Melt Density Research Articles

Surface morphological evolution and microhardness change of Cr12MoV steel by pulsed laser polishing

Laser polishing is gaining more and more attention because of its environmental friendliness and ability to process complex surfaces. It is considered as one of the technologies to replace traditional methods in the mold industry. In this work, the variation pattern of surface roughness with single pulse energy density and light-out overlap rate during nanosecond pulsed laser polishing of Cr12MoV steel was investigated. The surface roughness increased and then decreased with increasing single-pulse energy density and beam overlap rate, respectively. The lowest surface roughness was found to be 0.50 μm, which is a decrease of 79.34 % compared to the untreated sample. The effect of each parameter on the surface morphology during the laser polishing process is explained. There is minimal austenitization on the surface of the sample after laser polishing. The second phase of the remelting layer disappeared. The microhardness of the remelting layer increased by 36.17 % because the grain size was reduced by 30.65 %. This work further explains the mechanism transformation threshold for pulsed laser polishing of steel. With 95 % beam overlap, the melting and gasification energy density thresholds for Cr12MoV steel are respectively 1.269 J/cm2 and 2.937 J/cm2. The beam overlap rate of 95.5 % is the critical condition for continuous melt movement. It provides a new idea for the mechanism judgment of pulsed laser polishing.

Toward a real validation of passive microwave snowmelt detection algorithms over the Antarctic Ice sheet

Antarctic Ice Sheet (AIS) surface snowmelt can profoundly impact climatological and hydrological processes, as well as energy and mass balances. Passive microwave remote sensing has proven highly effective in detecting snowmelt, and the accuracy of the algorithms is typically assessed using air temperature (Tair). Nonetheless, the occurrence of snowmelt cannot be solely determined by Tair being above the melting point, it can only provide evaluation of the algorithms rather than a real validation. This study is the first to conduct a reliable validation of six commonly used algorithms by using the melt rate data from a surface energy balance model, including the cross-polarized gradient ratio (XPGR) algorithm, the diurnal amplitude variation (DAV) algorithm, the Microwave Emission Model of Layered Snowpacks based (MEMLS-based) algorithm, the constant threshold algorithm (CTA), the Ashcraft and Long Algorithm (ALA), and the M + 30 K (M30 hereafter) algorithm. Results suggest that the M30 algorithm and MEMLS-based algorithm have overall better performances, with high overall accuracies (with a value of 95.81 % and 95.56 %) and high kappa coefficient (with a value of 0.69 and 0.68). Though Tair is not reliable for evaluating the algorithms, a threshold of Tair = − 0.8 ℃ can serve as an alternative criterion in the absence of melt rate data. Additionally, two algorithms with the highest accuracy were selected to investigate interannual snowmelt trends in the Antarctic and the sub-regions. The results show a significant increase in melt extent and density in the Wilkes and Adelie sub-region during the period 1979–2020.

Partial melt composition of enstatite chondritic mantle around the rheological transition at 23 GPa: Implications for the chemical differentiation of the Earth's mantle

Terrestrial planets are thought to be made of chondritic materials. However, previous studies have suggested that the Earth's upper mantle is depleted in some incompatible refractory lithophile elements (RLEs), such as U and Th in comparison with chondritic values. To explain the compositional contradiction between the upper mantle and chondrites, a hidden reservoir for incompatible RLEs in the lower mantle has been proposed. These studies have invoked a possible formation of hidden reservoir for incompatible RLEs during magma ocean solidification and by subsequent mantle overturn due to the gravitational instability of Fe-rich dense cumulates. However, the density of partial melt and its fate in a crystallizing deep magma ocean is still debated. Here we report partial melt compositions of enstatite chondritic mantle at 23 GPa as a function of the extent of melting. The results show that bridgmanite crystallized in a chondritic magma ocean becomes enriched in Fe and Ca with decreasing extent of melting and prevents the crystallization of ferropericlase and CaSiO3 perovskite (Davemaoite). At the rheological transition (the extent of melting of 40% based on mass balance calculation), where the solid-liquid separation efficiently occurs even in the case of equilibrium crystallization, the solid part is mostly composed of bridgmanite with a trace amount of majorite, and the melt becomes enriched in FeO and CaO compared to the initial chondritic composition. The calculated density of the melt at the rheological transition is lighter than bridgmanite, but denser than majorite, suggesting that the melt may have ponded at the bottom of the mantle transition zone. The melt ponded around 660 km depth may have been enriched in incompatible RLEs and formed the hidden reservoir for missing RLEs if this melt-bearing region is stable against subsequent mantle convection.

