Coupled growth during three phase equilibrium solidification is well known from directionally solidified eutectic systems, and was recently generated in monotectic systems. Several theories predict a stationary peritectic reaction and coupled growth of the properitectic and the peritectic phases therefore should be possible. In spite of these theories coupled growth has not been observed up to now. The TiAl system was selected for further investigation on this topic because of its technical relevance and the fact that it meets the condition mentioned for coupled growth.

In a Bridgman laboratory furnace, TiAl with 53.4 at.% Al was directionally solidified with solidification rates v between 0.025 mm/min and 0.1 mm/min and a temperature gradient up to 20 K/mm. The resulting microstructures, analyzed using optical and scanning microscopy with EDX and WDX, consist of two phases parallel to the growth direction. At v = 0.05 to 0.1 mm/min, the alloy solidifies as properitectical α, which subsequently eutectoidally transforms to a substructure of α2-Ti3Al and γ-TiAl, and peritectic γ. The lamellar α2/γ-substructure is oriented parallel to the growth direction.

The experimental results were compared with the existing theoretical models of a stationary peritectic reaction and the possibility of metastable eutectic growth was discussed.

Conventional and high resolution electron microscopy have been applied to characterize the microstructure of the CrAs-GaAs eutectic. The CrAs-GaAs eutectic crystals were directionally solidified by the Czochralski method in order to produce an ordered array of CrAs rods embedded in a GaAs matrix. The CrAs rods of 2-3 μm in diameter align parallel to the growth axis of the ingot. Where the GaAs matrix is found to contain structural defects, the CrAs rods are effectively defect-free. The CrAs has an orthorombic structure with the parameters a=3.5±0.1 Å, b=6.2±0.1 Å,c=5.7±0.1 Å.The c-axis is close to the direction of solidification.

Analyses of a floating zone (FZ) crystal growth system with a radio frequency (RF) induction heating are carried out. The electromagnetic and temperature fields, and surface interfaces are solved for numerically. Finite element methods are used for the calculation of the temperature fields and interfaces and the hybrid finite element and boundary element methods are used for the calculation of the electromagnetic field. The calculation domain is divided into eleven regions, each of which, except for the RF coil region, require a coordinate transformation. In the present study, the silicon FZ growth system ,where the diameter of both feed-rod and single crystal were set to be 1cm was computed. The Lorentz force is found to play an important role in determining the melt free surface shape and the molten zone length. The effect of the current density, frequency of RF coil and crystal growth rate are investigated.

We present new results on the ion-assisted amorphous to polycrystal transition in silicon. The grain growth velocity vg and the nucleation rate r exhibit quite different behaviours: i) ion irradiation enhances r by a factor which is several orders of magnitude larger than that observed for vg; ii) irradiation with lighter ions produces a decrease of vg and a strong increase of r; iii) the increase of dose rate produces a decrease of both vg and r, but is particulary severe for r. These results have been explained by assuming that ion irradiation produces three fundamental effects: i) generation of long living defects which increase the free energy of the amorphous phase to the value of the fully unrelaxed amorphous silicon, causing the decrease of the thermodynamic barrier to nucleation; ii) generation of defects promoting the transition kinetics at concentrations well above the thermal equilibrium value; iii) prompt amorphization of a small volume at the surface of each crystalline grain.

We have examined the radiation resistance of magnesio-aluminate spinel by irradiating single crystals of MgAl2O4 with 400 keV Xe++ ions at 100 K. At low irradiation doses, the material transformed into a metastable crystalline phase with half the lattice spacing of the original crystal. Electron diffraction analysis revealed that this structural change can be explained in terms of the redistribution of cations among octahedral, tetrahedral, and three-fold coordinated interstitial sites of the close-packed anion lattice. Corresponding to this transformation, the hardness and elastic modulus increased with dose to values about 10% greater than those of unirradiated spinel. We believe that the formation of this metastable phase plays an important role in determining the radiation resistance of spinel.

High-energy electrons were used to study tensile property changes in simple Fe-Cu and Fe-Cu-Mn alloys irradiated at 288°C. A comparison was made with neutron irradiation data on the same alloys. An apparent effect of alloy chemistry was observed in which the presence of Mn affected embrittlement differently for electron and neutron irradiation. Comparison of previous experimental studies with the present experimental results indicates that electrons may be more efficient than fast neutrons at producing embrittlement.

The Zr-based intermetallic compounds ZrCr2, ZrFe2 and Zr3(Fei_x,Nix) were irradiated with high energy electrons at the HVEM/Tandem facility at Argonne National Laboratory to study their amorphization behavior. The results show that although ZrCr2 and ZrFe2 have the same Laves phase C15 fee crystal structure, their critical temperatures for amorphization under electron irradiation were 180 K and 80 K, showing that the substitution of Cr for Fe in the sublattice had a marked effect on the annealing characteristics of the material. The low temperature dose to amorphization was higher in ZrFe2 than in ZrCr2 by a factor of two. The presence of a high density of stacking faults had a strong effect on amorphization in both compounds causing the critical temperature to be increased by 10–15 K. By contrast, the addition of Ni to Zr3(Fei_x,Nix) had no effect on amorphization behavior for x=0. 1 and 0. 5. These results are discussed in terms of current models of amorphization based on defect accumulation and the attainment of a critical damage level, such as given by the Lindemann criterion.

