The technique of constant stress molecular dynamics (MD) has been used to study NaCI systems containing ⅀-5 [001] twist boundaries interacting with the rigid-ion model of Tosi and Fumi. While perfect coincidence ⅀-5 [001] twist boundaries were found to dissociate into free surfaces below 0. 125Tm, ⅀-5 [001] anticoincidence twist boundaries with Schottky defects on the boundaries were stable up to the bulk melting temperature. No grain boundary melting was observed below the bulk melting temperature. Grain boundary thermodynamic properties, including the excess entropy and excess free energy of stable boundaries with 464 and 944 ions were calculated by molecular dynamics from low temperatures to bulk melting.

Atomic structure of [001] tilt boundaries in an ordered A3 B compound with Ll2 structure has been studied using model pair-potentials Ko describe atomic interactions. The structural features related to the ordering tendency as well as structural changes associated with deviations away from stoichiometry have been investigated. The differences between boundary structures in pure metals and compounds have been found to be most pronounced when the stoichiometry is preserved at the boundary. Possible significance of these structural aspects for understanding very different fracture behavior of pure f.c.c. metals and L12 compounds is discussed.

In recent years, the influence of the establishment of long-range order in cubic alloys on the structure of grain boundaries in L12 alloys has been considered. Thus, for example, for the ⅀=5 (310) tilt boundary the various possible structures have been investigated that are generated upon ordering, starting from plausible structures in the disordered state. However, apart from some rough energy estimates based upon nearest neighbour interactions, no reliable energy calculations have been performed of these different possible structures. In this paper, computer calculations based upon interatomic pair potentials constructed in such a way that the L12 structure is stable with respect to disordering, are reported for the ⅀=5 (310) boundary. The relative stability of various possible structures, with associated different boundary compositions, has been investigated.

Phase transitions which are possible in internal interfaces are grouped into several classes. Simple examples of these transitions, corresponding to coherent interphase boundaries and antiphase boundaries, are given for each class. Lattice models may be used to study these transitions. Interfacial phase behavior closely connected with bulk phase transitions is also discussed. Implications for extended faults bounded by partial dislocations are suggested.

The equilibrium structure of a 2-dimensional ⅀5 symmetric coincidence site grain boundary has been analyzed theoretically as a function of temperature and composition using a lattice-gas model. Over a wide range of concentration, ranging from a pure single component to dilute impurities to concentrated solutions that undergo ordering, the boundary “melts” before bulk melting occurs. The entire temperature range can be described by two limiting descriptions and a smooth crossover between them: a low temperature one that with increasing temperature deviates from the ground state by increasing desorption with little structural change, and a high temperature behavior that culminates in an ever thickening layer that becomes liquid as the bulk melting point is approached. A slight change in the assumed interaction alters the location of the impurity atoms. The low temperature desorption behavior in general is a sum of Boltzmann factors; adsorption tends to decrease but, because these factors can have opposite signs, not always as a monotonic function of temperature. In the high temperature region, adsorption follows solid-liquid segregation behavior and increases with rising temperature as the grain boundary thickens.

ɣ(Ni)/ɣ'(Ni3Al) diffusion bonded specimens are prepared under experimental conditions whose effect is confirmed by modelling the bonding process. The migration of the interphase boundary and the chemical concentration profiles are described and analysed. Some T.E.M. observations of these synthetic phase boundaries and their structural features are also reported.

The discontinuous grain boundary (GB) precipitation behavior of rapidly solidified Fe-31.3wt. % Mn-8.7wt.%A1-2.0wt.%C on aging at 923K has been investigated by TEM and X-ray diffraction. The parent matrix was already composed of fcc ɣ phase and perovskite K' carbide of L'12 crystal structure even before the aging treatment. On aging, the matrix precipitation of K' phase out of the -ɣ-phase matrix progressed further, but eventually came to a standstill before the initiation of the DP reaction in the GB. Later on, cells or colonies made up of alternate lamellae of carbon depleted ɣ and carbon enriched K phases of L'12 structure continued to grow. Between the product ɣ and K phases in a cell, near perfect {111} interfaces were found.