Mass transport and thermal properties of liquid (melting to boiling point) tungsten: a molecular dynamics simulations

We present molecular dynamics simulation to obtain melting point, structural and atomic transport properties of liquid tungsten. We considered the second-neighbor extended Finnis-Sinclair (EFS) potential as an effective interaction. We find melting temperature (T M) and density in agreement with the reported values. EFS potential gives accurate information for structure factor S(q), pair correlation function g(r), and transport coefficients like self-diffusion coefficient and viscosity upto ∼1.5T M. Large viscosity proposes the rheological nature of liquid W. The self-diffusion coefficient follows the Arrhenius law giving the activation energy 1.22 eV. We find ’shoulder’ in the second peak of S(q), which disappears with temperatures. This characteristic is attributed to the high density and clustering of W-atoms at the near-neighbor distance. The asymmetric first peak in g(r) and shoulder in S(q) proposes that the liquid W exhibits non-normal metallic behaviour. Discrepancy observed in caloric properties for temperature >6000 K is also discussed. We propose that the ’softness’ and broad dip in EFS potential are responsible for these discrepancies, and necessitate the inclusion of angular forces. The single particle correlation is discussed in terms of the velocity autocorrelation function and the long-wavelength limit of S(q) is utilized to derive adiabatic sound velocity in confirmation with reported results.

Molecular dynamics simulation of homogeneous nucleation of melting in superheated sodium crystal

The melting process and nucleation behaviour of sodium (Na) crystals are crucial for the frozen start-up of high-temperature sodium heat pipes and the working performance of molten sodium batteries. Equilibrium melting and the homogeneous nucleation of melting in superheated Na crystal are studied by molecular dynamics simulation. The thermodynamic properties and nucleation of Na crystal during the heating process are investigated by the characterization of the density, radial distribution function (RDF), self-diffusion coefficient (D), nucleation rate (Ihom), etc. Firstly, the equilibrium melting of Na crystal is simulated and verified by the single-phase method. Results show that Na's equilibrium melting temperature, densities, and RDF simulated match well with experimental values, demonstrating that the EAM/FS potential is suitable for studying the melting processes of Na crystal. Secondly, the calculated D of liquid Na increases approximately linearly with rising temperature within 440 K∼540 K and fits with experimental data well. The activation energy for the diffusion of Na obtained by linear fitting is 1.6406·10-20 J, which agrees well with the observed values. Finally, the homogeneous nucleation of melting in superheated Na crystal is analyzed. It is found that the Ihom of melting in superheated Na crystal rises exponentially with increasing temperature, which is consistent with the prediction in literature. The kinetic stability limit of superheated Na crystal is 468.3 K, and the relation between the Ihom of melting in superheated Na crystal and temperature is given. The physical properties and nucleation theory of Na crystal simulated during the melting can provide theoretical support for sodium's heat and mass transfer-related applications, such as sodium heat pipes and sodium batteries.

