Short-range order in Al-rich γ-TiAl alloys studied by high-resolution transmission electron microscopy with image processing

The atomic arrangements of short-range order (SRO) in two Al-rich gamma-TiAl alloys with 62.5 and 60.0 at.% Al respectively have been investigated by high-resolution transmission electron microscopy with image processing. For both alloys, SRO structures are formed in Ti-rich (002) layers of the L1(0) ordered gamma-TiAl matrix, in the course of the phase transformation from the Al5Ti3 long-period superstructure phase at lower temperatures to the h-Al2Ti phase at higher temperatures. The SRO structures are composed of three types of ordered cluster: Ti4Al type (square shaped), Ti3Al type (fat rhombus shaped) and Ti2Al type (lean rhombus shaped). The ordered clusters form local microdomains of the Al5Ti3' and Al3Ti2 superstructures in addition to the Al5Ti3 and h-Al2Ti superstructures, by different methods of tiling the ordered clusters. Such ordered clusters or microdomains tend to be in contact with each other through common {310) boundaries. This results in peculiar diffuse streaks in diffraction connecting intensity maxima arising from the long-period superstructures.