Precipitation of an icosahedrally symmetric ordered phase in Zr-Ti-Cu-Ni-Al metallic glasses

Results from a study of the precipitation of an icosahedrally symmetric ordered phase in Zr_62-xTi_xCu₂₀Ni ₈Al₁₀ amorphous alloys are presented. The data indicate the development of icosahedral short-range order (similar to that in the ordered phase) in the undercooled melt during quenching, and in the amorphous phase with annealing in the supercooled liquid region. Precipitation is controlled by long-range diffusion in alloys made with less than 3 at.% Ti. The resulting lower nucleation rates and longer induction times lead to a fine microstructure that contains icosahedral phase (i phase) grains ranging from 50 to 100 nm in diameter. In the glasses made with 5-7.5 at.% Ti, the composition of the precipitating phase is more similar to that of the glass, lowering the kinetic nucleation barrier and leading to an extremely fine nanostructured devitrified material, with i phase grain sizes from 2 to 5 nm. The x-ray diffraction pattern from this phase is similar to that of the i phase. However, because the grain size is nearly the same as the edge length of the unit cell of a high-order crystal approximate phase, no distinction can be made between the i phase and crystal approximate phases in such nanostructured materials.