The effect of extrusion and injection molding on physical, chemical, and biological properties of PLLA/HAp whiskers composites

This paper deals with physical, chemical and biological properties of polymer-ceramic composites based on poly(l-lactide) (PLLA) with the addition of 10, 20 or 30 % (by weight) of hydroxyapatite (HAp) whiskers, produced by a combination of twin-screw extrusion and injection molding techniques. The materials obtained were tested for morphology (scanning electron microscopy), chemical composition (energy dispersive x-ray spectroscopy, infrared spectroscopy), processing properties (melt flow rates and density), thermal properties (differential scanning calorimetry, thermogravimetry), strength (static tensile test, impact strength) and surface properties (optical profilometry, contact angle measurements). The biological evaluation was performed by in vitro assessment of the manufactured materials’ cytotoxicity, level of colonization by osteo- and fibroblasts and level of adhesion displayed by a microbial pathogen, referred to as the Staphylococcus aureus. The results indicate an influence of the applied processing path on the properties of the filler particles − the apatite whiskers were significantly shortened. On the other hand, we have demonstrated the possibility of obtaining PLLA/HAp composites with good strength parameters, precise geometric shape and slightly improved surface properties. The results of biological evaluation revealed that all of the materials obtained displayed low cytotoxicity and a high level of colonization, irrespective of the content of hydroxyapatite. The only exception was the composite containing 10 % HAp, for which we observed weaker fibroblast adhesion compared to other materials. In turn, the level of colonization by S.aureus grew with the increase in HAp content in polymers. Results of biological evaluation indicate that although obtained polymers display favorable properties in terms of eukaryotic cell adhesion/colonization, they may also provide advantageous surface conditions for bacterial development. Therefore, inclusion of antimicrobial to the materials obtained may be necessary and should be further investigated. Nevertheless, the results obtained certainly provide useful information from the point of view of designing and manufacturing of biocomposites for bone tissue engineering applications.

Thermodynamic Insights into Variation in Thermomechanical and Physical Properties of Isotactic Polypropylene: Effect of Shear and Cooling Rates.

In order to elucidate the effect of shear and cooling process on structural, thermomechanical, and physical properties of polymer melt, excess entropy, a thermodynamic quantity is calculated from radial distribution function generated from equilibrated parts of the molecular simulation trajectories. The structural properties are calculated, which includes the density of polypropylene melt, end to end distance, radius of gyration of the polypropylene polymer chain, and monomer-monomer radial distribution function. Non-equilibrium molecular dynamics simulation was employed to investigate the role of the applied shear rate on the properties of polypropylene. Furthermore, a range of cooling rates were employed to cool the melt. Thermomechanical properties, such as Young's modulus, and physical properties, such as glass transition temperature, were determined for different cases. Results showed that slow cooling and high shear substantially improved the Young's modulus and glass transition temperature of the i-PP. Furthermore, a two-body contribution to the excess entropy was used to elucidate the structure-property relationships in the polymer melt as well as the glassy state and the dependence of shear and cooling rate on these properties. We have used the Rosenfeld excess entropy-viscosity relationship to calculate the viscous behavior of the polymer under a steady shear condition.

Spatial Organization of Slit-Confined Melts of Ring Polymers with Nonconserved Topology: A Lattice Monte Carlo Study.

We present Monte Carlo computer simulations for melts of semiflexible randomly knotted and randomly concatenated ring polymers on the fcc lattice and in slit confinement. Through systematic variation of the slit width at fixed melt density, we explore the influence of confinement on single-chain conformations and interchain interactions. We demonstrate that confinement makes chains globally larger and more elongated while enhancing both contacts and knottedness propensities. As for multichain properties, we show that ring-ring contacts decrease with the confinement, yet neighboring rings overlap more as confinement grows. These aspects are accompanied by a marked decrease in the links formed between pairs of neighboring rings. In connection with the quantitative relation between links and entanglements in polymer melts recently established by us [Ubertini M. A.; Rosa A.Macromolecules2023, 56, 3354-3362], we propose that confinement can be used to set polymer networks that act softer under mechanical stress and suggest a viable experimental setup to validate our results.

