Undercooling Conditions Research Articles

Surface wave phenomenon and microstructure evolution mechanism of electromagnetically levitated ternary Ti-Al-Fe alloy

Electromagnetic levitation technique was performed to investigate the surface wave phenomenon and microstructure evolution mechanism of undercooled ternary Ti50Al30Fe20 alloy. Under the condition of high undercooling, the formation process of surface waves on the levitated alloy surface were directly detected by high-speed camera. The cooling gas provided the external perturbation that promoted surface nucleation, providing the suitable site for surface wave generation. On the rotating semisolid alloy surface, the competition between centrifugal force and surface tension caused the surface wrinkling, leading to the formation of surface waves. The residual liquid phase was solidified in the vortex circulation flow, and the vortex-shape pattern was preserved during rapid solidification. With the increases of undercooling, the microstructure evolved from dendrites and interdendritic lamellar eutectics into uniform anomalous eutectics because of the independent nucleation and competitive growth of (βTi) and TiFe phase. The surface wave was formed only at high undercoolings, indicating this phenomenon was related to the fluidity in eutectic growth. In this case, the uniform microstructure was closer to satisfying the continuum medium hypothesis, and the homogeneous medium reduced the energy damage during wave propagation. In addition, the micromechanical properties were remarkably increased as undercooling rose owing to the microstructural morphology and solute trapping effect.

Identifying crystal nucleation mechanisms in a synthetic trachybasalt: a multimodal approach

To develop new criteria to distinguish different crystal nucleation mechanisms in silicate melts, we performed crystallization experiments using a synthetic hydrous (2 wt% H2O) trachybasalt and combined three-dimensional information from synchrotron X-ray computed microtomography with two-dimensional mapping of crystallographic orientation relationships (CORs) using electron backscatter diffraction. Crystallization experiments were performed at 400 MPa by cooling the melt from 1300 °C to resting temperatures of 1150 and 1100 °C and maintaining isothermal conditions for 30 min and 8 h. Three distinct titanomagnetite (Tmt) populations formed: (1) skeletal crystals, isolated or partially embedded in clinopyroxene (Cpx); (2) anhedral crystals, always attached to Cpx; (3) flattened needle-shaped crystals, embedded in Cpx. These morphologically different Tmt populations formed in response to one cooling event, with varying nucleation mechanisms and at different undercooling conditions. The clustered three-dimensional distribution of population 2 and 3 Tmt grains and the high proportion of Tmt-Cpx interfaces sharing CORs indicate that these Tmt grains heterogeneously nucleated on Cpx. The near-random three-dimensional distribution of (often isolated) population 1 Tmt grains, together with the low proportion of Tmt-Cpx interfaces sharing CORs, imply their isolated, possibly homogeneous nucleation, potentially followed by heterogeneous nucleation of Cpx on population 1 Tmt. Heterogeneous nucleation in slightly to moderately undercooled magmas should affect the sequence of crystallization as well as morphology and clustering of crystals, which may actively contribute to the variation of rheological parameters like viscosity. Finally, observed intra- and inter-sample variations in Tmt-Cpx COR frequencies indicate the potential for this parameter to record further petrological information.

Revealing the nucleation and growth modes upon rapid solidification of undercooled Co-24 at.% Sn eutectic alloy by the crystallographic orientation relations