Silicon oxide films ( > 1μm ) were grown at room-temperature after low-energy copper-ion implantation of Si(100) substrates. The structural properties of the silicon oxide layer and the implanted silicon were characterized by Rutherford backscattering spectrometry and transmission-electron microscopy. During room temperature oxidation a portion of the implanted copper resided on the surface and a portion moved with the advancing Si/SiOx interface. This study revealed that the oxide growth rate was dependent on the amount of Cu present at the moving interface. The resulting oxide formed was approximately stoichiometric silicon dioxide.

Metal fluorides are excellent candidates as high resolution inorganic resists in electron beam nanolithography. Hence it is important to study their damage characteristics under the electron beam to understand their behaviour as inorganic resists. In this paper the electron beam induced damage process in amorphous FeF3 films deposited by different techniques is investigated using selected area diffraction patterns and time resolved parallel electron energy loss spectroscopy. It is found that fluorine is lost from amorphous FeF3 during irradiation and that the material crystallises mainly to FeF2 with some crystalline FeF3. Time resolved energy loss spectroscopy studies show that the coordination polyhedra in amorphous FeF3 are randomly arranged FeF6 octahedra which arrange themselves to give a long range order in the electron beam during crystallisation to FeF2 and FeF3.

We report here the results of a study conducted to examine the effect of Kr ion irradiation on bubble formation in Zr alloys. We used the HVEM/Tandem facility at Argonne National Laboratory to irradiate several Zr alloys, including Zircaloy-2 and Zircaloy-4, at temperatures from 300 to 800 C and to doses up to 2 × 1016ion.cm−2. Both in-situ irradiation of thin foils as well as irradiation of bulk samples with an ion implanter were used in this study. For the thin foil irradiations, a distribution of small bubbles in the range of 30-100 Å was found, at all temperatures with the exception of the Cr-rich Valloy where bubbles of 130 Å were found. The irradiation of bulk samples at high temperature (700–800 C) produced large faceted bubbles (up to 300 Å) after irradiation to 2 × 1016ion.cm−2. The results are examined in the context of existing models for bubble formation and growth in other metals.

Under constraints and at the nanometer scale, transitory metastable states can be generated in multicomponents materials. Examples illustrating such specific states are presented. They concern i) The crystalline nucleation in a growing undercooled liquid droplet formed from a liquid parent phase, ii) The suppression of intermetallic nucleation in solid solutions or glasses subjected to sharp concentration gradients, iii) The nanocrystalline transitory state preceding amorphisation by ball milling. In connection with this latter example, a thermodynamic model for the nanocrytal to glass transition, based on an hypothesis of a topological disorder wetting at the nanograin boundaries, is proposed. Thermodynamics - Nucleation - Phase transitions - nanocrystals - metallic glasses

The Extended X-ray Absorption Fine Structure (EXAFS) technique has been employed to monitor the formation of metastable alloys by mechanical alloying in two positive-heat-of-mixing systems, Cu-Fe and Ag-Fe. With its unique atom selectivity and local atomic environment sensitivity, EXAFS provides clear evidence of atomic-level alloying forming bcc (Fe80Cu20) and fee (Fe50Cu50) solid solutions in the Cu-Fe system. In contrast, such alloying appears to be absent in Ag-Fe. The possible availability of a thermodynamic driving force for alloying in certain nanostructured mixtures of immiscible elements is discussed. A preliminary study was performed on the possibility of alloying and amorphization in vacuum-deposited Cu/Ta multilayer films.

We have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer films with a 1:3 molar ratio and various periodicities ranging from 320 nm to a codeposited film with an effective periodicity of zero. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that a reaction which results in the formation of an amorphous phase has taken place during the multilayer deposition process. This reaction substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. The mode of transformation for a film with 10 nm periodicity was investigated, in detail, by applying the Johnson-Mehl-Avrami analysis to data obtained from isothermal and constant heating rate differential scanning calorimetry, in combination with electron microscopy studies of the transformation microstructure.

The roles of irradiation temperature and interfaces (free surfaces and grain boundaries) in irradiation- and thermally- induced amorphization of ceramics (coesite, apatite, olivines and spinels) have been studied by transmission electron microscopy (TEM). The irradiations were performed with 1.5 MeV Kr+, 200 keV and 1 MeV electrons over a wide temperature range (20-700 K). The critical amorphization dose at which amorphization is complete, Dc, increased with increasing irradiation temperature for most materials except coesite (a high pressure polymorph of Si02) which showed a decreasing Dc with increasing temperature under 1 MeV electron irradiation. Although amorphization may occur directly within a displacement cascade or by cascade overlap, this study shows that free surfaces and grain boundaries are favorable sites for nucleation of amorphous volumes. Once the amorphous volume is formed at interfaces, it may grow rapidly under continued irradiation. Coesite which has a glass transition temperature higher than its melting temperature underwent spontaneous amorphization during thermal annealing at 1200 K. This thermally-induced amorphization also started at free surface and grain boundaries and propagated into the interior of the crystal. The interface-mediated amorphization is analogous to the process of thermodynamic melting.

