Size Effect in the Shear-Coupled Migration of Grain Boundaries Pinned by Triple Junctions
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Size Effect in the Shear-Coupled Migration of Grain Boundaries Pinned by Triple Junctions Javier Gil Sevillano1, Aitor Luque1, Javier Aldazabal1, José M. Martinez-Esnaola1 1 CEIT and TECNUN, University of Navarra, M. de Lardizabal 15, 20018 San Sebastian, Spain. ABSTRACT This paper presents molecular dynamics simulations of shear-coupled migration of tilt boundaries pinned by triple junctions in a simple model structure of columnar grains of different sizes. Simulations are for copper at 300 K. The phenomenon is of interest as a possible explanation of the Hall-Petch relationship breakdown in nano-grained polycrystals deformed at high or moderate strain rate and low temperature. INTRODUCTION In a stressed polycrystal, tilt boundaries under resolved shear stress experience a virtual pressure that promotes their shear strain-coupled migration (SCM), like similar tilt boundaries in bicrystals do [1-21]. However, in polycrystals the grain boundaries are constrained in their motion by their limiting triple junctions. The pinning effect of the junctions induces the bulging of migrating boundaries that cannot take place without additional accommodation by elastic distortion (for early bulging) or in general by elastoplastic accommodation, with dislocation emission or grain boundary sliding (GBS) simultaneous to SCM [22, 23]. Finite migration of pinned boundaries by bulging requires an increment of the applied resolved shear stress on the boundary above the shear stress needed for the migration of a free-to-move flat boundary. Such increase depends on the size of the boundary. This paper presents molecular dynamics simulations of shear-coupled migration of tilt boundaries pinned by triple junctions in a simple model structure of columnar grains of different sizes. Simulations are for copper at 300 K. The phenomenon is of interest as a possible explanation of the Hall-Petch relationship breakdown in nano-grained polycrystals deformed at high or moderate strain rate and low-temperature [23], Fig. 1. For copper the HP breakdown and the threshold stress for SCM both occur for equivalent tensile stresses of the order of 0.9 GPa. For many tilt boundaries SCM occurs in preference to GBS at low temperature and high or moderate shear strain rate, as attested by experimental or numerical simulations [1-23]. SIMULATION TECHNIQUE Molecular dynamics (MD) simulations of shear deformation of copper bicrystals and multicrystals of different sizes have been performed using the embedded atom method (MDEAM) as previously described [22, 23]. The potential used was the Mishin potential [24]. The bicrystals were of uniform thickness Y = 2.17 nm in the y // [001] direction and of rectangular xz section varying from 2.12 x 4.01 nm2 to 14.90 x 29.22 nm2 (X = Z/2). The bicrystals were constructed with a symmetrical Σ17(530) tilt boundary lying on the xy midplane. The
characteristics of the boundary are given in Table 1. The shear coupling factor β represents the shear displacement in x direction contributed by the migration of the bo
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