Computational Materials Science MATERIALS FOR STRUCTURAL APPLICATIONS CMAST

Computational Materials Science MATERIALS FOR STRUCTURAL APPLICATIONS CMAST (Computational MAterials Science & Technology) Virtual Lab www. afs. enea. it/project/cmast Undercooled Cu and Ni A. Di Cicco (Univ. Camerino), S. De Panfilis (IIT), A. Filipponi (Univ. L’Aquila), F. Iesari (Univ. Camerino), S. Giusepponi (ENEA), M. Celino (ENEA) Projects: • Scientific collaboration Comparison with experiments: Neutron Structure Factor and Pair correlation function Problem: Many metals can be found in the liquid phase even if their temperature is below the melting temperature: this phenomena is usually called undercooling. Undercooling allows to industrially work metals at lower temperatures, thus reducing costs and production times. However the undercooled phase is not well understood: recent experimental results provide indications about the presence of a large fraction of atoms in the icosahedral symmetry. Icosahedral short range order is postulated in the melts to explain the large undercoolings of pure metals. j = the number of neighbors common to both atoms k = the number of bonds between the common neighbors l = the number of bonds in the longest continuous chain formed by icosahedral the k bonds between 555 order common neighbors Perfect Icosahedra 421 fcc order 421 and 422 Open questions: 1) Why is it possible to undercool a metal ? 2) Which is the intrinsic atomic structures governing the undercooling process ? 3) Which is the effect of pressure on undercooling ? hcp order Snapshot of the undercooled liquid. Only icosahedra atoms are drawn Liquid Method: Classical molecular dynamics simulations are used to model both liquid and undercooled simple metals (copper and nichel). The results of the simulations are compared with experimental results. Common Neighbor Analysis Three indeces jkl specifies the local environment of a pair of atoms: Undercooled liquid Solid Atoms with icosahedral symmetry are the most stable configurations that could prevent solidification Results: 1) Numerical model in agreement with experimental results 2) Quantitative calculation of icosahedral order in liquids via common neighbor analysis 3) Pressure can increase the number of icosahedra in the liquid. Role of cellulose oxidation in the yellowing of ancient paper Projects: • Scientific collaboration with Ministry of cultural heritage Chemical structure of: (a) unaged cellulose (with the carbon atom numbering); (b)–(f) of the oxidized groups. Yellow, red and blue spheres represent, respectively, carbon oxygen, and hydrogen atoms. A. Mosca Conte, O. Pulci, C. Violante (Univ. of Rome. Tor Vergata) in collaboration with experimental groups: M. Missori and L. Teodonio (CNR -ISC), J. Lojewska and J. Bagniuk (Jagiellonian University of Krakow) Problem: The yellowing of paper on aging causes major aesthetic damages of cultural heritage. It is due to cellulose oxidation, a complex process with many possible products still to be clarified. Open questions: 1) Is it possible to identuify the oxidized groups inducing yellowing in ancient paper in a non-invasive and non-destructive way ? 2) Is it posible to quantify them in order to measure the level of degradation of ancient masterpieces ? 3) Is it possible to learn something about conservation conditions of single paper samples ? Method: ●By comparing ultraviolet-visible reflectance spectra of ancient and artificially aged modern papers with ab initio time- dependent density functional theory calculations, we identify and estimate the abundance of oxidized functional groups acting as chromophores and responsible of paper yellowing. This knowledge can be used to set up strategies and selective chemical treatments preventing paper yellowing. Results: 1) The theoretical optical absorption spectra are in agreement with the experiments. 2) We find that the presence of humidity accelerates the formation of chromofores, in particular, those absorbing the blue-violet region (LUVAG). 3) Our method has been applied to the Leonardo da Vinci self portrait and will soon be published. Eperimental crystal parameter as input for numerical modeling Mechanical properties of Cu. Zr alloys Projects: • National Swiss Foudation and scientific collaboration Stress-strain curve for uniaxial loading along the zaxis with a strain rate of 107 s-1. A deviation from ideal linear elastic behavior is observed starting at approximately 3% strain. A. Zemp (Univ. Of Zurich), B. Schonfeld (Univ. Of Zurich), J. F. Löffler (Univ. Of Zurich), M. Celino (ENEA) Problem: Metallic glasses are amorphous alloys with unique mechanical, electric, and magnetic properties. They are produced by rapid quenching from the melt. The critical cooling rate to avoid crystallization is on the order of 106 K s-1 for conventional binary metallic glasses and 102 K s-1 for bulk metallic glasses. Therefore, the critical casting thickness is limited to a few micrometers, while metallic glasses have a casting thickness larger than 1 mm. Binary Cu Zr can be cast into a metallic glass with a thickness of 2 mm, which is exceptional for a binary system. The advantage of a binary system is that the structural description becomes more feasible, when compared to multi-component systems, so that it is better suited for structural investigations. Open questions: 1) Which are the mechanical properties of metallic glasses ? 2) How mechanical properties are related to intrinsic atomic structure ? 3) What is the origin of shear bands ? Method: Classical molecular dynamics simulations are used to model both liquid and amorphous Cu. Zr metals. The results of the simulations are compared with experimental results. Total pair correlation function g(r) at different temperatures during cooling. With decreasing temperature the first peak narrows and becomes larger indicating a better defined nearest-neighbor position. Additional features appear in the second peak meaning that also the next nearest-neighbors are arranged more systematically. (a) Full Cu 6 Zr 7 icosahedron with a VI of <0 0 12 0>; (b) Distorted Cu 8 Zr 5 icosahedron with a VI of <0 2 8 2>. Copper atoms are colored orange and zirconium olive. The bonds are only drawn for better 3 -dimensional visibility and do not represent physical bonds. Atomic local shear strain after 9. 4% strain. Only atoms with a strain > 0. 2 are shown. Red corresponds to a high and blue to a low local strain. Considering the total macroscopic strain (left), extended regions are affected by the deformation. However, deformation only occurring between 8. 4% and 9. 4% strain is highly localized into one shear band (right).