There exists a growing body of literature that correlates the fraction of “special” boundaries in a microstructure, as described by the Coincident Site Lattice Model, to properties such as corrosion resistance, intergranular stress corrosion cracking, creep, etc. Several studies suggest that the grain boundary character distribution (GBCD), which is defined in terms of the relative fractions of “special” and “random” grain boundaries, can be manipulated through thermomechanical processing. This investigation evaluates the influence of specific thermomechanical processing methods on the resulting GBCD in FCC materials such as oxygenfree electronic (ofe) copper and Inconel 600. We also demonstrate that the primary effect of thermomechanical processing is to reduce or break the connectivity of the random grain boundary network. Samples of ofe Cu were subjected to a minimum of three different deformation paths to evaluate the influence of deformation path on the resulting GBCD. These include: rolling to 82% reduction in thickness, compression to 82% strain, repeated compression to 20% strain followed by annealing. In addition, the influence of annealing temperature was probed by applying, for each of the processes, three different annealing temperatures of 400, 560, and 800°C. The observations obtained from automated electron backscatter diffraction (EBSD) characterization of the microstructure are discussed in terms of deformation path, annealing temperature, and processing method. Results are compared to previous reports on strainannealed ofe Cu and sequential processed Inconel 600. These results demonstrate that among the processes considered, sequential processing is the most effective method to disrupt the random grain boundary network and improve the GBCD.

Electrostatic force microscopy and scanning surface potential microscopy are combined to quantify nanometer scale field variations in the vicinity of grain boundaries in donor doped σ15 SrTiO3 bicrystals. An analytical electrostatic model is used to develop a procedure for determining interface potential from measurements made above the surface. Grain boundary potentials and depletion widths determined by both techniques are in excellent agreement despite the fundamental difference in imaging mechanisms. The comparison confirms the analytical approach and illustrates use of scanning probes to image interface properties.

The topological theory of interfacial defects and the associated flux analysis are reviewed. It is shown that the shears and diffusive fluxes associated with the motion of disconnections can be determined directly from their crystallographic characteristics, and that the effects of changes in chemical composition, interfacial misfit and ordering can be incorporated into the analysis. The special conditions are identified for which there is conservation of atomic sites during the motion of disconnections. It is shown that, under these circumstances, disconnection motion may result in mixed-mode displacive-diffusive transformations whereby diffusion is required for the transformation to proceed but the interfaces exhibit crystallographic characteristics which one would normally associate with a martensitic transformation. It is shown that the growth of γ lamellae in TiAl-based alloys is an example of such a mixed-mode transformation.

Thermodynamic and kinetic aspects of interfacial decohesion are analyzed for a uniform separation along an interface subjected to a uniform tensile stress normal to the interface. During the separation process, an embrittling impurity can penetrate into the interface and reduce its cohesive strength. Rice(1976) and Hirth and Rice (1980) analyzed this process in great detail, and in particular considered two limiting cases: slow separation (constant chemical potential of the impurity) and fast separation (constant impurity concentration). Our work extends their analysis to intermediate, time-dependent situations, which correspond to the problem at hand. A phenomenological model is introduced to describe the free energy of the interfacial “solid solution”, stress-separation curves as functions of the impurity concentration, and kinetics of separation. The observed separation kinetics, as well as the work of decohesion, cohesive strength and other interfacial properties, depend on the interrelation between the strain rate and the impurity diffusion rate. An equation of stress-driven diffusion along the interface is derived, and the origin of the stress effect on diffusion is analyzed.

