% 12.5%), misfit is accommodated in two steps: formation of coincidence site lattice during prenucleation to accommodate the major misfit (fcsL) and the residual misfit (fr) is accommodated during heterogeneous nucleation by the dislocation mechanism if the residual misfit is less than 0 or by the vacancy mechanism if the residual misfit is larger than 0. Further analysis suggests that heterogeneous nucleation is spontaneous thus barrierless and deterministic rather than stochastic.EPSRC of the UKRI under the grant number EP/N007638/
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Heterogeneous Nucleation Mechanisms in Systems with Large Lattice Misfit Demonstrated by the Pb(l)/Cu(s) System
Copyright: © 2022 by the authors. Our current understanding of heterogeneous nucleation has been largely confined to the classical nucleation theory (CNT) that was postulated over 100 years ago based on a thermodynamic approach. Further advances in heterogeneous nucleation research requires detailed knowledge of atomistic activities at the liquid/substrate interface. In this work, using a classical molecular dynamics (MD) simulation, we investigated the atomistic mechanisms of heterogeneous nucleation in systems with a large lattice misfit (|f| > 12.5%) demonstrated by the liquid Pb and solid Cu system (denoted as the Pb(l)/Cu(s) system) with a misfit of 27.3%. We found that heterogeneous nucleation in systems with a large misfit takes place in two distinctive steps: (1) Prenucleation creates a coincidence site lattice (CSL) on the substrate surface to accommodate the majority (fcsl) of the initial misfit (f) and (2) Heterogeneous nucleation accommodates the residual misfit fr (fr = misfit − fcsl) at the nucleation temperature to create a plane of the new solid phase (a two-dimensional (2D) nucleus) through either a three-layer dislocation mechanism if fr 0, such as in the case of the Pb(l)/Cu(s) system.EPSRC of the UKRI under the grant number EP/N007638/1
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