Growth by Ledges

The prediction and observation of growth ledges at various moving interphase boundaries in solid-solid phase transformations has become increasingly widespread since first proposed in the early 1960's by Aaronson. The role of steps at the growth interfaces in several phase transformations is reviewed. For precipitate plates emphasis is placed on rationalizing experimental growth kinetics with mathematical models developed for the migration rate of a ledge. More complex transformations are employed to examine the possible role of growth ledges to account for microstructure development. In all cases, the growth ledge is shown to be necessary to fully understand solid-solid phase transformations.     

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.     

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.     

Dislocation transmission across the Cu/Ni interface: a hybrid atomistic–continuum study

The strengthening mechanisms in bimetallic Cu/Ni thin layers are investigated using a hybrid approach that links the parametric dislocation dynamics method with ab initio calculations. The hybrid approach is an extension of the Peierls–Nabarro (PN) model to bimaterials, where the dislocation spreading over the interface is explicitly accounted for. The model takes into account all three components of atomic displacements of the dislocation and utilizes the entire generalized stacking fault energy surface (GSFS) to capture the essential features of dislocation core structure. Both coherent and incoherent interfaces are considered and the lattice resistance of dislocation motion is estimated through the ab initio-determined GSFS. The effects of the mismatch in the elastic properties, GSFS and lattice parameters on the spreading of the dislocation onto the interface and the transmission across the interface are studied in detail. The hybrid model shows that the dislocation dissociates into partials in both Cu and Ni, and the dislocation core is squeezed near the interface facilitating the spreading process, and leaving an interfacial ledge. The competition of dislocation spreading and transmission depends on the characteristics of the GSFS of the interface. The strength of the bimaterial can be greatly enhanced by the spreading of the glide dislocation, and also increased by the pre-existence of misfit dislocations. In contrast to other available PN models, dislocation core spreading in the two dissimilar materials and on their common interface must be simultaneously considered because of the significant effects on the transmission stress.     

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.     

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.     

First-principles study of interface relaxation effect on interface and electronic structures of InAs/GaSb superlattices with different interface types

We have implemented first-principles relativistic pseudopotential calculations within general gradient approximation to investigate the structural and electronic properties of quaternary InAs/GaSb superlattices with an InSb or GaAs type of interface. Because of the complexity and low symmetry of the quaternary interfaces, the interface energy and strain in the InAs/GaSb superlattice system have been calculated to determine the equilibrium interface structural parameters. The band structures of InAs/GaSb superlattices with InSb and GaAs interfaces have been calculated with respect to the lattice constant and atomic position relaxations of the superlattice interfaces. The calculation of the relativistic Hartree–Fock pseudopotential in local density approximation has also been performed to verify the calculated band structure results that have been predicted in other empirical theories. The calculated band structures of InAs/GaSb superlattices with different types of interface (InSb or GaAs) have been systematically compared. We find that the virtual–crystal approximation fails to properly describe the quaternary InAs/GaSb superlattice system, and the chemical bonding and ionicity of anion atoms are essential in determining the interface and electronic structures of InAs/GaSb superlattice system.     

TEM and HRTEM study of influence of thermal cycles with stress on dynamic recrystallization in Ti46Al8Nb1B during creep

Short-term tension creep and thermal cycles under compressive stress were performed on Ti46Al8Nb1B in order to explore the dynamic recrystallization (DRX) grains formed during the creep and the impact of thermal cycles under stress to the DRX. After 1600 times' thermal cycles from 300 degrees C to 800 degrees C under 300 MPa compressive stress, high density of ledges and thick ledges are found in the interfaces. Two kinds of moiré fringes, instead of 9R structure, can be found in the thick ledges. Ti46Al8Nb1B sample and another sample which was treated by thermal cycles with stress were crept under 300 MPa compressive stress at 800 degrees C. DRX grains are found in the interfaces in those samples. Those grains, formed at the ledges, have an orientation relationship of [101](gamma)//[011](gammaR), (1 1 1)(gamma)//(1 11 )(gammaR) with the matrix of gamma phases. Thermal cycles with stress could lead to more DRX grains during creep.     