In the realm of undercooled eutectic alloys, the nucleation (i.e., copious vs. single) and the growth (i.e., coupled vs. decouple) modes, particularly the formation of anomalous eutectics, remain subjects of intense debate. This study delves into the crystallographic orientation relationships (ORs) during rapid solidification of an undercooled Co-24 at.% Sn eutectic alloy, where the high-temperature eutectic phases (β-Co3Sn2 and α-Co) persist to room temperature. A significant finding is the consistent observance of the same eutectic OR, i.e., 112̅0β−Co3Sn2 // 11̅1α−Co and 0001β−Co3Sn2 // 110α−Co, across various undercooling conditions from low to high and eutectic morphologies from lamellar to anomalous. This consistency underscores a unified growth mode of coupled eutectic growth. Furthermore, the observance of twin OR, i.e., 112̅1<11̅26>, 112̅2<1123̅> or 112̅4<2243̅>, between β-Co3Sn2 phases in diverse eutectic colonies, spanning from low to intermediate undercooling, and would also be the case of high undercooling, reinforces a single nucleation mode except under exceedingly low undercooling. Building on these insights, the anomalous eutectic colonies are suggested to be formed from a single origin where coupled eutectic growth of eutectic seaweeds or eutectic dendrites instead of dual origins. This study not only illuminates the underlying mechanisms of rapid solidification in undercooled eutectic alloys but also paves the way for innovative microstructure modulation strategies.

Fluid exsolution during disequilibrium crystallization in the border and wall zones of internally zoned pegmatites

Understanding processes that occur during pegmatite crystallization are important for understanding magma evolution and enrichment processes of e.g. strategic and rare metals in pegmatites. Several processes are involved in the formation of pegmatites, and among these the timing of fluid saturation and exsolution is a critical area of investigation. The primary question focuses on if the timing of these processes influences pegmatite crystallization and the concentration of elements. The understanding of the relationship between fluid dynamics and mineralization is essential to develop comprehensive models of pegmatite formation. The unidirectional tourmaline prisms and quartz-tourmaline intergrowths (QTIs), composed of a central tourmaline surrounded by skeletal tourmaline intergrown with quartz are anisotropic textures, linked to disequilibrium crystallization at high degrees of undercooling. This study aims to trace fluid saturation and evolution in the outer zones of internally zoned pegmatites and to provide more accurate temperature constraints for their crystallization, using the Emmons pegmatite (Maine, USA) as a case study. The methodology comprises detailed inclusion petrography, microthermometric analyses, Raman spectroscopy, and LA-ICP-MS analyses. The results indicate that localized fluid saturation occurs early during the magmatic stage of pegmatite crystallization. At this point, the crystallizing mineral assemblages are composed of feldspar, muscovite, quartz, tourmaline, and minor garnet. The exsolved fluid is similar throughout the host tourmalines and is characterized by an H2O-NaCl-CO2-(N2-CH4) composition. Temperatures, which are obtained from primary fluid inclusion isochores vary between 315 °C and 440 °C and confirm that the pegmatite crystallized at undercooled conditions. A decrease in temperature occurs on a centimeter scale between the central tourmaline and the skeletal tourmaline of the QTIs, which could be caused by a nucleation delay within individual boundary layers that are formed around the central tourmaline of the QTI. The higher concentration of water and other incompatible components, including fluxes, in the boundary layer led to fluid exsolution localized at the crystal-melt interface. The increased concentration of fluxes causes a nucleation delay, increasing the degree of undercooling and consequently leading to the skeletal morphologies seen in the QTIs. Furthermore, studied melt inclusions indicate that a compositionally modified melt was present during tourmaline crystallization, confirming the existence of a boundary layer at centimeter to decimeter scale.

Numerical Texture Analysis on NW Iran Dacite, Basalt, and Tephraphonolite: Implications to Constrain their Petrogenesis

ABSTRACT In this research, our focus has been on the quantitative calculations of the texture of volcanic rocks. For this purpose, the rocks with acidic to basic and ultrabasic combinations were selected, and the shape, size, and spatial distribution (CSD and SDP) of their phenocrysts were investigated. The results showed that in volcanic rocks, the ratio of nucleation to crystal growth (J/G) was several times higher, and crystal growth per time unit (Gt) was higher in rocks with fewer nuclei. It also turns out that to evaluate the magma mixing, CSD pattern analysis can be instrumental. The spatial distribution of grains in the extrusive rocks is clustered to random, probably due to the high degree of overstepping and under-cooling conditions. The mineral clusters may form before the emplacement of the flow, at depth during the residence time before the eruption flow, or throughout the rise and flow. The importance of these results displays that due to the progress of imaging methods and statistical-numerical work on crystals and textures; it is time that all petrologists from all over the world, along with geochemical work, pay attention to these types of studies to provide a complete petrological work.

