The crystallization of amorphous sputtered NiTi films was investigated for selected heat treatments. From x-ray diffraction patterns, when the films were annealed below the crystallization temperature, the intensity of the broad maximum centered at 2θ = 43.5° increased with increasing the annealing temperature and time. When the films were annealed at 550°C for 0.5hr, parent B2 phase and Ni4Ti3 precipitates appeared. For annealing temperature above 700°C, the films showed embrittlement and volatilization. Therefore the optimum heat treatment for the optimum shape memory effect is found. The average grain size increased slowly in the initial stage of annealing but remained almost unchanged when the films were annealed for more than 1hr. This is because the grain boundary grooving caused by the titanium at a certain temperature volatilization seriously hinder the grain growth.

A realistic computer model for polymorphic crystallization under isothermal and nonisothermal conditions, which takes proper account of time-dependent nucleation behavior and cluster-size-dependent growth, is presented. A new correction to the standard Johnson-Mehl-Avrami-Kolmogorov (JMAK) statistical analysis that takes account of finite sample size is incorporated to simulate data taken from fine particles and nano-structured materials. Model predictions compare well with experimental data obtained from calorimetric studies of the polymorphic crystallization of lithium disilicate glass. The computer model is employed to evaluate commonly used methods of analysis for calorimetric data and to suggest new approaches for extracting kinetic parameters.

The Kolmogorov-Johnson-Mehl-Avrami theory is an exact statistical solution for the expected fraction transformed in a nucleation and growth reaction in an infinite specimen, when nucleation is random in the untransformed volume and the radial growth rate after nucleation is constant until impingement. Many of these restrictive assumptions are introduced to facilitate the use of statistics. The introduction of “phantom nuclei” and “extended volumes” are constructs that permit exact estimates of the fraction transformed. An alternative, the time cone method, is presented that does not make use of either of these constructs. The method permits obtaining exact closed form solutions for any specimen that is convex in time and space, and for nucleation rates and growth rates that are both time and position dependent. Certain types of growth anisotropies can be included. The expected fraction transformed is position and time dependent. Expressions for transformation kinetics in simple specimen geometries such as plates and growing films are given, and are shown to reduce to expected formulas in certain limits.

Morphological evolution of coherent precipitates with arbitrary transformation strain is studied via a discrete atom method. With a purely dilatational misfit strain, a soft precipitate tends to have a plate-like equilibrium shape, whereas a hard precipitate takes up a shape of high symmetry such as a circle. If the stiffness is comparable between the matrix and precipitate, however, the equilibrium shape depends on the degree of anisotropy, misfit strain, size, and interfacial energy. Shape bifurcation phenomena from circular to rectangular shapes are also revealed during clustering. With either a tetragonal misfit strain of mixed signs or a pure shear misfit strain, a precipitate takes up a plate-like shape whose major axis lies along the direction containing an invariant line in accordance with the continuum elasticity prediction. Both elastic anisotropy and elastic inhomo-geneity exert little influence on the preferred orientation relationship. With a misfit strain of combined shear and dilatation, a precipitate follows an orientation and shape dictated by both components of the misfit strain.

At elevated temperatures, stresses enhance mass transport at the microstructural scale and under certain conditions, lead to microstructural instabilities. Using a linear stability analysis, we examine the effect of stresses on the morphological stability of plate-like phases and lamellar eutectic composites. These stresses can be either due to misfit strains and/or due to externally applied loads. We find that for misfitting plates, the nominally flat plate-matrix interface is unstable with respect to the growth of perturbations with wavelengths greater than a critical wavelength, provided that the reinforcing plates are elastically suffer than the surrounding matrix. On the other hand, for stresses generated by externally applied loads, the flat interface is always unstable as long as the plate modulus and the matrix modulus are not identical. In addition, the analysis reveals that misfit strains can either counteract or enhance the destabilizing influence of applied loads depending on the elastic properties of the plate and the matrix. Finally, we have developed stability diagrams that identify material properties and operating conditions required to maintain a stable interface in these lamellar eutectic composites.