Interfaces between Al2O3 and beta-Ni(1-x)Al(x) alloys: complex structures and Ab initio thermodynamics

A methodology to study the structural stability of binary alloy/Al2O3 interfaces is developed by expanding the conventional ab initio thermodynamics to include the dependence on alloy composition. Results on beta-Ni(1-x)Al(x)/Al2O3 interfaces predict the existence of two types of stable interfaces. The stable interface at equilibrium with Al-rich or strictly 1:1 alloy contains an Al2-terminated Al2O3 surface and continues with NiAl layers, and the interface at equilibrium with Ni-rich alloy has Al accumulation but continues with a Ni-rich and then NiAl layers. Works of separation for the two interfaces are close to each other.     

Rotation of Mo-Fibers in Directionally Solidified Ni-Al-Mo Eutectic Alloys

The interface structure of -Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)//(1 0 0) relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moiré and O-lattice models.     

Ultrafast strain engineering in complex oxide heterostructures

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.     

Molecular dynamics simulations of interfaces between NiO and cubic ZrO2

The structures of eight heterophase interfaces between cubic ZrO₂ and NiO simulated by molecular dynamics (MD) are compared with those of the corresponding isolated surfaces to better understand the factors influencing interface formation and coherency in ceramic oxides. The near coincident site lattice (NCSL) theory used to construct initial configurations with small lattice mismatch between low index crystal planes is extended to describe interface planes with rectangular symmetry. Interface energies, works of adhesion, expansion volumes, lattice strains and planar structure factors are reported for several bicrystals. Most interfaces are found to consist of disordered, open structures with correspondingly high energies as a consequence of the strong repulsive forces between like-charged ions brought into close proximity. The exception is the (111) NiO//(100) c-ZrO₂ interface, which exhibits good coherency across the interphase boundary and consists of a shared oxygen plane. The relatively high crystallinity of the interface explains its low energies and small excess volume. The results are discussed in the context of developing a simulation-based method for identifying interface configurations with high lattice coherency and strong cohesiveness for optimizing the properties of composites of technologically important materials.     

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.     

Lattice structures and electronic properties of CIGS/CdS interface:First-principles calculations

Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective （2VCu＋ Incu） CulnGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of （2VCu＋ InCu） interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the （2VCu＋InCu） interface the interface states near the Fermi level in CulnGaSe2 and CdS band gap regions are mainly composed of interracial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CulnGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.     

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.     

Modelling of mass-transfer induced instabilities at liquid–liquid interfaces based on molecular simulations

Interfaces and especially mass transfer across interfaces are of great importance in many fields of chemical engineering. Interfacial convection, which is generally called the Marangoni effect, may improve mass transfer across interfaces quite drastically and has not been investigated adequately in detail. In order to investigate the influence of mass transfer on a liquid–liquid interface molecular computer simulations have been performed. Since many molecules have to be considered for a significant modelling of the interface, cubic lattice systems have been chosen for the simulation which proceeds according to the Monte-Carlo scheme. The parameters that describe the thermodynamic and transport properties resemble those of realistic standard EFCE test systems for extraction. Results of various Monte-Carlo simulations show that under certain conditions mass transfer across interfaces induces the formation of nano droplets in the close vicinity of the interface. The different combinations of the nano droplet behaviour due to attractive or repulsive long-range forces together with the characteristics of coalescence may lead to different macroscopic interfacial instabilities such as spontaneous emulsification or eruptions. Based on diffusive and thermodynamic properties of the chosen lattice system a first stability criterion which allows the prediction of the onset of nano droplet formation is developed. The theoretical results compare well with experimental observations at a single drop and in a two-phase cell where the instabilities are investigated optically via Schlieren optics.     

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.     

The 4:5 Si-to-SiC atomic lattice matching interfaces in the system of Si(111) heteroepitaxially grown on 6H-SiC(001) substrates

Corresponding lattice planes of 4:5 Si-to-SiC atomic matching structures are observed at the Si(111)/6H-SiC(001) interface. The periodical Si/SiC interface structure is further illustrated and characterized by an atomic model derived from experiment results. It is discovered that there is a minor lattice mismatch of 0.26% in the structure. Moreover, the atomic structure of the interface and its stability are energetically investigated by molecular dynamics simulations. The results demonstrate that the atomic relaxations caused by lattice mismatch are slight to the growth of a perfect crystalline Si film and the interfaces are quite stable with the formation energy of ?22.452 eV.     

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

Abstract

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.