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

Prof Michel Rappaz - EPFL, Switzerland

Michel Rappaz

After a PhD in solid state physics (1979) at the Ecole Polytechnique Fédérale de Lausanne (EPFL) and a post-doc at Oak Ridge National Laboratory, Michel Rappaz joined the Institute of Materials in 1981. He was nominated Adjunct Professor in 1990 and Full Professor in 2003.

His field of research has been focused on solidification problems, in particular the coupling of macroscopic aspects of heat and mass transfer with microscopic aspects of microstructure and defect formation. His group has developed several new modelling tools, such as cellular automata and granular models of grain structure formation. Some of them have been commercialized by a spin-off company founded by his group in 1991 (Calcom-ESI). Michel Rappaz initiated in 1992 an annual postgraduate course on solidification which has been attended by more than 800 participants from all over the world.

Michel Rappaz has received several awards, in particular the Mathweson co-author award (1994) and author award (1997) of TMS, the Koerber foundation award with Profs Y. Bréchet and M. Asbby (1996), the Sainte-Claire Deville Medal (1996) and the Grand Medal (2011) from the French Materials Society, the Bruce Chalmers Award of TMS (2002), the Mc Donald Memorial Lecture award of Canada (2005) and the FEMS European Gold Medal (2013). He is a highly-cited author of ISI, a fellow of ASM and IOP, and has co-authored about 300 publications and two books.


Abstract

Enhancement of bulk nucleation in fcc-type liquid metals by icosahedral quasicrystals formation

Michel Rappaz and Güven Kurtuldu

Institute of Materials, Ecole Polytechnique Fédérale de Lausanne, Switzerland

In the 1980’s, 5-fold symmetry icosahedral quasicrystals (iQC) were discovered in Al-alloys: they inherit the local icosahedral arrangement of atoms in the liquid already predicted by Frank in the 1950’s, but without exhibiting translation invariance. This icosahedral arrangement of atoms is also retrieved in the building blocks of the unit cell of the corresponding approximant intermetallic phase. Recently, we have shown that minute Cr additions (typically 0.1 wt%) to Al-Zn alloys solidified in a uniform temperature field spontaneously lead to the formation of fine equiaxed fcc Al grains [1]. Furthermore, these grains exhibit an unexpectedly large number of twin relationships and some of them even show a 5-fold symmetry multi-twin relationship with a common ⟨110⟩ direction. Similar observations have been made for yellow gold alloys which are grain-refined with very small amounts of iridium (5-200 ppm) [2]. The grain refinement is accompanied by an increase of the fraction of the grain boundaries which are in fact twins. A configuration of 9 neighbor grains with multi-twin relationships has been found: six groups of 4 or 5 grains among them share a common ⟨110⟩ direction, and furthermore the angles between these ⟨110⟩ directions have angles corresponding to those between the six 5-fold symmetry axes of an icosahedron. These results become fully consistent when one considers that the primary fcc phase forms on facets of iQC’s, or alternatively on nuclei of the parent approximant phase, with the following heteroepitaxy relationship: {111}fcc ⊥ 3-fold i-QC symmetry axes, ⟨110⟩fcc ⊥ 2-fold i-QC symmetry axes. This new nucleation mechanism is consistent with the fact that the interfacial energy of iQC with the liquid is about 10 times lower than that of the fcc phase. It also brings an explanation for a long-standing problem of the origin of twinned dendrites in Al alloys, a morphology discovered more than 60 years ago and consisting in ⟨110⟩ dendrites split in their center by coherent {111} twin planes. Furthermore, from a practical point of view, it also sets guidelines for the quest of new solute elements acting as grain refiners for fcc, and possibly hcp metallic alloys.

References

  1. G. Kurtuldu, Ph. Jarry and M. Rappaz, Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al-Zn-Cr alloys: Can icosahedral solid clusters play a role? Acta Mater. 61 (2013) 7098-7108.
  2. G. Kurtuldu, A. Sicco and M. Rappaz, Icosahedral quasicrystal-enhanced nucleation of the fcc phase in liquid gold alloys. Acta Mater. (2014, to appear).