The Influence of Fields and Dopants on Grain Boundary Mobility
Wayne D. Kaplan
Department of Materials Science and Engineering
Technion – Israel Institute of Technology
Controlling grain size is a fundamental part of Materials Science and Engineering. While the
driving force for grain growth in fairly well understood, the mechanism by which grain
boundaries migrate, and how microscopic parameters affect grain boundary mobility are less
understood. This presentation focuses on the mobility of grain boundaries and how dopants
and external fields influence the kinetics of grain growth.
The first part of the talk will address the concept of solute-drag, where conventional wisdom
indicates that moving a solute cloud with a grain boundary should either slow down grain
boundary motion (e.g. Mg in Al2O3), or not affect it. Model experiments at dopant levels
below the experimentally determined solubility limit clearly show that some adsorbates
increase grain boundary mobility (e.g. Ca in Al2O3), whereas others reduce grain boundary
mobility. Reasons for the varying behavior are discussed within the framework of grain
boundary disconnections as the mechanism by which grain boundaries move.
The second part of the talk reviews model experiments designed to probe the influence of
external fields on grain boundary mobility. As a model system, polycrystalline SiC
underwent conventional annealing, and annealing using spark plasma sintering (SPS) without
pressure, and the grain size as a function of annealing time was characterized. From these
experiments, the grain boundary mobility of SiC at 2100°C under conventional versus SPS
annealing was determined. SPS annealing resulted in a grain boundary mobility which is
three orders of magnitude larger than that resulting from conventional annealing. This
indicates that the same (or similar) mechanism which promotes rapid sintering during SPS
also significantly increases the rate of grain growth.