Understanding Homogeneous Nucleation in Solidification of Aluminum by Molecular Dynamics Simulations

Homogeneous nucleation from aluminum (Al) melt was investigated by million-atom molecular dynamics (MD) simulations utilizing the second nearest neighbor modified embedded atom method (MEAM) potentials. The natural spontaneous homogenous nucleation from the Al melt was produced without any influence of pressure, free surface effects and impurities. Initially isothermal crystal nucleation from undercooled melt was studied at different constant temperatures, and later superheated Al melt was quenched with different cooling rates. The crystal structure of nuclei, critical nucleus size, critical temperature for homogenous nucleation, induction time, and nucleation rate were determined. The quenching simulations clearly revealed three temperature regimes: sub-critical nucleation, super-critical nucleation, and solid-state grain growth regimes. The main crystalline phase was identified as face-centered cubic (fcc), but a hexagonal close-packed (hcp) and an amorphous solid phase were also detected. The hcp phase was created due to the formation of stacking faults during solidification of Al melt. By slowing down the cooling rate, the volume fraction of hcp and amorphous phases decreased. After the box was completely solid, grain growth was simulated and the grain growth exponent was determined for different annealing temperatures.Comment: 41 page

Melting and Solidification Study of Indium and Bismuth Nanocrystals Using Reflection High-Energy Electron Diffraction

As technology begins to utilize nanocrystals for many chemical, biological, medical, electrical, and optoelectrical applications, there is a growing need for an understanding of their fundamental properties. The study of melting and solidification of nanocrystals is of interest to fundamental understanding of the effect of reduced size and crystal shape on the solid-liquid phase transition. Melting and solidification of as-deposited and recrystallized indium and bismuth nanocrystals were studied using reflection high-energy electron diffraction (RHEED). The nanocrystals were thermally deposited on highly oriented 002-graphite substrate at different deposition temperatures. The growth dynamics of the nanocrystals was studied using in situ RHEED while the morphology and size distributions were studied using ex situ real image technique (atomic force microscopy (AFM) or scanning electron microscopy (SEM)). RHEED observation during deposition showed that 3D nanocrystals of indium are directly formed from the vapor phase within the investigated temperature range, 300 K up to 25 K below the bulk melting point of indium. On the other hand, bismuth condensed in the form of supercooled liquid droplets at temperatures above its maximum supercooling point, 125 K below the bulk melting point of bismuth. Below the maximum supercooling point, bismuth condensed in the solid phase. Post deposition real images showed that the formed nanocrystals have morphologies and size distributions that depend on the deposition temperature, heat treatment, and the amount of the deposited material. As-deposited nanocrystals are found to have different shapes and sizes, while those recrystallized from melt were formed in similar shapes but different sizes. The change in the RHEED pattern with temperature was used to probe the melting and solidification of the nanocrystals. Melting started early before the bulk melting point and extended over a temperature range that depends on the size distribution of the nanocrystals. Nanocrystals at the lower part of the distribution melt early at lower temperatures. With the increase in temperature, more nanocrystals completely melt with the thickness of the liquid shell on the remaining crystals continuing to grow. Due to size increase after melting, recrystallized bismuth nanocrystals showed a melting range at temperatures higher than that of as-deposited. However, recrystallized indium nanocrystals showed an end melting point nearly equal to that of-the recrystallized ones except for the 1.5-ML film which showed an end melting point ∼10 K higher than that of as-deposited

Reflooding with internal boiling of a heating model porous medium with mm-scale pores

This paper presents a pore-scale experimental study of the reflooding of a two-dimensional model porous medium. The objective is to better understand the reflooding mechanisms in play in the context of nuclear reactor safety. The hot debris bed that forms in a nuclear reactor following a loss of coolant accident is comparable to a heat-generating porous medium. Its cooling by water reflooding involves intense boiling mechanisms that must be modeled properly to assess mitigation procedures. The experimental study presented in this paper focuses on the phenomenology of reflooding of a model porous medium composed of a bank of mm-scale heating cylinders placed between two ceramic plates. A Fluorinert™ liquid, HFE-7000, is injected at a temperature close to saturation into the dry and superheated porous medium. Each cylinder of the test section is used both as a heating element and a temperature probe, which enables to track the evolution of the three different macroscopic zones identified during cooling of the system. The reflooding dynamics, in particular the cooling fronts velocities, are thus determined thanks to pore-scale thermal measurements together with direct visualizations. The influence of the injection flow rate and of the heating power are studied in a parametric way

Processing and Phase Formation in Zr-Based Bulk-Metallic Glass-Forming Alloys

Bulk-metallic glasses have established a formidable presence in the scientific community in recent years, due to a number of properties that are uncharacteristic of metallically-bonded materials. One of the fundamental challenges facing researchers in this field is to develop new and improved processing methods with the ultimate goal of facilitating a large-scale industrial integration of the materials. The research described herein is directed toward the pursuit of developing and improving upon the current state-of-the art in the science of bulk-metallic glass processing. A number of research and development projects were undertaken in this pursuit. First, the technology to process bulk-metallic glasses at the University of Tennessee was developed and successfully implemented. Second, bulk-metallic glasses were produced using aerodynamic levitation, which showed an improvement over the accessible cooling rates achievable employing other containerless-processing methods. Third, erbium was found to be a superior dopant to other rare-earth elements to neutralize the oxygen in a Zr-based glass-forming alloy. The alloy was found to form a glass in the presence of up to 16,000-atomic-ppm oxygen by microalloying with Er, with a relatively minor effect on the thermal and mechanical integrity of the materials. Fourth, metastable intermetallic phases were identified in as-cast VIT-105 alloy materials that contained oxygen, using diffraction. The diffraction study included the whole pattern fitting of diffraction from crystalline species in a BMG, an analytical approach that, if existing at all in the literature, is quite rare. Furthermore, this study included a novel approach to fitting diffraction from the glass. Fifth, oxygen-stabilized analogues to intermetallic phases were found in the superheated-liquid state. The presence of Er was found to inhibit surface reoxidation, revealing its mechanism for the neutralization of oxygen. The results were used to propose a model for heterogeneous nucleation and the so-called overheating threshold in the alloy

