Abstract: 63Ni radiotracer diffusion in a well-compacted nanocrystalline (grain size d ∼ 80 to 100 nm) γ-Fe–40wt%Ni alloy was measured by the serial sectioning technique in an extended temperature range from about 610 to 1010 K. Since the material microstructure reveals two different length scales with nano-size grains forming micrometer-size clusters (or agglomerates), three main diffusion paths determine the diffusion behavior: the nanocrystalline grain interior, the nanocrystalline grain boundaries (GB), and the inter-agglomerate interfaces. The systematics of diffusion in a compacted nanostructured material with such a bimodal distribution of interface characteristics was elaborated and the experimental data were analyzed in dependence on the diffusion regime. The absolute values and the Arrhenius parameters of Ni GB diffusion in the nano-γ-Fe–40wt%Ni alloy (D 0 = 9.3 × 10−4 m2 s−1 and Q = 177 kJ/mol) are similar to the Ni GB diffusivity in coarse-grained poly-crystalline γ-Fe. Accordingly, the nanocrystalline GBs are concluded to have quasi-equilibrium structures, particularly because of a pronounced grain growth (from about 30 to about 100 nm) during the production stage of the nanomaterial. In contrast, the inter-agglomerate interfaces, which present the fastest diffusion path in the present investigation (D 0 = 1.9 × 10−3 m2 s−1 and Q = 134 kJ/mol), are likely to be in a non-equilibrium state due to specific features of the applied powder metallurgical process.

Abstract: The microstructure of a Cu-Ni alloy after static recrystallization was investigated using electron backscatter diffraction in a scanning electron microscope and the existence of orientationally related clusters of crystallites formed by multiple twinning has been established. Grain boundary and triple junction character within the clusters are analyzed. While the outer boundaries of the cluster are crystallographically random, all the inner boundaries have Σ 3n misorientations. A newly developed crystallographic theory of triple junctions and multicrystallite ensembles consisting of CSL boundaries is used to describe the structure of the cluster. The presence of an α ≠ 1 triple junction is confirmed. Apparently, the microstructure of recrystallized materials susceptible to annealing twinning consists of multiple-twinned clusters. The cluster size cannot be reduced to the “grain size excluding twins.”

Abstract: We have shown a connection among the three important properties of interfaces, namely, the free energy, diffusion and solute segregation through the conjecture that the interface free energy is the difference between those responsible for diffusion in the lattice and the interface itself. The interface energy is known to decrease upon solute additions. We discuss the methodology and the thermodynamical analysis of the diffusion parameters which enable extraction of the interfacial energies and illustrate them by results obtained in a wide variety of materials. Investigations carried out in pure polycrystalline metals have yielded grain boundary energies comparable to those directly measured. Furthermore, we discuss the role of solute segregation at grain boundaries in alloys in altering diffusion. From the perturbations caused, the solute segregation parameters—the enthalpy and the entropy of binding—have been extracted and levels of solute concentrations estimated. It is shown that similar analyses when applied to complex materials, e.g. the Pb–Sn eutectic alloy, several intermetallic compounds, and oxide systems, also result in acceptable values of interface energies and segregation factors. Finally, some ad-hoc guidelines are provided to alter diffusion in interfaces through solute additions in order to achieve some end use engineering objectives.

Abstract: This paper presents a topical overview of molecular-dynamics and Monte Carlo simulations for polymer systems close to solid interfaces. The simulations utilize simplified coarse-grained models: The polymers are represented by bead-spring chains, and the walls by a crystalline layer of Lennard-Jones particles or by a smooth impenetrable barrier. This approach has two advantages. First, it reduces the complexity of the simulation. Often, it is only then possible that the interesting length and time scales can be studied at all. Second, the approach concentrates on generic features that are believed to determine the physics of the problem under consideration. The results of the simulation can thus help to single out those features which should be incorporated in an analytical treatment. In this paper, we want to illustrate the versatility of these models by applying them to a broad spectrum of different problems. The situations considered range from the adsorption of a polymer from dilute solution onto a wall, over the importance of sub-monolayer monomeric or polymeric lubricants for kinetic friction, to the crystallization or glass transition of dense polymer films.