Investigation on solidification structure evolution of deeply undercooled single-phase Ni-Cu-Co alloys

In this work, the main focus was on exploring the effect of increasing the gradient content of Co element on the solidification process of Ni-Cu-Co alloy under deep undercooling conditions. The effects on its solidification rate, recalescence effect, and critical undercooling range were also explored. In the experiment, a combination of molten glass purification and cyclic superheating technology was mainly used to obtain gradient undercooling of Ni88Cu10Co2, Ni88Cu8Co4, and Ni88Cu6Co6 alloy samples at intervals of 10 K. By analyzing the microstructure evolution under different undercooling degrees through metallographic diagrams, and combining it with high-speed cameras to record the phenomenon of recalescence during the solidification process, the influence of the addition of Co element on the solidification rate, recalescence effect, and the critical undercooling range appearing in the microstructure during the overall solidification process was explored. Conduct EBSD analysis on the alloy samples under the maximum undercooling of Ni88CuCo6, and analyze the representative significance behind their characterization data. At the same time, TEM transmission analysis was conducted on the Ni88Cu6Co6 alloy sample with the highest Co content, and it was found that there are high-density dislocation and stacking fault networks as well as obvious twinning structures in the substructure area. The standard FCC diffraction pattern also represents that it is still a single-phase structure. Based on the above metallographic diagram, EBSD, and TEM data analysis, it is proven that the occurrence of grain refinement under high undercooling is mainly due to stress induced recrystallization.

Study on the microstructure evolution mechanism of copper single phase alloys under deep undercooling conditions

This article mainly used the deep undercooling technology to treat Cu60Ni40, Cu60Ni38Co2, and Cu60Ni35Co5 alloys to obtain different undercoolings. By using high speed cameras, the rapid solidification process was recorded to analyze the relationship between its evolution law and the magnitude of undercooling. When observing its microstructure, it was also found that there was a critical undercooling degree throughout its entire solidification process. The influence of Co element addition on solidification rate, recalescence effect, and critical undercooling of copper based single-phase alloys under undercooling conditions was explored. EBSD analysis was conducted on the refined microstructure of the alloy that achieved the maximum undercooling, and the significance of grain orientation and orientation difference distribution was found. TEM tests were conducted on a Cu60Ni38Co2 alloy with a maximum undercooling of 259 K, and it was found that there are high-density dislocation networks in some areas within them. Finally, the Vickers hardness of the above alloys was measured and recorded using a microhardness tester (HVS-1000Z), and the evolution between microhardness and undercooling, as well as the changes in microhardness under critical undercooling, were analyzed. Based on the analysis of EBSD, TEM, and microhardness data above, it is found that grain refinement at high undercooling is dominated by recrystallization.

Microstructure, precipitation behavior, and properties of quaternary Cu–Be–Co–Ni alloy under electromagnetic levitation

Electromagnetic levitation (EML) can provide a containerless environment and, consequently, large undercooling solidification for various metals. In this study, the microstructures, precipitated phase characteristics, properties, and strengthening mechanisms of Cu–0.2Be–1.6Co–1.6Ni alloy with different undercooling conditions were investigated using the EML method. The achieved undercooling range was 76–315 K. The microstructure of the alloy was mainly composed of dendrites and slender-strip precipitates. Under EML, the length and width of the precipitates decreased by an order of magnitude with increasing undercooling. The precipitates transformed from a α(Co) phase with a face-centered cubic structure to a metastable γ′ phase with a body-centered tetragonal structure. The microhardness increased almost linearly from 120 to 154 HV with enhanced undercooling. The electrical conductivity was higher than that of the master alloy and increased slightly with enhanced undercooling. Quantitative analyses of the strengthening mechanisms revealed the vital roles of solution and precipitation strengthening in the mechanical properties of EML alloys, whereby the former was weakened and the latter was strengthened as undercooling progressed. The simultaneous enhancement of the mechanical and electrical properties of Cu–0.2Be–1.6Co–1.6Ni alloy can be attributed to the increased solute precipitation and refined precipitates due to deep undercooling.

