Molecular Dynamics simulations of the melting of small crystalline clusters (≃800 atoms) in the liquid have been performed at various temperatures above the equilibrium melting point. The melting rates as functions of size and temperature are derived and compared to that predicted by Classical Nucleation Theory. It is found that the driving force for the melting of clusters does not follow the form assumed in the theory, and that this difference is most apparent for clusters containing less than 300 atoms. The implications of these findings on nucleation phenomenon and possible sources for the discrepancies are discussed.

Coarsening of a population of second-phase precipitates in a solid affects the physical properties of the solid. A numerical model of coarsening is introduced to treat both diffusion-controlled and interface reaction-controlled coarsening. The model is based on the escape probabilities of particles from precipitates. The experimental time dependences of precipitate coarsening are obtained.

The stability of phases in materials can be related to reactions occurring at their interfaces, for example fiber/matrix reactions in composites. The reaction can be modelled in a simple way, even for multicomponent systems, when it is diffusion controlled and when no intermediate phases form. In the current work a model is proposed that assumes local equilibrium at the interface and small concentration differences in each phase. The physical properties required for the model are the “square root diffusivity” for each phase and the applicable phase diagram. The model is applied to predicting concentration profiles and interface velocities for the reaction of a B2 aluminide containing Ni-34at%Cr-12at%Al with various Ni-rich, gamma solid solutions containing Cr and Al. The example shows how varying the gamma composition can alter the aluminide stability with respect to dissolution or growth. In addition, a simplified form of the model is given for extrapolating experimental interface velocity data.

The artificial layering of metals can change both physical and structural characteristics from the bulk. The stabilization of such phases often occurs when the component layers are constructed on a nanometric scale. The sequential layering of nickel and titanium has produced the formation of a previously unreported, face-centered-cubic (f.c.c.) phase of titanium.

The paper illustrates the benefits of a combined use of computer simulations of interfacial properties and experiments performed on macroscopic, well-defined bicrystals. The results from two studies are reported. Relative energies of inclined Cu Σ3 grain boundaries were determined by thermal grooving experiments on diffusion-bonded Cu bicrystals. A comparison with calculated energies for crystallographically identical interfaces points to the possibility of macrofaceting of inclined grain boundaries upon annealing. Grain boundary entropy effects are discussed as another possible reason for small differences between theory and experiment. The diffusion rate of Ag* radiotracer in heterophase boundaries of diffusion-bonded Ag-Cu bicrystals can qualitatively be correlated with computed structural details of crystallographically identical Ag-Cu and Ag-Ni phase boundaries. The (computed) site-to-site variation of vacancy formation enthalpy on the (110) phase boundary leads to an anisotropy of Ag* phase boundary diffusion.

A geometric model for intergranular stress corrosion cracking (IGSCC) is presented and used to evaluate the influence of grain boundary structure on the IGSCC resistance of polycrystalline materials. Preliminary observations regarding the structure of intergranular fracture paths in Alloy 600 C-ring specimens exposed to high temperature caustic media are noted to be consistent with the general predictions of the proposed geometric model and demonstrate that significant enhancement to bulk IGSCC resistance may be achieved through material processing considerations which result in (1) moderate increases in the frequency of structurally ‘special’ grain boundaries (i.e., interfaces close to low Σ CSL's) and (2) refinement in grain size.

Sputter deposition of ZnO films on GaAs substrates has been investigated. ZnO Alms were deposited using a ZnO compound target in Ar or Ar/O2 (95/5) ambient. Deposition parameters such as RF power, substrate-target distance, and gas composition/pressure were optimized to obtain highly c-axis oriented and highly resistive films. Deposited films were characterized by X-ray diffraction, scanning electron microscopy, capacitance, resistivity, and breakdown field strength measurements. X-ray diffraction measurements show a strong (0002)-plane peak with a full-width-half-maximum of less than 1°, indicating that the ZnO films deposited on GaAs substrates are highly c-axis oriented. DC resistivities of the films were measured to be on the order of 1011 Ω-cm.

Polycrystalline zinc oxide containing oxides of bismuth, cobalt, manganese and antimony is found to exhibit highly non-ohmic I–V characteristic arising due to the presence of schottky barriers at the grain boundaries. Defect states present at the grain–grain boundary interfaces store excess negative charge and give rise to such potential barriers. Any improvement in the I–V characteristics therefore demands the understanding of grain boundary properties of the material. Impedance spectroscopy is found to separate grain and grain boundary contributions in a polycrystalline material and is used to determine grain boundary contributions towards the electrical behavior of the zinc oxide varistors. It also helps in identifying stored charges and defect states present and elucidates different trapping phenomena occurring at the interface behavior of zinc oxide varistors.

The effect of incorporating a Yb diffusion barrier on the electrical and interfacial properties of In/Hg1−xCdxTe contacts has been examined. Measurements of the specific contact resistance, pc, as a function of Yb layer thickness were performed for a series of semiconductor compositions from x = 0.32 to 0.65. Large changes in pc were evident only at x 0.56 and above a Yb thickness of 2.5nm to 6.0nm, this critical level of layer thickness decreasing with x-value. Analytical examination of the interfaces by Auger electron spectroscopy, secondary ion mass spectroscopy and Rutherford backscattering spectrometry has shown a progressive reduction in the extent of In indiffusion with increasing thickness of the Yb interlayer.