Abstract: Ag grain boundary (GB) diffusion was measured in the Cu-02at%Ag alloy in a wide temperature range from 473 to 970 K The direct measurements of Ag GB diffusivity D alloy gb under conditions of the Harrison C regime revealed that D alloy gb is almost identical to D pure gb determined earlier for Ag diffusion in high-purity Cu (Divinski, Lohmann, and Herzig, 2001) The penetration profiles determined in the Harrison B regime showed a complex, multi-stage shape This diffusion behavior can be rationalized assuming that besides GBs significantly covered by segregated Ag atoms, some fraction of GBs remains almost free from Ag atoms in the studied temperature interval The total amount of “pure” GBs drastically decreases with decreasing temperature This hypothesis was proven by measurements of Ag GB diffusion in Cu near Σ5 bicrystals, which allowed us to analyze in detail the non-linear segregation of Ag in Cu GBs

Abstract: Kinetic mechanism of premature grain boundary (GB) failure under simultaneous action of tensile stress and wetting liquid metals (LME) is not clear yet. We summarize briefly the stress intensity, surface energy and GB segregation effects in LME crack kinetics and the explanations that have been put forward. A possible mechanism of LME is outlined. It is proposed that GB cracks under LME propagate by Mullins grooving controlled by bulk diffusion in liquid and dramatically accelerated by periodic blunting of the groove tip by local plastic flow. It is found that this GALOP (Grooving Accelerated by Local Plasticity) mechanism captures the major experimental observations of stress intensity, surface energy and GB segregation effects in LME fairly well. A parallel between the GALOP mechanism and the GB subcritical crack growth under creep in inert atmosphere is mentioned.

Abstract: Pores and cavities form at filler particle-polymer matrix interfaces, at polymer film-silicon substrate interfaces as well as in molding compounds of IC packages. Moisture diffuses to these voids. During reflow soldering, surface mount plastic encapsulated devices are exposed to temperatures between 210 to 260°C. At these temperatures, the condensed moisture vaporizes. The rapidly expanding water vapor can create internal pressures within the voids that reach 3–6 MPa. These levels are comparable to the yield strengths of epoxy molding compounds and epoxy adhesives, whose glass transition temperatures T g range between 150 to 300°C. Under the combined action of thermal stress and high vapor pressure (relative to the yield strength at T g), both pre-existing and newly nucleated voids grow rapidly and coalesce. In extreme situations, vapor pressure alone could drive voids to grow and coalesce unstably causing film rupture, film-substrate interface delamination and cracking of the plastic package. Vapor pressure effects on void growth have been incorporated into Gurson's porous material model and a cohesive law. Crack growth resistance-curve calculations using these models show that high vapor pressure combined with high porosity bring about severe reduction in the fracture toughness. In some cases, high vapor pressure accelerates void growth and coalescence resulting in brittle-like interface delamination. Vapor pressure also contributes a strong tensile mode component to an otherwise shear dominated interface loading. An example of vapor pressure related IC package failure, known as “popcorn” cracking, is discussed.

Abstract: This article discusses grain boundary diffusion in ceramics. It gives a brief review of the experimental data available for ionic oxides and the problems of interpretation associated with it. The fundamental differences between grain boundary diffusion in metals and ceramics are noted. Calculations of the segregation of defects and impurities to grain boundaries are discussed together with methods of calculating diffusion coefficients in these boundaries. New results for alumina and chromia are presented. The problem of defining a grain boundary width is discussed with respect to new calculations on nickel oxide.