Melting of Multilayer Colloidal Crystals Confined Between Two Walls

Video microscopy is employed to study the melting behaviors of multilayer colloidal crystals composed of diameter-tunable microgel spheres confined between two walls.We systematically explore film thickness effects on the melting process and on the phase behaviors of single crystal and polycrystalline films. Thick films (\u3e4 layers) are observed to melt heterogeneously, while thin films ( ≤ 4 layers) melt homogeneously, even for polycrystalline films. Grain-boundary melting dominates other types of melting processes in polycrystalline films thicker than 12 layers. The heterogeneous melting from dislocations is found to coexist with grain-boundary melting in films between 5- and 12-layers. In dislocation melting, liquid nucleates at dislocations and forms lakelike domains embedded in the larger crystalline matrix; the “lakes” are observed to diffuse, interact, merge with each other, and eventually merge with large strips of liquid melted from grain boundaries. Thin film melting is qualitatively different: thin films homogeneously melt by generating many small defects which need not nucleate at grain boundaries or dislocations. For three- and four-layer thin films, different layers are observed to have the same melting point, but surface layers melt faster than bulk layers. Within our resolution, two- to four-layer films appear to melt in one step, while monolayers melt in two steps with an intermediate hexatic phase

Study of small particles in the neighbourhood of the melting point

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Research study on materials processing in space experiment M512

A study program was conducted to clarify the role of gravity in the fluid mechanics of certain molten metal processes of potential significance to manufacturing in space. In particular, analyses were conducted of the M551 Metals Melting Experiment and the M553 Sphere Forming Experiment to be conducted in the M512 Facility onboard Skylab. The M551 experiment consisted of a study of electron beam welding of various metals, and the M553 experiment studied the formation of molten metal spheres by free-floating in a near zero-gravity environment. The analyses of these experiments and a comparison with ground-based and KC135 experimental results are presented

Dynamics of Phase Transitions on Low-Index Metal Surfaces

The surface superheating and phase transitions at the low-index surface of metal were investigated using conventional continuous and 100-ps time-resolved reflection high-energy electron diffraction. Three metal surfaces, In(111), Au(110) and Pb(111), have been investigated in this work. The high temperature behavior of the In(111) surface was investigated using reflection high-energy electron diffraction with 100-ps temporal resolution. The change of surface vacancy density on In(111) is observed from 300 K to near the bulk melting point. The vacancy-formation energy of In(111) surface is estimated from experimental results. The surface vacancy density is observed to increase with temperature; however, the average random step terrace width remains unchanged from its value at room temperature. When the indium surface is heated at a slow rate, the In(111) surface remains ordered up to the highest temperature studied, 4 K below the bulk melting point. When the surface is laser heated at a heating rate of ∼1012 K/s, the In(111) surface remains ordered up to 73 ± 9 K above its bulk melting pointing. The surface Debye temperature of indium is also obtained from the measurement. The top layer relaxation (or contraction) of In(111) surface is studied. The vicinal Au(110) surface morphology throughout the (1 × 2)-(1 × 1) reconstruction is studied by quantitative reflection high-energy electron diffraction. As the surface is heated from 300 K, the average terrace width remains unchanged up to 635 ± 10 K. Above that temperature, the average terrace width increases and at 683 K has a value (34 ± 10)% more than at 300 K. At higher temperatures, the average terrace width decreases. The average string length at step terraces on Au(110) remains unchanged up to ∼650 K and decreases sharply at higher temperatures indicative of a step-induced roughening transition. Thermal-induced adatom/vacancy generation on terraces is shown to increase significantly above ∼680 K. At room temperature, the spacing between the topmost two layers of the Au(110) surface is contracted by 0.31 ± 0.03 Å or ∼22% of the bulk-terminated structure. A laser-driven Photoemission electron microscope with 5 μm spatial resolution was developed and used to study surface morphology of laser heated Pb single crystal. The basic idea of this microscope is the use of μ50 ps ultraviolet laser pulse to photoemit electrons from the surface, while a well synchronized infrared laser pulse heats the surface and induces surface morphology change

An Enthalpy Landscape View of Homogeneous Melting in Crystals

A detailed analysis of homogeneous melting in crystalline materials modeled by empirical interatomic potentials is presented using the theory of inherent structures.We show that the homogeneous melting of a perfect, infinite crystalline material can be inferred directly from the growth exponent of the inherent structure density-of-states distribution expressed as a function of formation enthalpy. Interestingly, this growth is already established by the presence of very few homogeneously nucleated point defects in the form of Frenkel pairs. This finding supports the notion that homogeneous melting is appropriately defined in terms of a one-phase theory and does not require detailed consideration of the liquid phase. We then apply this framework to the study of applied hydrostatic compression on homogeneous melting and show that the inherent structure analysis used here is able to capture the correct pressure-dependence for two crystalline materials, namely silicon and aluminum. The coupling between the melting temperature and applied pressure arises through the distribution of formation volumes for the various inherent structures