Indentation Techniques for the Study of Deformation Across Grain Boundaries
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Indentation Techniques for the Study of Deformation across Grain Boundaries K. A. Nibur and D. F. Bahr Mechanical and Materials Engineering, Washington State University, Pullman Wa ABSTRACT The mechanism by which deformation is transferred across grain boundaries and ways in which boundaries of different misorientations impact this process has been studied using indentation testing. This information could be useful in designing texture of nanocrystalline materials to maximize their mechanical properties for specific applications. Atomic force microscopy (AFM) and orientation imaging microscopy (OIM) has been combined to identify slip systems activated around indentations. When indentations are placed near grain boundaries, slip steps can be imaged on both sides of the boundary and the associated slip systems of each grain can be determined. Dislocation pile ups have been observed around indentations near boundaries which do not share a common slip direction with the active slip planes of either grain, and slip steps have been seen to traverse boundaries when these shared slip directions are present. INTRODUCTION Nanoindentation testing is being examined as a technique for studying deformation on lengths scales which bridge the gap between atomistic studies, such as transmission electron microscopy (TEM), and bulk testing methods such as tensile/compression testing [1,2,3]. The recent insurgence of research on nanocrystalline materials triggers the need for a better understanding of the mechanisms by which deformation crosses grain boundaries. Because many nanocrystalline specimens are small, microhardness testing has become a popular means of characterizing mechanical properties, however this technique measures properties from many grains of various orientations [4,5,6]. The ability to link atomistic deformation models and bulk material properties requires an understanding of the mechanisms by which deformation crosses boundaries such as grain boundaries, phase boundaries and precipitates. Because of the ability to place indentations at precise locations, it should be possible to study how dislocations interact with these boundaries using indentation testing. This study looks at ways in which indentations smaller than the grain size of the material can be placed near grain boundaries to observe the deformation transferred to adjacent grains. A combination of atomic force microscopy (AFM) and orientation imaging microscopy (OIM) is used to characterize the indentations [7]. Slip steps around the indentation can be clearly imaged using AFM and can then be correlated to the responsible slip system using the grain orientations obtained through OIM. To demonstrate this technique, this paper will show Vicker’s indentations in large grain nickel specimens, however, the techniques shown here could be scaled down to the nanometer scale and repeated in finer grained materials. EXPERIMENTAL PROCEDURE Several experiments have been done with indentations near grain boundaries of various misorientations and deformation c
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