Kinetics and morphological instabilities of stressed solid-solid phase transformations

An atomistic model of the growth kinetics of stressed solid-solid phase transformations is presented. Solid phase epitaxial growth of (001) Si was used for comparison of new and prior models with experiments. The results indicate that the migration of crystal island ledges in the growth interface may involve coordinated atomic motion. The model accounts for morphological instabilities during stressed solid-solid phase transformations.     

Adatom binding at the surface ledges of a jellium metal

The nature of a surface ledge on a jellium metal has been simulated by sharply cutting the uniform positive charge into the prescribed ledge and allowing the negative electronic charge to show a compensating modulation, falling off tangentially to the surface. The size of the associated fall-off parameter was determined by energy minimization and turned out to be quite independent of step height and comparable to that for the charge fall-off perpendicular to the surface. To this ledge a model adatom was brought, consisting of a Topp-Hopfield ion core and a single valence electron distribution. When approaching the ledge from the lower terrace, the adatom binding increased by about 40% more than its flat surface value. Conversely, the adatom approaching the ledge across the upper terrace experienced a decrease in binding (balustrade effect) also of about 40% of the flat surface binding. A particular application of the results to condensation from the vapor phase is the prediction that the spacing between a series of ledges will tend to become more uniform as the condensation proceeds.     

Measurement of solid-solid transformation rates by positron annihilation

Positron lifetime measurements have been used to obtain the rates of solid-solid phase transformations at 21°C in certain binary mixtures of n-eicosane and n-docosane.     

Influence of temperature and anisotropic pressure on the phase transitions in alpha-cristobalite

The role of temperature and anisotropy of the applied load in the pressure-induced transformations of alpha-cristobalite is investigated by means of first principles molecular dynamics combined with the metadynamics algorithm for the study of solid-solid phase transitions. We reproduce the transition to alpha-PbO2 as found in experiments and we observe that the transition paths are qualitatively different and yield different products when a nonhydrostatic load is applied, giving rise to a new class of metastable structures with mixed tetrahedral and octahedral coordination.     

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particle's temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

LEED studies of UO2(∼111) vicinal surfaces

The ordering/disordering of terraces, ledges and kinks on three UO₂(~ 111) vicinal surfaces were examined by LEED. Optical transforms were used as an indication of the types of diffraction displays expected. On surfaces with high kink and ledge densities and unheated above 600 °C, the ledges on the average are equally spaced and parallel, but the kink-site positions are random. At higher temperatures of 700 < T < 900 °C, the ledges rotate and decompose irreversibly into {553} and {311} microfacets. By transform simulations it was inferred that ledges on the UO₂{553} surfaces are highly ordered and contain less than 7% kink-sites.     

Multi-phase field simulation of grain growth in multiple phase transformations of a binary alloy

This work establishes a temperature-controlled sequence function, and a new multi-phase-field model, for liquid–solid–solid multi-phase transformation by coupling the liquid–solid phase transformation model with the solid–solid phase transformation model. Taking an Fe–C alloy as an example, the continuous evolution of a multi-phase transformation is simulated by using this new model. In addition, the growth of grains affected by the grain orientation of the parent phase(generated in liquid–solid phase transformation) in the solid–solid phase transformation is studied. The results show that the morphology of ferrite grains which nucleate at the boundaries of the austenite grains is influenced by the orientation of the parent austenite grains. The growth rate of ferrite grains which nucleate at small-angle austenite grain boundaries is faster than those that nucleate at large-angle austenite grain boundaries. The difference of the growth rate of ferrites grains in different parent phase that nucleate at large-angle austenite grain boundaries, on both sides of the boundaries, is greater than that of ferrites nucleating at small-angle austenite grain boundaries.     

The theory of Ostwald ripening

Developments in the theory of Ostwald ripening since the classic work of I. M. Lifshitz and V. V. Slyozov (LS) are reviewed and directions for future work are suggested. Recent theoretical work on the role of a finite volume fraction of coarsening phase on the ripening behavior of two-phase systems is reformulated in terms of a consistent set of notation through which each of the theories can be compared and contrasted. Although more theoretical work is necessary, these theories are in general agreement on the effects of a finite volume fraction of coarsening phase on the coarsening behavior of two-phase systems. New work on transient Ostwald ripening is presented which illustrates the broad range of behavior which is possible in this regime. The conditions responsible for the presence of the asymptotic state first discovered by LS, as well as the manner in which this state is approached, are also discussed. The role of elastic fields during Ostwald ripening in solid-solid mixtures is reviewed, and it is shown that these fields can play a dominant role in determining the coarsening behavior of a solid-solid system.     

Recent topics in microstructure control by magnetic field in Ni<Subscript>2</Subscript>MnGa and CoPt and new transformation behavior in Ti-Ni-Fe alloys

Three topics related to solid-solid phase transformations are presented. The first topic is related to ferromagnetic shape memory alloys. The general condition for rearrangement of martensite variants by magnetic field is discussed quantitatively. The second topic is related to microstructure control of CoPt (tetragonal L1₀-type structure) during ordering heat-treatment under a magnetic field. We show that a single variant state is realized by magnetic field, and magnetic field is especially effective at the early stage of ordering. The third topic is related to the so-called precursor phenomena in Ti-Ni-Fe shape memory alloys. In the topic we will show the existence of a commensurate phase, which inherits the microstructure of the incommensurate phase and is probably different from the R-phase.     

