We investigate the phase transformation in non-stoichiometric GaAs films, which are doped with group V isovalent impurities, namely Sb and P. In contrast to Al and In atoms of group III, group V atoms may not only form substitutional alloys with the GaAs matrix, but also can be dissolved in As precipitates. Our experimental study based on transmission electron microscopy, x-ray diffraction and optical characterizations revealed an opposite impact of the P and Sb alloying on the nucleation, growth and coarsening of the second phase. While Sb enhances the precipitation rate, P retards it. Delta-doping with Sb causes two-dimensional precipitation, whereas such doping with P does not result in heterogeneous nucleation. The microstructure and strains have been found to be different around nanoinclusions in the Sb-doped and P-doped materials, indicating different thermodynamics and kinetics of segregation of the two isovalent impurities in As nanoclusters. We analyze the observed phenomena in terms of thermodynamic and kinetic models taking into account the underlying phase equilibriums and diffusion mechanisms.

The goal of the present study was to gain insight into the mechanisms and behavior of solute driven melting processes. Solute-driven melting refers to melting whose driving force originates from a change in composition, i.e., solute content, rather than a change in temperature. This kind of melting has been known to cause casting defects in superalloys and other metals.

An experimental apparatus was designed and a series of experiments were carried out on Sn-Bi alloys. The apparatus involved diffusing Bi into solid Sn cast inside a glass capillary to cause it to melt. The Bi source was an enriched liquid Sn-Bi alloy contained in a reservoir. The Sn in the capillary was kept at a constant temperature below its melting point so that the melting was caused by the increasing Bi composition. The progression of the interface was monitored by quenching the process at various times for the same conditions.

The apparatus was successful in delivering data for the displacement of the interface against time. It was found that interface position was approximately proportional to the square root of time, and so the process may be diffusion controlled as conjectured in previous literature. Rough calculations are made relating the temperature and supersaturation to the displacement coefficient A in the equation z = At½ where z is displacement and t is time. By comparing data at different temperatures with the same supersaturation, an activation energy Q of ~60,000J/mol is calculated. This value is between the activations energies for diffusion of Bi through the liquid and the solid. Suggestions for modification of the apparatus to include in-situ interface monitoring are made.

The crystallization process of Zr70Cu27.5Rh2.5 metallic glass was studied with Transmission Electron Microscopy (TEM). In contrast to previous studies where the precipitation of metastable icosahedral quasicrystalline (IQC) particles is of the interest, we designed the present work to focus on the nucleation process of the stable Zr2Cu crystalline phase. It has been found that the alloy consists of IQC particles distributed in amorphous matrix prior to the precipitation of the Zr2Cu stable crystalline phase and Zr2Cu nucleates from the amorphous matrix. The encounter of the IQC phase with Zr2Cu transforms the former into the latter so quickly that no interface between them was found in the present experiment. These insights provide the basis for a discussion of the stability of metallic glasses and the IQC particles.

Equilibrium-thickness, intergranular films (IGFs) have been observed in various ceramic materials. Recently, surficial amorphous films (SAFs) of similar character have also been found. Furthermore, a series of studies revealed the stabilization of disordered (quasi-liquid) IGFs and SAFs well below the bulk solidus or eutectic temperatures, wherein analogies to the phenomena of premelting and prewetting can be made. Accordingly, combined interfacial premelting and prewetting models have been developed using a diffuse-interface theory. This paper outlines the key results of two model experiments in support of the above theory: namely observation of quasi-liquid grain boundary films (metallic IGFs) in W-Ni and searching of a complete wetting transition for Bi2O3 on ZnO where SAFs become macroscopically thick. We propose that simple combined interfacial premelting and prewetting models apply to metallic IGFs, but only serve as a basis to understand IGFs and SAFs in ceramics where additional interactions, e.g. dispersion forces and space-charges, should be added separately and may result in more complex behaviors.