Metastable phase separation and duplex metallic glass formation of liquid Zr&lt;sub&gt;35&lt;/sub&gt;Al&lt;sub&gt;23&lt;/sub&gt;Ni&lt;sub&gt;22&lt;/sub&gt;Gd&lt;sub&gt;20&lt;/sub&gt; alloy

Duplex metallic glass with two amorphous phases has been extensively investigated for desirable strength and plasticity. In this paper, the metastable phase separation and dual amorphous phase formation of liquid Zr&lt;sub&gt;35&lt;/sub&gt;Al&lt;sub&gt;23&lt;/sub&gt;Ni&lt;sub&gt;22&lt;/sub&gt;Gd&lt;sub&gt;20&lt;/sub&gt; alloy under substantial undercooling condition and rapid cooling condition are studied by drop tube technology. The equilibrium solidification structure consists of three crystalline phases, while the critical undercooling temperature of metastable phase separation is determined to be 516 K (0.37&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;). The separated Zr-rich liquid phase undergoes amorphous transition and becomes amorphous AM-Zr phase with the composition of Zr&lt;sub&gt;45&lt;/sub&gt;Ni&lt;sub&gt;23&lt;/sub&gt;Al&lt;sub&gt;23&lt;/sub&gt;Gd&lt;sub&gt;9&lt;/sub&gt; when alloy undercooling is increased to 624 K (0.45&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;). After that, the Gd-rich liquid phase forms amorphous AM-Gd phase with the composition of Gd&lt;sub&gt;39&lt;/sub&gt;Al&lt;sub&gt;22&lt;/sub&gt;Ni&lt;sub&gt;20&lt;/sub&gt;Zr&lt;sub&gt;19&lt;/sub&gt; at larger undercooling of 714 K (0.52&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;). With the increase of liquid undercooling and cooling rate, the kinetic mechanism of metastable phase separation changes from nucleation and growth type to spinodal decomposition type, and consequently the microstructure of dual amorphous phases transforms from a spherical morphology to a reticular structure. The average hardness and Young’s modulus, which are influenced by free volume, phase volume fraction and structure of dual amorphous phases, exhibit a complex variation of first increasing and then decreasing with the decrease of alloy droplet size. The formation of dual amorphous phases is in favor of the energy dissipation and the generation of multiple shear bands during mechanical compression, which improves the plasticity for this kind of amorphous alloy.

Machine learning interatomic potentials for aluminium: application to solidification phenomena

In studying solidification process by simulations on the atomic scale, the modeling of crystal nucleation or amorphization requires the construction of interatomic interactions that are able to reproduce the properties of both the solid and the liquid states. Taking into account rare nucleation events or structural relaxation under deep undercooling conditions requires much larger length scales and longer time scales than those achievable by ab initio molecular dynamics (AIMD). This problem is addressed by means of classical molecular dynamics simulations using a well established high dimensional neural network potential trained on a set of configurations generated by AIMD relevant for solidification phenomena. Our dataset contains various crystalline structures and liquid states at different pressures, including their time fluctuations in a wide range of temperatures. Applied to elemental aluminium, the resulting potential is shown to be efficient to reproduce the basic structural, dynamics and thermodynamic quantities in the liquid and undercooled states. Early stages of crystallization are further investigated on a much larger scale with one million atoms, allowing us to unravel features of the homogeneous nucleation mechanisms in the fcc phase at ambient pressure as well as in the bcc phase at high pressure with unprecedented accuracy close to the ab initio one. In both cases, a single step nucleation process is observed.

