Quantitative Measurements of Grain Boundary Sliding in an Ultrafine-Grained Al Alloy by Atomic Force Microscopy
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INTRODUCTION
IN recent years, ultrafine-grained (UFG) materials that are characterized by grain sizes in the range 300 nm to 800 nm have been the subject of considerable interest for two primary reasons: (1) techniques for the fabrication of bulk UFG materials have been developed successfully[1] and (2) as a result of grain refinement, UFG materials exhibit superior properties over their coarse-grained counterparts. In this article, quantitative measurements of grain boundary sliding (GBS) in UFG 5083 Al by means of atomic force microscopy (AFM), are reported and analyzed. The alloy in this case was processed by cryomilling followed by hot isostatic pressing (HIP) and extrusion. The work was motivated by three primary considerations. First, in a previous investigation,[3] the deformation behavior of bulk UFG 5083 Al was studied in the temperature range of 523 K to 648 K (250 C to 375 C). An analysis of the experimental data provided evidence for the occurrence of GBS. Such evidence was inferred[2] from the presence of substructural features that, in general, accompany GBS and include voids, striated bands, and the emergence of new grains. Second, recent experimental data on thermal stability in UFG 5083 Al suggested[3] that the alloy can be used in manufacturing structural components for use at high temperatures in the range 473 K to 550 K (200 C to
277 C). However, to improve the safety of a structural component to be made from UFG 5083 Al, it is important that the phenomenon of GBS in the alloy be studied quantitatively. As reported elsewhere,[4–7] the occurrence of boundary sliding can lead to premature failure of structural components because of the nucleation of cavities at triple points and ledges. Finally, a major problem that was encountered in accurately monitoring GBS in UFG materials was that traditional techniques such as surface marker lines used in measuring sliding in micrograined materials[8–13] did not have sufficient resolution for the former materials, which have grain sizes as fine as 400 nm. Recently, a solution for the preceding problem has become available: Investigators demonstrated that AFM provides a powerful technique for obtaining high-resolution images of the surface topography of bulk materials. Examples of recent uses of AFM for investigating topography include the following: slip bands, initiation of cracks under fatigue conditions,[14,15] nanoindentation,[16] and the surface topography of a Zn-Al alloy[17] processed by equal-channel angular pressing (ECAP). The results of those investigations showed that AFM serves as a highresolution surface technique that can detect with a high accuracy surface offsets resulting from the occurrence of GBS. II.
EXPERIMENTAL PROCEDURE
A. Processing JUNG H. HAN, Graduate Research Assistant, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical Engineering and Materials Science, University of California—Irvine, Irvine, CA 92697-2575. Contact e-mail: [email protected] Manuscript submitted December 7, 2010. Article published online Sep
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