Statics and Dynamics of Grain Boundaries in Ni 3 Al
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STA TICS AND D YNAMICS OF GRAIN BOUNDARIES IN Ni3 A/ DIANA FARKAS Department of Materials Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
ABSTRACT Various modeling techniques have been used to study the structure of grain boundaries in ordered compounds, particularly Ni 3 AI. The techniques include computer simulation, group theoretical analysis and the cluster variation method for high temperature effects. A multiplicity of possible grain boundary structures was analyzed for different misorientations and grain boundary plane location. 1The influence of several alloy properties like atomic size differences and deviation from stochiometry on the occurrence of these different structures is analyzed. The implications of these results for grain boundary misorientation distribution, grain boundary plane location and faceting behavior are discussed and compared to experimental findings. The possibility of orderdisorder transitions in the grain boundary region is also discussed.
INTRODUCTION The present work is a summary of a basic effort in understanding the particular behavior of grain boundaries in ordered alloys, especially in NikAI. Technological applications envisaged for Ni 3 AI include high temperature structural applications which become possible for the polycrystalline material doped with boron to overcome grain boundary brittleness. These applications triggered a large research effort in the understanding of the reasons for this behavior. From the beginning, the problem was recognized to be a very complex one requiring a study from various standpoints, from a basic electronic structure study to dislocation studies and grain boundary plastic behavior. The work we sill describe in this paper concentrated on structural aspects. It seemed to us that a complete structural understanding of the grain boundary region is a necessary step for any further study. It also seems that the study should not be circumscribed to Ni 3 AI alone but rather, contribute to the general understanding of interfaces in ordered alloys. The basic point to be addressed in the present work is the structural particularities that may be expected in grain boundaries in ordered alloys as opposed to pure metals. The methods that can be useful in such a study are the traditional ones for grain boundary structure. These are geometrical methods at first, and then atomistic simulations. We discuss here a series of results of general geometrical methods for interphases in ordered alloys. It is important to point out that although hard sphere or variable sphere calculations are simple methods to use as examples, the important geometrical results are those derived only on symmetry arguments alone using a group theoretical analysis. These results of course, are more generally valid than those of computer simulation, since they do not rely on accurate interatomic potentials. In the present work, we present atomistic simulations that were carried out following the results of geometrical methods. The atomistic simulat
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