The ascent of subduction zone mélanges: Experimental constraints on mélange rock densities and solidus temperatures

Mélanges are mixtures of subducted materials and serpentinized mantle rocks that form along the slab-mantle interface in subduction zones. It has been suggested that mélange rocks may be able to ascend from the slab-top into the overlying mantle, as solid or partially molten buoyant diapirs, and transfer their compositional signatures to the source regions of arc magmas. However, their ability to buoyantly rise is in part tied to their phase equilibria during melting and residual densities after melt extraction, all of which are poorly constrained. Here, we report a series of piston-cylinder experiments performed at 1.5–2.5 GPa and 500–1050°C on three natural mélange rocks that span a range of mélange compositions. Using phase equilibria, solidus temperatures, and densities for all experiments, we show that melting of mélanges is unlikely to occur along the slab-top at pressures ≤ 2.5 GPa, so that diapirism into the hotter mantle wedge would be required for melting to initiate. For the two metaluminous mélange compositions, diapir formation is favored up to pressures of at least 2.5 GPa. For the peraluminous mélange composition investigated, diapir buoyancy is possible at 1.5 GPa but limited at 2.5 GPa due to the formation of high-density garnet, primarily at the expense of chlorite. We also evaluate whether thermodynamic modeling (Perple_X) can accurately reproduce the phase equilibria, solidus temperatures, and density evolution of mélange compositions. Our analysis shows good agreement between models and experiments in mélange compositions with low initial water contents and low-pressure (≤ 1.5 GPa) conditions. However, discrepancies between the thermodynamic models and experiments become larger at higher pressures and high-water contents, highlighting the need for an improved thermodynamic database that can model novel bulk compositions beyond the canonical subducting lithologies. This study provides experimental constraints on mélange buoyancy that can inform numerical models of mélange diapirism and influence the interpretations of both geophysical signals and geochemical characteristics of magmas in subduction zones.

Hyperactive learning for data-driven interatomic potentials

Data-driven interatomic potentials have emerged as a powerful tool for approximating ab initio potential energy surfaces. The most time-consuming step in creating these interatomic potentials is typically the generation of a suitable training database. To aid this process hyperactive learning (HAL), an accelerated active learning scheme, is presented as a method for rapid automated training database assembly. HAL adds a biasing term to a physically motivated sampler (e.g. molecular dynamics) driving atomic structures towards uncertainty in turn generating unseen or valuable training configurations. The proposed HAL framework is used to develop atomic cluster expansion (ACE) interatomic potentials for the AlSi10 alloy and polyethylene glycol (PEG) polymer starting from roughly a dozen initial configurations. The HAL generated ACE potentials are shown to be able to determine macroscopic properties, such as melting temperature and density, with close to experimental accuracy.

Synthesis and thermophysical property determination of NaCl-PuCl3 salts

Currently, a knowledge gap exists in the available data and understanding of thermophysical properties relating to fresh fuel salts, especially those containing plutonium. These data are necessary for designing, constructing, and licensing future commercial molten-salt reactors. Thermophysical properties, such as melting temperature, salt stability, density, and heat capacity were ascertained using NaCl-PuCl3 (36 mol% PuCl3) and a more sodium rich composition containing 25 mol% PuCl3. The NaCl-PuCl3 salt mixture was synthesized for this study and contained 63.4 mol% NaCl, 36.3 mol% PuCl3 and was 99.7% pure. Upon heating, the NaCl–PuCl3 eutectic was stable at temperatures up to 800 °C. The onset of melting occurred at 451 ± 3 °C, and the enthalpy of fusion was determined to be 23.0 ± 1.4 kJ/mol. Heat capacity measurements in the liquid phase ranged from 107.7 to 91.3 J/mol.K, with an average value of 104.6 ± 11.4 J/mol.K between 500 and 650 °C. Three independent trials of the molten NaCl-PuCl3 salt found the density to be ρ(T) = 3.8589 – 9.5342·10-4 T(°C). In addition, ab initio molecular dynamic simulations to calculate density and heat capacity values are included.

Determination of Strength Characteristics of Basalt Composite in COMSOL Multiphysics

Some strength characteristics of basalt melt, and basalt fiber are considered. The analysis of micromechanics and stress analysis is carried out on the example of composite cylindrical reinforcement based on multilayer and equivalent single-layer theory. The distributions of the stress wave field of three different values of natural frequencies and mode shapes in three samples of objects from the melt of basalt and basalt fiber are studied using the COMSOL Multiphysics software package. It has been established that three parameters of basalt fiber and basalt melt mainly influence the natural frequency and shape of the modes: these are the volume fraction, the density of the basalt fiber and the density of the basalt melt. It has also been found that the concentration of basalt melt, and basalt fiber affect the strength of the composite material.