The phase transformation between the α- and β-cristobalite modifications of SiO2 were studied using molecular dynamic simulations. The transformation was induced isothermally by control of the pressure within the structure, either through an externally applied constant stress or by changing the simulation box volume. The atomic scale mechanisms of the transformation can best be observed by means of a time-correlation function describing the spatial orientation of the planes containing the Si-O-Si bonds. These planes rotate by 90° over the course of the transition. Both, the bulk modulus, calculated for static structures, as well as the vibrational spectra of the Si-O-Si planes, reflect the softening of acoustic modes in the midst of the transition, which is characteristic of displacive phase transformations. Although the aperiodic shift of atomic positions upon passage of the transformation front could be responsible for a momentary softening of the structure, this is not the only reason, since this behavior persists when maintaining the structure at intermediate densities, which corresponds neither to α-, nor to β- cristobalite.

Two-dimensional cellular automaton computer simulations were carried out to model the geometric interaction between growing matrix grains and mobile, equiaxed particles, thus simulating crystallization (respectively, recrystallization, phase transformation or solidification) of a material containing mobile particles. As a result of particle pushing by growing grains, particle accumulation at grain boundaries and triple-points and concomitant particle depletion within grains occurs. A range of grain nucleation rate, particle area fractions and particle settling rate was investigated under continuous grain nucleation conditions. While these parameters do not change the crystallization kinetics or grain size or shape, they strongly affect the particle spatial distribution and clustering during and after crystallization.

Crystallization, precipitation, and phase transformation phenomena were observed in titanium suicide thin film samples during in situ heating experiments in a transmission electron microscope. The as-deposited TixSiy films were 110 nm in thickness with a composition of 1 Ti to 2.33 Si. Crystallization of the C49 phase was followed isothermally near the sputter deposition temperature. The movement of individual grain boundaries was recorded so that a “velocity of crystallization” could be calculated. The precipitation of excess silicon from the C49 phase was first observed in the 650°C to 750°C temperature range. The precipitates were predominantly of the incoherent type, with a smaller number existing at the grain boundaries. Ostwald ripening then occurred up to the C49 to C54 phase transformation which was accompanied by a dramatic increase in grain size. Grain boundary movement during the phase transformation was such that large precipitates, which were originally at C49-C49 boundaries, ended up within resulting C54 crystals. Many of these larger precipitates were found to exist as epitaxial “islands” at the TiSi2/Si substrate interface.

We introduce a high-sensitivity (∼1 J/m2) scanning microcalorimeter that can be used to perform direct calorimetric measurements on thin film samples at ultrafast heating rate (∼104 °C/s). This novel microcalorimeter is fabricated by utilizing SiN thin-film membrane technology, resulting in dramatically reduced thermal mass of the system. Calorimetric measurements are accomplished by applying a dc-current pulse to the thin-film metal (Ni) heater which also serves as a thermometer, and monitoring the real-time voltage and current of the heater. The temperature of the system and the energy delivered to the system are then determined. This calorimetric technique has been demonstrated by measuring the melting process of thin Sn films with thickness ranging from 13 to 1000 Å, and shows potential for calorimetric probing of irreversible reactions at interfaces and surfaces, as well as transformations in nanostructured materials.

Chemical reaction rate theory is used to model the kinetics of precipitation reactions in Al alloys, including the effects of continuous cooling and thermally generated point defects. The computational method models the processes of nucleation, growth, and coarsening within a single framework. Calculated time and temperature dependent precipitate number densities and sizes during the homogeneous precipitation of the A13Sc phase in an Al-.11 at% Sc alloy are shown to compare favorably with experimental observations.

The influence of coherency strains produced by the γ-γ’ lattice mismatch, δ, on the decomposition process of Ni-Al-Mo alloys with a bimodal size distribution is presented. Samples with δ ranging from positive to negative, were investigated in a double-step aging procedure. The evolution of the microstructure and the kinetics of coarsening were studied using transmission electron microscopy (TEM). The lattice mismatch between the matrix and the different classes of precipitates was determined by high-resolution high-temperature x-ray diffraction. It is shown that the strain fields produced by the lattice mismatch can influence dramatically the decomposition of metallic alloys. It is suggested that the reduction of the coarsening rate of the large precipitates, the fast coarsening rate of the small precipitates and the distortions detected in the matrix are all direct consequences of the elastic fields produced by the γ-γ’ lattice mismatch.

