Pavel Lejček

Kinetics of sulfur surface segregation in Fe-6at%Si

Abstract The kinetics of Si and S segregation to the (100) surface in Fe-6at%Si single crystals has been measured by Auger electron spectroscopy for the temperature range from 770 to 900 K. A rather unexpected behaviour for sulfur was found which was not controlled by bulk diffusion. Sulfur segregation kinetics can be explained by taking into account both sulfide formation as well as pipe diffusion. At higher sulfur surface coverage the reaction rate drops. The latter may be a consequence of a surface phase transition. Moreover, the model is extended to such cases where the history of the sample should also be considered. Thus, the experiments of the segregation sequence can be described.

On the determination of surface composition in auger electron spectroscopy

Abstract The methods of determination of surface composition from AES measurements are briefly reviewed. The derivation of an expression is shown based on the “ multilayer segregation ” model. This expression is compared with similar expressions on the basis of various assumed concentration depth profiles of an Fe-Si-P system. The use of AES data obtained during the initial stages of ion sputtering in the determination of surface composition is discussed. The expression is applied to recently reported data measured on the grain boundary fracture surfaces of Fe-Si alloy bicrystals containing traces of P and N.

On the fracture path and the intergranular phosphorus distribution in phosphorus-doped FeSi symmetrical bicrystals

Abstract A high-resolution scanning Auger micropobe analysis was carried out on matching fracture surfaces of symmetrical Σ = 5 (013) bicrystals of Fe-3 wt%Si doped with 270 ppm phosphorus which were grown from seeds by floating-zone melting. The phosphorus distribution on some regions of a fracture surface was found to alternate between high and low concentrations; these alternations were often associated with deformation bands, such as mechanical twins. The reverse pattern was found on the matching region on the opposite fracture surface. This confirms a hypothesis made earlier that the inhomogeneous phosphorus concentrations found on fracture surfaces are the result of off-center crack propagation through a laterally extended phosphorus distribution along the grain boundary and the switching of the crack path from one side to the other upon an encounter with a grain-boundary step caused by a deformation band. In the regions of unequal division of the P-enriched region by the fracture, a 2:1 ratio of phosphorus concentrations was found. The total width of the region was deduced to be about five atomic planes from observations of sputtering rates and an analysis based on simple logic.

Solute segregation to grain boundaries and free surfaces in an FeSi multicomponent alloy

Abstract Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si x N y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmanns theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmanns theory.

Orientation relationships in the secondary recrystallization of pure nickel

Abstract The preffered orientation relationship 19°, 〈100〉 was confirmed between the primary recrystallization texture and the large secondary grains grown during the unidirectional annealing of heavily deformed nickel. It was found that the rate of specimen movement affects the operational mode of the particular rotation axis. The twist rotaion about [dash100] corresponds to higher rates of specimen movement compared with the tilt rotation about the [001] axis which is effective at lower rates. The change in rotation axis might be explained by a higher value of the activation energy of migration for the twist-type grain boundary than for the tilt boundary.

Anisotropy of interfacial segregation: grain boundaries and free surfaces

Solute segregation was measured at both the {013} grain boundary and the (013) free surface of the same sample of an Fe-6at%Si alloy containing traces of P and S at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation whereas no sulfur was detected there. On the other hand, pronounced sulfur segregation was observed at the surface accompanied by weak enrichments of phosphorus and silicon. Qualitative differences between both types of segregation behavior are considered as a generalized anisotropy of interfacial segregation.

Crystal growth in thin nickel strips

Abstract The technique of single growth in thin strips of pure nickel based on unidirectional secondary recrystallization has been developed. The nickel single crystals with dimensions of 0.2 x 10 x 50 mm 3 have been prepared. The effects of purity, degree of deformation, rate of specimen movement and temperature gradient on the growth of single crystals have been investigated. The preferred orientations of single crystals have been determined and discussed. The dislocation density in as-grown crystals has been found to be less than 10 4 cm -2 .

A High-Resolution Aes Study of the Non-Uniform Distribution of Phosphorus on the Fractured Grain Boundaries of Doped Fe-Si Bicrystals

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 10 2 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.