HRTEM Observation of Partially Ordered Long-Period Superstructures in Al-Rich TiAl Alloys
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HRTEM Observation of Partially Ordered Long-Period Superstructures in Al-Rich TiAl Alloys S. Hata, K. Higuchi, T. Mitate, N. Kuwano1, M. Itakura, Y. Tomokiyo, T. Nakano2, Y. Nagasawa2 and Y. Umakoshi2 Department of Applied Science for Electronics and Materials, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 1 Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2 Department of Materials Science and Engineering & Handai Frontier Research Center, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan ABSTRACT Atomistic microstructures of long-period superstructures in Al-rich TiAl alloys were investigated by high-resolution transmission electron microscopy. The multi-slice image simulation revealed that Al-atoms in Ti-rich (002) layers of the superstructures are imaged as very bright dots under appropriate observing conditions. By enhancing the contrast of the very bright dots using an image processing technique, partially ordered long-period superstructures were clearly characterized. For the short-range order state, A4B, A2B and A3B type ordered-clusters are generated in the Ti-rich (002) layers. These ordered-clusters are in contact with each other, and form local microdomains of various superstructures, such as Al5Ti3, h-Al2Ti and so on. For the Al5Ti3 and h-Al2Ti ordered states, the ordered-clusters are tiled periodically to form the Al5Ti3 or h-Al2Ti domains and characteristic anti-phase boundary structures.
INTRODUCTION Non-stoichiometric γ-TiAl alloys that are rich in Al form metastable long-period superstructures in the L10 matrix, as illustrated in figure 1. These superstructures are characterized by the periodic arrangements of Al atoms in Ti-rich (002) layers. The periodic arrangements of Al atoms are described using three different ordered-clusters: A4B type (square-shaped), A2B type (lean rhombus-shaped) and A3B type (fat rhombus-shaped), as drawn by solid lines in figure 1. Some characteristic features of the long-period superstructures were reported previously. Firstly, the crystal structure and microstructure strongly depend on the Al concentration and the heat treatment condition [1-9]. Secondly, the long-period superstructure phases are often in partially ordered states, which exhibit, for example, short-range order (SRO) and anti-phase boundary (APB) structures [1-14]. Some research groups [2,3,6,8,9,13,14] focused on the partially ordered states for understanding the mechanism of the long-period ordering in Al-rich TiAl. However, their structural details are still controversial, because of insufficient experimental information in atomic level. A number of high-resolution transmission electron microscopy (HRTEM) studies for such partially ordered materials were reported [15-25]. Hata et al. [23], for example, succeeded in analysing the SRO structures in Al-rich TiAl alloys by HRTEM with an image processing technique [24,25]. This technique w
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