The understanding of electrical properties of grain boundaries in perovskites is essential for their application to capacitors, varistors and positive-temperature coefficient resistors. The origin of the electrical activity is generally attributed to the existence of charged defects in grain boundaries, usually assumed to be impurities, which set up a double Schottky barrier as they are screened by dopants in the adjacent bulk crystal. Microscopic understanding of the origin of the grain boundary charge, however, has not been achieved. It is not known yet if the charged grain boundary states are an intrinsic property of a stoichiometric grain boundary, arise from nonstoichiometry, or are caused by impurities. Here, the relation between atomic structure and electronic properties is studied by combining experiment with ab-initio calculations. The starting structures for theoretical calculations were obtained from Z-contrast images combined with electron energy loss spectroscopy to resolve the dislocation core structures comprising the boundary. Dislocation core reconstructions are typical of all grain boundaries so far observed in this material. They avoid like-ion repulsion, and provide alternative sites for cation occupation in the grain boundaries. Optimized atomic positions are found by total energy calculations. Calculated differences in vacancy formation energies between the grain boundaries and the bulk suggest that vacancy segregation can account for the postulated grain boundary charge.

The adhesion geometries of the (100) heterophase boundaries between the metals Al or Ag and spinel (MgAl2O4) were determined by density-functional calculations employing a mixed-basis pseudopotential method. The results compare well with high-resolution transmission-electronmicroscopy studies which were carried out in parallel. An analysis of the calculated electronic structures shows that the strong adhesion of Al on spinel is mediated by directional bonding. The comparatively weak interaction of spinel with Ag is dominated by polarization effects which can be classified as originating from an image-charge interaction. Additionally, the effects of changes in stoichiometry and of segregated substitutional atoms at the interface were analyzed in terms of the excess of the corresponding component at the interface. The implications for the geometric structure and the related mechanical properties at the metal-ceramic junctions are discussed.

Semi-crystalline polymeric materials exhibit metastable interfaces which must simultaneously feature molecular connectivity through chain bonding and disorder across the interface. To address this, off lattice molecular simulations have been constructed to capture the structure and properties of crystal-amorphous interfaces in polyethylene with freely rotating chains. Monte-Carlo simulations were performed on systems having {001 }, {201} and {502} interfaces (polymer chains tilted to the lamella normal by 0°, 34.4° and 41° respectively.)

Results of the simulations reveal changes in interfacial structure with variation in the chain-tilt angle. The degree of tight folding decreases with increasing tilt, from predominantly [110] loops in the {001} interface to [200] and [310] loops on the other interfaces. The topology shifts towards an increase in loosely folded loops along with longer bridges and tails. The results also compare well with a Gaussian chain length distribution model. The {201} interface exhibits the lowest interfacial energy, in apparent agreement with experimental data.

The elimination of the grain boundary liquid in liquid-phase sintered materials is examined for the case of anorthite liquid in alumina grain boundaries. It is shown that under suitable conditions the liquid can exude from the grain boundary to the free surface. The proposed driving force is provided by the difference in energies and wetting behavior of the grain boundary and the free surface at high temperatures. The results emphasize the importance of the crystallography of the boundary and the nature of free surfaces (i.e., the surface energies) on the exudation behavior.

The influence of the sintering additive system on the grain growth anisotropy of silicon nitride has been studied in the two supersaturated oxynitride glass systems M-Si-Al-O-N (where M = Yb, Gd, Nd, La, and Y) and M-Si-Mg-O-N (where M = Lu, Sm, La, Sc and Y). The glasses contained a low volume fraction of grains and offer the possibility to study grain growth without impingement. Both systems show an increase of the aspect ratio with an increasing cation radius of the rare earth elements. The aspect ratios for the Sc- and Y-based glasses are higher than expected from the ionic radius.

Analytical transmission electron microscopy of the bulk glass and grain boundary film between flocculated particles reveals a difference in chemical composition. The larger rare earth cations are enriched in the film and the smaller ones are depleted compared to the bulk composition. The enrichment of the larger cations can be considered as an adsorption layer which reduces the growth rate of the prism planes. Glass systems with smaller cations (Yb, Gd) reveal a pronounced growth of the prism planes and sialon is formed in the outer region of the grains. The interfacial strength which determines the fracture toughness increases with a decreasing cation radius, but the effect is mainly attributed to the sialon formation rather than by the cation itself.

