Investigation of grain boundary dynamics in metals

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Hereby we apply for funding of the second phase of the research project , which originally started in 02/2013. By combining experimental investigations of structurally different grain boundaries in specially grown bicrystals of aluminum and copper as well as molecular dynamics simulations, the proposed research will further advance the understanding of grain boundary migration induced by various driving forces during microstructure evolution in metallic materials. The proposed program consists of three parts. In the first one, our investigation of stress induced grain boundary migration shall be continued. During the first phase of the project, an experimental setup has been constructed, which allows the in situ investigation of bicrystalline samples subjected to a pure shear stress at elevated temperatures. Using this setup, it is planned to compare grain boundary migration induced by a shear stress with such induced by tensile stress. The acquired data will allow for verification, or if needed improvement, of the existing model of stress induced grain boundary migration coupled with shear deformation. The same setup will then be used at a higher mechanical stress to investigate rigid gliding of the grains along the boundary plane (grain boundary sliding). In the second part the investigation of special Sigma 3 CSL tilt grain boundaries, which have the same geometry as coherent or incoherent twin boundaries, is planned. During the first phase of the project boundaries of this type have been investigated under capillary driving force and a similar behavior as for symmetrical low angle tilt grain boundaries was observed. Now this study shall be completed by investigation of the same Sigma 3 CSL tilt grain boundaries under stress induced driving force and of mixed boundaries with tilt as well as twist components which still fulfill the Sigma 3 CSL relationship. Boundaries of mixed type are the most common in real microstructure and for low ange grain boundaries were found out to behave strongly different than those with pure tilt geometry. So, the similar effects on the kinetics of the Sigma 3 CSL boundaries are expected as well. The third part will deal with the investigation of isolated grains during shrinkage at elevated temperatures. A special bridgman type furnace, allowing the growth of bicrystals with such isolated grains embedded in a monocrystalline matrix, is under construction and will be completed until the end of the ongoing first project phase. Systematic experimental investigation of the bicrystals at elevated temperartures will then be conducted to verify MD simulations of shrinking grains which have been conducted already and imply a strong effect of the orientation between grain and matrix on shrinking rate, shape evolution and grain rotation.