Grain Boundary Segregation Transitions and Critical Phenomena in Binary Regular Solutions: A Systematics of Complexion Diagrams with Universal Characters

TL;DR

This paper develops a systematic framework for understanding grain boundary segregation transitions and critical phenomena, introducing a new descriptor and constructing complexion diagrams with universal features.

Contribution

It extends classical GB segregation theory by introducing normalized segregation strength and analyzing transition types and their relation to temperature and segregation strength.

Findings

Layering transitions occur at low temperatures for high segregation strength.

Transitions become continuous above GB roughening temperatures.

Universal complexion diagrams resemble classical bulk phase diagrams.

Abstract

A systematics of grain boundary (GB) segregation transitions and critical phenomena has been derived to expand the classical GB segregation theory. Using twist GBs as an example, this study uncovers when GB layering vs. prewetting transitions should occur and how they are related to one another. Moreover, a novel descriptor, normalized segregation strength (phi_seg), is introduced. It can represent several factors that control GB segregation, including strain and bond energies, as well as misorientation for small-angle GBs (in a mean-field approximation), which had to be treated separately in prior models. In a strong segregation system with a large phi_seg, first-order layering transitions occur at low temperatures and become continuous above GB roughing temperatures. With reducing phi_seg, the layering transitions gradually merge and finally lump into prewetting transitions without quantized layer numbers, akin to Cahn's critical-point wetting model. Furthermore, GB complexion diagrams with universal characters are constructed as the GB counterpart to the classical exemplar of Pelton-Thompson regular-solution binary bulk phase diagrams.