Calculation of the Electronic Structures of Advanced Intermetallic Compounds and Design for the Improvement of Ductility

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Electronic states of stoichiometric and non-stoichiometric TiAl compound in the course of deformation were calculated using a discrete variational (DV) CHI cluster method. For the stoichiometric TiAl the bond strength between atoms on the adjacent (111) slip planes decreased during deformation, but the decrease was relatively smaller in the [110] direction than in the [101] and [211] directions. This result indicates that the slip deformation is more active in the [110] direction than in the other two directions. The same slip system was expected to be active in the Tiーrich TiAl. However, this is no longer true for the Al-rich TiAl, where the (111), [101] slip system was found to be dominant in the Al-rich TiAl, in agreement with experiments.Alloying effects on the ductility was also investigated in this study. It was shown that the existence of directional Al p - Ti d covalent bonds causes this TiAl compound to be brittle. The addition of those elements into TiAl which weaken p-d interactions but enhance d-d interactions is most effective in improving the ductility.In addition, twin deformation was shown to be activated by substituting Cr or Mn atoms for Al atoms in TiAl.Furthermore, electronic structures of Nb_3Al and Mosi_2 were calculated, and the nature of the atomic interactions operating in these compounds was examined in detail.