Current understanding of the equilibrium thickness of interfacial films derives from models which balance a steric repulsion against an attractive force between abutting crystals such as the van der Waals forces [1]. Recent experimental observations indicate that, a) film thickness is a non-monotonic function of additions of impurities and b) impurities are non-uniformly distributed in the interfacial phases and triple junctions [2]. We have developed a model which couples the steric terms which affect the equilibrium thickness with composition for a semi-infinite film in equilibrium with a chemical reservoir at a fixed composition.

We present a method for assessing the relative vulnerabilites of distinct classes of grain boundaries to localized corrosion. Orientation imaging microscopy provides a spatial map which identifies and classifies grain boundaries at a metal surface. Once the microstructure of a region of a sample surface has been characterized, a sample can be exposed to repeated cycles of exposure to a corrosive environment alternating with topographic measurement by an atomic force microscope in the same region in which the microstructure had been mapped. When this procedure is applied to Ni and Ni-based alloys, we observe enhanced attack at random grain boundaries relative to special boundaries and twins in a variety of environments.

The early stages of dewetting of a reactive silicate (CaMgSiO4) film on a ceramic (MgO) substrate were studied by scanning probe microscopy. Subsequent interactions between the glass phase and an underlying single-crystal substrate, such as dissolution/re-precipitation of substrate material at the glass-ceramic interface and evaporation of silicate phase, were investigated by observing selected areas of the specimens through a series of annealing-quenching cycles. Implications deduced from the CaMgSiO4-MgO equilibrium phase diagram on the process of dissolution/re-precipitation of MgO are briefly discussed.

In the first part of the paper the microstructure of Ti-6Al-4V with laser embedded SiC particles is explained. The interfacial reaction between Ti and SiC is responsible for the largely improved wear resistance of the Ti alloy. In the second part the phase responsible for the intergranular corrosion susceptibility of an Al-Mg alloy when annealed above 65 °C is identified and possible mechanisms to solve this interfacial problem are proposed.

Silicon oxynitride films, fabricated by direct thermal growth and annealing in N2O or NO, were analyzed by Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS). It is seen that for the samples processed in N2O, N is bonded as Si3N4 only, irrespective of whether the fabrication was done on bare Si or on an oxide pre-grown in O2. But the films processed in NO depict additional bonding arrangements, namely, non-stoichiometric SiOxNy, (Si-)2-N-O, and Si-N(-O)2. These bonding states are found to be concentrated in a higher proportion above the oxynitride/substrate interface. Further, it is seen that annealing of a pre-grown oxide in NO for 30 min incorporates the same bonding states as by direct growth in NO for as long as 120 min. Also, a critical N concentration (between 1.9% and 2.3%) is required for the incorporation of the Si-N(-O)2 structure, observed at 400.7 eV. Besides enhancing the overall understanding of the progress of silicon oxynitridation process in N2O and NO, these findings can help significantly towards developing process-property relationships for incorporation of N with the desired bonding state(s) at specific positions within an oxynitride film.

Multilayer structures with depth graded spacing can present a high reflectivity in a broad energy bandpass for hard X-rays (>20 keV) if the interface roughness/diffuseness is controlled and minimized. We present an optimization study of several multilayer systems deposited by DC magnetron sputtering on <111> silicon wafer substrates. The material combinations presented in this work are W/Si, W/C, Wsi2/Si, Pt/C, Ni/C, and Mo/Si.

Liquid phase diffusion bonding was applied to the development of the joined thermoelectric material Pb-Sn-Te, which was expected to be superior to the monolithic Pb-Sn-Te. A Sn sheet 50μm thick was used for the inserted material. The sheet was sandwiched between Pb-Sn-Te segments with different hole concentrations, and subsequently the bonding was performed under 2MPa at 700K for 15min in Ar. The Sn sheet was changed to a SnTe interfacial layer of less than 10μm in thickness. The interfacial layer showed high toughness, and good ohmic and thermal contact up to 550K. Liquid phase diffusion bonding was an excellent technique for joining of thermoelectric material Pb-Sn-Te.

