Topological grain boundary segregation transitions

TL;DR

This study reveals how iron segregation at titanium grain boundaries induces phase transitions by stabilizing icosahedral units, leading to hierarchical boundary phases with distinct structures, advancing understanding of interface engineering.

Contribution

It uncovers the atomistic mechanisms of Fe-induced phase transitions at titanium grain boundaries using combined microscopy and simulation methods.

Findings

Fe segregation stabilizes icosahedral units at GBs

Hierarchical GB phases form with different arrangements

Fe excess is accommodated by phase transitions

Abstract

Engineering structure of grain boundaries (GBs) by solute segregation is a promising strategy to tailor the properties of polycrystalline materials. Theoretically it has been suggested that solute segregation can trigger phase transitions at GBs offering novel pathways to design interfaces. However, an understanding of their intrinsic atomistic nature is missing. Here, we combine atomic resolution electron microscopy atomistic simulations to discover that iron segregation to GBs in titanium stabilizes icosahedral units (cages) that form robust building blocks of distinct GB phases. Due to their five-fold symmetry, the Fe cages cluster and assemble into hierarchical GB phases characterised by a different number and arrangement of the constituent icosahedral units. Our advanced GB structure prediction algorithms and atomistic simulations validate the stability of these observed phases and the high excess of Fe at the GB that is accommodated by the phase transitions.