On the Interactions between Lattice Dislocations and Grain Boundaries in Ordered Compounds
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ON THE INTERACTIONS BETWEEN LATTICE DISLOCATIONS AND GRAIN BOUNDARIES IN ORDERED COMPOUNDS J. Th. M. DE of HOSSON*, B.J. PESTMAN*, V. VITEK** "Department Applied Physics, University of Groningen, Zernike Complex, Nijenborgh 4, 9747 AG Groningen, The Netherlands. ".'Departmentof Materials Science & Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. ABSTRACT The interaction of 1/2 screw dislocations with symmetric [1 1 0] tilt boundaries was investigated by atomistic simulations using many-body potentials representing ordered compounds. The calculations were performed with and without an applied shear stress. The observations were: absorption into the grain boundary, attraction of a lattice Shockley partial dislocation towards the grain boundary and transmission through the grain boundary under the influence of a shear stress. INTRODUCTION Polycrystalline Ni 3AI, with a LI 2 structure, is one of the ordered compounds that is a very promising candidate for various applications, such as turbine blades used by the aircraft industry. If useful structural materials based on these intermetallic ordered compounds are ever to be further developed, it is crucial to scrutinize the reasons for their brittle behaviour. A possible reason could be found in low cohesion of the grain boundaries. If the energy of a grain boundary is not much lower than the sum of the energies of the two surfaces that are created by fracturing along the grain boundaries, inter-granular fracture can occur by simple de-cohesion of the grain boundary. This paper, however, deals with a different approach by considering explicitly the plasticity near grain boundaries. Consequently, the ultimate objective of the work is to establish a link between inter-granular brittleness and the interaction between dislocations and grain boundaries. Indeed, there exists experimental evidence that in Ni3A1 the dislocation mobility in the vicinity of grain boundaries may be strongly enhanced when ductilization takes place [1][2] and that plastic flow precedes inter-granular fracture [3]. Considering these experiments, it might be reasoned that the passage of gliding dislocations arriving from the lattice might be hindered by grain boundaries. COMPUTER MODELLING EXPERIMENTS In the modelling study, Finnis-Sinclair potentials representing Ni3AI [4] were used for the description of interatomic forces. For the simulations, the following procedure was used. First, the grain boundary was relaxed, using a standard gradient method; details are described elsewhere [5]. Secondly, a computational block for the relaxation of the dislocation near the grain boundary was constructed. The computational block of the relaxed grain boundary was extended, according to the periodicity of the CSL, to form a block of more than 40 b x 40 b (b is the magnitude of the Burgers vector) perpendicular to the dislocation line. Next, the displacement field of a 1/2[1 1 0] dislocation was imposed with its elastic center initially positioned at such a distance from the grain boundary th
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