Unsupervised topological learning for identification of atomic structures.

We propose an unsupervised learning methodology with descriptors based on topological data analysis (TDA) concepts to describe the local structural properties of materials at the atomic scale. Based only on atomic positions and without a priori knowledge, our method allows for an autonomous identification of clusters of atomic structures through a Gaussian mixture model. We apply successfully this approach to the analysis of elemental Zr in the crystalline and liquid states as well as homogeneous nucleation events under deep undercooling conditions. This opens the way to deeper and autonomous study of complex phenomena in materials at the atomic scale.

Quantitative simulations of grain nucleation and growth at additively manufactured bimetallic interfaces of SS316L and IN625

This paper provides a quantitative understanding of grain nucleation and growth at the interface of SS316L and IN625 bimetallic structures during directed energy deposition (DED) through multi-physics simulations, including phase-field models and computational fluid dynamics analysis. The main finding is that the flow behaviors would lead to composition redistribution and the change of liquidus temperature in the mixing zone at the interface, which further influences the solidification sequence and the final grain structure that are different from general single-metal counterparts without the mixing zone. The results show that during depositing IN625 on SS316L, a gradual transition in composition distribution and liquidus temperature in the well-mixing zone caused by simple clockwise flow leads to epitaxial grain growth from the SS316L substrate and the prevalence of columnar grains with a low undercooling (<1 K) condition. However, when depositing SS316L on IN625, it turns out that initial solidification can occur due to abrupt compositional change at the interface where the well-mixing breaks are caused by two opposite (clockwise and counterclockwise) flow behaviors. Such an abrupt compositional change results in high liquidus temperatures that may trigger the grain nucleation in the middle melt pool due to a high undercooling (>30 K); thus, a mixed grain microstructure is present.

Microporphyritic and microspherulitic melt grains, Hiawatha crater, Northwest Greenland: Implications for post-impact cooling rates, hydration, and the cratering environment

Abstract Sand-sized impactite melt grains hand-picked from a glaciofluvial sample proximal to the Hiawatha impact crater in Northwest Greenland contain new information about the crystallization and cooling history of this impact structure, which is concealed by the Greenland Ice Sheet. Of course, the original locations of the individual sand grains are unknown, but this is offset by the substantial number and wide variety of impactite grains available for study. A detailed investigation of 16 melt grains shows that post-cratering crystallization took place under very variable conditions of strong undercooling with temperatures that dropped rapidly from high above their solidus to far below. A distinct event of near-isochemical hydration at above or ~250 °C is recorded by intense perlitic fracturing and the growth of closely packed mordenite spherulites only 1–3 μm across in felsic melt grains, which was followed by lower temperature hydrothermal alteration along the pre-existing perlitic fractures. The formation of abundant mordenite microspherulites appears to be very rare or not previously recorded in impactite melts and suggests the rapid infilling of the Hiawatha crater by a hydrous source. The infilling did not occur immediately after the impact as in submarine impacts, but soon thereafter, and before the establishment of a low-temperature hydrothermal alteration system common to the waning stage of cooling in many impact structures. These observations and previous documentation of terrestrial organic matter in the impactites are consistent with an impact into a water-rich terrestrial environment, such as through the Greenland Ice Sheet or into a forested, lacustrine–fluvial region.

