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Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the...
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Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK

A special issue of Metals (ISSN 2075-4701). This special issue belongs to the section "Metal Casting, Forming and Heat Treatment".

Deadline for manuscript submissions: closed (31 July 2022) | Viewed by 42182

Special Issue Editor

Prof. Dr. Zhongyun Fan

E-Mail Website
Guest Editor
1. Director of BCAST (Brunel Centre for Advanced Solidification Technology), Brunel University, London, UK
2. Director/Principal Investigator, the Liquid Metal Engineering (LiME) Research Hub, Brunel University, London, UK
Interests: heterogeneous nucleation; grain initiation; grain refinement; solidification; phase transformation; alloy development and solidification processing

Special Issue Information

Dear Colleagues,

Heterogeneous nucleation and grain initiation are critical processes at the early stages of solidification, which largely determine the finally solidified microstructures and have significant influence of the performance of metallic materials. However, due to experimental difficulties, our current understanding of the subjects has been dominated by the classical nucleation theory (CNT), which was postulated over 100 years ago. With financial support from the EPSRC (Engineering and Physical Science Research Council, UK) in the last 6 years, substantial progress has been made on the early stages of solidification by the LiME (liquid metal engineering) Research Hub (www.LiME.ac.uk). The LiME Research Hub is a national centre of excellence in liquid metal engineering based in BCAST at Brunel University London supported by research groups in Oxford, Leeds and Manchester Universities and Imperial College London. Although the LiME Hub Research covers a wide range of activities, spanning heterogeneous nucleation, grain refinement, solidification processing, alloy development and industrial applications, in this Special Issue, we will focus on progress made on our understanding of heterogeneous nucleation, grain initiation, grain refinement and their application to practical casting processes, such as direct chill (DC) casting, twin roll casting (TRC), die casting and metal recycling. While the Special Issue is primarily focused on work from the LiME Research Hub, we also welcome submissions from other researchers in the field.

Prof. Dr. Zhongyun Fan
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Metals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Heterogeneous nucleation
  • Grain initiation
  • Grain refinement
  • Metals and alloys
  • Solidification
  • Metal casting
  • Liquid metal engineering
  • Oxides
  • Recycling

Published Papers (18 papers)

