Materials Frontier No.147
Title: KEY ROLES OF IMPURITIES DURING SOLIDIFICATION
Speaker: Prof. Peter Schumacher
Inviter: Prof.Jianguo LI
Prof. Peter Schumacher was born in 1964BremeninGermany. He graduated with a Diplomingenieur (Dipl.-Ing.) atUniversityofBraunschweig(Germany) in 1989 and obtained his Ph.D. degree fromUniversityofCambridge,DarwinCollegein 1993. Subsequently he continued to work as postdoctoral researcher at Department of Materials Science and Metallurgy, University ofCambridge, and moved in 1995 to Department of Materials, University of Oxford, where he gained a prestigious EPSRC Advanced Fellowship in 1997. Since 2002 he holds the Chair of Casting Research at Montanuniversität Leoben, and became the Managing Director of Austrian Foundry Institute inAustria.
Prof. Peter Schumacher acts as a referee in several international magazines and as a co-editor for many international magazines. He is the organiser of the annual Austrian Foundry Symposium and is the member of international advisory boards for conferences and funding agencies. His scientific output portrays him as an international expert on grain refinement and he has obtained the Cook-Ablett award for his work in Al alloy and the TMS Magnesium Application Award for his work in Mg alloy in 2006. To date, his research focuses mainly on (i) melting metallurgy of Al, Mg, Cu and cast iron, i.e. grain refinement of Mg, Al and Cu alloys, modification of eutectic Si of Al-Si alloys, inoculation of cast iron, (ii) high performance Mg and Al alloy development, (iii) solidification principles of metal alloys, i.e. nucleation kinetics, and (iv) advanced casting technologies. A very close collaboration with Austrian Foundry Institute facilitates advanced casting technologies of high pressure die casting, low pressure die casting, and ablation casting in applied research and development of cast components for automotive applications. For more details, please see www.mu-leoben.at, and www.ogi.at
Impurities, even with very low amounts (ppm), can result into a significant effect on the solidification microstructure via nucleation processes. For example, (i) traces of P (about 10 ppm) can greatly refine the primary Si and also affect the nucleation of eutectic Si in Al-Si based alloys, (ii) traces of Sr (about 200 ppm) and or Na (about 50 ppm) can significantly modify the eutectic Si in Al-Si based alloys, (iii) traces of free Ti (about 10 ppm) can enhance the heterogeneous nucleation of TiB2 particle and thereby the grain refinement potency of Al alloys, (iv) traces of Fe and Si (about 1000 ppm) can not only enhance the heterogeneous nucleation but also change the growth mode of Al3(Sc,Zr) in Al alloys. However, there is still a lack of the detailed investigation on the key roles of impurities, due to very low concentration level and very strong interactions with other elements. In this contribution, advanced electron microscopy (i.e. high resolution transmission electron microscopy, high resolution scanning transmission electron microscopy and atom probe tomography) has been used to elucidate the key roles of impurities in Al alloys. Three different cases will be highlighted. Firstly, the distribution of modified agents (Sr, Na and Eu) within eutectic Si and along the interface between eutectic Si and eutectic Al will be discussed. On the basis of this investigation, a revised modification mechanism has been proposed. Secondly, the impurities effect of free Ti on the nucleation potency of TiB2 particle will be discussed. Trace amounts of free Ti were found to play a dominant role for grain refinement of Al alloys, even with a defined growth restriction. Thirdly, impurities effect of traces of Fe and Si on the nucleation and growth of Al3(Sc,Zr) in Al alloys will be discussed. The presence of Fe and Si in commercial purity Al alloys was found to enhance the formation of Al3(Sc,Zr) with a higher number density and a narrower size distribution, when compared with the corresponding high purity Al alloys. This contribution provides some novel insights in the heterogeneous nucleation, grain refinement and modification of Al based alloys.