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U3.05 - Atomistic Study of Twinning-Associated Boundaries in HCP Metals 

HCP metals are widely used as structural materials in many industries, ranging from transport and energy to biomedical applications due to their low density, high specific strength. However, HCP metals show pronounced anisotropy in their mechanical properties and less number of slip systems compared to FCC and BCC metals. In the absence of sufficient number of slip systems in HCP metals, twining is found to be one of the important deformation modes during plastic deformation. At present, we do not have very clear knowledge of properties of twin boundaries in HCP metals at atomic length scale. Understanding atomic structure and chemistry of twin-associated boundaries is crucial to improve mechanical properties of HCP metals. In this work, using first-principles density function theory, we study twinning-associated boundaries (TBs), coherent twin boundaries (CTBs) and coherent basal-prismatic boundary (CBP) in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be), with a focus on structure and solute’s solubility at twin boundaries. We find that the formation of TBs is associated with creation of an excess volume. All the six metals show positive excess volume associated with and CTBs, but the excess volume associated with CTBs and CBP can be positive or negative depending on metal. To understand solubility at TBs, we calculated solubility of solute atoms in Mg, Ti, and Zr for solute positions in bulk, CTB and CBP boundaries and show that, in general, solute atoms have better solubility at CTB and CBP than in bulk. We also found solubility of solute atoms linearly changes with normal strain at CBP, increasing with the normal strain for solute atom with a greater metallic radius than the matrix, and decreasing with the normal strain for solute atom with a smaller metallic radius than the matrix. This suggests that the distribution of solute atoms in bulk, CTB, and CPB varies with stress state, and in turn affects mobility of CTB and CPB boundaries. 

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