Inverse melting of bcc Nb4sCr55 is investigated using transmission electron microscopy, high-resolution TEM and electron diffraction. It is shown that the transformation to the amorphous phase initiates at the bcc grain boundaries. The transformation results in an increase in incoherence, evidenced by a loss of bend contours. Some anisotropy is found in the amorphous phase produced by inverse melting, which is associated in HRTEM with preferentially oriented but discontinuous and distorted fringes. The results are consistent with the production of an amorphous phase by inverse melting.

We have investigated the solid-state reaction of Ti/Al multilayer films by x-ray diffraction (XRD) and differential scanning calorimetry (DSC), with focus on the early stages of the reaction provided by samples with pair thicknesses in the range 5 - 40 nm. This reaction, which results in formation of TiAl3 with metastable Ll2 structure, can be modeled by a nucleation and growth process on the basis of the Johnson-Mehl-Avrami theory, with a reaction-order parameter n ≈1. These observations indicate the significance of nucleation barriers even at early stages of solid-state reactions, and suggests that the phenomena of phase selection and formation of metastable phases can result from the presence of nucleation barriers.

The interfacial reaction between Al2O3 (alumina) and a β-Ti alloy has been characterized by transmission electron microscopy, scanning electron microscopy, and X-ray energy-dispersive spectroscopy. Diffusion bonding single-crystal alumina and a β-Ti alloy was found to produce three interfacial regions: a region of intermetallics (Tl3Al and TiAl) located near the alumina interface, an α-Ti region, and a β-Ti region (rich in Mo, the β-phase stabilizer). Of the intermetallics to form, Ti3Al was found to form first and have an aligned, planar interface with the alumina. TiAl formed second and was found to separate grains of Ti3Al and the alumina. Reaction products observed in the diffusion-bonded alumina/β-Ti couples are compared with those observed in metal-matrix composites (MMCs), where a β-Ti alloy matrix is reinforced with alumina fibers. Different coatings used in MMCs are investigated for their ability to prevent the reaction between the matrix and fibers.

The formation of the first phase in the reaction of sputter-deposited Nb/Al multilayer thin films has been studied by power-compensated and heat-flux differential scanning calorimetry, x-ray diffraction and transmission electron microscopy. The modulation periods of the films are in the range of 10-500 nm. Both types of calorimetrie measurements, performed at a constant heating rate, show the presence of two peaks (A and B) for the formation of the single product phase, NbAl3. Isothermal calorimetrie scans show that peak A is associated with a nucleation and growth type transformation. The formation of NbAl3 is thus interpreted as a two-stage process of nucleation and lateral growth to coalescence (peak A) followed by normal growth until the consumption of one or both reactants (peak B). Transmission electron microscopy investigations of samples annealed into the first stage of NbAl3 formation show the presence of this phase at the Nb/Al interface and its preferential growth along the grain boundaries of the Al layer. The latter highlights the role of reactant phase grain structure in product phase formation.

A fundamental aspect of martensitic or order-disorder phase transformations is the maintainence of correspondences between atoms or atomic sites in parent and precipitate phases. The present work concerns a structural relationship between initial and final phases in a transformation which is neither martensitic nor order-disorder. A structural relationship between the C14 Laves compound structure and body-centered cubic (bcc) is presented based on electron diffraction recorded in the course of the electron irradiation-induced transformation of the TiCr2 Laves compound to the bcc (β) phase. The diffraction evidence indicates a preferential orientation of β with respect to the initial Laves phase. Consistent with the diffraction data, it is shown how the structures can be formed from one another via small displacements of atom positions. It is suggested that structural relationships in first-order transformations may be significant as paths of minimal structural rearrangement by which such transformations may occur.

Interfacial reactions between A1N substrate and 4A-family elements including Ti, Zr and Hf were studied. The samples were prepared by Physical Vapour Deposition and annealed at different temperatures from 200°C to 800°C. X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) were employed to detect the compounds formed at the interfaces between them. For Ti/AIN system, when the samples were annealed from 600°C to 800°C for 1 hour, it was found from XRD patterns that TiAl3, TiN, and Ti4N3.x including Ti2N were formed at Ti/AIN interface. With the temperature increasing, the intensities of Ti, TiN(200), Ti2N and Ti4N3.x diffraction peaks decreased while that of TiAl3 and TiN(111) increased. For Zr/AlN system, it was found that the reactions between Zr and AlN resulted in the formation of Al3Zr at about 300°C and Al2Zr at about 500°C. According to RBS spectra, it can be assumed that Al3Zr was the direct product by the reaction between AlN and Zr and Al2Zr was formed by the reaction between Al3Zr and Zr. For Hf/AlN system, however, even the sample was annealed at 800°C, no compound resulted from interfacial reactions was detected.