2016 Van Horn Distinguished Lectures: 3 (boundary mobility)
Channel: bhadeshia123
Date: 2016-09-29

The Kent R. van Horn Lectureship is an endowed Lectureship at the Case Western Reserve University and dates from 1974. It honours Kent R. van Horn, an alum, who had a distinguished career as a metallurgist, director of research, and ultimately corporate vice-president of Alcoa. Three lectures on varied topics are to be delivered over three successive days. The 2016 lectures were delivered by Professor Wayne Kaplan of the Department of Materials Science and Engineering Technion – Israel Institute of Technology. This one is about grain boundary mobility.

These videos are reproduced courtesy of the Materials Science and Engineering Department of Case Western Reserve University.

http://engineering.case.edu/emse/node/121
http://www.phase-trans.msm.cam.ac.uk

Abstract: 
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 behaviour 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 characterised. 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.