MATERIALS FOR STRUCTURAL APPLICATIONSCMAST (Computational MAterials Science & Technology) Virtual Lab

www.afs.enea.it/project/cmast

Computational Materials Science

Undercooled Cu and Ni Projects:• Scientific collaboration

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.

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 ?

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 .

A. Di Cicco (Univ. Camerino), S. De Panfilis (IIT), A. Filipponi (Univ. L’Aquila), F. Iesari (Univ . Camerino), S. Giusepponi (ENEA), M. Celino (ENEA)

Results:1) Numerical model in agreement with experimental results2) Quantitative calculation of icosahedral order in liquids via common neighbor analysis3) Pressure can increase the number of icosahedra in the liquid.

Liquid

Solid

Undercooled liquid

555 icosahedral order

421 fcc order

421 and 422 hcp order

Common Neighbor AnalysisThree indeces jkl specifies the local environment of a pair of atoms:

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 the k bonds between common neighbors

Perfect Icosahedra

Snapshot of the undercooled liquid. Only icosahedra atoms are drawn

Comparison with experiments: Neutron Structure Factor and Pair correlation function

Atoms with icosahedral symmetry are the most stable configurations that could prevent solidification

Mechanical properties of CuZr alloysA. Zemp (Univ. Of Zurich), B. Schonfeld (Univ. Of Zurich), J. F. Löffler (Univ. Of Zurich), M. Celino (ENEA)

Projects:• National Swiss Foudation and scientific

collaboration

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 CuZr 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 CuZr 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 Cu6Zr7 icosahedron with a VI of <0 0 12 0>; (b) Distorted Cu8Zr5 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.

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.

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).

Role of cellulose oxidation in the yellowing of ancient paper

Projects:•Scientific collaboration with Ministry of cultural heritage

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.

A. Mosca Conte, O. Pulci, C. Violante (Univ. of RomeTor Vergata) in collaboration with experimental groups: M. Missori and L. Teodonio (CNR-ISC), J. Lojewska and J. Bagniuk (Jagiellonian University of Krakow)

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

Chemical structure of: (a) unaged cellulose(with the carbon atom numbering); (b)–(f) of the oxidizedgroups. Yellow, red and blue spheres represent, respectively, carbon oxygen, and hydrogen atoms.