Abstract: The effect of the interplay between bulk and surface free energy terms on surface segregation in miscible blends is probed by comparing angle-dependent x-ray photoelectron spectroscopy (ADXPS) measurements for polystyrene/polyvinylmethylether (PS/PVME) blends of with those for perdeuteropolystyrene/polyvinylmethylether (dPS/PVME) blends. The magnitudes of the bulk interaction parameters for the two systems differ markedly while the surface interactions are essentially identical. Experimental concentration depth profiles are almost identical for the two systems indicating that their surface properties are little affected by bulk interactions and dominated by surface energy effects. These data and previous data from our group are compared to the predictions of the square gradient theory developed by Schmidt and Binder in order to gain a more quantitative understanding of the factors that control surface segregation in miscible blends. While there is general qualitative agreement between theory and experiment, predicted surface compositions fall significantly below experimental values and predicted composition depth profiles decay more gradually than what is observed experimentally, especially for low PVME contents. The use of the more appropriate Sanchez-Lacombe-Balazs equation of state does not yield any significant improvement over the use of the Flory-Huggins lattice model for representing the bulk free energy terms. Careful analysis of the experimental behavior suggests that configurational effects associated with the flattening of surface adsorbed chains and differences in mer-mer interaction parameters in the bulk and near surface regions are possible origins for the discrepancies between theory and experiment.

Abstract: Plasticity is a significant contributor to the interfacial fracture resistance of multilayer thin-film structures containing ductile layers. Salient parameters affecting plasticity contributions to interfacial fracture energy including the ductile layer thickness, yield strength, and the maximum cohesive stress governing interface separation, have been reported. However, the effects of residual stresses in the thin-film layers on such plasticity contributions have not been considered. We explore the effect of residual stresses on debonding with particular attention to the relationship between the stress state in both ductile and elastic layers and the resulting macroscopic debond energy. Using multiscale simulations it is shown that residual thin-film stresses can alter plasticity in the ductile layer and significantly influence the macroscopic fracture energy. A simple yield-based model to account for this behavior is proposed.

Abstract: Molecular dynamics simulations of high-energy twist and tilt bicrystals of fcc palladium reveal a universal, liquid-like, isotropic high-temperature diffusion mechanism, characterized by a rather low self-diffusion activation energy that is independent of the boundary type or misorientation Medium-energy grain boundaries exhibit the same behavior at the highest temperatures; however, at lower temperatures the diffusion mechanism becomes anisotropic, with a higher, misorientation-dependent activation energy Our simulations demonstrate that the lower activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures We demonstrate that the existence of such a transition has important consequences for diffusion creep in nanocrystalline fcc metals In particular, our simulations reveal that in the absence of grain growth, nanocrystalline microstructures containing only high-energy grain boundaries exhibit steady-state diffusion creep with a creep rate that agrees quantitatively with that given by the Coble-creep formula Remarkably, the activation energy for the high-temperature creep rate is the same as that characterizing the universal high-temperature diffusion in high-energy energy bicrystalline grain boundaries

Abstract: Interactions have been calculated between cracks induced in thermal barrier coatings (TBCs) upon thermal cycling, motivated by displacement instability in the thermally grown oxide (TGO). The results indicate that the energy release rate G cycles as the temperature changes, with the largest value arising at ambient temperature. It increases on a cycle-by-cycle basis. The trends in G predict two responses. (i) Isolated cracks should be confined to the vicinity of the imperfection, as observed experimentally. (ii) Cracks that converge from neighboring imperfections exhibit a minimum G prior to convergence, providing the possibility that they might coalesce. Equating this minimum to the toughness of the TBC provides a criterion for coalescence and failure. Imposing this criterion allows the change in crack length upon cycling and the number of cycles to failure to be ascertained. The results are consistent with experimental measurements obtained from furnace cycle tests.

