Atomic Scale Simulation of the Effect of Hydrogen on Dislocations in Zr
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ATOMIC SCALE SIMULATION OF THE EFFECT OF HYDROGEN ON DISLOCATIONS IN ZR C. Domain1 and A. Legris2 1
EDF − R&D Département EMA, Les Renardières, F−77818 Moret sur Loing Cédex, France Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Université de Lille I, F−59655 Villeneuve d’Ascq Cédex, France
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ABSTRACT In nuclear power plants, Zr−based cladding is corroded by the primary coolant. Concomitantly, it undergoes hydrogen pick−up which induces modifications of its mechanical properties, especially creep and recrystallization rates. Below 600K, the deformation in hcp Zr is partially controlled by screw dislocations, which due to their intricate core structure have reduced intrinsic mobility. Here, we address the possible hydrogen induced modifications of the core structure of screw dislocations. We used first−principle calculations based on the density functional theory to evaluate the interaction between hydrogen and screw dislocation cores and also to evaluate the hydrogen induced modifications of the prismatic and basal gamma surfaces. We show that the presence of hydrogen results in significant reductions of the stacking fault energies. INTRODUCTION The deleterious effect of hydrogen on mechanical properties of metallic alloys was documented since the end of the 19th century, with an increasing interest for an understanding of the mechanisms involved at the atomic scale. A first distinction of the hydrogen effects can be made between systems that undergo hydrogen−induced phase transitions and those that do not. In Zr alloys, hydrogen is known to modify the mechanical behaviour both in solid solution and as hydride precipitates [1]. Experimental and theoretical studies of hcp Zr alloys show that, at temperatures of interest (i.e. below 600 K), the plastic behaviour is controlled by the mobility of screw dislocations. Since they have a core spread in the basal and prismatic planes, their mobility is low. Whatever the type of mechanism mentioned (locking−unlocking, Peierls−like [2]) to explain the low−temperature plastic deformation, the lattice friction that results from the intricate core structure seems to be the key−phenomenon governing plasticity. To understand the influence of hydrogen on the mechanical behaviour of Zr alloys, it is therefore necessary to know the energies of intricate ionic and electronic configurations involving Zr and H atoms. This requires accurate atomic−scale numerical simulations. Despite their high computational efficiency for metals, empirical energetic models cannot be used as they deal only implicitly with the electronic density and consequently suffer from poor transferability, specially in alloys. Electronic structure calculations can be performed at various levels, going from simple quantum mechanics formulations (diffusion of free electrons by a central potential) to self−consistent fully ab initio quantum mechanics models, through intermediate tight−binding models with firm quantum mechanics foundations but involving fitted parameters [3]. In this work, we pr
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