Materials Frontier No.151
Title: Constitutional Supercooling Promotes, Prevents and Protects Nucleation and Grain Formation
Speaker: Prof. David StJohn, Centre for Advanced Materials Processing and Manufacturing (AMPAM), SchoolofMechanicaland Mining Engineering, TheUniversityofQueensland,Australia
Inviter: Prof Jun WANG
Professor StJohn is a graduate of The University of Queensland with a BSc(Hons) and PhD in Physical Metallurgy. From 1994 to 2008 he was with the CAST Cooperative Research Centre being CEO from August 2002. He initiated the Centre of Advanced Materials Processing and Manufacturing (AMPAM) at UQ in 2009. He has over 300 publications in journals and conference proceedings with research interests covering a broad range of topics including grain refinement and the formation of casting defects, in particular hot tearing. His work on grain refinement covers Al, Mg and Ti alloy systems which enabled the verification of a new theoretical perspective on the mechanisms of refinement encompassed by the Interdependence Theory developed in 2011. Prof StJohn was awarded the MaterialsAustraliaSilver Medal in 2011 for “outstanding contributions to the advancement of metallurgy, metallurgical engineering, materials science or materials engineering through management, teaching, innovation, development or research”. His team was awarded the ASM Henry Marion Howe Medal in 2006 for the best paper published in Metallurgical and Materials Transactions in 2005, and the Magnesium Technology Award by TMS in 2003.
The concept of constitutional supercooling (CS) was first described and discussed qualitatively by Rutter and Chalmers (1953), in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller, Jackson, Rutter, and Chalmers (1953). On that basis, Winegard and Chalmers (1954) further considered ‘supercooling and dendritic freezing of alloys’ where they described how CS promotes the formation of an equiaxed zone due to heterogeneous nucleation of new crystals. The importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified to date. The Interdependence Theory is the culmination of work by the authors which takes into accoun both CS and nucleant particle characteristics for the prediction of grain size. This analysis has revealed that in addition to promoting refinement, CS also prevents nucleation within a nucleation-free zone surrounding each nucleated and growing grain. A detailed account of the role of CS in promoting and protecting nucleation and grain formation, and the factors that limit refinement and the effectiveness of master alloy additions is presented.
These phenomena will be evaluated using the Interdependence Theory to provide a comprehensive description of the mechanisms affecting nucleation and grain formation and, thus, the degree of refinement achieved in an alloy casting.