Stress Assisted Grain Growth in Ultrafine and Nanocrystalline Aluminum Revealed by in-situ TEM
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1086-U09-04
Stress Assisted Grain Growth in Ultrafine and Nanocrystalline Aluminum Revealed by insitu TEM Frederic Mompiou, Marc Legros, and Daniel Caillard CEMES-CNRS, Toulouse, 31055, France ABSTRACT In-situ straining experiments on nanocrystalline (nc) and ultra fine grain (UFG) Aluminum were performed at room and intermediate temperatures. Both materials exhibit significant stress assisted grain growth. The strain induced by grain boundary motion has been measured in UFG Al, and was found to be on the order of a few percents. These results cannot be interpreted solely within the framework of Displacement Shift Complete (DSC) dislocation motion. We propose here that GB motion occurs via both shuffling and secondary DSC dislocation motion. INTRODUCTION Ultrafine grains (UFG) and nanocrystalline (nc) metals have exceptional mechanical properties such as very high yield stresses. However their structure is unstable and tends to evolve by grain growth even at relatively low temperature. Grain growth seems also favoured by the application of a mechanical stress as shown during fatigue [1] or monotonic tensile experiments [2]. However few studies have focused on the elementary mechanisms that promote this instability and on the role of grain growth in the overall deformation process. Such mechanisms should indeed be crucial in the plasticity of materials where grain boundaries constitute a large volume fraction. For the last few years, in situ transmission electron microscopy (TEM) deformation experiments have provided a well designed tool for probing these mechanisms at the nanoscale. Indeed, it is supposed that, according numerical simulations [3], plasticity mechanisms in nc metals which should imply extensively fast grain boundaries and dislocations motion, could be probe at a timescale and length scale only reached by in-situ TEM. In this article we present the results of in situ straining experiments on self standing nc-Aluminium films at room temperature and on aluminium ultra fine grains (UFG) at moderate temperature. The choice of UFG Aluminum at this given condition was dictated by first, the fact that it should be easier to analyze grain boundary (GB) motion in UFG than in nc materials and second, by the fact that grain boundary motion should be strongly enhanced at higher temperature in association with a lower dislocation activity. EXPERIMENT 180nm thick nc aluminum films were deposited on Si substrate by DC pulsed magnetron sputtering. Once patterned and released from the substrate by reactive ion and XeF2 etching, they form electron transparent free standing films. Aluminum UFG were produced by equal channel angular pressing (ECAP) after 8 passes. The mean grain size was around 40-90nm for nc and 800nm for UFG. In situ experiments were carried out in JEOL 2010 microscope operated at 200kV. Rectangular UFG samples were cut in massive ingots and prepared by electro-chemical
polishing for TEM observations. Grain boundary motion was monitored by means of DVD/HD recording using a MEGAVIEW II camera.
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