The Structure, Role and Flexibility of Grain Boundaries
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The Structure, Role and Flexibility of Grain Boundaries M. Samaras1*, M. Victoria1, W. Hoffelner1 1 Nuclear Energy and Safety, Paul Scherrer Institute, CH-5232 Villigen, Switzerland *Corresponding author: [email protected] ABSTRACT The structure and role of grain boundaries is investigated using an atomic analysis of the grain boundary movement during Molecular Dynamics displacement cascade simulations of bcc Fe. The results show the grain boundary to be a flexible entity. Local restructuring of the GB accommodates the incoming self interstitial atoms with local kinks, or small movements of a few atomic spacings occurring when the grain boundary is engulfed in the displacement cascade. The damage created is investigated using two potentials: the Ackland (non-magnetic) and the Dudarev- Derlet (magnetic) to study the role and influence of magnetism on the results obtained. INTRODUCTION To obtain a better life-time assessment of ferritic steels [1] it is necessary to understand the behaviour of single defects at the nanoscale [2]. Such an understanding is possible through the modelling of their microstructure under certain deformation processes. Simulation of displacement cascade evolution during the initial stages of radiation damage using Molecular Dynamics (MD) computer simulations is a method of introducing defects into the sample in a manner which mimics the irradiation damage that materials undergo when subjected to the extreme conditions in current fission, and future Gen III, Gen IV and fusion reactors. Pre-existing defects and sinks present in the sample will affect the damage that will be produced under such conditions. Introduction of GBs and other interfaces influences and alters the structural and mechanical properties of a material [3,4]. In nanocrystalline (nc) materials a large ratio of atoms present in a sample belong to the GB region. Under irradiation, various results have been observed in nc samples: grain growth has been seen in nc materials both in experiment [5,6] and in simulation [7]; alternatively, grain refinement [6] as well as no change in grain size [8] have also been seen. Past simulations of radiation damage of nc samples showed no significant GB movement [9] with maximal GB movement seen in the form of kinks during cascade overlap [9]. In these simulations it was shown that the role of the GB during displacement cascades plays a major role in the defects remaining in the grain [10-15]. It is important to note that MD simulations are dependent on the empirical potentials that are used to describe the forces between the atoms and determine the movement of these atoms in the sample. Recently, it was acknowledged that the insufficient understanding of magnetism in ferritic/martensitic steels is a limiting factor in determining the lifetime of such materials. Indeed it is well known that that magnetism stabilises bcc α −Fe [16]. Furthermore, ab initio calculations have shown that magnetism affects the stability configuration and mobility of defects present in Fe [17]. Such ch
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