The current understanding of mass transport at interfaces in solids is reviewed. The materials covered are metals, semiconductors and ionic compounds and the interfaces are mainly grain boundaries.

In metals and semiconductors grain boundary diffusion is always faster than the bulk and both experiments and theory support the concept of narrow (about 1 nm) pathways in which fast diffusion occurs by a point defect mechanism. In ionic compounds, however, experiments have indicated that in some materials grain boundary diffusion is faster in the bulk whereas in others it is slower. Furthermore, different workers have reached different conclusions in, ostensibly, the same materials. The possible reasons for this lack of convergence are discussed.

In the author's opinion no satisfactory link between interface structure and mass transport has yet been established in any material.

Anion diffusivities in the lattice and along grain boundaries have been measured on sintered polycrystals of Cr2 O3 and Cr2O3-0.09 weight percent Y2 O3at 1100°C and at the oxygen partial pressure corresponding to that of the Cr/Cr2O3 equilibrium at thattemperature. Results show that under the same conditions, anion diffusion is significantly faster than cation diffusion. The results are discussed in terms of their implications for the reactive element effect on high temperature oxidation of chromia forming alloys.

Kinetics of grain boundary migration have been investigated in prestrained monocrystalline specimens of aluminum in situ at temperatures ranging from 415 to 610° C by synchrotron (polychromatic) x-ray topography (SXRT). In general, new (recrystallized) grains, which nucleate at prepositioned surface indentations and subsequently grow into the prestrained matrix, transform progressively from random to faceted configurations. Analysis of corresponding migration rates as a function of temperature for several faceted boundaries yields activation energies ranging from about 56 to 105 kcal/mole, in contrast to previously reported values, obtained by repetitive cooling and reheating cycles, ranging from about 15 to 35 kcal/mole. This disparity in measured activation energy is attributed to variation of grain boundary mobility with grain boundary inclination, and ultimate survival of low-mobility (faceted) grain boundary inclinations, as a natural consequence of growth selection.

An instability at solid-liquid interfaces induced by coherency strain has been studied in the Mo-Ni system. When the lattice parameter of a solid solution varies with compositions, a coherency strain develops if there is diffusion in the solid adjacent to the liquid. As a result of this strain, the originally planar solid-liquid interface becomes unstable by dissolution of the initial solid and precipitation of the new solid, forming a sinusoidal interface shape. In a previous study, the coherency strains have been shown to be the driving force for the instability. In this paper we analyze the evolution of the instability with time, from initiation to healing of the instability. Using the equations developed for liquid film migration (LFM) the change of microstructure is explained qualitatively. In addition, the relationships between this instability, diffusion induced grain boundary migration (DIGM) and LFM are discussed.

In a TEM study of diffusion induced grain boundary migration (DIGM) we have found that it is very common for lattice defects to be ejected into the grains adjoining a migrating boundary. The defects may be point defects or dislocations, or both. Dislocation ejection occurs both ahead of and behind moving boundaries, but vacancy deposition appears to occur only behind a moving boundary.

High temperature anelastic relaxation measurements have been made on Ni-20 wt% Cr alloys containing 0 to 180 at.ppm Ce. Anelastic relaxation mechanisms for grain boundary sliding are presented and observed high temperature relaxation peaks are discussed. A large hysteresis in the high temperature background between heating and cooling cycles is observed, which is attributed to a grain boundary phase transformation.

The interdiffusion between Au/Ni/Cr multilayer metallization and silicon substrate was studied by using the measurement of sheet resistance and AES profiling technique. Interdiffusion was found to increase with temperature and resulted in the increase of sheet resistance. The effect of Cr and Ni as diffusion barrier was also verified.

