Creep in Polycrystalline Aluminum
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EXPERIMENTAL The material used in this study was five-nines pure aluminum. It had been annealed for several hours near its melting point to produce grains with diameters greater than 1 mm. The creep experiments were performed by heating the sample in the SEM and then holding it in tension at a fixed load. A flat tensile samples was used and the surface was cleaned by ion-milling before the test. In this experiment the temperature of the sample was approximately 250 0C and the load was held at a value of 38N; near the end of the test the temperature was increased to 300'C. Both temperature and load fluctuated during the test and had to be readjusted manually. Relative displacements were measured using the DISMAP system by comparing photographs made at various times during the experiment to one made at zero time. These measurements allowed determination of the strain tensor, which in turn could be used to determine the principal strains, the maximum shear strains, and the effective strain. We could also use this information to make a Mohr's circle construction. More details of this system are given in references 3-6. Electron back scattering diffraction patterns were obtained in a scanning electron microscope. A Nordif system was used that included software prepared by HKL, Inc. The sample was examined using this technique after the creep test. We will present the results in the form of inverse pole figures, which use the standard stereographic triangle
231 Mat. Res. Soc. Symp. Proc. Vol. 586 ©2000 Materials Research Society
to present the data; the crystallographic direction parallel to a chosen direction in the sample is plotted as a point in the standard triangle. In this paper we present only the zinverse pole figure. The z-axis was taken as normal to the sample surface, and therefore, the points plotted in the standard triangle represent the crystallographic direction normal to the sample surface at each place the specimen was analyzed. In this application our goal is not to determine specific crystallographic orientations but rather to use this method to determine where changes in crystallographic orientation have occurred. An abrupt change in the orientation would indicate that we have encountered a grain of different orientation, whereas gradual shifts would indicate that internal deformation or sub-boundary formation has taken place. RESULTS The first set of results that we consider are those obtained in the in-situ straining stage in the scanning electron microscope. In this experiment we focused on one triple point in the large grain aluminum sample. A micrograph of its initial state is shown in Figure la; note that the three grains are identified as A, B, and C. At the completion of the test, the same region had the appearance shown in Figure lb. As the test progressed it was clear that the grain boundaries had migrated and developed ridges. Note also that there appears to be a separation between grains A and C. We used features on the surface to analyze the strain developing in the sample. One way in
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