The properties (electrical and structural) and the defect levels dominating cadmium telluride (CdTe) films prepared by radio frequency (rf) planar magnetron sputtering, and electrochemical deposition have been determined and compared. The properties of the deposited CdTe film and the behavior of its interface with cadmium sulfide (CdS) depend strongly on the method of depositing the CdTe film, and on postdeposition heat treatments. These treatments determine various parameters crucial to the device performance, including the type and concentration of the dominant defects, interface states, and deep trap levels. Photoluminescence (PL) emission from p-type CdTe polycrystalline thin films and single crystals is reported at low temperatures (9–50 K) as well as at room temperature. The room temperature PL peak at 1.58 eV due to band-to-band recombination was observed for the first time in polycrystalline thin films. The peak location of the exciton emission is indicative of the structure perfection of the film, which is also related to the deviation from stoichiometry. The as-grown polycrystalline films are composed of a close-packed array of preferentially oriented (the [100], [110], and [111] axes aligned perpendicular to the substrate) single-crystal grains of a size in the range of 0.25–2.0 μm for the electrochemically deposited films (E-film) and 0.3–4.0 μm for rf sputtered material (rf-film). The resistivity of the heat treated E-film was substantially lower than that of the rf-film (ρ = 1.0–5.0 and 300–500 Ωcm). The performance of the CdS/(E-CdTe) devices is limited by deep levels, while the performance of die rf-CdTe device is affected by a high density of interface states.

Ferromagnetic resonance (FMR) measurements on Co films of thicknesses 10–100 Å grown on GaAs (110) and Au (111) substrates have been made as a function of dc magnetic field orientation in a plane perpendicular to the film. Experimental data were fitted by writing the energy density expression including uniaxial perpendicular anisotropy terms up to second order and bulk anisotropy terms. Co films grown on Au (111) substrates show consistently a higher uniaxial perpendicular anisotropy compared to those grown on GaAs (110) for the same thickness of Co. This is attributed to stress induced anisotropy due to a lattice mismatch between Au (111) and Co layers.

Sapphire substrates, mechanically polished to an optical finish, were annealed for two days at either 1000°C or 1350°C. The near surface condition of as-polished and of the annealed substrates was analyzed by Rutherford backscattering/channeling (RBS-C) and by scanning electron microscopy/channeling (SEM-C), by transmission electron microscopy (TEM), and by scanning tunnelling microscopy (STM). The polished substrates were found to be RBS-amorphous up to 100 nm, and heavily damaged at larger depths. In agreement with these results, no electron channeling was obtained from polished samples. TEM, however, showed that the damaged region was crystalline, and the only defects detected were microtwins. Both RBS-C and SEM-C analyses revealed that the damage is removed when the sapphire substrates are annealed for 48 hrs. at 1350°C. The condition of the near-surface region, viz., as-polished or annealed, is found to strongly affect the morphology of the laser-irradiated copper films deposited on sapphire substrates. A correlation is found between the threshold for film evaporation and for film rupture upon laser irradiation, both being a function of the substrate condition. It is concluded that the near-surface damaged layer acts as a thermal barrier for heat transport across the substrate.

The conditions established in disordered FCC systems for predicting the slip system that will be activated by a grain boundary to relieve a local stress concentration have been applied to the ordered FCC alloy Ni3Al. The slip transfer behavior in hypo-stoichiometric Ni3Al with (0.2 at. %B) and without boron was directly observed by performing the deformation experiments in situ in the transmission electron microscope. In the boron-free and boron-doped alloys, lattice dislocations were incorporated in the grain boundary, but did not show evidence of dissociation to grain boundary dislocations or of movement in the grain boundary plane. The stress concentration associated with the dislocation pileup at the grain boundary was relieved by the emission of dislocations from the grain boundary in the boron-doped alloy. The slip system initiated in the adjoining grain obeyed the conditions established for disordered FCC systems. In the boron-free alloy, the primary stress relief mechanism was grain-boundary cracking, although dislocation emission from the grain boundary also occurred and accompanied intergranular crack advance. Because of the importance of the grain boundary chemistry in the models for explaining the boron-induced ductility in hypo-stoichiometric Ni3Al, the chemistry of grain boundaries in well-annealed boron-doped and boron-free alloys was determined by using EDS. No Ni enrichment was found at the grain boundaries examined. These observations are discussed in terms of the different models proposed to explain the ductility improvement in the boron-doped, hypo-stoichiometric alloy.

The low ductility and toughness of β-NiAl alloys near room temperature pose major problems in their potential application as structural materials. The inability of the material to generate and move a sufficient density of dislocations at applied stresses below the fracture stress is the major cause for this inherent brittleness. A directionally solidified β+(γ+γ') composite of nominal composition Ni50Fe30Al20 (at.%) has been used to investigate the effect of interphase boundaries on the mechanical behavior of β phase. The composite exhibits 10% tensile elongation to fracture at room temperature. Observation of slip traces and dislocation substructures shows that the normally brittle β phase undergoes extensive plastic deformation afforded by slip transfer from the plastically soft (γ+γ') phase mixture across the semi-coherent β/(γ+γ') interface. The effect of the orientation relationship between the two phases and the interface strength on the transfer of slip across the interphase boundary is discussed.

