Direct Observations of Grain Boundary Phenomena during Indentation of Al and Al-Mg Thin Films
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Direct Observations of Grain Boundary Phenomena during Indentation of Al and Al-Mg Thin Films W.A. Soer1, J.Th.M. De Hosson1, A.M. Minor2, E.A. Stach3, J.W. Morris, Jr.2,4 1 Dept. of Applied Physics, Netherlands Institute for Metals Research, University of Groningen, Groningen, The Netherlands 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 4 Dept. of Materials Science and Engineering, University of California, Berkeley, CA, USA ABSTRACT The deformation behavior of Al and Al-Mg thin films has been studied with the unique experimental approach of in-situ nanoindentation in a transmission electron microscope. This paper concentrates on the role of solute Mg additions in the transfer of plasticity across grain boundaries. The investigated Al alloys were deposited onto a Si substrate as thin films with a thickness of 200-300 nm and Mg concentrations of 0, 1.1, 1.8, 2.6 and 5.0 wt% Mg. The results show that in the Al-Mg alloys, the solutes effectively pin high-angle grain boundaries, while in pure Al considerable grain boundary motion is observed at room temperature. The mobility of low-angle grain boundaries is however not affected by the presence of Mg. In addition, Mg was observed to affect dislocation dynamics in the matrix. INTRODUCTION The recently developed technique of in-situ nanoindentation in a transmission electron microscope (TEM) [1,2] bears some significant advantages over conventional nanoindentation [3,4] and post-mortem TEM analysis of indented specimens. Most importantly, it allows for direct observation of dislocation nucleation and motion, as well as other phenomena that occur during indentation. Reports of relaxation of indentation damage after unloading [5] indicate that direct observation is indeed useful. Furthermore, as the indenter can be positioned on the specimen accurately, regions of interest (e.g. specific crystal orientations or grain boundaries) can be specifically selected for indentation. Several classes of materials, both amorphous and crystalline, have been studied using in-situ nanoindentation. Measurements on Al thin films have quantitatively related dislocation behavior to force-displacement data [5,6]. Furthermore, extensive grain boundary movement was observed to be an important deformation mechanism [7]. Less in-situ work has been devoted to the influence of impurities on the dislocation dynamics and therefore we have focused on in-situ observations of Al-Mg alloys in this study. A significant part of the research on Al-Mg has traditionally been focused on the repeated yielding that occurs during plastic deformation of these alloys. This phenomenon is known as either the Portevin - Le Châtelier (PL) effect or serrated yielding [8] and leads to a negative strain rate sensitivity caused by interaction between dislocations and mobile solute atoms [9]. In recent years the PL effect has been investigated in several deformation modes, incl
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