Plastic straining and concomitant microstructure recrystallization of Ni-Cu alloy in the undercooled condition

Microstructure and microtexture of rapidly solidified undercooled Ni-Cu alloys were investigated. The characteristic undercooling of Ni80Cu20 alloy was determined as 45K, 90K and 160K. Dendrite deformation due to rapid solidification led to strong deformation microtexture. Due to recrystallization upon annealing after recalescence, many subgrains were formed in the microstructure. Further, annealing the quenched alloy at 900°, new microtextures and subgrains were formed, which was due to recrystallization and dislocation network rearrangement. The results of comparative experiment proved the recrystallization mechanism of the microstructure refinement in the non-equilibrium solidification structure of the undercooled binary alloy

Unveiling the occurrence of transient, multi-contaminated mafic magmas inside a rhyolitic reservoir feeding an explosive eruption (Nisyros, Greece)

The investigation of heterogeneous magma systems enhances the understanding of magma differentiation and transfer processes in active volcanoes, thus constraining the dynamics driving the eruptions and the related hazard. Magma heterogeneity is generally preserved in the coeval juvenile products of explosive eruptions, as it occurs in the Upper Pumice sequence, emplaced by the last sub-Plinian explosive eruption at Nisyros volcano (Greece). The deposit comprises a basal fallout, overlaid by pyroclastic density current units, followed by a lag-breccia level. White-yellow, porphyritic, rhyolitic pumices constitute the main juvenile component. Grey, crystal-rich juvenile clasts (CRCs) are less abundant (up to 10–15%), and are characterised by three different texture types (Type-A, -B and -C), with specific recurrence in the different depositional units and well correlated to the magma evolution. In the basal unit CRCs occur as andesitic to dacitic lapilli with Type-A and -B vesicular textures associated with highly variable trace element and isotopic compositions. In the lag-breccia deposit, the juvenile clasts occur as bombs with crenulated or bread-crust surfaces, displaying diktytaxitic Type-C textures and less evolved andesitic compositions, covering a larger Nd-isotope range at lower Sr-isotopes compared to the others.The CRCs are interpreted as the result of the rapid cooling of more mafic magma blobs sequentially intruded in the cooler rhyolitic host magma, in which they attained variable textures by different undercooling conditions, due to their variable compositions. We suggest that a two-stage AFC (Assimilation plus Fractional Crystallisation) process occurred at different pressures, before intrusion in the host magma, accounting for their heterogeneous chemical and isotopic characteristics. Firstly, the most primitive melts variably assimilated gneissic wallrock at depth, acquiring a variable Nd-isotope signature. On the way to the surface, they later experienced shallow AFC processes within different small magma reservoirs, involving heterogeneous carbonate-rocks such as pure limestone, metasomatised marble and skarn. Sequential dynamics of ascent and intrusion into the rhyolitic magma chamber lead the more evolved and skarn-contaminated Type-A and -B melts to firstly move in the upper part of the reservoir to be erupted in the early fallout deposits. Type-C more mafic melts later intruded the rhyolitic reservoir and were erupted in the lag-breccia deposit. The lowest Nd-isotopes recorded by CRCs, with respect to all the volcanic products of the Kos-Nisyros volcanic field, reveal the peculiar transient history for these magmas at relatively shallow levels in the crust. The CO2 release from the carbonate-rock assimilation has also possibly contributed to trigger the explosive eruption, discharging a large amount of CO2 into the atmosphere.

Study on Microstructure Refinement Mechanism in Deeply Supercooled Solidification of Ni-25at.%Cu Alloys

Maximum undercooling degrees were obtained by glass purification and superheating. Rapid solidification in an undercooled alloy was systematically studied. As the undercooling increases, the solidification front appears randomly. The microstructure undergoes a transformation from coarse dendrite grains to granular grains to oriented dendrite to equiaxed crystal. The results show two grain refinement under high and low undercooling. No high strength textures are found under high undercooling conditions, and many grain boundaries of high angle nature are observed, which confirms recrystallization. In the TEM characterization of high undercooling, high density dislocation strain indicates that accumulated stress in the rapid solidification process induces microstructure plastic strain and promotes recrystallization. The results show that the grain boundary of refined grain is coarser at low undercooling, but narrower at high undercooling. Twins and subgrains observed at large undercooling indicate that the grain refinement mechanisms are very different.

