Anisotropy Of Grain Boundary Segregation Research Articles

The Significance of Entropy in Grain Boundary Segregation.

The role of entropy in materials science is demonstrated in this report in order to establish its importance for the example of solute segregation at the grain boundaries of bcc iron. We show that substantial differences in grain boundary chemistry arise if their composition is calculated with or without consideration of the entropic term. Another example which clearly documents the necessity of implementing the entropic term in materials science is the enthalpy-entropy compensation effect. Entropy also plays a decisive role in the anisotropy of grain boundary segregation and in interface characterization. The consequences of the ambiguous determination of grain boundary segregation on the prediction of materials behavior are also briefly discussed. All the mentioned examples prove the importance of entropy in the quantification of grain boundary segregation and consequently of other materials properties.

Potential barrier at grain boundaries in polycrystalline silicon: influence of grain-boundary character and copper/iron contamination

We used Kelvin probe force microscopy to measure the potential barrier associated with grain boundaries in B-doped p-type polycrystalline silicon. The potential barrier at non-contaminated grain boundaries varied approximately from 5 to 90 meV according to grain-boundary character. The random boundaries had a higher potential barrier than the coincidence boundaries. This barrier increased with increasing twist component of the grain boundary when the misorientation angle of two adjoining grains was almost the same. Copper and iron contamination caused a significant increase in the grain-boundary potential barrier, making the potential barrier depend more strongly on misorientation, probably because of anisotropy of grain-boundary segregation.

Thermodynamics of Grain Boundary Segregation and Applications to Anisotropy, Compensation Effect and Prediction

Interfacial segregation has long been studied because of its impact on various properties of materials. Usually, theoretical descriptions are based on thermodynamic terms such as segregation enthalpy and entropy. Comparisons of published papers of different authors reveal fundamental differences in our understanding and application of these thermodynamic characteristics. To clarify this situation, the report reviews several types of segregation isotherms and their derivations. Individual sets of thermodynamic state functions appearing in segregation isotherms—Gibbs energy, enthalpy and entropy—are interpreted in detail. We identify the sources of controversial interpretations and the physical meaning of particular sets of thermodynamic state functions. It is shown that for a correct interpretation an unambiguous and systematic use of the term excess in thermodynamic terminology is necessary, as well as a correct evaluation scheme for data obtained with Auger Electron Sepctroscopy (AES). Successful applications of segregation isotherms based on standard thermodynamic state functions of segregation are demonstrated for three examples: anisotropy of grain boundary segregation and its consequences for the classification of grain boundaries, compensation effect applied to grain boundary segregation, and quantitative prediction of grain boundary segregation (including anisotropy).

Some aspects of the anisotropy of grain boundary segregation and wetting

A recently developed model of grain boundary (GB) segregation, in terms of the five macroscopic parameters of GB orientation, has been exercised to explore the anisotropy of GB segregation. The five macroscopic GB orientation parameters are defined by means of the orientations of the two crystallographic planes that terminate the crystals on either side of the GB, and a twist angle. Some important conclusions include the following: (a) the composition of a boundary depends on all five parameters of GB orientation, (b) the segregation profile across a GB depends on the two planes which terminate the adjacent crystals, (c) the composition profile across GB’s terminated by identical crystallographic planes is symmetric, but is asymmetric when GB’s are terminated by different planes, and (d) the strength of the segregation on one side of a GB influences the extent of segregation on the other. Some experimental results on Nb-doped TiO2 are presented in order to verify above predicted trends. In addition, it is shown that the model predicts the possibility of anisotropic GB wetting transitions as two-phase coexistence is approached.

On the relationship between entropy and enthalpy of grain boundary segregation

An extensive study of anisotropy of grain boundary segregation in α-iron, performed in the last decade, confirmed the existence of a linear relationship between entropy and enthalpy of solute segregation at individual grain boundaries. A thermodynamic analysis of this relationship is performed in the present work. It is shown that this dependence defines a “compensation temperature” and an entropy-like parameter. The compensation temperature is not necessarily connected with a phase transformation, but it represents exclusively a mean temperature at which all boundaries approach to a mean value of the Gibbs free energy of segregation. The most important consequences of this dependence are outlined: a reversed anisotropy of grain boundary segregation at temperatures above the compensation temperature, and a new method for the prediction of the enthalpy and entropy of solute segregation at individual grain boundaries.

