Volumen II - SAM

Congreso SAM/CONAMET 2009 Buenos Aires, 19 al 23 de Octubre de 2009
MIGRATION OF SMALL INTERSTITIALS CLUSTERS IN BCC METALS: A MODEL
FOR MOLYBDENUM
R. C. Pasianot (1,2,3) and V.P Ramunni (2,4)
(1) Departamento de Materiales, CNEA-CAC
Avda. Gral. Paz, 1499 (1650) San Martín, Argentina.
(2) CONICET
(3) Instituto Sábato, UNSAM/CNEA
(4) Instituto de Ciencias, UNGS
E-mail (contact author): pasianot@cnea.gov.ar
ABSTRACT
The migration mechanisms of small self-interstitial clusters in bcc Molybdenum are studied by computer
simulation. This is a continuation of our previous work {SAM07) where only the single interstitial was
considered; here, the sizes up to four units being tackled, assure correct extrapolation of the general trends.
The research is pertinent to the post-collapse cascade stages of radiation damage, when defects diffuse to
sinks and agglomerate into bigger units. Two interaction models are considered, one rather standard manybody
central forces potential, and another one including angle contributions to the energy, which in
principle is a more consistent representation of a bcc transition metal. Also, two techniques are employed:
Molecular Dynamics and Monomer. The former provides global parameters of the diffusion process; the
latter is a static technique to search the potential energy surface for saddle points, thus providing detailed
information on transition events. Reasonable agreement is found among the two, though the connection
between the static parameters with the dynamic ones may not be straightforward. The extent to which the
interaction model affects the diffusion parameters turns out to be cluster dependent. Notably, in some cases
where such an influence is small, the underlying microscopic mechanisms are nevertheless different.
Keywords: Interstitials clusters, migration mechanisms, computer simulation..
1. INTRODUCTION
Since about a decade ago there have been numerous studies that employ the molecular dynamics technique
(MD) furnished with semi-empirical interatomic potentials, to investigate the damage structure of collision
cascades. Several metals and crystallographies have been considered and new knowledge has been acquired
with respect to the pioneering works of the 1960s and 1970s [1]. These new insights include that the
production of Frenkel pairs amounts to about 1/3 of the classical NRT estimation [2], and that small point
defect clusters are formed, besides single ones, right after cascade collapse. A great body of those studies
focused on Fe, steered by the need to better understand the irradiation damage processes taking place in
reactor pressure vessel steels. Also remarkable is the result that a sizable fraction of the small interstitials
clusters in Fe, is predicted to be highly mobile along the atomic rows, executing essentially a onedimensional
(1D) motion. Such a behavior would notably diminish the recombination with vacancies,
implying a biased production [3] of the latter type of defect, strongly affecting the understanding of radiation
damage and the models devised for its prediction.
Due to the high computational demands, i.e. simulation crystallites of some 10 6 atoms are needed, this kind
of MD studies are only possible by using simplified, semi-empirical, interaction models. The majority of the
studies have employed interatomic potentials of the embedded atom method (EAM) type [4,5], despite the
fact that only central forces can be derived from them, whereas it is accepted that the d-orbitals contribute
angle-dependent terms.
Low temperature electron irradiation experiments show that for most metals of this group but Fe, the selfinterstitial
(SIA) is already mobile at a few kelvins [6], comprising Mo the objective of the present study. To
the authors' knowledge, currently there is no interatomic potential for Mo reproducing SIA properties
consistent with both, a very low migration energy, E m , and the experimentally found [7] dumbbell as
most stable configuration. Recent ab initio calculations however [8,9], support the belief that, except for Fe,
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