Grain size effect on microstructural properties of 3D nanocrystalline magnesium under tensile deformation Amitava Moitra ⇑ Thematic Unit of Excellence on Computational Materials Science, S.N. Bose National Centre for Basic Sciences, Sector-III, Block – JD, Salt Lake, Kolkata 700 098, India article info Article history: Received 14 March 2013 Received in revised form 22 April 2013 Accepted 31 May 2013 Keywords: Polycrystal Magnesium Molecular dynamics Microstructure abstract The strength of nanocrystalline (NC) hexagonal closed packed (hcp) magnesium has been studied using computer simulations. Three dimensional NC magnesium materials are developed using Voronoi tessel- lation in which a random distribution of grains is generated. The microstructural properties and mechan- ical behaviors under tensile loading are investigated using molecular dynamics simulations. A size scale effect related to the yield stress in the specimen is evidenced. A transition from grain size softening to grain size hardening has been observed for a 10 nm average grain size. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction One view of the materials science approach is to understand the properties of materials or condensed matter starting from infinite continuum theoretical treatments and then adding on the effects of multi-dimensional defects. During the last two decades a tre- mendous effort has been made to study the deformation process for single crystal and bi-crystal materials [1,2]. Due to extraordi- nary improvements in modern computing facilities, atomistic sim- ulations [2–9] for three dimensional nanocrystalline (NC) materials have recently become far more feasible. The grain size effect on the deformation mechanism of NC materials has been qualitatively understood: the optimum grain size for optimum strength of the NC materials can vary for different materials, depending on their stacking fault energies, as reported by Yamakov et al. [10,4,5]. It is well known that the deformation mechanism of hexagonal closed pack materials are fundamentally different from that of their fcc counterparts, because of their different slip systems and related anisotropyness. Studies [11,12] related to the deformation process also speculates a major role of deformation twinning for hcp materials. However, for magnesium NC materials in particular, the influence of grain size on the strength of the material has not yet been confirmed. Nanocrystalline materials consist of networks of triple points/ lines connecting their grain boundaries. It is assumed that suffi- cient diffusion always takes place through the interfaces, which reorient themselves to satisfy the energy minimization [13–15]. The chemical reactivity enhances the most at these triple lines due to the dangling bonds of frustrated atoms [16]. Understanding this deformation mechanism through experimental observation of nanocrystalline material is especially challenging because of the material’s smaller grain sizes and the large volume fraction of atoms that are situated in grain boundary (GB) or triple points (TP). Although molecular dynamics (MD) strain rate is orders of magnitude higher than the real experiments, the deformation mechanisms, evolution of defects, diffusion through grain bound- aries, can be identified more clearly at different strain level with MD [17–19,14,20]. Thus MD simulations stand alongside experi- ments as an integral part of the modern approach to enhance our understanding of the atomistic deformation mechanisms of nano- crystalline materials [19,17,21]. In fact, these simulations have re- vealed that, depending on the generalized stacking fault energy curve, the deformation mechanism for fcc NC material changes from a dislocation driven to a grain boundary mediated process for those cases where the grain interior (GI) is comparable to the grain boundary (GB) region [10,22]. Thus, with an MD simulation approach, we estimate the optimum grain size for maximum strength of magnesium nanocrystalline material. 2. Method and model development For the purpose of present study, five nanocrystalline samples corresponding to five different grain sizes (5, 10, 15, 20 and 30 nm) are geometrically created in three dimensions using Voro- noi construction. In this construction, these grains are randomly nucleated within a cube with sides 60.8 nm. Each grain grows with 0927-0256/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2013.05.051 ⇑ Tel.: +91 8442841861. E-mail address: moitra@bose.res.in Computational Materials Science 79 (2013) 247–251 Contents lists available at SciVerse ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci