Grain-boundary segregation

Three techniques (FEGSTEM/EDX, Auger spectroscopy and APT) provide information on grain boundary chemistry. There are advantages and disadvantages of each as summarised in Table 9.3. Assumptions are involved in the interpretation of data from each of the techniques and there have been relatively few studies in which two or more of these techniques have been applied to the same material conditions. 

Ratio of height of peaks associated with Fe and P in differentiated Auger electron spectrum. 

Strictly, conversion of combined PFIRs for all elements, weighted by escape depths, etc. 

Some monolayer calculations do not incorporate contributions from elements other than P and Fe. 

Composition of region analysed when incident probe is centred on boundary image, and boundary plane is parallel to incident beam direction. 

Auger specimens 10 x 2 x 1 mm were heat-treated and then notched to control the location of fracture. Two of these notched samples at a time were loaded into specimen grips such that they could be broken independently under UHV and then examined by a SAM instrument. Scanning electron micrographs allowed different grain boundaries and transgranular cleavage areas to be selected for analysis. 

Suzuki et al. studied phosphorus segregation in iron by AES using a similar technique to that described above. Additionally, the orientation of each grain relative to the specimen axis was determined by means of selected area channelling patterns (SACP), and the crystallographic orientation of the boundary plane was 

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Application of AES to zirconia ceramics has been reported by Moser et al. [2.146]. Elemental maps of Al and Si demonstrate the grain boundary segregation of small impurities of silica and alumina in these ceramics. 

It is claimed that since S, Ca and A1 will be present as precipitates they would not in general be detected as grain boundary segregants and their ineffectiveness is indicated by the brackets in equation 8.10. Lea and Hondros do not consider the possible roles of carbon or nitrogen in the cracking of their steels, but from the data obtained phosphorous had the most deleterious effects. 

Figure 1.10. Grain boundary segregation in neutron irradiated weld from Russian WER 440 nuclear reactor position of individual atom is represented by a dot. Enrichment of P, Mo and Mn atoms are visible at boundary (arrowed), and three 2 nm diameter Cu-enriched precipitates near the boundary are also visible. (Reproduced by permission of Thomson and Miller 2000.)...

If the composition of the outermost layer is different from that of the bulk (for example, as a result of surface or grain boundary segregation), a different approach has to be made. For example, if a partial overlayer of element A of fractional coverage 9a covers a substrate of element B, the spectrum contains three contributions that from the overlayer, that from the exposed part of the substrate, and that from the covered part of the substrate. 

Depth Profiling. The quantitative interpretation of AES data requires a knowledge of the distribution of the grain boundary segregants both laterally on the... 

A careful, quantitative study on grain boundary segregation, boron segregation in austenitic stainless steels of types 316L (with 206 at. p.p.m. of boron, and with <5 at. p.p.m. of boron) and Mo-free 316L (75 at. p.p.m. boron), has been reported by Karlsson, Norden, and Andren36 using a combination of TEM, FIM, AP and IAP techniques. Concen-... 

Nakatani, K., Nagayama, H., Yoshida, H., Yamamoto, T., and Sakuma, T., The effect of grain boundary segregation on superplastic behavior in cation-doped 3Y-TZP , Scrlpta Mater., 2003, 49, 791-5. 

K. Przybyiski, A. J. Garratt-Reed, and G. J. Yurek, Grain-boundary Segregation of Yttrium in Chromia Scale, Journal of Electrochemical Section, 135,1988, p.509. 

Grain Boundary Segregation of Impurity in Polycrystalline Silicon... 

For the SoG-Si polycrystalline silicon, both the XGB and X are far less than unity. Furthermore, if the impurities follow Henry s law, e.g., (13.6), the effect of the excess Gibbs energy on the grain boundary segregation can be neglected. Equation (13.20) may, thus, be simplified to pcpc... 

A more sophisticated approach for determination of the grain boundary segregation is similar to the determination of the surface tension of silicon melt. The novel approach of surface tension simulation has been successfully implemented in the thermochemical database. Hence, the assessment of the parameters for impurity segregation in solid silicon phase may greatly extend the application of the thermochemical database. The calculation results for C and O segregation are shown as dashed lines in Fig. 13.28. The McLean segregation isotherm can be reproduced using the approach similar to the surface tension simulation. 

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