Variable Elastic-Plastic Properties of the Grain Boundaries and Their Effect on the Macroscopic Flow Stress of Nanocryst
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Variable Elastic-Plastic Properties of the Grain Boundaries and Their Effect on the Macroscopic Flow Stress of Nanocrystalline Metals Malgorzata Lewandowska1, Romuald Dobosz1, Krzysztof J. Kurzydlowski1 1
Warsaw University of Technology, Faculty of Materials Science and Engineering,
Woloska 141, 02-507 Warsaw, POLAND.
ABSTRACT The paper reports new experimental results describing properties and microstructure of nanocrystalline metals. Nano- and sub-micron aluminium has been produced by hydrostatic extrusion at ambient tempearture. The structures have been quantified in terms of size of grains and misorientation of the grain boundaries. Different average size of grains, variable normalized width of grain size distribution and changing grain boundary misorientation distribution functions have been revealed depending on processing parameters. The results of the tensile tests showed that the average grain size, grain size distribution and the distribution function of misorientation angles influence the flow stress of obtained nano-metals. In order to explain the observed difference in the properties of nano- and micro-sized aluminium alloys, a Finite Element Method models have been developed, which assumes that both grain boundaries and grain interiors may accommodated elastic and non-linear plastic deformation. These models assumed true geometry of grains (which differed in size and shape). Also, variable mechanical properties of grain boundaries have been taken into account (elastic modulus, yield strength and work hardening rate). The results of modelling explain in a semiquantitative way macroscopic deformation of nano-crystalline aggregates. In particular, they illustrate the importance of the interplay between properties of grain boundaries and grain interiors in elastic and plastic regime. INTRODUCTION A vast majority of engineering materials are used in polycrystalline form which makes grain boundaries one of the most important microstructural elements. Their role is particularly important in the case of fine grained and nanocrystalline materials as their surface area per unit volume is substantially greater than in conventional micro-structured polycrystalline counterparts. This can be supported by simple geometric considerations which indicate that for an average grain size of 10 nm the volume fraction of atoms located at grain boundaries is 25% assuming 1 nm thickness of grain boundaries. As a result, the properties of nanocrystalline materials are largely governed by grain boundaries. The effect of the grain boundaries on flow stress of metals has been a subject of numerous experimental and theoretical studies [1-2]. It is usually described by the Hall-Petch relationship which predicts linear dependence of the flow stress on the inverse square root of grain size. Hall-Petch relationship was experimentally proven for a wide range of materials with average grain sizes ranging from several hundreds of microns to dozens of nanometres.
However, it has been found, in this context, that below a
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