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Highlight Lecture

Effect of solute segregation on the stability and evolution of microstructures

Thursday (27.09.2018)
16:45 - 17:00 S1/03 - 223
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If the microstructure is defined in a more general way as the density and distribution of discontinuities, it coincides partly with its generally accepted definition including dislocation densities and dislocation structures as well as grain boundary densities (grain size) and densities of related one- or two-dimensional defects. The advantage of the general definition is first getting rid of the negative term “defect” and second allowing a very general treatment of the solute/discontinuity-interaction including point defects as vacancies or surfaces in pores. Thus the interaction of mobile solutes with a stationary microstructure can be treated in Statistical Mechanics independent of the nature of solutes and solvents [1]. More important, the formation energy of discontinuities and its decrease by solute segregation can be treated in Thermodynamics in the same general way leading to a generalized Gibbs Adsorption Isotherm [2]. Thus the lowering of the surface energy of water by surfactants is based on the same principle as the lowering of the dislocation formation energy by hydrogen segregation [3]. Besides the latter example it will be shown that (i) the vacancy concentration in pearlitic steels is increased by interstitial carbon [4], (ii) dislocation densities will be much higher, if deformation occurs in the presence of hydrogen [5] and (iii) grain growth is inhibited in both metals [2] and ceramics [6] in the presence of segregating solute. In all cases the formation energy of discontinuities is decreased by solute segregation leading to an easier formation of the discontinuities resulting in a larger density and a reduced driving force for their annihilation. Thus the evolution of the microstructure is controlled by solutes and being formed it is also thermally stabilized by the solutes.

[1] R. Kirchheim, In Solid State Physics, eds. H. Ehrenreich and F. Spaepen, Elsevier, Amsterdam (2004), Vol. 59, 203-305

[2] R. Kirchheim, Acta Mater. 55 (2007) 5129-5138 and 55 (2007) 5139-5148

[3] A. Barnoush et al., Scripta Mater., 108 (2015) 76-79

[4] T. Kresse et al., Scripta Mater. 69 (2013) 424-427

[5] M. Deutges et al., Acta Mater., 82 (2015) 266–274

[6] N. Nafszin et al., Acta Mater.,136 (2017) 224-234

 

Speaker:
Prof. Dr. Reiner Kirchheim
University of Göttingen