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Research

Jump to: Review

21 pages, 8270 KiB  
Article
Degassing of Aluminum Alloy Melts by High Shear Melt Conditioning Technology: An Overview
by Jaime Lazaro-Nebreda, Jayesh B. Patel, Ewan Lordan, Yijie Zhang, Erdem Karakulak, Kawther Al-Helal, Geoff M. Scamans and Zhongyun Fan
Metals 2022, 12(10), 1772; https://doi.org/10.3390/met12101772 - 21 Oct 2022
Cited by 5 | Viewed by 4582
Abstract
The search for more efficient methods for degassing aluminum alloy melts has always been of great interest for the metal industry because the presence of hydrogen and oxides in the melts’ prior casting was detrimental to the integrity and properties of the final [...] Read more.
The search for more efficient methods for degassing aluminum alloy melts has always been of great interest for the metal industry because the presence of hydrogen and oxides in the melts’ prior casting was detrimental to the integrity and properties of the final products. In this work, we present an overview of the progress and key findings from the research and development of an innovative High Shear Melt Conditioning (HSMC) degassing technology during the Liquid Metal Engineering (LiME) Research Hub project. Compared to conventional rotary degassing, this novel technique was capable of working at higher rotor speeds to efficiently break and disperse the naturally occurring oxide bifilms in the melt and to capture and disperse each supplied inert gas bubble into many tiny bubbles throughout the whole melt. This resulted in the elimination of the need to degas fluxes to remove the oxides in the melt, the reduction in the gas flow required to reach the same level of hydrogen removal rate, and the minimization of the regassing effect after processing. The increased process efficiency allowed for reduced melt processing costs and, at the same time, improved the melt quality, which resulted in fewer defects and improved mechanical properties. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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20 pages, 7714 KiB  
Article
Heterogeneous Nucleation Mechanisms in Systems with Large Lattice Misfit Demonstrated by the Pb(l)/Cu(s) System
by Hua Men and Zhongyun Fan
Metals 2022, 12(10), 1583; https://doi.org/10.3390/met12101583 - 23 Sep 2022
Cited by 10 | Viewed by 1145
Abstract
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 [...] Read more.
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 or a three-layer vacancy mechanism if fr > 0, such as in the case of the Pb(l)/Cu(s) system. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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16 pages, 4736 KiB  
Article
Effect of Segregation of Sc, Y and La Atoms on Prenucleation at the Liquid-Al/γ-Al2O3{1 1 1} Interfaces
by Changming Fang and Zhongyun Fan
Metals 2022, 12(10), 1550; https://doi.org/10.3390/met12101550 - 20 Sep 2022
Cited by 2 | Viewed by 1242
Abstract
γ-Al2O3 particles form inevitably in liquid Al- alloys during liquid-handling and casting processes. Such oxide particles may act as potential nucleation sites during solidification. Recent research revealed that native γ-Al2O3 particles exhibit different potency for nucleating solid [...] Read more.
γ-Al2O3 particles form inevitably in liquid Al- alloys during liquid-handling and casting processes. Such oxide particles may act as potential nucleation sites during solidification. Recent research revealed that native γ-Al2O3 particles exhibit different potency for nucleating solid Al, which may reduce the number of potential nucleation sites in the liquid. Chemical segregation at the liquid/oxide interface may modify the substrates’ nucleation potency. In this paper, we investigated prenucleation at the Al(l)/γ-Al2O3 interface with segregation of Sc, Y and La (Group 3) atoms using an ab initio molecular dynamics simulation technique. Our results revealed that the segregation of Sc, Y and La results in a reconstruction of the Al atoms adjacent to the substrates and consequently a rough substrate surface. Present investigation opens a new path for manipulating solidification processes via chemical segregation at the liquid/substrate interface. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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14 pages, 3590 KiB  
Article
Molecular Dynamics Simulations on Effect of Surface Roughness of Amorphous Substrate on Nucleation in Liquid Al
by Hua Men and Zhongyun Fan
Metals 2022, 12(9), 1529; https://doi.org/10.3390/met12091529 - 15 Sep 2022
Cited by 3 | Viewed by 1408
Abstract
In this study, we used molecular dynamics (MD) simulations to investigate the atomic ordering in the liquid aluminum (Al) adjacent to the amorphous substrate with smooth and rough surfaces. This study revealed that the liquid exhibited layering within about 5 atomic layers but [...] Read more.
In this study, we used molecular dynamics (MD) simulations to investigate the atomic ordering in the liquid aluminum (Al) adjacent to the amorphous substrate with smooth and rough surfaces. This study revealed that the liquid exhibited layering within about 5 atomic layers but no visible in-plane atomic ordering at the interface with the smooth amorphous surface, and neither layering nor in-plane atomic ordering with the rough surface of the amorphous substrate. However, the smooth amorphous surface induced some local ordered structure in the liquid at the interface by a structural templating mechanism, which promoted heterogeneous nucleation by creating a 2-dimensional (2D) nucleus in the third layer. The amorphous substrate with a rough surface had no effect on the nucleation in the liquid, leading to the occurrence of homogeneous nucleation with an undercooling 100 K larger than that of heterogeneous nucleation on the smooth amorphous substrate. This study confirmed that structural templating is a general mechanism for heterogeneous nucleation. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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17 pages, 3483 KiB  
Article
Competition for Nucleation and Grain Initiation during Solidification
by Feng Gao and Zhongyun Fan