Abstract: Mapping a phenomenological diffusion problem onto a lattice permits lattice-based random walk theory to address the problem The solution is generally carried out using Monte Carlo methods This paper reviews the substantial progress that has been made with this method for tracer diffusion associated with grain boundaries

Abstract: We couple a morphological study of an immiscible binary AB mixture with a micromechanical simulation to determine how the spatial distribution of the A and B domains and the interfacial region (interphase) affects the mechanical behavior of the blend. The morphological studies are conducted through a three-dimensional Cahn-Hilliard (CH) simulation. Through the CH calculations, we obtain the size and structure of the domains for different blend compositions. The output of the CH model serves as the input to the Lattice Spring Model (LSM), which consists of a three-dimensional network of springs. In particular, the location of the different phases is mapped onto the LSM lattice and the appropriate force constants are assigned to the LSM sites. A stress is applied to the LSM lattice and we calculate the elastic response of the material. We find that the local stress and strain fields are highly dependent on the morphology of the system. By integrating the morphological and mechanical models, we can isolate how modifications in the composition of the mixture affect the macroscopic behavior. Thus, we can establish how choices made in the components affect the ultimate performance of the material.

Abstract: The paper summarizes recent experiments on diffusion at migrating grain boundaries (GBs) occurring during discontinuous reactions, like discontinuous precipitation (DP) and diffusion induced grain boundary mi- gration. Analytical electron microscopy was used for measurements of the solute concentration across individual solute-depleted α lamellae. These data combined with information on the growth velocity and the thickness of an individual lamella allowed the determination of the local values of the diffusivities of the moving reaction front of the DP cell in Al-Zn, Ni-Sn, Cu-In and Co-Al alloys. The obtained diffusivities and activation energies are very similar to the relevant parameters of stationary GBs. This allows us to conclude that there is no significant difference in the rates of diffusion along migrating and stationary GBs in the systems investigated. It is therefore believed that the diffusivity values of the moving reaction front of the DP reaction can be a source of reliable information on interfacial diffusion characteristics, especially in systems and/or at temperatures where radiotracer data are not readily available.

Abstract: King [1] established that due to the discrete nature of their dislocation structure, finite length grain boundaries (GBs) in polycrystalline materials possess discrete values of misorientation angle. For a GB with a length that is not a multiple of the GB period, this leads to the formation of specific disclinations at their junctions with neighboring GBs, which compensate the difference between the misorientations of finite and infinite boundaries. In the present paper the origin of these compensating disclinations within GB triple junctions is elucidated and their strength is calculated using the disclination-structural unit model. It is shown that for a GB with length of about 10 nm the junction disclinations can have a strength value not more than 1°, in contrast to King's calculations that indicate much larger values. Elastic energies of triple junctions due to compensating disclinations are calculated for both equilibrium and non-equilibrium structures of a finite length GB, which differ by the position of the grain boundary dislocation network with respect to the junctions. The calculations show that triple junction energies are comparable to dislocation energies, and that compensating disclinations can play a significant role in the properties of nanocrystalline metals with grain sizes less than about 10 nm.

Abstract: The influence of intragranular slip on grain boundary sliding is studied in originally compatible zinc bicrystals with symmetric tilt boundary. The experiment is designed to separate different effects of intragranular slip on the boundary sliding and establish their mechanisms. Grain boundary sliding with and without development of intragranular slip is observed. The rate of sliding accompanied by slip is more than five times of that without slip. A good correlation between the boundary sliding and intragranular slip prior to slide hardening is established. Slide hardening followed by the negative sliding near one end of the boundary and strain hardening in the boundary vicinity, are observed at the last stages of deformation. For the case of formation of slip induced glissile grain boundary dislocations of opposite signs the possibility of their contribution to total grain boundary sliding, is analyzed. The effect of the increase in the rate of sliding is explained in terms of the accommodation of sliding by slip and appearance of additional glissile grain boundary dislocations of one sign due to strain incompatibility. Contribution of these different dislocation mechanisms to the increase in the sliding rate is determined for the stage of deformation preceding slide hardening. It is supposed that the effect of slide hardening and negative sliding as well as boundary curving is created by non-smooth boundary and small degree of incompatibility caused by straining.

