Ordering process of Al 5Ti 3, h-Al 2Ti and r-Al 2Ti with f.c.c.-based long-period superstructures in rapidly solidified Al-rich TiAl alloys

Abstract

Change in microstructure and stability of superstructural phases in Al-rich TiAl alloys containing 58.0–62.5 at.% Al were investigated using melt-spun ribbons. Ordering processes of long-period ordered phases such as Al 5Ti 3, h-Al 2Ti and r-Al 2Ti in the L1 0 matrix during annealing were examined. The presence of Al 5Ti 3 and h-Al 2Ti phases in the L1 0 matrix was confirmed in melt-spun Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons by electron diffraction patterns, while diffuse scattering corresponding to the Al 5Ti 3 superstructure appeared in Ti–58.0 at.% Al ribbon. In Ti–58.0 at.% Al ribbon, the Al 5Ti 3 phase developed as an island in the L1 0 matrix having an obscure coherent boundary at and below 800°C, while it dissolved during annealing above 800°C. Although the r-Al 2Ti phase was finally formed as an equilibrium phase, the ordering of Al 5Ti 3 and metastable h-Al 2Ti phases in Ti–60.0 at.% Al and Ti–62.5 at.% Al ribbons occurred prior to the precipitation of the r-Al 2Ti during annealing below 800°C. The priority for the ordering process is discussed on the basis of crystal symmetry and periodicity of Al layers parallel to the (002) plane. The anti-phase boundaries (APBs) based on the Al 5Ti 3-type ordering were observed along {110) planes in Ti–62.5 at.% Al ribbon annealed at 700°C and their energies were calculated using the interaction energy between neighbouring atoms.