Mineral–Geochemical and Geotectonic Features of the Lotmvara-II Ultrabasic Sill, Serpentinite Belt (Kola Peninsula)

Abstract —In this paper, we present a description of the characteristics of the Lotmvara-II sill, which is a representative of the Serpentinite Belt (SB) composed of a series of shallowly emplaced ultrabasic intrusive bodies. The Paleoproterozoic SB complexes were derived from a large-scale mantle plume of komatiitic melt. The sill consists predominantly of fine-grained (locally nearly micrograined) harzburgites with subordinate zones of dunites and orthopyroxenites, located in the central and marginal parts, respectively. It formed from an Al-undepleted komatiitic magma of extremely high Mg content and may represent a near-surface laccolithic “ridge.” In general, the sill is comparatively homogeneous and does not have distinct zoning in the distribution of Mg# values in rock compositions (Mg# = 84.2–88.9, average 86.7). Detailed studies show that olivine, chromian spinel, and ilmenite are the most strongly magnesian in the central part of the body. The maximum values of Mg# equal to 90.7–91.4 in the compositions of olivine at the center of the sill are interpreted as “centers of initial crystallization”. The low values of Mg# equal to 73.4–76.4 are attributed to manifestations of the recurrent generation of olivine. The values of Mg# of orthopyroxene in the sill are within the range 84.6 to 92.3. Orthopyroxene grains in a porphyritic texture are surrounded by a rim of calcic amphibole (autometasomatic in origin); they do not differ compositionally from normal grains. The Zn content of the chromian spinel generally decreases toward the marginal parts of the sill. There is an insignificant degree of magmatic differentiation in the sill with respect to the principal components, but incompatible elements (REE and HFSE) locally show increased levels of their relative enrichment, which is reflected in the nature of the mineral associations described. Thus, the sill has a cryptic zonal structure, which is consistent with its overall crystallization from the center to the edges. The data gathered suggest the presence and significant development of volatile components, halogens, CO2, and especially magmatic H2O, which are capable of strongly lowering the liquidus and reducing the density and viscosity of the high-magnesium melt, thereby improving its mobility during ascent from the mantle to the near-surface level of the crust. An increase in fO2 is observed during in situ subvolcanic crystallization of the sill, as noted earlier in the related complexes of the belt. The relatively small volume of the komatiitic magma in the sill crystallized fairly quickly, resulting in unusual mineral intergrowths. Thus, the Lotmvara-II sill is a novel member in the Serpentinite Belt–Tulppio Belt (SB–TB) in the Paleoproterozoic SB–TB megastructure of the Fennoscandian Shield.

Experimental Study on the Interaction of an Impulse Water Jet with Molten Metal

The impingement of a short-duration water jet on a pool of molten Rose’s metal is studied experimentally herein. Short-duration water jet impacting on the free surface of a molten metal pool with a temperature of 300 °C are generated with a pneumatic water delivery system, with two-camera high-speed video registration. A total of 14 experimental series, each containing 5 repeated tests, are performed for a water volume of 0.2–1 mL and a jet impact velocity of 4.1–9.0 m/s. The cavity development in the melt layer is studied, with the main stages described herein. Despite the significantly higher density of melt in comparison with water, the cavity can reach the melt pool bottom; furthermore, its further collapse results in the formation of a central jet rising to the height of a few centimeters. The maximum height of the central jet is shown to depend linearly on the total momentum of the water jet, and a semi-logarithmic correlation is found for the maximum diameter of the cavity. Repeatability analysis is performed within each experimental series, and the relative standard deviation for the melt splash height is shown to be from 8.8% to 26.8%. The effects of the pool depth, the vessel shape, and the water temperature are weaker in the range of the experimental parameters used here.