We have investigated compliance with the σ-product rule at triple junctions with a common [111] axis using computer simulations and in real thin films, using electron microscopy. Large discrepancies exist between the simulations, which assume randomly distributed misorientations, and the experiments. Although for both cases the rule fails at most triple junctions, the real material shows a much higher than expected occurrences of compliance. This result can be explained in two distinct ways: first, there might be a preference for triple junctions that obey the rule (implying a distinct influence of the triple junction itself). Alternatively, the misorientations in the real crystal might not be randomly distributed: if there is a preference for boundaries that have only small deviations from coincidence, there will be an increase in the compliance with the rule.

Recent advances in our understanding of the role of the chemistry of grain boundaries in controlling the mechanical properties of materials (in particular intergranular brittle fracture) are reviewed. It is now possible in a modem field-emission gun (FEG) analytical transmission electron microscope (AEM) to measure the chemistry of sub-nanometer films of GB segregants while at the same time observing the effect (if any) on the bonding of the atoms within a nanometer of the boundary plane. This has been accomplished by the development of X-ray mapping (XRM) a powerful new tool for the study of segregation. For the first time, in the same instrument, on the same grain boundary, any changes in the boundary chemistry can be correlated with the occurrence or absence of brittle failure, which is often associated with boundary segregation. There is strong evidence that boundary segregation is extremely nonuniform, even in some strongly embrittling systems (e.g. Cu-Bi) and in these same systems, embrittling segregants introduce subtle but consistent changes in the bonding. Non-embrittling segregants (e.g. Ag in Cu) do not introduce detectable bonding changes.

Alloys designed for high-temperature applications need to form a protective oxide scale to withstand further degradation by oxidation. The importance of the interfacial chemistry for the adhesion of the protective scale was demonstrated in experiments with β-NiAl, iron-aluminides, NiCrAlYs and Ni-based superalloys. Interfaces between alloys both with and without thermal barrier coatings and their thermally grown alumina scales were studied as well as interfaces between alloys and deposited alumina coatings. It was found, that in all cases improved oxidation performance and scale adhesion could be achieved by doping the alloys with reactive elements. The improvement was shown to be due to interfacial segregation of the reactive element. Segregation of sulfur to internal interfaces on the other hand caused scale spallation.

A procedure for obtaining interface and grain boundary multiplication, without externally applied deformation, in alloy systems is described. It is shown that an essential prerequisite is the occurrence of, and the possibility of controlling, discontinuous precipitation (DP) reactions in the alloy. Several alloy systems have been studied and experimentally generated, in a controlled volume fraction, DP lamellar products in the following model alloys: Al%Ag; Al-22at%Zn; Cu-7at%In and Ni-8at%Sn. The resulting microstructures have been observed in detail through conventional and analytical electron microscopy, confirming that DP is a transformation controlled by solute diffusion along moving grain boundaries. Although the precipitates correspond to the equilibrium phase, the transformation, as a whole, does not reach thermodynamic equilibrium since high-resolution microanalysis of aged and quenched microstructures reveal that a significant amount of supersaturation is retained in the depleted lamellar matrix. In addition, TEM observations have revealed that the DP product, besides generating a high density of interfaces, is able to incorporate a significant amount of strain energy. On the other hand, the dissolution of the lamellar microstructures has been observed to proceed in both continuous and discontinuous fashions. The former, dominating at high dissolution temperatures, is controlled by volume diffusion and gives rise to the development of new microstructures in the original DP colonies: the generation of dislocations and their dynamic re-arrangement into a cellular sub-structure in Al – base alloys; the formation of new grains, in Cu-In and in Ni-Sn alloys. Operating mechanism are proposed and the nature of the driving force for the observed phenomena is discussed in terms of its chemical, interfacial and strain energy components.