Evidence of multi-step nucleation leading to various crystallization pathways from an Fe-O-Al melt

New Insights into the Mechanism of Nucleation of ZrO2 Inclusions at High Temperature

It is difficult to observe the nucleation mechanism of inclusions in real-time. In this study, the nucleation process of zirconium oxide inclusions was systematically studied by classical nucleation theory and first principles. Zr deoxidized steel with 100 ppm Zr addition was processed into metallographic samples for scanning electron microscopy energy-dispersive spectroscopy observation. The electrolytic sample was analyzed by micro X-ray diffraction and transmission electron microscopy, and the zirconium oxide in the sample was determined to be ZrO₂. The nucleation rate and radius of the ZrO₂ inclusions were calculated by classical nucleation theory, and they were compared with the experimental values. There was a considerable difference between the experimental and theoretical values of the nucleation rate. The effect of the nucleation size was analyzed by first-principles calculation, and the thermodynamic properties of ZrO₂ clusters and nanoparticles were analyzed by constructing (ZrO₂)_n (n = 1-6) clusters. The thermodynamic properties of ZrO₂ calculated by first principles were consistent with the values in the literature. Based on two-step nucleation theory, the nucleation pathway of ZrO₂ is as follows: Zr_atom + O_atom → (ZrO₂)_n → (ZrO₂)₂ → core (ZrO₂ particle)-shell ((ZrO₂)₂ cluster) nanoparticle → (ZrO₂)_bulk.

Thermodynamic insight into the growth of nanoscale inclusion of Al-deoxidation in Fe-O-Al melt

Products of Al-deoxidation reaction in iron melt are the most common inclusions and play an important effect on steel performance. Understanding the thermodynamics on nano-alumina (or nano-hercynite) is very critical to explore the relationship between Al-deoxidation reaction and products growth in iron melt. In present study, a thermodynamic modeling of nano-alumina inclusions in Fe-O-Al melt has been developed. The thermodynamic results show that the Gibbs free energy changes for the formation of nano-Al2O3 and nano-FeAl2O4 decrease with the increasing size and increase with the increasing temperature. The Gibbs free energy changes for transformation of nano-Al2O3 into bulk-Al2O3 increase with the increasing size and temperature. The thermodynamic curve of nano-alumina (or nano-hercynite) and the equilibrium curve of bulk-alumina (or bulk-hercynite) obtained in this work are agree with the published experimental data of Al-deoxidation equilibria in liquid iron. In addition, the thermodynamic coexisting points about Al2O3 and FeAl2O4 in liquid iron are in a straight line and coincide with the various previous data. It suggested that these scattered experimental data maybe in the different thermodynamic state of Al-deoxidized liquid iron and the reaction products for most of the previous Al-deoxidation experiments are nano-alumina (or nano-hercynite).

Effect of Mg Treatment on the Nucleation and Ostwald Growth of Inclusions in Fe-O-Al-Mg Melt

This study aimed to investigate the effect of Mg treatment on the nucleation and ostwald growth of inclusions. Deoxidized experiments with Al (0.05%Al) and Al-Mg (0.05%Al + 0.03%Mg) were carried out at 1873 K, and the composition, number, and size of inclusions were studied as a function of holding time. Homogeneous nucleation theory and ostwald ripening were utilized to calculate the nucleation rate, the critical size of nuclei, and coarsening rate of inclusions. The results show that small inclusions were more easily found in the steels with Al-Mg complex deoxidation, and the number of inclusions with Al-Mg complex deoxidation is larger at an early stage of deoxidation. The critical size of nuclei increases in the order of MgAl₂O₄ (0.3-0.4 nm) < Al₂O₃ (0.4-0.6 nm), and the nucleation rate increases in the order of Al₂O₃ (1100 cm^-3 s^-1) < MgAl₂O₄ (1200 cm^-3s^-1), which is consistent with the experimental results. Moreover, the coarsening rate of MgAl₂O₄ inclusions was smaller than Al₂O₃ inclusions in both the value of k_d(cal.) from ostwald growth and the value of k_d(obs.) from inclusion size. The effect of Mg addition on coarsening of inclusion was analyzed and their mechanism was discussed based on ostwald ripening theory and Factsage calculation.

