Effects of Grain Boundary Constraint on the Constitutive Response of Tantalum Bicrystals
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Effects of Grain Boundary Constraint on the Constitutive Response of Tantalum Bicrystals A. Ziegler1, G. H. Campbell1, M. Kumar1 and J. S. Stölken2 1
Material Science and Technology Division, Chemistry and Materials Science Directorate New Technologies Engineering Division, Engineering Directorate Lawrence Livermore National Laboratory, University of California, Livermore, CA 94551, USA 2
ABSTRACT The role of grain boundary constraint in strain localization, slip system activation, slip transmission, and the concomitant constitutive response was examined performing a series of uniaxial compression tests on tantalum bicrystals. Tantalum single crystals were diffusion bonded to form a (011) twist boundary and compressed along the [011] direction. The resulting threedimensional deformation was analyzed via volume reconstruction. With this technique, both the effective states of stress and strain over the cross-sectional area could be measured as a function of distance from the twist boundary, revealing a highly constrained grain boundary region. Post-test metallurgical characterization was performed using Electron Back-Scattered-Diffraction (EBSD) maps. The results, a spatial distribution of slip patterning and mapping of crystal rotation around the twist-boundary, were analyzed and compared to the known behavior of the individual single crystals. A rather large area near the grain boundary revealed no crystal rotation. Instead, patterns of alternating crystal rotation similar to single crystal experiments were found to be some distance away (~1mm) from the immediate grain boundary region, indicating the large length scale of the rotation free region. INTRODUCTION Crystal plasticity models rely on essential information obtained from experiments analyzing the materials response to mechanical deformation. Such information is critical when considering the rather complex nature of polycrystal deformation that, besides developing heterogeneous patterns of local slip and grain boundary-dislocation interactions, also shows pronounced length scale effects, i.e., particle hardening, nano-indentation hardening, Hall-Petch effect. This length scale effect is crucial when attempting to understand and realistically model plastic deformation mechanisms at small scales. Finite element calculations [1,2] have demonstrated that heterogeneous intergranular deformation patterns are a result of the combination of compatibility and equilibrium criteria of neighboring grains. However, the grain boundary itself with its characteristic and dynamic phenomena such as slip transmission and blocking, dislocation nucleation and dislocation source activation [3] has been ignored as such in crystal plasticity models. Efforts to incorporate grain boundaries into such crystal plasticity models have typically regarded the grain boundary as merely a surface of displacement continuity. An alternative approach is required for theoretical models to capture complex, grain boundary mediated phenomena, such as hardening of a polycrystalline material
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