4th International Conference on Advances in Solidification Processes
8-11th July 2014, Beaumont Estates, Old Windsor, UK

Prof Lindsay Greer - University of Cambridge, UK

Lindsay Greer
A. L. (Lindsay) Greer is Professor of Materials Science and was (2006‒2013) Head of the Department of Materials Science & Metallurgy at the University of Cambridge. He has also held faculty positions at Harvard University, the Centre d’Etudes Nucléaires de Grenoble, and the Centre for Materials Innovation, Washington University (St Louis). He is an editor of Philosophical Magazine (founded in 1798, publishing papers on the structure and properties of condensed matter). His research interests are metallic glasses and crystal nucleation, grain refinement in casting, chalcogenide thin films for phase-change data storage, and electromigration. He has published more than 380 scientific papers and 14 book chapters, is the author (with K. F. Kelton) of Nucleation in Condensed Matter: Applications in Materials and Biology (Elsevier 2010), and he has edited 13 volumes of proceedings. He is a Fellow of the Institute of Materials Minerals and Mining (IoM3), and holds several awards for his work, including: the Honda Kotaro Medal of Tohoku University, Sendai, Japan; the Hume-Rothery Prize and the Griffith Medal and Prize of the IoM3; and the 2012 Bruce Chalmers Award of TMS (USA) “for his seminal contributions in the area of nucleation, in particular for the role of inoculants in aluminum alloys and for the formation of bulk metallic glasses.”

Abstract

Principles of Alloy Grain Refinement in Solidification Processing

A. L. Greer

University of Cambridge, Department of Materials Science & Metallurgy, UK

The development of grain structure in cast alloys is reviewed. The development involves the initiation of new grains, their growth, and possibly subsequent solid-state changes such as recrystallization and grain growth. The analysis of the grain size resulting from solidification itself involves consideration of the kinetics of both grain initiation and grain growth, and in essence the grain size is determined by a competition between the rates of initiation and growth. There are many possible origins of grain initiation: homogeneous crystal nucleation in the melt; heterogeneous nucleation on extrinsic particles (intentionally added to the melt, or otherwise); and seeding, for example through dendrite fragmentation. The various forms of grain initiation are compared, focusing on the required degree of supercooling. The mechanisms of growth restriction are also reviewed, considering the principles for the selection of solutes suitable for promoting grain refinement. The supercooling at which solidification is initiated is very significant. In conventional solidification processing, that is shaped casting and casting of wrought alloys, the solidification is initiated at small supercooling. In such cases, grain initiation is by seeding or by potent heterogeneous nucleation. The conditions for successful grain refinement by seeding are narrowly defined, and heterogeneous nucleation is the more likely practical method for achieving a fine grain size. The requirements for successful heterogeneous nucleation will be considered. Grain refinement through inoculation by extrinsic particles is widely practised for aluminium alloys, and key findings will be reviewed, focusing on the importance of nucleant particle size. The same principles of grain refinement apply in other alloy systems, and some practical examples will be compared. At the other extreme, much interest has arisen in melts taken to extremes of large supercooling. In such melts, on the border of glass formation, grain refinement can also be achieved. The origins and potential applications of such refinement will be surveyed. The talk will conclude with consideration of how such studies at large supercooling may be of practical interest in a wide range of alloy systems and applications.