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

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

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

GPU-based ultra-fast direct aperture optimization for online adaptive radiation therapy

Online adaptive radiation therapy (ART) has great promise to significantly reduce normal tissue toxicity and/or improve tumor control through real-time treatment adaptations based on the current patient anatomy. However, the major technical obstacle for clinical realization of online ART, namely the inability to achieve real-time efficiency in treatment re-planning, has yet to be solved. To overcome this challenge, this paper presents our work on the implementation of an intensity modulated radiation therapy (IMRT) direct aperture optimization (DAO) algorithm on graphics processing unit (GPU) based on our previous work on CPU. We formulate the DAO problem as a large-scale convex programming problem, and use an exact method called column generation approach to deal with its extremely large dimensionality on GPU. Five 9-field prostate and five 5-field head-and-neck IMRT clinical cases with 5\times5 mm2 beamlet size and 2.5\times2.5\times2.5 mm3 voxel size were used to evaluate our algorithm on GPU. It takes only 0.7~2.5 seconds for our implementation to generate optimal treatment plans using 50 MLC apertures on an NVIDIA Tesla C1060 GPU card. Our work has therefore solved a major problem in developing ultra-fast (re-)planning technologies for online ART

Ultrafast treatment plan optimization for volumetric modulated arc therapy (VMAT)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134981/1/mp1675.pd

Foci of Schistosomiasis mekongi, Northern Cambodia: II. Distribution of infection and morbidity.

In the province of Kracheh, in Northern Cambodia, a baseline epidemiological survey on Schistosoma mekongi was conducted along the Mekong River between December 1994 and April 1995. The results of household surveys of highly affected villages of the East and the West bank of the river and of school surveys in 20 primary schools are presented. In household surveys 1396 people were examined. An overall prevalence of infection of 49.3% was detected by a single stool examination with the Kato-Katz technique. The overall intensity of infection was 118.2 eggs per gram of stool (epg). There was no difference between the population of the east and west shore of the Mekong for prevalence (P = 0.3) or intensity (P = 0.9) of infection. Severe morbidity was very frequent. Hepatomegaly of the left lobe was detected in 48.7% of the population. Splenomegaly was seen in 26.8% of the study participants. Visible diverted circulation was found in 7.2% of the population, and ascites in 0.1%. Significantly more hepatomegaly (P = 0.001), splenomegaly (P = 0. 001) and patients with diverted circulation (P = 0.001) were present on the west bank of the Mekong. The age group of 10-14 years was most affected. The prevalence of infection in this group was 71.8% and 71.9% in the population of the West and East of the Mekong, respectively. The intensity of infection was 172.4 and 194.2 epg on the West and the East bank, respectively. In the peak age group hepatomegaly reached a prevalence of 88.1% on the west and 82.8% on the east bank. In the 20 schools 2391 children aged 6-16 years were examined. The overall prevalence of infection was 40.0%, ranging from 7.7% to 72.9% per school. The overalls mean intensity of infection was 110.1 epg (range by school: 26.7-187.5 epg). Both prevalence (P = 0.001) and intensity of infection (P = 0.001) were significantly higher in schools on the east side of the Mekong. Hepatomegaly (55.2%), splenomegaly (23.6%), diverted circulation (4. 1%), ascites (0.5%), reported blood (26.7%) and mucus (24.3%) were very frequent. Hepatomegaly (P = 0.001), splenomegaly (P = 0.001), diverted circulation (P = 0.001) and blood in stool (P = 0.001) were significantly more frequent in schools of the east side of the Mekong. Boys suffered more frequently from splenomegaly (P = 0.05), ascites (P = 0.05) and bloody stools (P = 0.004) than girls. No difference in sex was found for the prevalence and intensity of infection and prevalence of hepatomegaly. On the school level prevalence and intensity of infection were highly associated (r = 0. 93, P = 0.0001). The intensity of infection was significantly associated only with the prevalence of hepatomegaly (r = 0.44, P = 0. 05) and blood in stool (r = 0.40, P = 0.02). This comprehensive epidemiological study documents for the first time the public health importance of schistosomiasis mekongi in the Province of Kracheh, Northern Cambodia and points at key epidemiological features of this schistosome species, in particular the high level of morbidity associated with infection

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