The solidification path of highly undercooled refractory metals and their alloys solidified in a ultra high vacuum drop-tube is shown to involve transitory metastable phases. First-principles calculations of structural stability are used to determine the possibility of obtaining metastable phases in these systems, with a view to relate this metastability with the site occupation in the σ phase in the case of the ReTa system.

We have studied the effect of phosphorus content on the formation and properties of bulk Pd-Cu-P and Pd-Ni-P metallic glasses. It was found that the molar volumes of the Pd-Ni-P bulk glasses plotted as a function of P content follow a straight line whereas those for the Pd-Cu-P glasses follow two straight lines of different slopes, intersecting at a critical P concentration. The transition in molar volume behavior is attributed to a change in the alloy's short-range order with increasing P content. The origin of this structural change is discussed based on x-ray photoelectron spectroscopy (XPS) measurements.

Phase transitions are often accompanied by pre-transition effects. These effects have been studied on an atomic scale by high resolution electron microscopy. For the diffusion controled phase transitions we discuss short range ordering effects in Ni-Mo and Cu-Pd. For the displacive transitions we discuss the martensitic nucleation in Ni-Al and the nanoscale domain formation in the Ni-Ti R phase.

Both x-ray diffuse reflectivity and laser light scattering have been used to investigate the temperature-dependent surface behavior of a Ni63Al37 single crystal on different length scales. In-situ experiments were performed above the conventional martensitic start temperature Ms to search for premartensitic phenomena. X-ray experiments showed the presence of a surface precursor with second-order (continuous) character several 10 K above Ms. This premonitory effect corresponds to a height-height-correlation function which changes on the nanometer scale as the martensitic transformation is approached. At the martensitic transformation, the surface morphology changed from nanoscopic roughness to macroscopic relief within a temperature interval of less than 1 K via intermediate stages. Laser light scattering was employed to study time-dependent aspects of the athermal martensitic transformation above Ms. The occurrence of a martensitic transformation on isothermal holding after a certain incubation period was observed in Ni-Al for the first time. The measured incubation times increased by four orders of magnitude within a temperature interval of 0.5 K.

A mechanistic model of the nucleation and growth of martensitic products that is consistent with the phenomenological theory of martensite crystallography is described, and applied to the case of NiTi. In addition to the lattice invariant deformation specified in the phenomenological theory, an array of interfacial transformation dislocations, or disconnections, is present. The motion of these disconnections across the interface between the nucleus and matrix cause the growth of the precipitate, and the two defect arrays in combination relieve the strain in the nucleus. For such processes to be diffusionless, special crystallographic circumstances must arise, and these are explored in the paper.

The diffusion controlled growth of a compound phase AnB between two thin films of material A and B is studied with the nonlinear Kirkendall effect included. Previous models of the growth rate do not solve the diffusion equation, and thus do not fully utilize the predictive capability. This paper describes a self-similar transformation that reduces the nonlinear, time-dependent diffusion equation with two free boundaries into a nonlinear ordinary differential equation, which is solved numerically by a shooting method. It is found that the intrinsic diffusion coefficients of A and B in AnB can be determined from the positions of the interfaces without using the concentration profile. This provides a simpler method for measuring intrinsic diffusion coefficients.

Particle Coarsening in ternary, multiphase -multicomponent, systems is quite different than its binary counterpart in that the presence of one phase can affect the coarsening behavior of another. We have shown by analytic analysis and numerical simulation that the interaction between two coexisting phases depends on: solution thermodynamics, diffusivities, and equilibrium concentration differences between matrix and precipitate phases. Therefore, different ternary systems create different cross phase interactions which affect coarsening behavior. For example, spatial correlations between the two coarsening phases are a strong function of this interaction and can lead to spatial ordering between the two phases. Also, specific ternary systems can produce kinetic instabilities and pattern formation.