Metallurgical applications of AES: anisotropy of grain boundary segregation

Recent metallurgical application of methods of surface analysis on investigation of internal interfaces are discussed. The fundamental requirements necessary for such studies (fracture possibility, lateral and depth resolution, detection limit and possibility of depth profiling) are listed and compared to characteristics of Auger electron spectroscopy. A way of transformation of AES spectra of fracture surfaces to the chemical composition of grain boundaries is suggested. The adaptability of AES to solve current metallurgical problems is documented by studies of the temperature dependence of grain boundary composition in NiAl base intermetallics and of anisotropy of grain boundary segregation in Fe–Si alloy bicrystals. Copyright © 2000 John Wiley & Sons, Ltd.

Metallurgical applications of AES: anisotropy of grain boundary segregation

Recent metallurgical application of methods of surface analysis on investigation of internal interfaces are discussed. The fundamental requirements necessary for such studies (fracture possibility, lateral and depth resolution, detection limit and possibility of depth profiling) are listed and compared to characteristics of Auger electron spectroscopy. A way of transformation of AES spectra of fracture surfaces to the chemical composition of grain boundaries is suggested. The adaptability of AES to solve current metallurgical problems is documented by studies of the temperature dependence of grain boundary composition in NiAl base intermetallics and of anisotropy of grain boundary segregation in Fe-Si alloy bicrystals.

Thermodynamic aspects of anisotropy of grain boundary segregation

La dependance de l'entropie envers l'enthalpie en fonction des donnes thermodynamiques de segregation de Si, P et C sur les joints de grains differents dans α-Fe a ete presentee. La linearite de cette dependance a ete demontre pour chaque segregant. L'analyse de cette relation principale ainsi que de la capacite differente de Si, et de P et C former des alliages ordonnes permet d'expliquer: (i) les valuers positives de l'entropie de segregation de P et C et les valeurs negatives de Si, (ii) les valeurs elevees de l'entropie de segregation dans le cas de joints de grains speciaux en comparison des joints de grains generaux, (iii) l'anisotropie inverse de segregation des joints de grains pour les temperatures elevees.

Anisotropy of grain boundary segregation in Σ = 5 bicrystals of α-iron

Based on AES measurements of temperature dependencies of P, Si and C segregation at the Σ = 5, 36.9° [100] tilt grain boundaries of well-defined bicrystals of an Fe-3.5at%Si alloy, the anisotropy of grain boundary segregation was determined. It was found that not only the two {013} and {012} symmetrical grain boundaries exhibit special behavior but also the (001)/(03 4 ) and (0 1 7)/(01 1 ) asymmetrical grain boundaries. All these boundaries are characterized by high values of the effective interplanar spacing. The measured data allow the construction of the first experimental grain boundary segregation diagram in terms of segregation enthalpy, solid solubility and boundary orientation.

Anisotropy of grain boundary segregation in as-grown Fe-6at%Si alloy bicrystals

The chemical composition of several grain boundaries in well-defined bicrystals of an Fe-6at%Si alloy containing phosphorus was studied by means of scanning Auger microscopy on in-situ fractured samples. The observed segregation of Si and P shows a strong dependence on the grain boundary type. Whereas P. segregates to all grain boundaries, Si segregation is only observed at Σ3 grain boundaries. In the case of both symmetrical and asymmetrical Σ5 grain boundaries a depletion of Si is detected. Some quantitative differences also exist in the segregation at individual grain boundaries corresponding to the same orientational relationship 37 ° [100]. The anisotropy of the segregation behaviour is explained on the basis of both the different atomic structures of individual grain boundaries and the mutual interaction between P and Si atoms. The main topographical features observed at the fracture surfaces are also classified and their effect on the measured segregation data is demonstrated.

On the anisotropy of grain boundary segregation in multicomponent systems

On the anisotropy of grain boundary segregation in multicomponent systems