A fixed grid based accurate phase-field method for dendritic solidification in complex geometries

In this paper, an easily implementable immersed interface method based high-resolution phase-field model is proposed for simulating dendritic growth related problems in complicated geometries. At first, resolution issues of different Fourier Pseudo-spectral based dealiasing schemes are shown for the anisotropic phase field equations at high thermal undercooling conditions using just adequate spatial grid resolution. The problems of aliasing error and spectral leakage are studied using high-order based exponential smoothing spectral filters, which generally occur at low sampling rate situations. A fully dealiased zero padding scheme based anisotropic phase-field method is also developed in conjunction with an optimal third order strong stability preserving Runge–Kutta (SSPRK3) time integration scheme for obtaining more accurate and stable numerical solutions. In the later part, a modified phase-field method is implemented with the indicator function which simulates different crystal growth problems in complex geometries without using non-trivial mesh refinement algorithm. Effect of non-interacting obstacles present in the computational domain can also be readily incorporated in the present numerical model by using the single order parameter based phase-field equations.

Grain refinement mechanism of rapidly solidified nickel alloys

Abstract The solidification microstructure evolution of Ni-25 at.% Cu alloys under different undercooling degrees were studied by the cladding method and cyclic superheating method. Two grain refinement phenomena were observed in the obtained undercooling. In the low undercooling condition, dendrite remelting is the main reason for grain refinement in the recalescence process, while in the high undercooling condition, the stress accumulated in the recalescence process leads to recrystallization in the later stage of recalescence. Under the condition of high undercooling, the solidification structure is composed of complete equiaxed grains with relatively uniform grain size, which indicates that grain boundary migration occurs during grain growth.

Local structure of supercooled liquid Ga90In10 alloy

The local structure of liquid and supercooled liquid Ga90In10 alloy was studied by the X-ray absorption fine-structure (XAFS), X-ray diffractometer (XRD) and the ab initio molecular dynamics (AIMD) simulation. The ab initio FEFF8 code was applied to simulate the In k-edge X-ray absorption near edge structure (XANES) spectrum of the melt. The local atomic arrangement models have been obtained. The results indicate that the structural evolution from the liquid to supercooled liquid Ga90In10 alloy is mainly caused by the change of the In atom local structure. As the temperature decreases, the nearest neighbor coordination number decreases and the nearest neighbor interatomic distance increases. The nearest neighbor coordination number around In atom decreases from 13 to 12, and there is obvious Ga aggregation under the undercooled condition.

Homogenous nucleation rate of CO2 hydrates using transition interface sampling.

Carbon dioxide and water can form solid clathrate structures in which water cages encapsulate the gas molecules. Such hydrates have sparked much interest due to their possible application in CO2 sequestration. How the solid structure forms exactly from the liquid phase via a homogenous nucleation process is still poorly understood. This nucleation event is rare on the molecular timescale even under moderate undercooling or supersaturation conditions because of the large free energy barrier toward crystallization, rendering a brute force simulation of hydrate nucleation unfeasible for moderate undercooling or supersaturation. Here, we perform transition interface sampling simulations to quantify the homogenous nucleation rate for CO2 hydrate formation using accurate atomistic force fields at 500 bars for three different temperatures between 260 and 273 K. Collecting more than 100 000 pathways comprising roughly two milliseconds of simulation time, we computed a nucleation rate in the amorphous phase of ∼1021 nuclei s-1 cm-3 for a temperature of 260 K and a rate of ∼1012 nuclei s-1 cm-3 for a temperature of 265 K. For a temperature of 273 K, we find that the hydrate forms an sI crystalline phase with a rate of order of ∼101 nuclei s-1 cm-3. We compare these rates to classical nucleation theory estimates as well as experiments, and to nucleation rate estimates for methane hydrates and discuss possible causes of the observed differences. Our findings shed light on the kinetics of this important clathrate and should assist in future hydrate formation investigation.