Metal-ceramic microstructures were formed in situ by partial reduction of the spinel compound NiAl2O4. At 1300°C the reduction reaction passed through an intermediate stage involving the formation of Ni and metastable “defect spinel” phases, before the appearance of the equilibrium phases, Ni + α-Al2O3. At 1100°C, depending on the reduction time, the products of the reduction reaction were Ni and the metastable “defect spinel” for less than 24 hours; Ni + θ-Αl2Ο3 for 24-48 hours; and Ni + GI-AI2O3 for 144-168 hours. The effects of dopants, such as Cr2O3, MgO, TiO2 and Y2O3, on the reduction mechanism and kinetics were examined and shown, in the case of TiO2, to have a controlling influence. A reduction reaction mechanism is proposed.

A systematic study on phase formation and stabilisation in the Ba-Cu-C-O system in the temperature range 20-500°C, under various atmospheres, by traditional thermal analysis techniques, high temperature X-ray diffraction and high resolution electron microscopy, has permitted to identify and characterise the formation kinetics of a new copper containing phase isomorphic to γ-BaCO3.

High pressure synthesis of SrGeO3 perovskite and its properties are reported. The metastable material recovered to room pressure shows a high dielectric constant (>100), due to a “stretched” crystal lattice.

A finite-difference implicit numerical model was used to study the diffusion-controlled α-γ solid-state phase transformation in the Fe-Cr-Ni system. The influence of a non-uniform initial composition distribution was examined in order to assess the impact that non-uniform solute profiles resulting from solidification may have on subsequent transformation behavior in weldments and castings. A significant impact on the transformation kinetics and transformation path was found in some cases. The factors that affect the degree of influence are discussed.

An internal friction peak associated with the viscous motion of interfaces was measured in the 50–150kHz range for the Cu-Al-Zn-Ni sample. The internal friction formula based on the viscous motion of interfaces has been derived. Also, corresponding parameters, which play an important role in shape memory effect such as viscosity coefficient, pinned force constant etc., were obtained.

Effects of high magnetic field and tensile stress on martensitic transformation behavior and microstructure at 4 K have been studied in Fe-31Ni-0.4C and Fe-27Ni-0.8C shape memory alloys. It was found that the critical magnetic field to induce martensitic transformation is between 7.5 T and 10 T. In the case of Fe-27Ni-0.8C, martensitic transformation is stress-induced at lower level of stress in magnetic field than in the case when no magnetic Field is applied. The amount of martensite formed by increasing the magnetic field under constant stress is larger than that formed by increasing the stress in the constant magnetic field.

PbSnF4 is the highest performance fluoride ion conductor known to date and is starting to be used for the fabrication of chemical sensors. Although several phase transitions have been reported, with conflicting results from different groups, the exact sequence of phase transitions versus temperature and the kinetics of the phase transformations remain unclear. We have prepared the three phases that can be stabilized at ambient temperature, i.e. o-, α- and β-PbSnF4, and studied the stability of each phase versus temperature. It appears that some of the phases transform very slowly to another under prolonged heating at constant temperature. This is very important since the conductivity is not necessarily the same for each phase, and therefore, a phase transition taking place slowly in the PbSnF4 used for the fabrication of a device, might not be detectable by a short time evaluation but, it could well alter its properties after prolonged use.

We report on Molecular Dynamics (MD) studies of free energy and vibrational entropy differences of the binary system Ag2Se-Ag2S as function of temperature. We have employed the methods of adiabatic switching and temperature integration over a reversible path to obtain the phase diagram. We discuss the nature of the entropy difference in terms of ion mobility and previous MD calculations of diffusion rates in these systems.

A new, quantitative model was developed to describe the martensite transformation kinetics of thermoelastic shape memory alloys (SMAs). In addition, a series of experiments were conducted to study the Kinetics of thermoelastic martensite transformation in four SMA systems: NiTi, NiTi-15at%Hf, NiTi-20at%Zr and NiMn-7.5at%Ti alloys. Comparisons between data of the kinetic of martensite transformation with the present theoretic models show that the proposed model is in good agreement and concurs with the experimental data. Also, a comparison of data from the proposed model with data from existing kinetic models, such as Liang's and Magee's [1,7], indicates that the proposed model can better describe the experimental data, including the relationship between dξ(T)/dT and ξ, and dξ(T)/dT and T.