Uncovering The Properties of Triple Lines in Polycrystalline Materials

Abstract

Triple lines are a fascinating defect which occur where three grain boundaries intersect. They form an essential part of the defect structure of a polycrystalline material and yet many of their properties are not well understood. To investigate triple lines in more detail the concept of a coincident site lattice (CSL) bicrystal is extended to a multiple CSL and in particular the tricrystal. The Sigma Factor Theorem is derived which finds all the possible CSL superlattices for a given CSL. Triple lines do have their own properties which is demonstrated using molecular dynamics. The relaxed atomic structures showed that in general regions of atoms near a triple line have a higher mean excess energy and mean excess volume that their adjoining grain boundaries. A Gibbsian method is presented to define and calculate the formation energy of a triple line and approximately 83% of the simulations produced a negative mean triple line formation energy. The formation energy of a triple line also varies with triple line position and this defines a force per unit length on the triple line which can affect how they migrate. Triple lines can restrict grain growth and this is demonstrated using CSL tricrystals and com- paring their mobility to the superlattice bicrystals. To make an accurate comparison a total driving force is defined which is the change in potential energy per unit volume of the shrinking grain and includes the changes in potential due to the migration of grain boundaries and triple lines. The results showed that tricrystal systems had a lower mobility than their superlattice bicrystals and that triple lines do impose a mobility drag. In addition triple lines can also inhibit the antithermal mobility of grain boundaries. The dynamic simulations of tricrystals showed that triple lines can nucleate new grains and twin grain boundaries and the orientation of the new grain forms a superlattice CSL as shown by The Sigma Factor Theorem. As triple lines migrate the motion can stagnate on some atomic planes and their shape becomes contorted. The analysis of CSL and close CSL points of a tricrystals simulation demonstrated some correlation with the degree of contortion and the spatial periodicity of the tricrystal.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joao Quinta Da Fonseca (Supervisor) & Christopher Race (Supervisor)