The microstructure developed in a first order phase transformation is obtained using a populational extension of the Kolmogorov, Johnson-Mehl and Avrami (KJMA) model, PKJMA. PKJMA allows one to determine the grain size distribution resulting from nucleation and growth kinetics. PKJMA is grounded on the mean field hypothesis that the free space around the growing grains is randomly distributed, independent of the grain radius. Although this approach is perfectly valid for the case of constant nucleation, a detailed analysis of the model shows that this hypothesis does not hold in the case of time dependent nucleation protocols or pre-existing nuclei. In this work, the PKJMA model has been improved by estimating the average free surface around the grains as a function of their radius and the time elapsed since nucleation. The resulting model gives quantitative determination of the microstructure developed under a variety of nucleation and growth processes: pre-existing nuclei, constant nucleation, and the combination of both mechanisms, constant and radius dependent growth rates. Comparison with Monte Carlo simulations, showing a quantitative agreement will be presented.

Precipitation of γ′ particles from a disordered γ matrix in a Ni-Al alloy is studied. A continuum phase-field model is employed in three-dimension(3D). The focus is on the evolution of γ′ particle morphology and development of spatial correlations during coherent nucleation, growth and coarsening. The effect of lattice mismatch on the rate of coarsening is investigated. The results are compared with those obtained from two-dimension(2D) simulations.

On the basis of the model of atomic cluster jump theory, a microdynamical model of crystal growth rate was developed under an electric field. Theoretical analysis indicates that the crystal growth rate is dependent on not only the nature of the metal, the undercooling and so on but also the applied electric fields. Moreover, a novel model, which shows the effect of an applied electric field on the solute distribution, the constitutional undercooling and the temperature distribution at solid/liquid interface during unidirectional solidification, has been established theoretically. Analytical results show that the electrotransport of solute strongly influences the solute distribution and then changes the constitutional undercooling and the temperature distribution. But all changes are connected with the electric current direction of the applied electric field (positive or negative).

The growth stress of metal films was measured continuously both during as well as after their deposition under UHV-conditions with a cantilever beam technique. The metal films were deposited onto 10 rim thick alumina substrate films prepared by reactive evaporation of Al in an oxygen atmosphere. The substrate temperature for the metal deposition was varied from -20°C to 500°C.

The growth stress of both titanium and aluminum films deposited at room temperature and above is characteristic of island growth and the formation of a polycrystalline film. The film stress built up in these films decreases with increasing substrate temperature. Below RT the growth stress of titanium films indicates a transition from island growth to layer by layer growth due to a reduced adatom mobility. The temperature range in which this transition in the growth mode occurs is strongly affected by impurities in the Ti-evaporation source material and gas ambient.

In the last part of this paper we present results of experiments in which the above metals were evaporated simultaneously from separate sources to form alloy films with TiAl3-stoichiometry. Sudden changes in the incremental film stress are tentatively attributed to segregation and phase formation phenomena.

A systematic study is undertaken on the effect of growth conditions, on the microstructure and other properties of radio frequency (RF) magnetron sputter deposited SrBi2Ta2O9 (SBT) and SrBi2TaNbO9 (SBTN) films. Polycrystalline SBT and SBTN films were deposited on Si (100), platinized Si, and MgO substrates using stoichiometric targets. Cross sectional scanning electron microscopy reveals the tendency for columnar growth in the films deposited at high substrate temperatures and at high partial pressures of sputtering gas mixtures. AFM studies show that the grain size and the homogeneous surface morphology depends on the post annealing time. XPS studies reveal that oxygen vacancies lie in the Bi2O2 planes. The compositional dependence of the rigid-sublattice mode and the symmetric stretching mode of oxygen octahedrons in the Raman spectra of these materials have been discussed.

We employed in situ ellipsometry to monitor and study the nucleation and growth processes of diamond thin films fabricated by chemical vapor deposition. The films were grown on Si substrates in a hot filament chemical vapor deposition (HFCVD) system. We monitored the effective extinction coefficient (k) at 1.96 eV of the diamond films during growth through ellipsometry. The behavior of this parameter was found to be reproducible, making it suitable as a basis for dividing the deposition process into intervals. The film growth was aborted at various k values yielding diamond film samples that represent snapshots of the growth process at different stages. These films were removed for ex situ characterization using Raman spectroscopy and scanning electron microscopy (SEM). These characterizations were used to correlate the ellipsometric data with film microstructure, enabling us from now on to monitor the diamond film growth in real time and to design experiments targeted at modifying the film microstructure by changing growth parameters in the middle of film fabrication.