Abstract: The statics and dynamics of vacancies and adatoms on different surface orientations in two hcp materials are studied by using static relaxation techniques and many-body potentials. Formation and migration energies and entropies as well as attempt frequencies are evaluated and used in the random walk approach to obtain correlation factors and diffusivities. It is found that the main features of surface diffusion are dominated by jumps on and between a few atomic layers, so that a consistent comparison between the two mechanisms is feasible. The activation energies and the diffusivities for different environments, namely, bulk Q b, D b, symmetric grain boundaries Q gb, D gb, and surfaces, Q s, D s, calculated using the same simulation technique and interatomic potentials, fulfil the expected relationships Q s < Q gb < Q b and D s > D gb > D b. It is also found that generally adatoms are faster surface diffusers than vacancies.

Abstract: P grain boundary segregation in an Fe-0.6wt%P alloy quenched from the melt was quantified by X-ray Mapping (XRM) in a Scanning Transmission Electron Microscope (STEM). The misorientation across the boundaries was determined by using ACT (Automatic Crystallography for TEM (Transmission Electron Microscopy)) and CBED (Convergent Beam Electron Diffraction). A significant range of the degree of P segregation to individual grain boundaries was found. Combination of chemical and structural studies provides evidence that P segregation to low-angle grain boundaries is reduced.

Abstract: Analyses are reviewed where plastic flow in the vicinity of an interfacial crack is represented in terms of the nucleation and glide of discrete dislocations. Attention is confined to cracks along a metal-ceramic interface, with the ceramic idealized as being rigid. Both monotonic and fatigue loading are considered. The main focus is on the stress and deformation fields near the crack tip predicted by discrete dislocation plasticity, in comparison with those obtained from conventional continuum plasticity theory. The role that discrete dislocation plasticity can play in interpreting interface fracture properties in the presence of plastic flow is discussed.

Abstract: Bulk and grain boundary (GB) diffusion of 14C in Nb has been studied by the radiotracer serial sectioning technique. B and C kinetic regimes were realized for GB diffusion in the temperature range from 800 to 1173 K. The values of P = sδDgb, Dgb and s follow the Arrhenius dependencies: P = 5.15 × 10−15 exp[−(83.1 kJ/mol)/RT] m3/s (973–1173 K), Dgb = 2.3 × 10−6 exp[−(133.0 kJ/mol)/RT] m2/s (800–950 K), and s = 4.7 exp[(49.9 kJ/mol)/RT].

Abstract: The structure of Σ = 3, {112} lateral twin boundaries in polycrystalline GaP has been investigated by transmission electron microscopy. The orientation of the polar bonding along the lateral twin boundary was characterized by convergent-beam electron diffraction and found to be mirror symmetric across the {112} interface. Rigid-body lattice translations and grain boundary dislocations along the boundaries were also characterized. The direction of the translation state between adjacent twin-related grains was studied by the α-fringe contrast technique. Models of the {112} interface in the GaP lattice are proposed and compared with the experimental observations.

Abstract: The role of defects introduced by both quenching and plastic deformation on the acceleration of surface segregation kinetics in the nickel-sulphur system is discussed. An estimation of the diffusion coefficient of sulphur in as-quenched single-crystal of nickel is given. Regarding cold-worked structures, the annihilation of the vacancies produces very quick segregation of sulphur atoms on the dislocation network at the beginning of the annealing treatment. However, the diffusion acceleration becomes truly significant when the pipes inside sub-grain boundaries resulting from the deformation are rearranged properly to form a percolated network. These results demonstrate that the micro-structural evolution of diffusion short circuits should be taken into account when describing complex diffusion kinetics.

