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 γ-TiAl alloys with 62.5 and 60.0at.%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 L1₀ ordered γ-TiAl matrix, in the course of the phase transformation from the Al₅ Ti₃ long-period superstructure phase at lower temperatures to the h-Al₂ Ti phase at higher temperatures. The SRO structures are composed of three types of ordered cluster: Ti₄ Al type (square shaped), Ti₃ Al type (fat rhombus shaped) and Ti₂ Al type (lean rhombus shaped). The ordered clusters form local microdomains of the Al₅ Ti₃ and Al₃ Ti₂ superstructures in addition to the Al₅ Ti₃ and h-Al₂ Ti 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.