CRYSTALLINE APPROXIMANTS TO QUASI-CRYSTALLINE TI-MN

Icosahedral-phase (i-phase) Ti62Mn36Si2 was transformed by annealing at temperatures between 700 and 1150-degrees-C for times ranging from 15 min to 1 week to produce three new crystalline phases. We present the results of transmission electron microscopy of these phases using selected-area diffraction (SAD), convergent-beam electron diffraction and energy-dispersive X-ray spectroscopy. The Bravais lattices of all three phases are reported. Intense modulations in the SAD patterns of all three crystalline phases produce patterns that strongly resemble the i-phase SAD patterns. These modulations demonstrate the existence of local clusters of atoms with icosahedral symmetry within the crystallographic unit cells. We present structural models based on crystalline packings of icosahedral clusters of atoms that closely predict the observed SAD modulations, the experimentally determined Bravais lattices, the unit-cell sizes and the internal symmetries of two of the phases. Our model for the third phase is incomplete; although consistent with all experimental data, the model can only predict one of the three lattice parameters. Detailed study of these icosahedrally ordered crystalline transformation products may guide efforts to determine the atomic locations in the i-phase.