Технологические особенности ультразвуковой сварки разнородных пластмасс

The paper considers technological features of heterogeneous plastics ultrasonic welding, which specificity is manifested in the volumetric interaction nature. Results of studying the mechanism of a welded joint formation showed that micro-volumes mixing, polymer molecules mutual diffusion in the contacting surfaces and emergence of the new chemical bonds with the block copolymers formation were the simultaneous processes. Their common feature was that all these processes proceeded in the temperature range corresponding to the polymer viscous-flow state. It was experimentally confirmed that one of the criteria for weldability of the heterogeneous plastics was their compatibility in terms of acoustic properties being estimated by the melt density and viscosity. Technological methods were proposed making it possible to compensate for differences in the welded materials acoustic properties, and thereby obtain the high-quality welded joints. Four stages of the welded joints formation mechanism were determined during ultrasonic welding of the heterogeneous plastics. It was established that physical and mechanical properties of the heterogeneous plastics were playing a decisive role in the mechanism of heat generation during the ultrasonic welding. An ultrasonic welding unit for welding the heterogeneous plastics in vacuum was developed.

CFD modeling of the multiphase flow in an SKS furnace—The effect of melt density and viscosity

A previously reported numerical method using the multi-fluid VOF model to simulate an SKS furnace multiphase flow was further verified to provide a good reference for the macroscopic flow field simulation of industrial vessels with similar geometry. Using the verified numerical model, CFD simulation of a full-scale SKS furnace multiphase flow was conducted, targeting the agitation performance with different melt densities and viscosities, under constant furnace operating conditions. According to this simulation, the melt density and viscosity in a common applied range in industry is not able to significantly affect the bath agitation efficiency like other parameters, such as the tuyere diameter and bath depth. More specifically, the results indicate that an increased melt density or viscosity slightly weakens the melt flow motion, and increases the wall shear stress. Since variation in melt density and viscosity is common in industry, the results from this simulation offer a good basis for the estimation of SKS furnace operating performance and the adjustment of the physical properties of the melt in SKS furnaces or other similar industrial vessels.

Toward a sustainable laser powder bed fusion of Ti 6Al 4 V: Powder reuse and its effects on material properties during a single batch regime

Titanium alloy Ti6Al4V is the most used titanium alloy due to its high specific strength, ductility, corrosion resistance, and weldability. Currently, this alloy is produced with a density above 99% with laser welding-based additive manufacturing processes. However, the costs of titanium powder are high, and for critical components, the excess powder is usually not reused. In this work, we characterized the powder and additively manufactured Ti6Al4V parts by reusing the powder of a single batch to determine the impact on morphology, chemical composition, melt pool viscosity, density, microstructure, and tensile and impact strength. We aimed to determine the limits of powder reuse to increase sustainability and reduce the carbon footprint of Laser Powder Bed Fusion (L-PBF). We found that the powders pick up oxygen after each cycle to form an oxide layer up to 20 nm, causing a change in the optical and thermal properties of the titanium powders. This change affected the laser absorption and, with this, the melt pool properties. The powders reused after 18 cycles were responsible for a drop in the relative density of 0.1%. However, the tensile properties remained unchanged. The reused powders also presented a small shift in particle size distribution that did not affect the sphericity and increased flowability. In this work, we proved that the powders of Ti6Al4V alloys could be reused up to 18 cycles without any modification to the tensile properties but with a decrement in the impact strength of 30% for vertical- and 12% for horizontal stress-relieved specimens and 19% and 16% for furnace-annealed specimens, respectively.