In bicrystals with CSL orientation relationships two kinds of coincidence site lattices form, resulting in two kinds of grain boundaries. We examine the thermodynamic properties, using a lattice gas model, and make microstructural predictions about the dihedral angles at junctions between these boundaries. The behavior of these boundaries is examined for low temperatures and through the critical disordering temperature. Some aspects of nonstoichiometry are presented. A wetting transition is found at low temperatures, in which one kind of grain boundary is converted into the other by expelling antiphase domain boundary. No wetting transition is found at the critical temperature because an unusually high critical exponent of 3 is found for the difference in grain boundary energies between the symmetric and antisymmetric cases.

In many commercial iron and nickel base alloys used in structural applications, complicated grain boundary chemistry changes can occur during the heat treatments that these materials experience during fabrication or service. These chemistry changes can be associated with the segregation of solute or impurity elements to the boundaries, or with solid state precipitation processes occurring at the boundaries. Auger electron spectroscopy and analytical electron microscopy are useful tools for the quantitative examination of these phenomena. In this paper, experimental studies of the thermodynamics and kinetics of segregation and precipitation processes at grain boundaries in Fe and Ni base alloys are reviewed. Site competition between segregants, and changes in the type of grain boundary precipitation, as a function of time and temperature of annealing are described. The interaction between segregants and precipitates at grain boundaries is considered. These results are related to changes in the macroscopic properties of the material, including corrosion resistance, ductility, and strength.

Thermodynamic equations are developed to express segregation in multicomponent alloys. These equations use concepts of excess partial free energies and activity coefficients and can be used in a wide variety of situations, including those where the segregating elements, such as sulfur, form strong chemical compounds in the bulk. A lower surface tension of the segregating element in relation to that of the alloy promotes its segregation. The segregation factor is also directly related to the activity/activity coefficient of the segregating element. The validity of our equations is checked by comparing the results of calculated segregation factors with the published values on sulfur segregation in Ni-S, Cr-S, Al-S, and Y-S alloys and on lead segregation in Pb-In and Pb-Sn alloys.

Symmetrical {013}, ⅀=5, bicrystals of Fe-3wt%Si doped with 270 ppm phosphorus, grown from seeds by floating-zone melting, were fractured in UHV, and the fracture surfaces were analyzed in a high-resolution scanning Auger microprobe. The phosphorus distribution was nonuniform over areas on the scale of 102 µm. The non-uniformities were sometimes fund to be associated with deformation bands, mainly twins, which formed in advance of the brittle crack as the specimens fractured. We have rationalized these observations in terms of fracture not along the central core of an extended phosphorus distribution at the grain boundary, but rather toward one side of the distribution. The deflection of the crack path at steps in the boundary would then produce the observed patterns of phosphorus distribution.

It is well established that the grain boundary strength (and thus the ductility) of polycrystalline Ni3Al is a function of the level of boron segregated to su h boundaries. This paper presents some Auger Electron Spectroscopy (AES) results obtained as part of an extensive investigation into the details of this segregation behavior. Specifically, the level of segregated boron was obtained in three substoichiometric alloys containing 0.048, 0.24 and 0.48 at.% (100, 500 and 1000 wppm, respectively) boron in the bulk. This level was correlated with the accruing fracture morphology for samples subjected to widely different thermal histories. Using McLean's approach, these segregant levels were used to arrive at the effective binding energy of boron to such boundaries. The observed trend of decreasing effective binding energy with increasing bulk boron level is more consistent with a “spectrum of binding energies” model of grain boundary segregation than with the single binding energy model of McLean.

A two dimensional lattice gas model of a binary ordering alloy system was used to study the segregation and the atomic configuration near a ⅀ = 5 grain boundary. The cluster variation calculation was performed to study the segregation of alloying elements in the grain boundary region. The present model allows the study of different ordering configurations in the grain boundary region as well as the segregation effect in the vicinity of the grain boundary. The results show that there will be segregation effects and that they can be significant. It is essential that the segregation and disordering effects be considered when high temperature properties of grain boundaries in ordered alloys are studied. The type of segregation observed depends on energetic assumptions used in the calculation and on the rigid body translational state of the boundary.