CONVERGENT-BEAM ELECTRON-DIFFRACTION ANALYSIS OF THE OMEGA PHASE IN AN AL-4.0 CU-0.5 MG-0.5 AG ALLOY

Convergent-beam electron diffraction (CBED) was employed to determine the point group of the OMEGA-phase in an Al-4.0 Cu-0.5 Mg-0.5 Ag (wt%) alloy. The [100]OMEGA-parallel-to <112>alpha-CBED pattern had 4 mm symmetry and not 2 mm as expected for the orthorhombic-OMEGA structure. The CBED symmetries along the [010]OMEGA-parallel-to <110>alpha and [001]OMEGA-parallel-to <111>alpha-zone axes were 2 mm and the only point group consistent with these symmetries is 4/mmm. Thus, the crystal structure of the OMEGA-phase is not hexagonal, monoclinic, or orthorhombic as proposed by others, but tetragonal. Electron diffraction patterns of the OMEGA-phase can be indexed on the basis of a tetragonal unit-cell with parameters a = b = 0.6066 nm, c = 0.496 nm. In comparison, the equilibrium-theta-(Al2Cu) phase in Al-Cu alloys has a tetragonal structure with unit-cell parameters a = b = 0.6066 nm, c = 0.4874 nm. The 1.76% increase in the c-parameter is sufficient for the tetragonal phase (designated theta-M) to attain nearly perfect atomic matching on the {111}alpha planes and still maintain 4-fold symmetry along the [001]theta-M parallel-to <112>alpha direction. In addition, the number of variants, hexagonal morphology and preferred growth of the theta-M phase along <112>-alpha are consistent with symmetry considerations, the proposed theta-M crystal structure and experimental TEM observations.