The Icosahedron in Materials Science
Event details
| Date | 18.05.2015 |
| Hour | 13:15 › 14:15 |
| Speaker | Prof. Michel Rappaz, Institute of Materials, EPFL |
| Location | |
| Category | Conferences - Seminars |
The icosahedron, a regular polyhedron with 12 vertices, 20 triangular facets and 30 edges, plays an important role in materials science. In the 1950’s already, Frank postulated that atoms in the liquid may exhibit an icosahedral short range order (ISRO) symmetry, which could explain the large undercoolings measured by Turnbull in close-packed metals. ISRO was later confirmed by X-ray diffraction measurements on levitated droplets. In the 1960’s, Ino explained the dark-field images of nanoparticles by the fact that they are made of 20 (or 5) fcc tetrahedra packed in multi-twinned icosahedra (or decahedra) exhibiting low-energy {111} external planes. In the 1980’s, Shechtman et al discovered quasicrystals (QC) in Al-Mn alloys which exhibit a 5-fold symmetry typical of the icosahedron, but without translation symmetry.
After reviewing these interesting past discoveries, we will see how recent results[1,2] indicate that nucleation of the fcc phase in certain liquid metallic alloys can be mediated, rather than prevented, by the formation of icosahedral QC’s in the melt. QC’s can easily nucleate in the melt because of their similarity with ISRO postulated by Frank, with an associated very low interfacial energy. After some growth, QC’s then act as precursors for the formation of the fcc phase via two mechanisms that will be discussed in this presentation: heteroepitaxy on QC facets or peritectic transformation of the outer layer of QC’s[3]. Such a QC-mediated nucleation mechanism of the fcc phase, which tends to rule out standard homogeneous nucleation theories, shares similarities with nucleation of precipitates in solids via intermediate metastable phases such as Guinier-Preston zones.
[1] G. Kurtuldu, Ph. Jarry and M. Rappaz, Acta Mater. 61 (2013) 7098.
[2] G. Kurtuldu, A. Sicco and M. Rappaz, Acta Mater. 70 (2014) 240.
[3] M. Rappaz and G. Kurtuldu, J. Metals (2015), to appear.
Bio: 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 of EPFL in 1981. He was nominated Adjunct Professor in 1990 and Full Professor in 2003.
His field of research has been focused on phase transformations and solidification, 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 approaches and tools, such as cellular automata and granular models of grain structure formation, models for microporosity and hot tearing formation in castings. Some of them have been commercialized by a spin-off company founded by his group in 1991 (Calcom SA), now part of the French company ESI. His activity also encompasses many fundamental aspects of nucleation, growth kinetics and phase/morphology competition in metallic alloys. 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 Mathewson co-author award (1994) and author award (1997) of the American Mineral, Metals and Materials (TMS) society, 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), the FEMS European Materials Gold Medal (2013) and the Brimacombe Prize of TMS (2015). He is a highly-cited author of ISI, a fellow of ASM, IOP and TMS, and has co-authored more than 200 publications and two books.
After reviewing these interesting past discoveries, we will see how recent results[1,2] indicate that nucleation of the fcc phase in certain liquid metallic alloys can be mediated, rather than prevented, by the formation of icosahedral QC’s in the melt. QC’s can easily nucleate in the melt because of their similarity with ISRO postulated by Frank, with an associated very low interfacial energy. After some growth, QC’s then act as precursors for the formation of the fcc phase via two mechanisms that will be discussed in this presentation: heteroepitaxy on QC facets or peritectic transformation of the outer layer of QC’s[3]. Such a QC-mediated nucleation mechanism of the fcc phase, which tends to rule out standard homogeneous nucleation theories, shares similarities with nucleation of precipitates in solids via intermediate metastable phases such as Guinier-Preston zones.
[1] G. Kurtuldu, Ph. Jarry and M. Rappaz, Acta Mater. 61 (2013) 7098.
[2] G. Kurtuldu, A. Sicco and M. Rappaz, Acta Mater. 70 (2014) 240.
[3] M. Rappaz and G. Kurtuldu, J. Metals (2015), to appear.
Bio: 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 of EPFL in 1981. He was nominated Adjunct Professor in 1990 and Full Professor in 2003.
His field of research has been focused on phase transformations and solidification, 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 approaches and tools, such as cellular automata and granular models of grain structure formation, models for microporosity and hot tearing formation in castings. Some of them have been commercialized by a spin-off company founded by his group in 1991 (Calcom SA), now part of the French company ESI. His activity also encompasses many fundamental aspects of nucleation, growth kinetics and phase/morphology competition in metallic alloys. 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 Mathewson co-author award (1994) and author award (1997) of the American Mineral, Metals and Materials (TMS) society, 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), the FEMS European Materials Gold Medal (2013) and the Brimacombe Prize of TMS (2015). He is a highly-cited author of ISI, a fellow of ASM, IOP and TMS, and has co-authored more than 200 publications and two books.
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Practical information
- General public
- Free
Organizer
- Fabien Sorin
Contact
- Fabien Sorin