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Cayron

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Dr. Cyril Cayron (1) & Dr. Xavier Maeder (2)Laboratoire de Métallurgie ThermoMécanique (LMTM), Institut des Matériaux (IMX) Ecole Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel, Microcity.EMPA, to completeThe research plan must not exceed 20 pages and 80,000 characters (with spaces); this includes title or front page, summary, footnotes, illustrations, formulae, tables (and, if applicable, the table of contents), but not the bibliography. A minimum of point 10 font size and 1.5 line spacing must be used. In general, the research plan should not contain any annexed documents.SEE http://www.snf.ch/en/funding/documents-downloads/Pages/regulations-requirements-for-the-research-plan.aspxSUMMARY (max 1 page)RESEARCH PLANCurrent state of research in the fieldFace centered cubic (fcc) and body centered cubic (bcc) metals are deformed plastically by dislocations multiplication and movements; however, when deformed at high speeds or low temperatures, the dislocation activity is reduced and deformation twinning appears [1][2][3]. Hexagonal close-packed (hcp) metals are different because of the lowest number of slip systems; they are deformed by twining even at medium and room temperatures. As magnesium and titanium are two hcp materials that have important applications in automotive, airplane and airspace industries, a better understanding of deformation twinning could help the development of new alloys with increased ductility. There are many fundamental questions related to deformation twinning that remain unsolved in fcc, bcc and hcp metals, and the most important one to our point of view is: how do the atom move during twinning? Answering this question would allow answering many other questions in a bottom-up way: how is distorted the crystallographic lattice? How this distortion is accommodated by the surrounding matrix? How can it induce cracking and what are the most deleterious twinning modes? How can it affect the recrystallization modes and kinetics? Which twinning modes and twinning variants are created in a stress field? What is the resulting macroscopic deformation? The project aims at answering the first questions of this list by adopting a new theoretical approach and recent powerful experimental methods.The main characteristics of deformation twinningThere are two types of twins, the annealing twins that are formed at medium/high temperatures in low-stacking fault energy fcc metals during post-deformation recrystallization heat treatments, and deformation twins that appear in hcp metals at medium/low temperatures during mechanical deformations, or in fcc and bcc metals at low temperatures and very high strain rates. In metallography, the formers take the form of straight grains with coherent {111} interfaces (Fig. 1), and the latter are lenticular plates (Fig. 2). Both are actually new grains of the same phase but with new orientations in comparison with the initial grains in which twins are formed. Annealing and deformation twins formed in fcc metals show disorientation of 60° around <111> with their parent grains. There are more twinning modes and thus more twin specific misorientations in magnesium. These specific disorientations are the main characteristics of twins, and are actually taken as definition by Friedel his seminal book [4]. This definition is still used in crystallography nowadays [5][6] probably because another definition that would be based on the mechanism is still out of reach due the lack of knowledge on the exact its nature and details. Indeed, there is not yet a consensus on the theory of mechanical twinning, and worse than that, a cleavage appeared these last decades between two metallurgist communities, as detailed in the sections hereafter.