Metals 2022, 12(9), 1512; https://doi.org/10.3390/met12091512 - 13 Sep 2022
Cited by 5 | Viewed by 1424
Abstract
Without the addition of any grain refiner, the inclusion particles in a melt will induce heterogeneous nucleation and grain initiation during the solidification of metallic materials. However, with grain refiner addition, the exogenous particles (from the grain refiner) and the native inclusions (e.g., [...] Read more.
Without the addition of any grain refiner, the inclusion particles in a melt will induce heterogeneous nucleation and grain initiation during the solidification of metallic materials. However, with grain refiner addition, the exogenous particles (from the grain refiner) and the native inclusions (e.g., oxide particles) will co-exist in the melt, and there will be competition for nucleation and grain initiation among different types of solid particles. In this paper, we analyze such competition in Al and Mg alloys using a numerical solidification model that we have developed previously. The numerical calculations show that the competition for nucleation is strongly dependent on nucleation undercooling of the different types of particles, while the competition for grain initiation is closely related to the sizes of solid particles. Based on the numerical results, the general rules of competition for nucleation and grain initiation have been developed: nucleation starts with particles of minimum nucleation undercooling, followed by particles with progressively larger nucleation undercooling; and grain initiation starts with solid particles of the largest size, followed by solid particles with progressively smaller sizes. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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19 pages, 9812 KiB  
Article
Heterogeneous Nucleation and Grain Initiation on a Single Substrate
by Zhongyun Fan and Hua Men
Metals 2022, 12(9), 1454; https://doi.org/10.3390/met12091454 - 30 Aug 2022
Cited by 8 | Viewed by 4353
Abstract
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 [...] Read more.
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, the spherical cap formation is constrained by the curvature of the liquid/solid interface, produces a dormant cap, and further growth is only made possible by increasing undercooling to overcome an energy barrier. However, 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. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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24 pages, 1914 KiB  
Article
Towards a Physically Consistent Phase-Field Model for Alloy Solidification
by Peter C. Bollada, Peter K. Jimack and Andrew M. Mullis
Metals 2022, 12(2), 272; https://doi.org/10.3390/met12020272 - 2 Feb 2022
Cited by 2 | Viewed by 1678
Abstract
We give an overview of contributions made to the computational phase-field modelling of alloy solidification from the University of Leeds as part of the LiME project (EPSRC Advanced Manufacturing Hub in Liquid Metal Engineering). The broader look at the more salient features from [...] Read more.
We give an overview of contributions made to the computational phase-field modelling of alloy solidification from the University of Leeds as part of the LiME project (EPSRC Advanced Manufacturing Hub in Liquid Metal Engineering). The broader look at the more salient features from our research allows the individual contributions to be seen in a wider context than can be seen from each contribution separately. We begin with a general introduction to phase-field and then reference the numerical issues that arise from the solution of the model before outlining contributions to phase-field modelling that we found most interesting or significant. These range from controlling and developing interface-width independent modelling; controlling morphology in both single and multiphase settings; generalising from single to multiphase models; and creating a thermodynamically consistent framework for modelling entropy flow and thereby postulating a temperature field consistent with the concepts of, and applicable in, multiphase and density-dependent settings. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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18 pages, 63227 KiB  
Article
Al-Mn Intermetallics in High Pressure Die Cast AZ91 and Direct Chill Cast AZ80
by Liuqing Peng, Guang Zeng, Di Wang, Jingwei Xian, Shouxun Ji, Hongyi Zhan and Christopher M. Gourlay
Metals 2022, 12(2), 266; https://doi.org/10.3390/met12020266 - 31 Jan 2022
Cited by 1 | Viewed by 2401
Abstract
Manganese-bearing intermetallic compounds (IMCs) are important for ensuring adequate corrosion performance of magnesium-aluminium alloys and can be deleterious to mechanical performance if they are large and/or form clusters. Here, we explore the formation of Al-Mn IMCs in Mg-9Al-0.7Zn-0.2Mn produced by two industrial casting [...] Read more.
Manganese-bearing intermetallic compounds (IMCs) are important for ensuring adequate corrosion performance of magnesium-aluminium alloys and can be deleterious to mechanical performance if they are large and/or form clusters. Here, we explore the formation of Al-Mn IMCs in Mg-9Al-0.7Zn-0.2Mn produced by two industrial casting processes, high-pressure die casting (HPDC) and direct chill (DC) casting. As Al8Mn5 starts forming above the α-Mg liquidus temperature in this alloy, we consider its formation during melt handling as well as during casting and heat treatment. In HPDC, we focus on sludge formation in the holding pot, partial solidification of IMCs in the shot chamber, and Al-Mn IMC solidification in the die cavity. In DC casting, we focus on interactions between Al-Mn IMCs and oxide films in the launder system, Al-Mn IMC solidification in the billet, and the partial transformation of Al8Mn5 into Al11Mn4 during solution heat treatment. The results show that minimising pre-solidification in the shot sleeve of HPDC and controlling pouring and filtration in DC casting are important for ensuring small Al-Mn intermetallic particles in these casting processes. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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Review

