The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied
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The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied by Nanoindentations
E. Schweitzer, K. Durst, D. Amberger and M. Göken Department of Materials Science & Engineering, University Erlangen-Nuremberg 91058 Erlangen, Martensstrasse 5, Germany ABSTRACT The strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations. INTRODUCTION The strength of structural materials depends strongly on the structure and properties of grain boundaries. Interfaces in crystalline solids usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the wellknown Hall-Petch relation. In addition, segregation effects and the nucleation of precipitates are more likely to occur at grain boundaries. Although the width of a grain boundary extends only over a few interatomic distances, the effect arising from the presence of a boundary may extend over a much greater distance (1-100 µm). Grain boundaries have also an important influence on corrosion cracking of austenitic stainless steels used as tubing materials. One of the easiest and perhaps most promising approaches to studying the influence of interfaces is to measure their mechanical properties and in particular plastic deformation. And indeed the hardness in the vicinity of grain boundaries has been studied intensively in early work by Westbrook et al. [1, 2]. They used Vickers hardness measurements to study hardness variation at grain boundaries in different metallic and intermetallic materials and found in many cases a strong distance dependence of the hardness. However, the measurements on intermetallic materials strongly depend on the chemical composition. Thus a significant decrease of hardness with incr
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