The repeatable growth of high quality thick layers of GaN by hydride vapor phase epitaxy generally requires buffer layers or other surface pretreatments to produce morphologically smooth films suitable for further device growth. Of the conditions for high quality GaN growth, proper film nucleation invariably contributes most to the overall growth of high quality films and the exclusion of impurities. We have studied a two step growth process which incorporates a low temperature HVPE grown nucleation layer and have compared it to films grown without buffer layers to determine how the impurity content and the dislocation networks change the overall film quality. Additionally we observe the formation of macroscopic steps during growth with step heights of ∼0.75 μm and terrace widths of ∼ 7 μm, which we attribute to the initial conditions of the low temperature nucleation layer prior to growth and to kinetically limited growth.

Results are presented for the coarse grained distribution function and Ginzburg-Landau free energy function for coexistence of liquid and gas phases. These distribution functions were obtained by two different methods: 1) the compilation of particle density information from different coarse-grained cells using the canonical ensemble, and 2) the compilation of energy and density information from a single simulation cell by tuning the chemical potential using the grand-canonical ensemble. Both methods permit the calculation of a coarse-grained free energy functional which links the atomic and mesoscopic length scales.

The evolution of sulfide morphology in the Fe- (0.3 to 18) mass% Cr -0.3 mass% S alloys during solidification and its modification by additions of elements such as Mn, Ti, Zr and C have been investigated by means of optical microscopy, scanning electron microscopy and X-ray diffraction. The sulfide morphologies in the Fe-Cr-S ternary alloy are classified under three types: (i) cellwall (ii) fine particle and (iii) globular sulfides. The iron-rich cellwall sulfide is formed in the Fe-(0.3-1) %Cr-0.3 %S alloys. This sulfide morphology changes to the globular type with increasing Cr content and the size of that is more than 51gm. Fine sulfide particles of size smaller than 2 gtm also coexist in the Fe-(0.3-5) %Cr-0.3 %S alloys. Zr or Ti additions change this morphology from the globular type to the rod-like type in the Fe-18 %Cr-0.3 %S alloy, but Mn addition does not have the same effect. The mechanism of the formation of different sulfide morphologies is discussed on the basis of phase diagram information.

Using atom probe field ion microscopy (APFIM) we examined the local chemical compositions of Fe-7Zr-5B-lAu and Fe-14B amorphous alloys in the course of primary crystallization. Au rich clusters are formed during primary crystallization of Fe-7Zr-5B-lAu alloy. However, these clusters do not act as nucleation sites for α-Fe particles. In the early stage of primary crystallization, heterogeneities of Fe and B evolve in the amorphous phase. Heterogeneous distribution of B was found in the as-melt-spun Fe-14B amorphous alloy. During primary crystallization, B is highly supersaturated not only in the nanometer sized α-Fe particles in Fe-7Zr-5B-lAu alloy, but also in those of large diameters in Fe-14B alloy. From the results it is concluded that presence of B lowers the nucleation barrier for primary crystallization.

We have examined the effect of Ti content on the crystallization kinetics of Zr-based amorphous alloys. In Zr59Ti3Cu20Ni8Al10, annealing above the glass transition temperature produces 50-100 nm crystalline precipitates, as seen in transmission electron microscope images. In contrast, TEM images and diffraction patterns from annealed Zr54.5Ti7.5Cu20Ni8Al10 show no evidence of crystalline phase formation. Structural changes upon annealing do occur in this alloy, however, as revealed by field ion microscopy. The effect of Ti is to favor the formation of clusters of short-range order; this tendency for clustering is apparently the cause of difference in crystallization behavior between the two alloys.

A metastable crystalline state was observed in amorphous SiGe/Si layers under illumination with a cw laser beam (514.5 nm) with power density below the crystallization threshold. The observation of this state was temperature dependent and it was reversible. The structural properties of this metastable crystallization were studied by Raman spectroscopy.

The statistics of nucleation are studied for different Co-Pd alloys by electromagnetic levitation. Each alloy was undercooled approximately 100 times under the same experimental conditions. A model developed by Skripov is employed to statistically analyze the distribution of maximum undercoolings in order to obtain information on the prevailing nucleation mechanism. The results of these investigations indicate that nucleation is influenced by magnetism when the nucleation temperature is approaching the Curie temperature.