Jump to: Research

27 pages, 7706 KiB  
Review
Grain Initiation and Grain Refinement: An Overview
by Zhongyun Fan and Feng Gao
Metals 2022, 12(10), 1728; https://doi.org/10.3390/met12101728 - 15 Oct 2022
Cited by 14 | Viewed by 2587
Abstract
Heterogeneous nucleation and grain initiation are two different processes in early-stage solidification (ESS), although both are deterministic. Heterogeneous nucleation refers to the formation of a 2-dimensional (2D) nucleus (a crystal plane of the solid) that can template further growth, while grain initiation is [...] Read more.
Heterogeneous nucleation and grain initiation are two different processes in early-stage solidification (ESS), although both are deterministic. Heterogeneous nucleation refers to the formation of a 2-dimensional (2D) nucleus (a crystal plane of the solid) that can template further growth, while grain initiation is the formation of a hemispherical cap (3D) from which isothermal growth is possible. It is both theoretically and practically beneficial to separate heterogeneous nucleation from grain initiation. This paper provides an overview of our recent understanding of grain initiation behaviour under different conditions and its consequences on grain refinement. After a brief review of the processes involved in the ESS, we present the grain initiation behaviour on a single substrate. This is followed by grain initiation behaviour in systems with a population of nucleant particles with varying particle types (corresponding to varying nucleation undercoolings), where we give detailed descriptions of progressive grain initiation, explosive grain initiation, hybrid grain initiation, grain initiation maps and grain refinement maps. We then provide a brief introduction to the rules that govern competition for heterogeneous nucleation and grain initiation among multiple types of nucleant particles with varying particles’ nucleation undercoolings and sizes. Finally, we present the practical implications of grain refinement maps to grain refinement. A key finding from this work is that more significant grain refinement can be achieved by promoting explosive grain initiation using impotent nucleant particles, which is opposite to the traditional approach for grain refinement where potent particles are used to enhance heterogeneous nucleation. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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30 pages, 11938 KiB  
Review
Prenucleation at the Liquid/Substrate Interface: An Overview
by Hua Men, Changming Fang and Zhongyun Fan
Metals 2022, 12(10), 1704; https://doi.org/10.3390/met12101704 - 12 Oct 2022
Cited by 11 | Viewed by 1428
Abstract
Prenucleation refers to the phenomenon of substrate-induced atomic ordering in the liquid adjacent to the liquid/substrate interface at temperatures above the nucleation temperature. We investigated the effects of the physical and chemical properties of the substrate on prenucleation, using the classical molecular dynamics [...] Read more.
Prenucleation refers to the phenomenon of substrate-induced atomic ordering in the liquid adjacent to the liquid/substrate interface at temperatures above the nucleation temperature. We investigated the effects of the physical and chemical properties of the substrate on prenucleation, using the classical molecular dynamics (MD) and ab initio MD simulations. We found that the physical origin of prenucleation is structural templating, which is affected significantly by the lattice misfit between the solid and the substrate, chemical interaction between the solid and the substrate, and the substrate surface roughness at the atomic level. Prenucleation ultimately determines the nucleation potency of a substrate and provides a precursor for heterogeneous nucleation at the nucleation temperature. In this paper, we provide an overview of the recent advances in the understanding of prenucleation made by the LiME Research Hub. After a brief review of the historical research on atomic ordering at the liquid/substrate interface in the literature, we present an overview of the recent advances in understanding prenucleation, covering the concept of prenucleation, the effect of temperature, lattice misfit and substrate chemistry, and substrate surface roughness at the atomic level. Our discussions will be focused on the effect of prenucleation on heterogeneous nucleation and its consequences on grain refinement. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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33 pages, 24747 KiB  
Review
Understanding Fe-Containing Intermetallic Compounds in Al Alloys: An Overview of Recent Advances from the LiME Research Hub
