Electron Diffraction Study on the Crystal Structure of a Ternary Intermetallic Compound Co3AICx

Intermetallic compound Co3A1Cx, or called ic-phase, has been reported to assume the E21, or Perovskite structure. In the present work its crystal structure is critically reinvestigated in the two-phase alloys containing κ-phase in the Co primary solid solution matrix, α(Co), using conventional electron diffraction. It is shown that the crystal structure of κ-phase is a derivative of E21 being a cubic structure composed of eight E21 sub-unit cells in a half of which the body center sites are not occupied by carbon atoms. As a result, its space group is Fm3m and the chemical formula should be Co3AIC0.5. It is also found that the lattice parameter of the phase is about twice as large as that of α(Co). Orientation relationship of the κ-phase with the matrix α(Co) is found to be similar to the case for the γ' phase with the fcc γmatrix in a Ni-base superalloy since three orthogonal axes of κ-phase are parallel to those of α(Co). Lattice misfit under this orientation relationship between the two phases is found to be about 2.5%.