Influence Of Solute Segregation Research Articles

The Harrison diffusion kinetics regimes in solute grain boundary diffusion

Knowledge of the limits of the principal Harrison kinetics regimes (Types A, B and C) for grain boundary diffusion is very important for the correct analysis of depth profiles in a tracer diffusion experiment. These regimes for self‐diffusion have been extensively studied in the past by making use of the phenomenological lattice Monte Carlo (LMC) method with the result that the limits are now well established. However, the relationship of these self‐diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregation on the limits was investigated with the LMC method for the well‐known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type‐B kinetics regime may differ from the global segregation factor.

Influence of solute segregation and drag on properties of migrating interfaces

Recent models on solute segregation and drag search for steady-state solutions of the diffusion equation in the region of a migrating interface and adjacent semi-infinite grains. Such solutions are limited to model massive transformations, for which the chemical composition of the parent grains and that of the product grains are the same. Simulation of diffusive transformations in transient systems with grains of different chemical composition is a hot issue. In the present model the steady-state solution of the diffusion equation is investigated only in the interface. The coupling with the diffusion equation in the adjacent grains is ensured by proper boundary conditions. For simulation of transient diffusive phase transformations it is very convenient if the interface can be treated as a sharp interface with a known effective mobility and prescribed boundary conditions at the interface. In this paper it is shown that solute segregation and drag can be taken into consideration by the effective mobility of a sharp interface and by proper boundary conditions at the sharp interface. The effective interface mobility reflects Gibbs energy dissipation due to rearrangement of solvent atoms and to trans-interface diffusion and drag of solute atoms in the migrating interface. Trans-interface diffusion and drag of solute atoms are also reflected by the jump of the chemical potential of the solute across the interface, which represents a boundary condition for coupling with the diffusion equation in the adjacent grains. Finally, it is shown that the incorporation of the effects of trans-interface diffusion and drag of solute atoms does not make the simulation more complicated provided that some necessary calculations are performed in preprocessing.

The influence of solute segregation on void swelling in Fe-Cr based alloys

Electron irradiations were performed on austenitic and ferritic low activation model alloys in the range 623 to 723 K. The microstructural results of these irradiations are consistent with similar studies conducted with fast neutrons. Radiation-induced elemental segregation is shown to shown to occur in both austenitic and ferritic alloys, and signals the potential for both radiation-induced phase instabilities and segregation-affected swelling.

High resolution detection of solute segregation to the cores of dislocations in grain boundaries

The segregation of solute atoms to grain boundaries in polycrystalline solids can be responsible for embrittlement of the boundaries. Auger electron spectroscopy and analytical electron microscopy (AEM) have verified the occurrence of solute segregation to grain boundaries, however, there has been little experimental evidence concerning the distribution of the solute within the plane of the interface. Au segregation causes a change in the primary dislocation structure of small angle (001) twist boundaries in Fe. (001) twist boundaries have been produced in bicrystals of both pure Fe, and Fe containing a small amount of Au. The boundary structure characteristic of pure Fe, which is square dislocation network, changes to two different square dislocation networks, and -type, when 0.17 at% Au is added to the bicrystal and Au segregates to the boundary at its midplane. Rutherford backscattering spectroscopy was used to demonstrate that there was Au segregation to the interface, and analytical electron microscopy showed that the network was Au-rich relative to the network. The present study has clearly established that Au segregates extensively to the a dislocation cores, it remains to understand why the presence of Au has a stronger influence on the core structure and energymore » of the a rather than the a/2 dislocation, leading to the stabilization of the a dislocation network. Theoretical calculations are needed to examine the influence of solute segregation on the core energy of dislocations in Fe and Fe alloys. Work is in progress to obtain quantitative information on the amount of Au at the cores.« less

The influence of solute segregation on the dislocation structure of small and large angle [001] twist boundaries in Fe-Sb alloys

In alloy systems where there is limited solubility in the solid solution solute atoms segregate to grain boundaries giving local compositions at the interface typically at the monolayer level. The practical consequence of such segregation in Fe-base alloys is frequently the embrittlement of the grain boundary region, making it susceptible to fracture at relatively low stresses. It is interesting to note that in polycrystalline Ni/sub 3/Al, which is intrinsically brittle, B segregation to grain boundaries and changes in stoichiometry, make the material ductile. It is of considerable importance to learn how the presence of solute changes the fracture resistance of grain boundaries. Briant and Messmer suggested that segregation causes a change in the nature of the electronic bonding at the interface, which is believed to influence its cohesive strength. Hart suggested that segregation causes a change in the structure of the boundary and in this manner controls its resistance to fracture. At present there is no experimental evidence concerning the bonding at grain boundaries in Fe-Te, and in Cu-Bi. Others have reported a characteristic grain boundary morphology in the presence of solute segregation, in the form of faceting. While both sets of observations lead to the suggestion that faceting resultsmore » from solute segregation, neither study examined the structure of the same boundary in both the clean and segregated states.« less

Grain boundary structural transformations induced by solute segregation

The influence of solute segregation on the structure of small angle [001] twist boundaries was studied in Fe-rich Fe-Au alloys which were in the form of thin bicrystal specimens. It was shown that Au segregation at the boundary causes a change from the single dislocation array seen in pure Fe to two dislocation arrays with different Burgers vectors. In the Au-poor regions of the boundary, the dislocation structure is a 〈110〉 square network with Burgers vector, b = a 2 〈111〉 , similar to that seen in pure Fe, while in the Au-rich regions of the boundary, the dislocation structure is a 〈100〉 square network with b = a〈100〉 . Analytical electron microscopy and Rutherford backscattering spectroscopy were used to obtain information on the distribution of the Au in the vicinity of the boundary. The observation of a change in dislocation structure is considered evidence for the occurrence of a structural transformation induced by solute segregation. The influence on the structural transformation of variables such as composition and boundary misorientation angle was studied, and the results are presented in the form of a grain boundary phase diagram, with each dislocation network considered as a two dimensionally periodic phase with a small extent along the direction normal to the interface.

Diffraction and microscopy studies of the atomic structure of grain boundaries

The structure of grain boundaries and the influence of solute segregation on boundary structure were studied using diffraction and microscopy techniques. The determination of the projected atomic structure of a large angle [001] twist boundary by use of X-ray diffraction techniques was demonstrated. Using an electron diffraction technique it was shown that the local change in plane spacing at a large angle [001] twist boundary was dependent on the type of bond (ionic, metallic, covalent) in the material. It was shown that solute segregation causes a major change in the dislocation content of a small angle [001] twist boundary in Fe containing a small amount of Au. These observations are evidence for the occurrence of a two-dimensional phase transformation in the grain boundary. As a result of these studies, generalizations were made concerning the structure of large-angle twist boundaries and the influence of bond type on boundary structure.

Influence of solute Segregation on grain-boundary energy and self-diffusion

The difference in the free energy of activation for the host atoms diffusing in the lattice and in the grain-boundary has been found to agree with the absolute grain-boundary energy within a few percent in the fcc metals and alloys where such data are available. The lowering of the grain-boundary energies in the dilute alloys from their values in pure hosts were compatible with the Gibbs adsorption isotherm applied to grain-boundaries and lead to the determination of the solute enrichment factors in a quantitative manner. The solute-grain-boundary enrichment factors thus computed from the solvent self-diffusion data in several alloys compare well with those determined by direct techniques notably the Auger Electron Spectroscopy by other investigators. The energy and entropy for the grainboundary-solute interaction have also been determined and are discussed.