by Zhongping Que, Yun Wang, Chamini L. Mendis, Changming Fang, Junhai Xia, Xiaorong Zhou and Zhongyun Fan
Metals 2022, 12(10), 1677; https://doi.org/10.3390/met12101677 - 6 Oct 2022
Cited by 11 | Viewed by 2813
Abstract
Control of Fe in Al alloys is a severe challenge for the full metal circulation to produce the recycled alloys with mechanical and physical performance as high as the primary alloys. The high restriction of Fe content is mainly due to the deterioration [...] Read more.
Control of Fe in Al alloys is a severe challenge for the full metal circulation to produce the recycled alloys with mechanical and physical performance as high as the primary alloys. The high restriction of Fe content is mainly due to the deterioration caused by the large-scale Fe-containing intermetallic compounds (FIMCs) in Al alloys. In this paper, recent knowledge gained regarding nucleation, formation, and technical developments on microstructural control and refinement of FIMCs are overviewed. Specific characteristics of the multiple types of FIMCs in Al alloys are presented in two- and three- dimensional (2D and 3D) form. Phase relationships between the FIMCs in different structures, such as primary phase, binary eutectic, and ternary eutectic, formed at different solidification stages are studied. Phase transformations between the FIMCs with or without intermediate phases during the solidification process are examined in different Al alloys, with the mechanisms being clarified. Various approaches to microstructural control of FIMCs are proposed and validated. Significant refinement of FIMCs has been achieved through inoculation of TiB2 particles that had been previously modified with deliberately interfacial segregation of desirable alloying elements, leading to the development of the novel “compositional templating” concept. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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38 pages, 16211 KiB  
Review
Manipulating Nucleation Potency of Substrates by Interfacial Segregation: An Overview
by Yun Wang, Shihao Wang, Zhongping Que, Changming Fang, Teruo Hashimoto, Xiaorong Zhou, Quentin M. Ramasse and Zhongyun Fan
Metals 2022, 12(10), 1636; https://doi.org/10.3390/met12101636 - 29 Sep 2022
Cited by 8 | Viewed by 1552
Abstract
During solidification of metallic materials, heterogeneous nucleation occurs on substrates, either endogenous or exogenous. The potency of the substrates for nucleation is mainly dependent upon the atomic arrangements on the substrate surface, which are affected by the lattice misfit between the substrate and [...] Read more.
During solidification of metallic materials, heterogeneous nucleation occurs on substrates, either endogenous or exogenous. The potency of the substrates for nucleation is mainly dependent upon the atomic arrangements on the substrate surface, which are affected by the lattice misfit between the substrate and the nucleated solid, the surface roughness at atomic scale, and the chemical interaction between the substrates and the melt. Extensive examinations on metal/substrate (M/S) interfaces at atomic scale by the state-of-the-art aberration (Cs) corrected STEM and associated EDS and EELS have shown that alloying elements in liquid melts tend to segregate at the interfaces, leading to the formation of various 2-dimensional compounds (2DCs) or 2-dimensional solutions (2DSs), depending upon segregation behavior of the elements. For instance, Al3Ti 2DC and Ti2Zr 2DC at the Al/TiB2 interface, Y2O3 2DC at the Mg/MgO interface, and a Si-rich 2DS layer at Al-Si/TiB2 interface have been identified. Such interfacial segregations significantly affect nucleation potency of the substrates, resulting in either promoting or impeding the heterogeneous nucleation process during solidification. In this paper, we present an overview of the current studies of interfacial segregation behavior, the structure and chemistry of interfaces, and their impacts on the subsequent heterogeneous nucleation and grain initiation processes. Our focus is on the advances made in the understanding of the mechanisms for nucleation and grain refinement. It is demonstrated that it is feasible to manipulate heterogeneous nucleation by modifying nucleation potency of a substrate through deliberate interfacial segregation of desirable elements, achieving effective control of the grain structure of cast metallic materials. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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20 pages, 5808 KiB  
Review
Ab Initio Molecular Dynamics Investigation of Prenucleation at Liquid–Metal/Oxide Interfaces: An Overview
by Changming Fang and Zhongyun Fan