Carbides, nitrides, and carbonitrides have great influence on the properties of steel but are relatively little studied at a fundamental level: experimentally because of small dimensions of the precipitates and other difficulties, theoretically because of complex structures and poorly defined compositions. Modern density functional calculations reveal significant trends that can be partially summarized in terms of atomic size, carbon affinity, and hybridization de-magnetization concepts. Phase stability and order-disorder phenomena in widely occurring Cr-based M23C6 carbides and Mo-based M6C carbides are used to illustrate predictions relevant to low alloyed steel.

The synthesis of metal nanoclusters in a polymeric environment has been shown to yield nearly monodisperse particles, whose size is controlled by the strength of the polymer/metal interactions. Although the phenomenon is quite general, little is known regarding the mechanism by which the polymer controls nanocluster size. Previous studies of the kinetics of nanocluster growth in polymeric melts above the glass transition temperature (Tg) showed that the nanoparticle size is set by the critical cluster size rather than the rate of metal precursor transport. In this paper we examined the kinetics of iron-oxide (Fe2O3) nanocluster formation in two polymer melts below their Tg: polystyrene (PS) and poly(methyl methacrylate) (PMMA). We found that the rate of particle formation is nucleation-controlled for both PS and PMMA, similarly to the same process above Tg, while the growth stage is diffusion limited. Transmission electron microscopy (TEM) revealed needlelike particles morphology with larger average dimensions as the temperature is elevated towards polymer matrix Tg. The stabilizing effect of the polymer chains on the growth of the nanoparticles was dominant when the synthesis was performed in solution or in the melt, allowing the polymer to adsorb on the cluster. However, below its Tg the chains were not free to move and conform to the nanocluster surface, and hence, capping occurred when the cluster growth limitation was imposed by the geometric confines set by the voids in the polymer matrix.

All stages of phase transformations in materials, nucleation, growth, and coarsening, are subjected to thermal effects that stem from the redistribution of energy in the system like, release of latent heat and heat conduction. The thermal effects change the rate and outcome of the transformation and may result in appearance of unusual states or phases, in particular in nanosystems. This paper is not a complete account of the theory of thermal effects; it is rather a guide through many seemingly unrelated effects in different phase transformations, which in fact have unified origin. Although the dynamical Ginzburg-Landau approach will be used for the analysis of the effects, they are robust and independent of the employed theoretical methods. Another purpose of this review is to bring these effects to the attention of experimenters and motivate them on conducting new experiments in the area of phase transitions.

We have used a transmission electron microscope (TEM)-based method to extract grain size information for 〈111〉 surface normal grains in lithium fluoride (LiF) thin films, and applied this to analyze textures as a function of substrate temperature and annealing time. The size distributions of grains diffracting into the (111)+(200) and (220) rings were measured separately using dark field (DF) TEM images. From these data, we deduce the distribution of 〈111〉 surface normal grain sizes based on the assumption that only 3 principal textures (100), (110) and (111) exist in films. The (111) texture formation was also observed by x-ray diffraction (XRD). For all deposition and annealing conditions, the grain size data can be matched to lognormal distributions within an acceptable error, but at longer annealing times the distribution becomes bimodal. A novel feature of the LiF films is that the (111) texture component strengthens with annealing and substrate temperature, through the nucleation of new grains rather than the growth of existing ones.

The purpose of this paper is to show some examples of phase transformations induced by grinding molecular materials. These materials are considered because they are extremely sensitive to external disturbances and are generally very good glass formers. This allows investigating more easily a broad range of the parameters which are open to influence the nature of the end product namely the temperature and intensity of grinding. Such an investigation has also a practical interest in pharmaceutical science. It is shown that the position of the grinding temperature with regard to the glass transition temperature of the compound is a key parameter. Comparison of the effects of temperature and intensity of grinding demonstrates that the driven material concept offers a framework to rationalize all the observed transformations.