Quasi-wave shape of an interface upon explosion welding (copper–tantalum,copper–titanium)

The interface relief of copper–titanium compounds and its evolution upon the intensification of explosion welding are investigated. Typical structures are revealed: splashes, waves consisting of ledges, interrupted waves, and transient states generated by interrupted waves combined into bands. The effect the mutual solubility of the initial elements has on the formation of a welded joint’s structure is determined. Intermetallic compounds are detected both on ledge surfaces and inside areas of local melting.     

Diffusion en surface de l'argent sur les plans (001), (111), (110) et des surfaces vicinales du cuivre

The diffusion coefficients of silver on singular and vicinal surfaces of copper have been measured using an oxidation method to determine the concentration profiles. The diffusion was carried out under ultra high vacuum at low temperature (250 to 500 °C) with a low concentration of the diffusing species (less than 0.5 monolayers). Comparison of the results with the terrace-ledge-kink model shows that the diffusion occurs along the ledges and also explains the diffusion on the vicinal surfaces. The differences between this model and the diffusion observed on the singular surfaces are explained by the presence of separated ledge loops on these surfaces.     

Molecular Dynamics Simulation of Icosahedral Transformations in Solid Cu-Co Clusters

We study the icosahedral transformations of solid Cu Co clusters with different initial configurations by using molecular dynamics with the embedded atom method. It is found that the formation of symmetric icosahedral cluster is strongly related to the atomic number and initial configuration. The transformation originates from the surface into the interior of the cluster and is a structural change which is rapid and diffusionless. The icosahedral clusters with any composition and configuration, such as core-shell or three-shell duster, can be prepared by the means of solid-solid phase transition in bimetallic clusters.     

Monte Carlo simulation of ion beam-assisted solid phase epitaxy

Abstract

We have developed a computer simulation of ion beam-induced epitaxial crystallisation (IBIEC) based on the familiar kink-and-ledge model which describes thermally activated solid phase epitaxy. In the latter, kinks are assumed to form (via thermal activation) on ledges. Here, we assume that the deposited ion energy induces kink formation on the terraces as well as on the ledges. We then derive, with minimal restrictions on the growth laws, the interface morphology evolution. The latter is correlated to the growth speed, and an interpretation of the IBIEC temperature dependence is proposed. The power laws which characterise the simulated interface evolution are those of the Kardar-Parisi-Zhang universality class.     

High-temperature morphology of stepped gold surfaces

Molecular dynamics simulations with a classical many-body potential are used to study the high-temperature stability of stepped non-melting metal surfaces. We have studied in particular the Au(111) vicinal surfaces in the (M + 1,M− 1,M) family and the Au(100) vicinals in the (M,1,1) family. Some vicinal orientations close to the non-melting Au(111) surface become unstable close to the bulk melting temperature and facet into a mixture of crystalline (111) regions and localized surface-melted regions. On the contrary, we do not find high-temperature faceting for vicinals close to Au(100), also a non-melting surface. These (100) vicinal surfaces gradually disorder with disappearance of individual steps well below the bulk melting temperature. We have also studied the high-temperature stability of ledges formed by pairs of monatomic steps of opposite sign on the Au(111) surface. It is found that these ledges attract each other, so that several of them merge into one larger ledge, whose edge steps then act as a nucleation site for surface melting.     

A theory of fusion of molecular crystals—II Phase diagrams and relations with solid state transitions

The theory of melting of molecular crystals with orientational degrees of freedom developed in a previous paper⁽¹⁾ is extended to cover disordering solid-solid transitions and complete phase diagrams for transitions under pressure. It is predicted that solid-solid and melting transitions should become second-order under certain conditions and also that a solid transition may appear at high pressures in some materials which show only a melting transition at zero pressure. The theory is compared with available experimental data.     

Infrared reflectivity spectrum of KTiOPO4 single crystal

The kinetic theory of overlapping phase transformations is a valuable tool in the characterization of diffusion-controlled precipitation reactions. Using this approach, one can obtain the kinetic parameters (n,?k) for simultaneous diffusion-controlled precipitation reactions, based on the knowledge of a specific macroscopic parameter, P(t), chosen to study the ongoing reaction. Both large and small process overlaps are considered. This methodology calculates the kinetic parameters for defined intervals of the fraction-transformed curve rather than for the entire curve. The approach used in this study permits an evaluation of which are the best protocols for the precipitate nucleation and growth rates during tempering reactions in low-alloy steel, whereas an analysis of the total combined transformations data set could lead to erroneous conclusions.     

Synthesis of silica supported titania nanocomposite in controllable phase content and morphology

The silica supported titania nanocomposite thin films with controllable particle size and phase content were successfully prepared by a convenient post annealing approach involving in solid-solid interfacial reaction. The effects of growth conditions, such as the annealing temperature and silicon concentration on the particle size and phase content, were systematically studied by using Atomic force microscopy (AFM), Raman spectroscopy, X-ray diffraction (XRD), and X-ray spectroscopy (XPS). The results indicate that the silicon concentration is a dominant factor in the morphology, crystallization and phase transformation of these nanocomposites. A mechanism for the high temperature phase transformation is also proposed based on the migration of the oxygen vacancies.