Metals 2022, 12(10), 1618; https://doi.org/10.3390/met12101618 - 27 Sep 2022
Cited by 5 | Viewed by 1588
Abstract
Prenucleation refers to the phenomenon of atomic ordering in the liquid adjacent to a liquid/solid interface at temperatures above its nucleation temperature. It produces a precursor for heterogeneous nucleation in the liquid and thus has a strong influence on the nucleation process. Oxide [...] Read more.
Prenucleation refers to the phenomenon of atomic ordering in the liquid adjacent to a liquid/solid interface at temperatures above its nucleation temperature. It produces a precursor for heterogeneous nucleation in the liquid and thus has a strong influence on the nucleation process. Oxide particles, including magnesia, spinel, and alumina, are inevitably formed in the liquid during liquid–metal handling and casting. They may act as nucleation sites for potential grain refinement. Knowledge about prenucleation at liquid–metal/oxide (M(l)/oxide) interfaces is important for an understanding of heterogeneous nucleation during casting. Here, we present an overview of the recent studies on the prenucleation at the M(l)/oxide interfaces using ab initio molecular dynamics simulation techniques. We observed a wide variety of interfacial chemistry and identified the formation of an ordered metal layer terminating the oxide substrates, such as MgO{1 1 1} (denoting MgO with {1 1 1} surface termination), α-Al2O3{0 0 0 1}, MgAl2O4{1 1 1} and γ-Al2O3{1 1 1} in liquid light metals. The terminating metal atoms are positively charged and form topologically rough layers, which strongly impact the prenucleation at the interfaces. We suggest modification of nucleation potency of the substrate surfaces via elemental segregation to manipulate the solidification processes. This is demonstrated by the segregation of La atoms at the Al(l)/γ-Al2O3 interfaces. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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18 pages, 5967 KiB  
Review
De-Ironing of Aluminium Alloy Melts by High Shear Melt Conditioning Technology: An Overview
by Jaime Lazaro-Nebreda, Jayesh B. Patel, Kawther Al-Helal, Feng Gao, Ian Stone, Isaac T. H. Chang, Geoff M. Scamans and Zhongyun Fan
Metals 2022, 12(10), 1579; https://doi.org/10.3390/met12101579 - 23 Sep 2022
Cited by 3 | Viewed by 2311
Abstract
The main problem of recycling aluminium scrap is the gradual accumulation of impurities, especially iron, which tend to form undesired intermetallic compounds that affect the integrity and the mechanical performance of the castings. In this paper, we aim to provide an overview on [...] Read more.
The main problem of recycling aluminium scrap is the gradual accumulation of impurities, especially iron, which tend to form undesired intermetallic compounds that affect the integrity and the mechanical performance of the castings. In this paper, we aim to provide an overview on the topic of iron removal from aluminium melts through primary intermetallic precipitation and the progress made during the LiME Hub project to understand the process and to develop a more efficient procedure. We cover both thermodynamic analysis and experimental validation. We found that high shear melt conditioning technology enhances the typically slow nucleation and growth of the dense primary intermetallics, speeding up their sedimentation and allowing a faster removal of Fe from the melt by simple gravity sedimentation. It also promotes the formation of smaller and more compact Fe-rich intermetallics, allowing an increased volume fraction recovery and mitigating their effect of being present in the final castings. The technology is not limited to batch processing, with a 90% efficiency, but can also be applied to continuous melt treatment of aluminium scrap, with currently 60% efficiency, and could be combined with other solid–liquid separation techniques to increase the purification efficiency even more. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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17 pages, 4673 KiB  
Review
High-Pressure Die Casting: A Review of Progress from the EPSRC Future LiME Hub
by Ewan Lordan, Yijie Zhang, Kun Dou, Alain Jacot, Chrysoula Tzileroglou, Shihao Wang, Yun Wang, Jayesh Patel, Jaime Lazaro-Nebreda, Xiaorong Zhou, Teruo Hashimoto and Zhongyun Fan
Metals 2022, 12(10), 1575; https://doi.org/10.3390/met12101575 - 23 Sep 2022
Cited by 10 | Viewed by 3345
Abstract
This article provides an overview of high-pressure die casting (HPDC)-related research undertaken at the EPSRC Future LiME Hub between 2015–2022. The project aimed to identify the cause of variability in the tensile ductility of die-cast structures, and to develop novel processing techniques to [...] Read more.