We seek to explain 1) the effectiveness of substituting Sc with Dy, Er, Y and 2) the loss of properties when Sc is replaced with Yb, Gd and Sm [1]. For a preliminary insight into the stability of structures we utilize the concept of Atomic Environment Type (AET) as pertaining to trialuminides. Electronic density functional total energy calculations at zero temperature are performed to obtain the enthalpy of mixing of quasi-binary Al-Sc-X structures. Estimates of the entropy are used to compute the stability regions of Al3Sc1-αXα L12 phase. We show that Sc is completely miscible with X=Dy, Er, Y and that there is a miscibility gap for X=Yb, Gd and Sm at temperatures near the aging temperature of Al-Sc alloys.

Our current understanding of martensitic transformations has been based on the Phenomenological Theory of Martensite Crystallography developed in the 1950s. Recently, a Topological Model of martensitic transformations has been presented wherein the habit plane is a semi-coherent structure, and the transformation mechanism is shown explicitly to be diffusionless. This approach is used here to model phase transformation crystallography of lath martensite in ferrous alloys. A range of network geometries is predicted corresponding to orientation relationships varying from Nishiyama-Wasserman to Kurdjumov-Sachs. Experimental observations from the literature of the dislocation and disconnection arrays, habit plane and orientation relationship are in good agreement with the model.

Tantalum thin films were deposited by RF (13.56 MHz) magnetron sputtering on silicon and aluminum substrates with ion irradiation (~ 0.3 mA per sqaure cm) controlled by applying different DC bias voltages (0-300 V) to the substrate. The presence of two main crystallographic phases of Ta in deposited films was investigated. Results showed that only the tetragonal â-phase formed in thin Ta films on both Si <100> and Al substrates at zero bias voltage (ion energy ~10 eV), and only bcc α-phase of Ta formed at the ion irradiation energy to 150 eV. Both phases were present at the ion energy of 100 eV. With increase of the ion energy to 250 eV, or higher, â-phase Ta dominated the film structure on Si <100> but not on Al substrate. Ta films were deposited in compressive stress with the lowest stress measured for α-phase Ta films deposited with ion energy of 150 eV. A new set of deposition parameters, significantly different than those previously reported, for low temperature growth Ta films with bcc crystallographic structure, desired for most applications, was found. An advantage of the new deposition parameters is relatively simplicity of the required equipment, which can be easily scaled up for an industrial process.

Recently, we discovered that a two-way shape memory effect can be realized on NiTi shape memory alloy surfaces by spherical indentation. Either thermomechanical cycling or slip-plastic deformation under contact loading conditions is effective in inducing reversible indent depth changes: deeper indents in the low temperature martensitic phase and shallower indents in the high temperature austenitic phase [1, 2]. By planarization of these spherical indents to restore a flat surface, circular reversible surface protrusions appear upon heating and disappear upon cooling. Nano-protrusions are also realized in sputter deposited NiTi thin films. Furthermore, two-way reversible line protrusions are made by planarizing scratch tracks on a NiTi shape memory alloy. Shape memory surfaces can be exploited for a wide range of potential applications, including information storage, optical devices, and smart tribological surfaces.

Extensive experimental research work has been carried out in order to develop an Mg-Zn-Sn based creep resistant alloy. It was found that the addition of Y improves the structural stability of this alloy. The main goal of this work was to investigate precipitation sequence in Mg-Zn-Sn-Y alloys during aging at different temperatures. In order to elucidate the sequence of phase formation, XRD, TEM, SEM and EDS analyses were applied. It was found that, firstly, an hcp-MgZn2 phase nucleates homogeneously, and grows in the form of needles having coherent interphase boundaries with Mg-matrix. High-resolution TEM images in combination with Fourier transformed images reveal a topotaxial orientation relationship which is given by {110}MgZn2 <100>MgZn2∥(002)α-Mg<110>α-Mg. The cubic fcc Mg2Sn-phase nucleates heterogeneously, mainly at the tips of MgZn2-needles when the needle's width reaches a certain critical value. The orientation of Mg2Sn-precipitates is usually perpendicular to the one of MgZn2-needles. Two-phase T-like particles are very typical of alloys aged for 1-16 days at 175-225C. The width/length ratio of MgZn2-needles inside T-like particles differs substantially from the one found in single needles. The elastic/surface energy balance of needles and its influence on the morphology and coarsening behaviour has been analyzed.

