The strengthening effect of grain boundaries is one of the key components in the development of modern structural materials which is responsible for beneficial effects arising from ultra fine grained and nanostructured materials, and grain boundary engineering. The precise nature of the often beneficial effects of grain boundaries, however, has not been understood to a level which would allow for theory-guided optimization of microstructures and accelerated alloy development. In this research, based on collaboration between Michigan State University (MSU) and the Max Planck Institut für Eisenforschung (MPIE) in Düsseldorf, Germany, the investigators combine, improve, and apply recently developed approaches to examine and quantify the micromechanical behavior of grain boundaries using techniques based upon microscopy and conical indentation tests. Combined use of electron backscattered electron pattern mapping, atomic force microscopy, 3-D x-ray characterization, and focused ion beam/electron imaging methods provide quantified measures of activated slip behavior. These methods are used to assess deformation caused by conical indentation to relate detailed indentation topographies to the plastic anisotropy of single crystals. The methods provide an efficiency advantage over traditional single and bi-crystal experiments. Initially, ?single crystal? characterization will occur using indents in grain interiors, and as modeling fidelity is developed using dislocation density based constitutive models implemented in crystal plasticity finite element analyses, indentations near and on grain boundaries will be used to assess the grain boundary transmissivity of slip and mechanical twinning behavior. From observed transmissivity behavior, models for grain boundary transmissivity will be implemented into the constitutive models.
The implemented approach, combined with state of the art characterization and simulation methods, can lead to a quantitative understanding of the interplay between grain boundaries and heterogeneous plasticity in titanium polycrystals. Complementary skills in characterization (MSU) and simulation (MPIE) are available to reach the stated goals. Extensive exchanges between the two laboratories occur principally during summers in order to integrate experimental and analytical outcomes.