The results of our initial efforts at measuring the fracture strengths of grain boundaries In Ni3Al using a miniaturized disk-bend test are presented. The samples tested were 3 mm in diameter and between 150 and 300 μm thick. An Ingot of directlonally-solidlfled, boron-free Ni3Al containing 24% Al was annealed between 1300 and 1350 °C to induce grain growth, producing many grain boundaries In excess of 1.5 mm in length. Specimens were cut from these In such a way that one long grain boundary was located near a diameter of the specimen. The relative orientations of the grains on either side of the boundary were determined from electron channeling patterns. Low-angle boundaries are so strong they do not fracture; Instead the samples deform In a completely ductile manner. High-angle boundaries always fracture, but only after considerable plastic deformation of the two grains flanking them. Fracture is Indicated by a load drop in the load vs. displacement curves. A method involving extrapolation of the elastic portion of these curves to the displacement at fracture is used to estimate the fracture stresses. This procedure yields consistent values of the fracture strengths of high-angle boundaries. The measured stresses are large (∼2 to 3 GPa), but considerably smaller than those required for the fracture of special boundaries, as predicted by computer simulations. No correlation was found between the fracture stresses or loads and the geometry of the high-angle boundaries, many of which are close to, but deviate from, coincident site lattice orientations.

In this study, molecular dynamics simulations were used to fracture Σ9 tilt boundaries in nickel lattices containing a range of trap site hydrogen concentrations. These lattices were created in a previous study using Monte Carlo simulations and the Embedded Atom Method to duplicate room temperature exposure to a hydrogen environment. The molecular dynamics simulations were run at absolute zero to immobilize the hydrogen distributions for determination of trap site occupancy effects on grain boundary fracture. In all lattices, fracture began by the breaking of bonds next to polyhedral defect sites that characterize the boundary structure followed by rapid failure of the remaining bonds. The effect of hydrogen was to lower the stress for fracture from 18 GPa to a lower limiting value of 8 GPa as the trap sites along the boundary plane filled. The simulations showed that the atoms at these sites were the only atoms involved in the fracture process. Within the constraints imposed on these calculations, the results of this study showed that the ‘inherent’ effect of hydrogen in the absence of plastic deformation is to reduce the cohesive force between atoms across the boundary.

It is well known that the atomic structure of an interface has some influence on its properties and thus on the properties of a poly crystalline ensemble as a whole. However, with the enormously large number of interface structures possible and no cohesive framework within which to understand them, it seems a hopeless task to search for structure-property relationships for all interfaces. Rather, we have adopted the viewpoint that interfaces may be grossly categorized into low-misorientation angle/special boundaries and random boundaries with a subsequent two-state division in their properties. We will discuss a simple methodology for examining stress-voiding and electromigration failures in thin metallic interconnects that takes the differing abilities of the interface classes to contribute to failure into account. We thus provide one of the first attempts to account consistently for the atomic structure of an interface and the macroscopic behavior observed in a poly crystalline system.

The Scanning Tunneling Microscope was used as a surface profilometer to image fractured interfaces of a model metal/ceramic system, Au/sapphire. By characterizing the metal side of interface fracture, features related to plastic deformation were quantified. The spatial resolution of STM allowed these measurements to be made down to the nanometer scale. Mathematical techniques were developed to characterize surface features on several size scales and to relate them to the mechanisms from which they were produced.

This research deals with a mode of brittle intergranular fracture in which a surface-adsorbed embrittling element is driven into a grain boundary as a result of the application of a tensile stress across the boundary. A Cu-8%Sn alloy has been employed to explore this phenomenon, since tin is a surface-active element, and this alloy is known to suffer intergranular weakness at elevated temperatures. Intergranular cracking occurred by brittle, discontinuous crack advance at 265°C in vacuum with an average rate of 0.1μm/sec. This behavior is analogous to sulfur-induced stress-relief cracking in steels and several cases of liquid-metal embrittlement, suggesting that this phenomenon has a generic nature.

Directionality of interfacial cracking resistance in bicrystals has been predicted by a modified Rice-Thomson model and supported by experimental results on Σ9 [110]/(221) Cu bicrystals in Wang and Anderson's recent work. With the intention of extending this work to the behavior of interfacial cracks in dissimilar materials, Rice, Suo and Wang predicted that this directionality also occurs in bimaterials. Reported here are experimental results on Cu/sapphire specimens, which again support the prediction of the theory. A crack along the interface in a Cu/saprjhire bimaterial is brittle and decohesion of the interface occurs when the crack propagates in the [114]Cu direction, whereas it is ductile and dislocation blunting operates if the crack is intended to propagate in the [114]Cu direction. This finding is significant theoretically and is of helpful in understanding interfacial cracking in composites and film spalling in packaging materials.