Selective segregation at grain boundaries

Abstract

Experimental observations provide support for two different types of solute enhancement at interfaces, namely equilibrium and non-equilibrium segregation. It is shown that these segregation processes are dependent upon the structures of different large-angle grain boundaries. Experimental data clearly indicate that this selective grain boundary segregation influences the mobility and energy of grain boundaries.

A fundamental relationship is found to exist between the energy of coincidence boundaries and the density of shared atom sites (or the size of the periodic unit) in the boundary. The energy and mobility results provide strong support for the boundary coincidence and “relaxed” coincidence models of grain boundary structure.

A selective solute segregation to grain boundaries can lead to the formation of preferred orientations in annealed materials and enhanced corrosion at grain boundaries.

Résumé

Les études expérimentales apportent des arguments aux deux types differents d'enrichissement en soluté aux interfaces, c'est-à-dire la ségrégation d'équilibre et la ségrégation de non-équilibre. Ces deux modes de ségrégation dépendent de la structure des différents joints de grains à forte désorientation. Les résultats expérimentaux indiquent clairement que cette ségrégation sélective aux joints de grains influence la mobilité et l'énergie des joints de grains.

Une relation fondamentale existe entre L'energie des joints en coincidence et la densité des sites atomiques partagés (ou la taille des unités périodiques) dans le joint. Les résultats relatifs à l'énergie et à la mobilité amènent de fortes preuves au modèle de joints de grains basé sur la coincidence ou la coincidence «relaxée».

Une ségrégation sélective du soluté aux joints de grains peut conduire à la formation d'orientations préférentielles dans les matériaux recuits et favoriser la corrosion aux joints de grains.     

Anisotropy of segregation at grain boundaries and surfaces

The purpose of this article is to review analytical models of the anisotropy of segregation to grain boundaries (GBs) and surfaces, and to evaluate their predictions. A summary of Gibbsian interfacial thermodynamics is provided as an introduction to the topic. This is followed by a historical overview of previous analytical models. A recently developed model of the dependence of GB segregation on the five macroscopic parameters of GB orientation is outlined, and illustration of how this formulation reduces to the particular cases of segregation to simpler types of interfaces is provided. In addition, some specific aspects of interfacial segregation, which have either been problematic or have lacked satisfactory explanation, are addressed. These include (a) the relationship between the compositions on the two sides of a given GB; (b) the difficulty of meaningful definitions of segregation-free energy (and related thermodynamic quantities such as enthalpy and entropy); (c) the so-called compensation temperature, at which the anisotropy of interfacial segregation seems to vanish; (d) the relationship between surface and GB segregation; and finally (e) an attempt to determine whether segregation increases or decreases interfacial energy anisotropy, and the consequences thereof on the equilibrium crystal shape of alloys. Where possible, comparisons are made with the results of experiments or computer simulations.     

The effects of hydrogen and segregation on fatigue crack nucleation at defined grain boundaries in nickel bicrystals

In order to explore the role of impurity segregation in intergranular fatigue crack initiation and propagation, tests have been made on nickel bicrystals, variously heat-treated to induce increasingly severe degrees of sulfur segregation, and using air, vacuum and hydrogen as the environment. The grains of the bicrystals were oriented to yield two types of boundaries: type I, with strong elastic-plastic incompatibility and type II, a compatible tilt boundary. If the boundaries were clean, persistent slip band cracking in the grains occurred in preference to intergranular cracking. Although equal degrees of sulfur segregation, as measured by Auger-spectroscopy, could be produced at the two types of boundaries by the heat treatments, the incompatible one was much more susceptible to intergranular cracking than the other, which could be made to crack intergranularly only by high partial pressures of hydrogen. The results show that high stresses associated with incompatibility coupled with a lowering of cohesive forces at the boundary produced by the segregant are the main factors controlling intergranular fracture.     

The influence of grain boundaries on mechanical properties

The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.     

SIMS study of hydrogen at the surface and grain boundaries of nickel bicrystals

The distribution of deuterium at the surface and at grain boundaries of cathodically charged nickel bicrystals was studied using Secondary Ion Mass Spectrometry. Both high energy and low energy boundaries were studied under conditions where deuteride formation occurred during charging. The formation of the deuteride along the grain boundaries was compared to that in the adjacent grains and interpreted in terms of the deuterium diffusion in the lattice and along the grain boundaries. Decomposition of the deuteride was also studied. The results are interpreted in terms of the dependence of diffusion along grain boundaries on the structure of the boundary.     

A simultaneous analysis of segregation and diffusion of impurities in the grain boundary of a bicrystal. Application to sulphur segregation and diffusion in nickel bicrystals

A new technique is described to study diffusion and segregation in a bicrystal; it is based on a radiochemical microanalysis through a narrow slit, slowly moving on the surface. It is used to study sulphur (³⁵S) behaviour in a Ni bicrystal at 700°C (〈100〉 tilt grain boundary; angular desorientation: 20°). The segregation coefficient (α = 7000), the bulk diffusion coefficient (D_b = 4.10⁻¹²cm² · s⁻¹) and the grain boundary diffusion parameter αδD_gb are obtained from one sole experiment in one sample.     

