Entropy-Driven Grain Boundary Segregation: Prediction of the Phenomenon

Pavel Lejček,Siegfried Hofmann

doi:10.3390/met11081331

Abstract

The question is formulated as to whether entropy-driven grain boundary segregation can exist. Such a phenomenon would be based on the assumption that a solute can segregate at the grain boundary sites that exhibit positive segregation energy (enthalpy) if the product of segregation entropy and temperature is larger than this energy (enthalpy). The possibility of entropy-driven grain boundary segregation is discussed for several model examples in iron-based systems, which can serve as indirect evidence of the phenomenon. It is shown that entropy-driven grain boundary segregation would be a further step beyond the recently proposed entropy-dominated grain boundary segregation as it represents solute segregation at “anti-segregation” sites.

Highlights

Grain boundary segregation is a phenomenon of increasing importance, e.g., in connection with the stabilization of nanocrystalline structures [1,2,3]
A prediction of the grain boundary segregation—entropy-driven grain boundary segregation—is given. It should be a part of the entropy-dominated grain boundary segregation introduced recently
Entropy-driven grain boundary segregation is expected for systems exhibiting positive values of segregation energy if the product of segregation entropy and temperature is higher than the value of the segregation energy

Summary

Introduction

Grain boundary segregation is a phenomenon of increasing importance, e.g., in connection with the stabilization of nanocrystalline structures [1,2,3]. Despite the admirable progress in theoretical calculations, its fundamental drawback is that the segregation energy can only be calculated for 0 K This means that the Gibbs energy of segregation, ∆GI , which is formed by the segregation enthalpy, ∆HI ; and by the product of temperature, T, and the segregation entropy, ∆SI ,. Segregation enthalpy is generally considered as characterizing the tendency of the solute to segregate, segregation entropy plays an additional important role in establishing the concentration level of the segregant at the grain boundaries. This is the case when the entropy term, T∆SI , is larger (in absolute values) than the enthalpy, ∆HI , i.e.,. If a solute segregates at the grain boundaries, even if its segregation energy/enthalpy is positive, such a phenomenon is called entropy-driven grain boundary segregation

Proposal of Entropy-Driven Grain Boundary Segregation

Indirect Support of Entropy-Driven Grain Boundary Segregation

Details

Conclusions