Study on the growth and morphology evolution of titanium oxide clusters in molten iron with molecular dynamics simulation

Formation of nano-scale titanium oxides is a desirable result in the deoxidation process of steelmaking. However, the nucleation of nano-scale titanium oxide inclusions remains unknown up to now because of the difficulty in observing and detecting inclusions in steel melt. In this work, we studied the formation and evolution of titanium oxygen clusters in molten iron by molecular dynamics (MD) simulation using empirical atomic interaction potentials. The structures of small titanium oxygen clusters in iron are reasonable compared to the first-principles simulation results. The growth process of small clusters into larger clusters was simulated and it is found the clusters grow through the collision mechanism, with the intermediate products exhibiting chain structures. The iron environment was found to play an important role in the structural form of the titanium oxygen clusters. This study is useful to provide the details of formation and the growth mechanism of titanium oxygen clusters and to provide a valuable picture for the nucleation mechanism of titanium oxide in molten steel.

The aggregation and growth of titanium oxygen clusters before nucleation were simulated by molecular dynamics.

Structural evolution of calcia during calcium deoxidation in Fe-O-Ca melt

The crystallization process of CaO in iron melt begins with nucleation, which determines the structure and size of the CaO inclusions; thus, it is important to investigate the mechanism of inclusion modification by calcium treatment. In this study, a two-step nucleation method was used to investigate the behavior during the early stages of CaO inclusion crystallization. The first principles method was applied to calculate the structures and properties of CaO crystals and CaO clusters. Then, the nucleation mechanism of CaO in the Fe-O-Ca melt has been discussed. The numerical results show that CaO clusters with cubic structures and appropriate variations are the lowest energy structures and are more stable than other isomers. The stability of the cubic (CaO)_n clusters increases with the increase in size and gradually approaches that of the CaO crystal. CaO clusters can form spontaneously in the Fe-O-Ca melt, while the transformation reaction of the CaO clusters in the Fe-O-Ca melt deeply depends on the supersaturation ratio of [Ca] and [O]. CaO clusters may remain as suspended CaO inclusions in the iron melt for a long time, and these suspensions of CaO clusters are the source of excess oxygen in the iron melt.

Thermodynamic insight into the growth of calcia inclusions at the nanoscale: the case of Fe-O-Ca melt

A thermodynamic model was developed to investigate the relationship between the thermodynamics of nano-CaO as a deoxidation reaction product and their size in an Fe–O–Ca melt. The results of thermodynamic model coupling with DFT (density functional theory) calculation prediction showed that the solubility product of calcium and oxygen for nanoscale CaO decreased with the increase of calcia product size in an Fe–O–Ca melt. The existing experimental data about the Ca-deoxidation equilibrium in liquid iron are covered by the region between the bulk-calcia equilibrium curve and the nano-CaO of 2 nm size curve. This result indicates that the partial product in most of the Ca-deoxidation experiments could be nanoscale CaO particles. Most of the Ca-deoxidation experimental equilibrium states are not reaching the equilibrium state between bulk calcia and liquid iron but a multi-equilibria between bulk- and nano-CaO and liquid iron.

Thermodynamics of the formation of nano-CaO in liquid iron is important to explore the relationship between deoxidation reaction and size of inclusions, and is very useful in the size controlling of inclusions.

Thermodynamic Modelling on Nanoscale Growth of Magnesia Inclusion in Fe-O-Mg Melt

Nano-magnesia is the intermediate product during the growth of magnesia inclusion in Mg-deoxidized steel. Understanding the thermodynamics on nano-magnesia is important to explore the relationship between magnesia product size and deoxidation reaction in molten steel. In this work, a thermodynamic modeling is developed to study the Mg-deoxidation reaction between nano-magnesia inclusions and liquid iron. The thermodynamic results based on the first principle method show that the Gibbs free energy change for the forming magnesia product decrease gradually with the increasing nano-magnesia size in liquid iron. The published experimental data about Mg-deoxidation equilibria in liquid iron are scattered across the region between the thermodynamic curves of 2 nm magnesia and bulk-magnesia. It is suggested that these scattered experimental data of Mg-deoxidized liquid iron are in different thermodynamic states. Some of these experiments are in equilibrium with bulk-magnesia, while most of these experiments do not reach the equilibrium state between bulk magnesia and liquid iron, but in quasi-equilibria between nano-magnesia and liquid iron. This is the reason that different researchers gave different equilibrium constants. Furthermore, the behavior of the metastable magnesia is one of the most important reasons for the supersaturation ratio or the excess oxygen for MgO formation in liquid iron.