Abstract: Micron thick silver films, vapour deposited onto high purity polycrystalline nickel substrates, dewet the substrate after high temperature annealing in oxygen rich atmospheres, while the films remain stable after annealing at the same temperature in a nitrogen atmosphere. Dewetting occurs when a nickel oxide layer is formed at the silver-nickel interface as a consequence of oxygen diffusion through the silver film. The sensitivity of the dewetting process on various parameters such as the annealing: temperature, time and oxygen partial pressure has been determined. Scanning Electron Microscopy (SEM) of cross-sections reveal that the main mechanism of dewetting at short annealing time is the nucleation of cavities at the Ag-NiO interface which grow towards the free surface of the Ag film. They are formed not only at Ag grain boundaries and triple junctions but also in the core of Ag grains. Such cavities do not occur when the Ag film is deposited onto a NiO single crystal. We propose a simple model for the cavitation: a vacancy supersaturation is sustained in Ag, at the Ag-NiO interface, as a result of oxygen consumption by the oxidation reaction. In regions of fast oxidation, the vacancy supersaturation is large enough to promote the nucleation and growth of interfacial cavities. The model qualitatively accounts for all the observed trends; quantitatively, on top of the vacancy supersaturation, extra-contributions to the driving force for cavitation must be invoked.

Abstract: Interfacial fracture mechanics is a relatively new field with many issues that have not yet been resolved. One such issue is the ability to accurately predict whether or not a bimaterial interface crack will propagate along the interface, kink into the film or substrate, or not propagate at all. In the present work, a crack driving force criterion is proposed in order to predict the level of applied load required to propagate a pre-existing interface crack in thin-film composites subjected to thermal loading. A primary objective is to predict the critical length of an interface crack at which it kinks into the substrate. The phenomenon of interface cracks kinking into the substrate is frequently observed when the film is under tensile loading and the substrate is brittle. An interface crack that advances into the substrate eventually propagates parallel to the interface at a certain steady-state depth. Ultimately, the portion of the structure that remains above the crack may spall, resulting in catastrophic failure. The crack driving force criterion is applied to a two-dimensional, plane strain interface crack problem, and the results compare favorably with available experimental results.

Abstract: In many experimental studies, curved penetration profiles are observed for grain boundary diffusion performed in the B kinetics regime in contrast to the shape expected from the solutions of the second Fick's equation. To explain these curvatures the effects of grain boundary structure, grain boundary migration, and grain boundary segregation have been successively proposed in the literature. Using previous data for Cu–Ag and Cu–Ni and new ones on Cu–Fe and Cu–Zn systems we will show how it is possible to separate all these possible contributions and how, knowing the true origin of the curvature, one can deduce much quantitative information impossible (or very difficult) to obtain by other techniques.

Abstract: In the processing of cross-ply fiber reinforced materials, residual stresses, as well as possible transverse cracking may arise. These affect the stress field about a delamination between two layers. In this investigation, the effect of residual stresses resulting from curing and transverse cracks is examined. A 0°/90°/0° ply system is considered with a delamination assumed between one of the 0° and 90° layers. The residual stresses along the interface without the delamination are calculated. First, this analysis is done neglecting the transverse cracks in the 90° layer. Then, the transverse cracks are included and several methods are employed to calculate the residual stresses. These include the shear lag method, a semi-analytic method and the finite element method. It is seen that the latter two methods produce similar results. By means of the superposition principle, the stress intensity factors resulting from the residual stresses are obtained for the delamination. Use is made of the conservative M-integral with tractions along the crack faces.

Abstract: For crack growth along an interface joining an elastic-plastic solid to an elastic substrate the effect of a non-singular stress component in the crack growth direction in the elastic-plastic solid is investigated. Conditions of small scale yielding are assumed, and due to the mismatch of elastic properties across the interface the corresponding oscillating stress singularity fields are applied as boundary conditions on the outer edge of the region analysed. The fracture process is represented in terms of a cohesive zone model. It is shown that the interface fracture toughness is significantly increased by a negative T-stress in the elastic-plastic solid, while a positive T-stress in the elastic-plastic solid leads to a reduced fracture toughness.