Abstrct:
    Iron based alloys play central roles in metallurgical and nuclear technology, in particular ferritic steels containing Cr are nowadays the strongest candidates as structural materials for  future fusion reactors. Although their mechanical properties have been extensively investigated, relatively little is known about the atomistic origin of their macroscopic behaviour, e.g. the interplay between magnetic properties and behavior of defects in FeCr alloys. First principles calculations within the Density Functional Theory (DFT) provide such information, which is not directly accessible through experiments. The main objective of our work is to reconcile theoretical and experimental results by DFT calculations. 

First, to validate our DFT-SIESTA approach [1], we have performed a systematic study of effects of exchange-correlation functionals, pseudopotentials (norm-conserving, ultrasoft , PAW) and basis sets (plane-wave vs. localized functions)  on various collinear and non-collinear structural-magnetic phases of Fe and Cr. Indeed, we note that the magnetic description of Cr not only depends on the exchange correlation functional, but also varies sensitively with the pseudopotentials, e.g. norm-conserving pseudopotentials tend to overestimate the magnetism in bulk Cr, which induces overestimation of solution energy of Cr in bcc-Fe.      

We have then focused our study on point defects created by irradiation in fission and fusion environments. We have determined a three-dimensional migration mechanism for self-interstitials (elementary radiation defects) in iron [2,3], which is consistent with experimental evidences and radically different from the mechanisms in other non-magnetic bcc transition metals.

In addition to vacancies and self-interstitials, large amount of He can also been created by transmutation under fast neutron irradiation. Although mechanisms of He diffusion and clustering with vacancies are rather well established [4,5], how the presence of Cr may modify such mechanisms remain to be understood. Considering He in FeCr alloys, we note e.g. that Cr in solid solution modifies substantially the migration barrier of interstitial He, from 0.06 eV in pure iron to 0.12 eV in dilute FeCr alloys, which may lead to percolation effects.    

Interactions between solutes and extensive defects, such as segregation behaviors of Cr in Fe(110) and Fe (100) surfaces have been studied and correlated to Fe-Cr magnetic coupling. Cr-Cr interaction and Cr migration near these surfaces will also be reported in detail.

Keywords: localized basis sets, defects, magnetism, steels

[1] J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejon and D. Sanchez-Portal, J. Phys. Condens. Matter, 14  2745  (2002)

[2] C.C. Fu, F. Willaime and P. Ordejon, Phys. Rev. Lett.. 92, 175503 (2004)

[3] C.C. Fu, J. Dalla Torre, F. Willaime, J.L. Bocquet and A. Barbu, Nature Mater.. 4, 68 (2005)

[4] C.C. Fu, and F. Willaime, Phys. Rev. B. 72, 064117 (2005)

[5] C.J. Ortiz, M.J. Caturla, C.C. Fu, and F. Willaime, Phys. Rev. B. 75, 100102(R) (2007)