Grain Boundary Responses to Local and Applied Stress: An In Situ TEM Deformation Study
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0976-EE02-01
Grain Boundary Responses to Local and Applied Stress: An In Situ TEM Deformation Study Bryan Miller, Jamey Fenske, Dong Su, Chung-Ming Li, Lisa Dougherty, and Ian M. Robertson Materials Science and Enigneering, University of Illinois at Urbana-Champaign, 1304 W. Green St., Urbana, IL, 61801
ABSTRACT Deformation experiments at temperatures between 300 and 750 K have been performed in situ in the transmission electron microscope to investigate dislocation interactions and reactions with grain boundaries and other obstacles. Dislocations, both partial and perfect, as well as deformation twins have been observed being emitted from grain boundaries and, in some cases, even the same grain boundary. The ejection of dislocations from the grain boundary can result in its partial or total annihilation. In the latter case, the disintegration of the grain boundary was accompanied by grain growth and a change in misorientation. INTRODUCTION Controlling the grain size and manipulating the grain boundary misorientation distribution are common methods for enhancing and tailoring the mechanical properties of materials[1, 2]. For example, the reduction of grain size through equal channel angular processing (ECAP) results in an increase in strength which is accompanied by a decrease in ductility. However, it has been shown that by increasing the number of passes, which changes the distribution of the type of grain boundaries to one containing a high fraction of high-angle grain boundaries, the ductility can largely be recovered with only a small reduction in strength. This effect has been attributed to the deformation within the grain, and the sliding and rotation of the high-angle grain boundaries [1, 3, 4]. Further refining of the grain size produces nanograined metals which also have high strength and low ductility [2, 5-8], although there are recent reports of improved ductility with some loss of strength through dynamic grain growth [8]. In systems with nanosized grains, the deformation mechanisms are still being debated although there is growing evidence supporting dislocation processes at all but the smallest grain sizes (
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