Nucleation and growth for magnesia inclusion in Fe-O-Mg melt

The crystallization process of magnesia in iron melt begins with nucleation, which determines the structure and size of magnesia inclusions. Thus, it is necessary to have a deep insight into the crystallization of magnesia by two-step nucleation mechanisms. In this work, the two-step nucleation method was used to investigate the behavior during the early stages of magnesia inclusions crystallization. A first principles method was applied to calculate the thermodynamic properties of magnesia crystal from various cluster structures for the formation of magnesia inclusions. Based on the numerical results, the nucleation mechanism of magnesia in liquid iron has been discussed. The magnesia clusters appear as the structural units for Mg-deoxidation reaction in the liquid iron, and the residual magnesia clusters are the reason for the supersaturation ratio or the excess oxygen for MgO formation in the liquid iron. Based on the comparison between Mg-deoxidation equilibrium experiments and numerical results, the previous experiments may be in a different thermodynamic state. The equilibrium reaction product should be not only magnesia clusters but also bulk-magnesia in those equilibrium experiments.

The crystallization process of magnesia involves two steps.

A two-step mechanism in nucleation of solid silica from Fe-O-Si melt

In order to control the structure and size distribution of silica inclusions in high purity steel, it is necessary to understand the nucleation mechanism of solid silica in molten steel. In high temperature reactions, crystallization begins with nucleation, which plays a crucial role in determining the structure and size of solid products. Nucleation originates in the formation of product intermediates. The structure and thermodynamic properties of silica clusters as the intermediate of solid silica products during nucleation were calculated by density functional theory. Comparison of thermodynamic properties of silica clusters and silicon deoxidation equilibrium experiment in liquid iron results shows the silica clusters with most of the dissolved silicon and oxygen in equilibrium; the molten iron silicon deoxidation reaction ([Si] [Formula: see text] [O] [Formula: see text] cannot reach thermodynamic equilibrium state, and some deoxidation products could only exist in the form of silica clusters but not the solid silica. Therefore, the nucleation process of solid silica in Fe-O-Si melt can be considered as a two-step process with silica clusters as intermediates. Finally, there are two paths of solid silica inclusion formation: one is that in the molten iron, the dissolved silicon reacts with the dissolved oxygen to form silica clusters, and clusters further nucleate and grow up; the second one is that silica clusters directly crystallized during the cooling process of the melt.

Structures and infrared spectra of calcium phosphate clusters by ab initio methods with implicit solvation models

Since the first detection of pre-nucleation clusters during the formation of calcium phosphate minerals, determining such clusters' compositions and structures has become crucial for understanding the early-stage nucleation of these minerals in solutions. In previous experimental studies, the composition and sizes of pre-nucleation clusters have been calculated, but their structural information has been difficult to determine because they are very small (<1 nm). In this study, we examined the structures and infrared spectra of small- and medium-sized calcium phosphate clusters using ab initio calculations combined with implicit solvation models. Adding solvent effects increased the possibility of the existence of alternative configurations of calcium phosphate clusters other than their compact configurations. The calcium atoms had a tendency to be located outside of the clusters to coordinate with water molecules in the aqueous environment. The computed infrared spectra of extended small calcium phosphate clusters captured some of the features measured in the in situ infrared spectra, which supports the network structures proposed by large-scale molecular dynamics studies and X-ray adsorption near-edge spectra. The relative stabilities of medium-sized Ca₉(PO₄)₆ clusters with respect to the stability of Posner's cluster in water were also reviewed. We found that in water, alternative structures with low symmetry or large dipole moments had lower energies than Posner's cluster.

Nucleation and Ostwald Growth of Particles in Fe-O-Al-Ca Melt

Tremendous focus has been put on the control of particle size distribution which effects the grain structure and mechanical properties of resulting metallic materials, and thus nucleation and growth of particles in solution should be clarified. This study uses classical nucleation theory and Ostwald ripening theory to probe the relationship between the compositions of Fe-O-Al-Ca melts and the behavior of particles under the condition of no external stirring. Our experimental data suggest that decreasing the initial Ca addition and Al addition is conductive to the increase of nucleation rate for calcium aluminate particles, which exhibits a same change trend with that predicted from classical nucleation theory. Based on the experimental evidence for particles size distribution in three-dimensional, we demonstrate that Ostwald ripening is the predominate mechanism on the coarsening of particles in Fe-O-Al-Ca melt at early stage of deoxidation under the condition of no external stirring but not at later stage.

Learning about the nucleation pathway of MgO·Al2O3 spinel from an Fe–O–Al–Mg melt using a two-step nucleation mechanism

MgO·Al₂O₃ spinel nucleation is attributed to the organization of clusters into ordered lattice structures by a multi-step nucleation pathway.