TEM Analysis of Long-Period Superstructures in TiAl Single Crystal with Composition Gradient
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TEM Analysis of Long-Period Superstructures in TiAl Single Crystal with Composition Gradient S. Hata, K. Shiraishi, N. Kuwano1, M. Itakura, Y. Tomokiyo, T. Nakano2 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 Art, 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 The ordering mechanism of long-period superstructures (LPSs) in Al-rich Ti-Al alloys was studied using a TiAl single crystal with a composition gradient. A TiAl single crystal with gradient compositions from 55 to 75 at.% Al was prepared by annealing in a molten Al at 1234°C. The single crystal exhibits long-period ordering into different LPSs depending on the Al concentration as follows: an Al5Ti3 type short-range order, h-Al2Ti and one-dimensional antiphase domain structures. These LPSs show an orientation relationship in which Al (002) layers of the LPSs are parallel to those of the TiAl matrix. The atomic arrangements of the LPSs are characterized in common as the alternate stacking of the Al (002) layers and Ti-Al (002) layers. It is thus concluded that the ordering of this type of LPSs and the phase transition between these LPSs are explained as structural changes in Ti-Al (002) layers of the Al-rich L10-TiAl crystal. INTRODUCTION Non-stoichiometric TiAl alloys that are rich in Al exhibit ordering into long-period superstructures (LPSs), such as Al5Ti3 [1], h-Al2Ti [2] and so on. This ordering phenomenon is explained as ordering of excess Al atoms in Ti (002) layers of the L10-TiAl matrix [3-10]. According to the Ti-Al phase diagram in figure 1 [10], there are some other LPS phases near the TiAl phase region: r-Al2Ti, a one-dimensional antiphase domain structure (1d-APS), Al3Ti, and so on. They are L12-based long-period structures and often formed with the Al5Ti3 and/or h-Al2Ti LPSs in Al-rich Ti-Al alloys [6-14]. Many research groups studied phase equilibria in the Al-rich part of the Ti-Al system. However, ordering processes into such a mixed state of the different LPSs are not understood well. This may be due to the fact that the long-period ordering is quite sensitive to the Al concentration and annealing conditions [1-14]. The purpose of the present study is to reach a general understanding of the ordering mechanism of LPSs in Al-rich Ti-Al alloys. For this purpose, we prepared a TiAl single crystal with gradient compositions and examined the formation of LPSs systematically as a function of Al concentration. Such a specimen preparation is known as a diffusion couple method to study a phase formation in a wide composition range under a fixed annealing condition [15,16]. Our original point is using a single crystal. The composition-gradient TiAl single crystal is effectiv
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