Experimental design and characterization of Eu-doped tellurite matrix glassy composite for medical and ionizing-radiation sensing applications

This study experimentally presents the synthesis and characterisation of a new transparent Eu-doped tellurite matrix glassy composite with the view of predicting its UV and radiation sensing applications. This glassy composite, described by 50TeO2–15Na2O – 15ZnO – (20-x)PbO – xEu2O3 for x = 0 (TPEu-0), 1 (TPEu-1), 2 (TPEu-2), and 3 (TPEu-3) mol%, was prepared by the well-known melting-and- quenching technique. The glassy nature of the prepared samples was verified by the X-ray diffraction (XED) technique, and the density of the glasses was measured by the Archimedes method. The glasses’ optical features were determined from optical transmission and absorption spectra. In addition, the gamma radiation sensing and shielding prowess of the TPEu-X samples was determined from Monte Carlo simulation data using FLUKA code and direct calculations from the NISTXCOM database. The response of the glasses under UV-excitation was also analysed. The melting point and density of the glasses increased as the glasses was enriched with Eu2O3. The density of TPEu-X increased from 5.51 to 5.58 while the melting point grew from 750 °C to 1100 °C Eu2O3 concentration grew from 0 to 3 mol%. The increase in the concentration of Eu2O3 activated crystallization in the glass system. TPEu-1 has the highest optical bandgap energy (3.0 eV) and the lowest refractive index (2.39) among the TPEu-X glasses. The Eu2O3 content of the glasses affected their responses and appearance under UV light. Generally, the glass with the lowest Eu2O3 content possessed the least capacity to shield gamma radiation. The prepared TPEu-3 glass displayed the ability to displace many conventional and non-traditional shields in design of efficient radiation protection systems.

Molybdenum disulfide under extreme conditions: An ab initio study on its melting

Crystalline molybdenum disulfide has become a central actor in the 2D-materials community due to its promising optoelectronic and thermoelectric properties. Despite the extensive work made in investigating these properties, a vast area of knowledge remains unknown on the structure and dynamics of its disordered phases such as liquid and amorphous. Thus, the goal of this work is to investigate the melting of bulk molybdenum disulfide using ab initio molecular dynamics based on density functional theory. We employ the two-phase and Z-methods to evaluate the melting in a number of conditions. Our results at 1 bar reveal that the two-phase procedure is preferred since it predicts a melting point of 2266.92 K that is directly computed using simulations at constant pressure and energy. In contrast, this temperature is indirectly estimated at 2154.01 K with the Z-method using an interpolation of simulations at constant volume and energy. Nevertheless, we find that both methods are complementary as they allow computing different thermodynamic and structural properties. For instance, we estimate a melting heat of 0.67 eV/atom with the two-phase coexistence route, which shows very good agreement with the value of 0.75 eV/atom obtained from the difference of the internal energies of separate crystalline and liquid ensembles at the same conditions of 1 bar and 2266.92 K. In contrast, the Z-method allows us to determine the influence of pressure on the melting temperature, density, and coordination number with a lower computational cost.

The motion and growth behaviors of nucleuses in Al melt solidified under supergravity condition—molecular dynamics simulation

That supergravity can refine grains is verified in many materials. However, the underlying mechanism is still an open question. Although some convincing theories have been proposed, including the ‘crystal rain’ theory and the dendrite fragmentation theory, there is a lack of solid evidence, especially from the atomic scale. Based on the presetting nucleuses method, this study investigates the motion and growth behaviors of nucleuses during the solidification process of Al melt under supergravity condition with molecular dynamics simulation. It is found that supergravity builds a gradient pressure in the samples along the direction of supergravity, and the gradient pressure results in the gradient distribution of sample density. The preset nucleuses move directionally along the direction of supergravity forming ‘crystal rain’, while their directional moving velocity decreases due to the increase of buoyancy, which is caused by the increase of melt density in the motion path of the nucleuses. The supergravity-induced pressure not only decreases the critical size of nucleuses but also increases the growth velocity of nucleuses. The research results also indicate that larger nucleuses grow much faster than smaller ones at the same pressure. Owing to the gradient distribution of pressure, the nucleuses grow much faster along the direction of supergravity than other directions and evolve into an ‘inverted cone’ shape. Therefore, these findings show that supergravity can change the nucleation, motion and growth of nucleuses by establishing a gradient pressure in the melt, thus affecting the microstructure of the casting. Our results provide solid support for the ‘crystal rain’ theory and the nucleation rate rising theory from atomic scale.