Recent trends and open questions in grain boundary segregation
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Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Praha 8, Czech Republic
Monika Všianská and Mojmír Šob Central European Institute of Technology, Masaryk University, CEITEC MU, 625 00 Brno, Czech Republic; Institute of Physics of Materials, Academy of Sciences of the Czech Republic, 616 62 Brno, Czech Republic; and Department of Chemistry, Faculty of Science, Masaryk University, 611 37 Brno, Czech Republic (Received 19 April 2018; accepted 21 June 2018)
Recently, significant progress in the field of grain boundary segregation was achieved, for example, in better understanding and modeling the stabilization of nanocrystalline structures by grain boundary segregation, searching for more advanced approaches to theoretical calculation of segregation energies and development of the complexion approach. Nevertheless, with each progress, new important questions appear which need to be solved. Here, we focus on two basic questions appearing recently: How can be the experimental results on the grain boundary segregation compared reliably to their theoretical counterparts? Is the preferred segregation site of a solute in the grain boundary core substitutional or interstitial? We also show that the entropy of grain boundary segregation is a very important quantity which cannot be neglected in thermodynamic considerations as it plays a crucial role, for example, in prediction of thermodynamic characteristics of grain boundary segregation and in the preference of the segregation site at the boundary.
I. INTRODUCTION
The phenomenon “grain boundary segregation” is known for decades. Although the first indirect evidence of the effect of changed composition of grain boundaries of copper on interfacial cohesion has been reported already in 19th century,1 probably the first direct reference in the literature comes from 1950s when Stewart et al.2 showed by autoradiography that polonium (i.e., radioactive isotope 210Bi) enriches grain boundaries in lead. After the starting period of indirect detection, extended studies of grain boundary segregation have been facilitated by the development of surface analytical techniques such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS or ESCA— electron spectroscopy for chemical analysis), etc.3 In this respect, the publication of Kalderon explaining the reasons for catastrophic damage of the rotor in the Hinkley Point power station in 1968 represents the very first application of AES in the field of grain boundary segregation.4 Since that time, numerous studies of grain boundary segregation of various solutes in different host metals were published. Somewhat later, molecular dynamics (MD)5 and tight binding (TB)6 calculations of the energy of grain boundary segregation have been
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2018.230 J. Mater. Res., 2018
started. Besides them, various other approaches to theoretical calculations have been later develo
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