Time-dependent Ginzburg-Landau (TDGL) formulation on a continuous medium has been developed for ordering processes of B2 and D03 types in binary alloy systems. The results of 2-dimensional simulations reproduce well the formation of B2 and D03 ordered domains in the processes in an Fe3Al alloy.

It has long been recognized that thin film polycrystalline microstructures are determined by the thermodynamics and kinetics associated with physical vapor deposition, but it is quite process dependent and not easily quantified. We have examined the microstructure in polycrystalline Au films obtained by pulse laser deposition (PLD) under various conditions and interpret the results in terms of three fundamental parameters common to all physical vapor deposition: Flux kinetic energy, substrate temperature, and deposition rate. With this model, it is predicted that nanocrystalline films are formed in the limits of low temperature, flux, and high deposition rate. The deposited films are analyzed with X-ray diffraction and SEM to determine texture and grain morphology, which are found to fit well within the process maps.

Thin Si films on SiO2 that are completely melted by pulsed laser irradiation cool rapidly and eventually solidify via nucleation and growth of solids. It has been observed that a variety of solidified microstructures can be obtained, depending primarily (but not exclusively) on the degree of supercooling achieved prior to the onset of nucleation. This paper focuses on investigating one particular and unusual polycrystalline microstructure that consists of “flower-like” grains, the interiors of which can be described as being made up of two distinct regions: (1) an extremely defective core region consisting of fine-grained material, and (2) an outer region consisting of relatively defect-free crystal “petals” that radiate outwards. After considering the microstructural details and experimental behavior of the microstructure, we have formulated a growth-based physical model to account for the formation of the microstructure. The model is found to be also capable of accounting for the other complex and unusual microstructures obtained via nucleation and growth in the complete melting regime.

Misfit strain connected with the formation of large internal stresses may drastically influence the precipitation kinetics. A new model for the simultaneous precipitate growth and misfit stress relaxation in binary systems has been developed based on the idea that the internal stresses are relaxed by generation or annihilation of vacancies at the matrix/precipitate interface. The model is applied to the Fe-C system by considering the growth of the cementite precipitate in the ferritic matrix. The influence of the stress relaxation on the precipitate growth kinetics is demonstrated.

Pairs of ?' particles in different Ni base alloys are investigated in order to determine the mechanism responsible for the formation of several particle arrays. Special attention is given to symmetrically arranged particles or the split patterns i.e., they have been believed to result from the splitting of large particles. High resolution electron microscopy is used to determine the translation domains of coherent ?' particles. The [001] zone axis allows determination of all possible translations due to differently nucleated domains. About 72 % of particle pairs forming splitting configurations are in the out-of-phase relationship (different translation order domains), indicating that most adjacent pairs are not formed by the splitting of a large particle. On the other hand, particles in symmetrical array (quartets) always have at least one particle with a different translation order domain. Additionally, the frequency of out-of-phase pairs is 78 % in quartet configurations. Calculation of probabilities shows that the quartet split patterns are not formed by splitting but most likely by means of migration due to diffusion.

We have investigated excimer laser irradiation of 2000-Å-thin as-deposited Al films on SiO2. Microstructural analysis of the irradiated films conducted with AFM and EBSD techniques reveals that there exists a wide energy density interval over which large equaxed grains with a strong (111) texture are obtained. Based on thermal, transformational, and microstructural considerations, we propose a heterogeneous nucleation model to account for the observed behaviors, and discuss the implication of the model on the phenomenon of heterogeneous nucleation of crystalline solids in condensed systems as regards the thermodynamic role played by the orientation of subcritical clusters.