MATERIALS FOR STRUCTURAL APPLICATIONS

1. ComputationalMaterials Science MATERIALS FOR STRUCTURAL APPLICATIONS CMAST (ComputationalMAterials Science & Technology) VirtualLab www.afs.enea.it/project/cmast Undercooled Cu and Ni • Projects: • Scientificcollaboration A. Di Cicco (Univ. Camerino), S. De Panfilis (IIT), A. Filipponi (Univ. L’Aquila), F. Iesari (Univ . Camerino), S. Giusepponi (ENEA), M. Celino (ENEA) Comparisonwithexperiments: NeutronStructureFactor and Paircorrelationfunction Common NeighborAnalysis Three indecesjklspecifies the local environment of a pair of atoms: Problem: Manymetals can befound in the liquidphaseeveniftheir temperature isbelow the melting temperature: thisphenomenaisusuallycalledundercooling. Undercoolingallowstoindustrially work metals at lowertemperatures, thusreducingcosts and production times. However the undercooledphaseisnotwellunderstood: recentexperimentalresultsprovideindicationsabout the presence of a largefraction of atoms in the icosahedralsymmetry. Icosahedral short rangeorderispostulated in the meltstoexplain the largeundercoolings of pure metals. • Open questions: • Whyisitpossibletoundercool a metal ? • Whichis the intrinsicatomicstructuresgoverning the undercoolingprocess ? • Whichis the effect of pressure on undercooling ? • Method: • Classicalmoleculardynamicssimulations are usedtomodelbothliquid and undercooledsimplemetals (copper and nichel). The results of the simulations are comparedwithexperimentalresults . j = the number of neighbors common tobothatoms k = the number of bonds between the common neighbors l= the number of bonds in the longestcontinuouschain formedby the k bondsbetween common neighbors 555icosahedralorder PerfectIcosahedra 421fccorder 421 and 422hcporder Snapshot of the undercooledliquid. Onlyicosahedraatoms are drawn Liquid Undercooledliquid Atomswithicosahedralsymmetry are the moststableconfigurationsthatcouldpreventsolidification Solid Results: Numericalmodel in agreement withexperimentalresults Quantitative calculation of icosahedralorder in liquids via common neighboranalysis 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 hydrogenatoms. A. Mosca Conte, O. Pulci, C. Violante (Univ. of RomeTorVergata) 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 Mechanicalproperties of CuZralloys • Projects: • National SwissFoudation and scientificcollaboration Stress-strain curve for uniaxial loading along the z-axis 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: • Which are the mechanicalproperties of metallicglasses ? • Howmechanicalproperties are relatedtointrinsicatomicstructure ? • Whatis the origin of shearbands ? • Method: • Classicalmoleculardynamicssimulations are usedtomodelbothliquid and amorphousCuZrmetals. The results of the simulations are comparedwithexperimentalresults . 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. 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). (a) Full Cu6Zr7icosahedron with a VI of <0 0 12 0>; (b) Distorted Cu8Zr5icosahedron 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.