Prof David StJohn - University of Queensland, Australia
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 is currently the Director of the Centre for Advanced Materials Processing and Manufacturing. 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 defects in castings, in particular hot tearing. Prof StJohn was awarded the Materials Australia Silver 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 role of constitutional supercooling in promoting grain refinement
D.H. StJohn1, A. Prasad1, M. Qian2, and M.A. Easton2
1Centre for Advanced Materials Processing and Manufacturing (AMPAM), School of Mechanical and Mining Engineering, The University of Queensland, Australia
2 RMIT University, School of Aerospace, Mechanical and Manufacturing Engineering, Australia
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. Without nucleation caused by CS columnar crystals would continue to grow unimpeded and CS has been shown to be the predominant facilitator of nucleation in most casting processes. 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 account 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.
This paper presents a detailed account of the role of CS in promoting nucleation and the factors that limit refinement and the effectiveness of master alloy additions. For example, the factors affecting master alloy efficiency are: the formation of the nucleation-free zone where the size of this zone is larger for alloys with low values of the growth restriction factor; a broad distribution of particle sizes and thus nucleation potencies; and solute accumulation from overlapping diffusion fields that reduces the amount of CS between the initially nucleated grains. These phenomena will be evaluated using the Interdependence Theory to provide a comprehensive description of the mechanisms affecting the degree of refinement achieved in an alloy casting.