This article provides an overview of high-pressure die casting (HPDC)-related research undertaken at the EPSRC Future LiME Hub between 2015–2022. The project aimed to identify the cause of variability in the tensile ductility of die-cast structures, and to develop novel processing techniques to address this issue. Variability in tensile ductility was related to the size of large pores and non-metallic inclusions. It was proposed that these non-metallic inclusions formed during the pyrolysis of commercial plunger lubricants in the shot sleeve, and that these large pores derived from dilatational strains introduced during semi-solid deformation. Processing parameters and die design were found to significantly influence the microstructure of die-cast products, and the subsequent variability in tensile ductility. To close, recent progress on the application of intensive melt shearing to HPDC is reviewed. Intensive melt shearing was found to induce significant grain refinement in both Al and Mg alloys due to the effective dispersion of native oxide particles, and the use of these particles as heterogeneous nucleation substrates. The presence of native oxide particles also enabled the use of novel heat treatment procedures that avoided conventional issues such as surface blistering and geometrical distortion. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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38 pages, 53916 KiB  
Review
An Overview on Atomistic Mechanisms of Heterogeneous Nucleation
by Zhongyun Fan and Hua Men
Metals 2022, 12(9), 1547; https://doi.org/10.3390/met12091547 - 19 Sep 2022
Cited by 13 | Viewed by 2658
Abstract
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 [...] Read more.
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% < f < 0), misfit is accommodated by dislocation mechanism; (2) for systems with small positive misfit (0 < f < 12.5%), misfit is accommodated by vacancy mechanism; and (3) for systems with large misfit (|f| > 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. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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16 pages, 2930 KiB  
Review
Solute Effect on Grain Refinement of Al- and Mg-Alloys: An Overview of the Recent Advances Made by the LiME Research Hub
by Feng Gao and Zhongyun Fan
Metals 2022, 12(9), 1488; https://doi.org/10.3390/met12091488 - 8 Sep 2022
Cited by 1 | Viewed by 1306
Abstract
Grain refinement is of importance for metallic materials since it provides multiple benefits, such as improved castability, reduced casting defects and improved mechanical properties. From extensive research carried out in the past decades, it has been widely accepted that solute is one of [...] Read more.
Grain refinement is of importance for metallic materials since it provides multiple benefits, such as improved castability, reduced casting defects and improved mechanical properties. From extensive research carried out in the past decades, it has been widely accepted that solute is one of the crucial factors for achieving grain refinement. However, grain refinement is a complex phenomenon, depending on not only solutes in the melt to provide growth restriction but also the physical and chemical nature of the nucleant particles (either endogenous or exogenous). Although significant progress has been made on the subject, some critical questions still remain open, and a comprehensive understanding of the mechanisms of solute effect on grain refinement is still desirable. In this paper, we present an overview of the solute effect on grain refinement based on our recent advances made in the LiME Research Hub. This covers the effect of solute on nucleation potency of nucleant particles due to interfacial segregation, columnar to equiaxed transition (CET), growth restriction and eventually on the overall grain refinement. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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19 pages, 18640 KiB  
Review
Investigating Metal Solidification with X-ray Imaging
by Shikang Feng, Insung Han, Andrew Lui, Robin Vincent, Gideon Ring, Patrick S. Grant and Enzo Liotti
Metals 2022, 12(3), 395; https://doi.org/10.3390/met12030395 - 24 Feb 2022
Cited by 6 | Viewed by 2785
Abstract
In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key [...] Read more.
In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described. Full article
(This article belongs to the Special Issue Heterogeneous Nucleation, Grain Initiation and Grain Refinement: Reports from the LiME Research Hub in the UK)
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