Surface and grain boundary segregation of deuterium in nickel

The segregation of deuterium to surfaces and grain boundaries in nickel has been studied using SIMS (secondary ion mass spectroscopy) techniques. Conditions for optimizing the detection of deuterium are discussed. Segregation of deuterium to external surfaces and grain boundaries was shown to occur at temperatures near 300 K The segregation of sulfur to the external surfaces was also studied and co-segregation effects for sulfur and deuterium were examined. In the presence of sulfur, segregation of deuterium to surfaces was shown to occur at temperatures as high as 845 K.     

Statistical and structural effects during solute-atom segregation to grain boundaries

Mean field theory of a rigid grain boundary segregation in binary mixture is developed on the basis of local equilibrium and geometrical considerations. The application of the Guggenheim adsorption isotherm equation to the problem of internal interfaces is proved to be valid for the case of regular substitutional solid solution. The segregation coefficient is found to be a function of a new interaction constant which yields the relative stability of an interface between a crystal made of pure solute and a crystal of pure solvent, with respect to the stability of grain boundaries in solvent and solute materials. Three typical structures are obtained when a local miscibility gap is formed and two other structures when the bulk tends to order. Segregation can stabilize new boundary structures which may involve changes in the intergranular translation, faceting, etc.     

Trapping of hydrogen and helium at grain boundaries in nickel: An atomistic study

Atomistic computer simulations of the trapping of hydrogen and helium at defect free grain boundaries in nickel are presented. Three symmetrical tilt boundaries that encompass a number of compact polyhedra of atoms are considered as regions of potential trapping sites. By employing the structural unit model, these boundaries are shown to be representative of a wide range of grain boundary structures. A general correspondence of trap locations in regions of expansion for both hydrogen and helium has been found; however, the binding energy for helium trapping is much greater than that for hydrogen. Consequently, clean grain boundaries in nickel appear to be important trapping sites for helium, but not significant sites for hydrogen binding. These results are consistent with experimental autoradiography, thermal desorption, and transmission electron microscopy observations. They imply that grain boundary trapping plays an important role in mechanisms of helium embrittlement, but not in hydrogen embrittlement of nickel.     

Effect of boron segregation at grain boundaries on heat-affected zone cracking in wrought INCONEL 718

Susceptibility to heat-affected zone (HAZ) cracking during electron-beam welding was studied in two INCONEL 718-based alloys doped with different levels of boron. By lowering the carbon, sulfur, and phosphorous concentrations to be “as low as possible,” the occurrence of HAZ cracking was related directly to the level of segregation of boron at grain boundaries, which occurred by nonequilibrium segregation during a preweld heat treatment. The study has demonstrated a direct correlation between the amount of boron segregated at grain boundaries and their susceptibility to HAZ cracking, in terms of the total crack length and number of cracks observed in the HAZ. The analysis of results suggests that both the melting and resolidification temperatures of the boron-segregated grain boundaries can be about 100 °C to 200 °C lower than those of the grain boundaries that were susceptible to constitutional liquation of Nb carbides on them, making boron more deleterious in causing HAZ cracking.     

Triple junctions of inclined and mixed grain boundaries in nickel

The structure of triple junctions of large-angle inclined boundaries 〈111〉 and 〈100〉 and mixed grain boundaries in nickel is investigated by a molecular-dynamic method. The equilibrium triple junction has no structural features that would identify it as a special defect against the background of the corresponding grain boundaries. The effective radius of the triple junctions and the width of the grain boundaries are determined.     

Plane coalescence at grain boundaries

The atomic structures of four symmetric tilt or twin boundaries in face centred cubic crystals have been investigated using an empirical inter-atomic potential representing copper. In each case the boundary energy corresponding to the relaxed structure and the associated volume increase have been determined. As anticipated the (111) boundary exhibits little relaxation but the predicted structures for the (113), (120) and (112) boundaries all involve translations away from the conventional mirror image orientation relation. In addition one pair of planes at the (120) boundary coalesce completely and four pairs of planes at the (112) boundary coalesce partially. The resulting structure in the latter case is broad and asymmetric. The (111) boundary has by far the lowest energy followed by (113), (112) and (012). Some aspects of the results, particularly the phenomenon of coalescence, the relative energies, and the breadth of the (112) boundary are very different from earlier computer simulation results on these interfaces, obtained using an aluminium potential. The reasons for this and the implications of the results are discussed.     

Embrittlement of copper due to segregation of oxygen to grain boundaries

The tensile and creep properties of oxygen free OF- and oxygen saturated OS-polycrystalline copper have been investigated in the temperature range 25 to 500 °C. Oxygen increases the yield strength by a factor of 2 or 3, by solid solution hardening, and for coarse-grained copper, causes a severe embrittlement, particularly under creep conditions. Both direct and indirect evidence indicate that the embrittlement is caused by the segregation of oxygen to the grain boundaries in copper, thus promoting grain boundary decohesion and intergranular fracture. Auger electron spectroscopy is used to indicate the presence of oxygen at the grain boundaries in OS-Cu. The embrittling effects of oxygen are reversible in the sense that both tensile and creep ductility are restored when oxygen is removed.