In Al-rich TiAl crystals, several long-period superstructures may appear depending on Al composition, annealing temperature and annealing time. Amongst these, Al5Ti3 and h-Al2Ti contain pure Al (002) layers, as in the L10 structure of the matrix, alternating with Ti (002) layers that exhibit an ordered arrangement of the Al atoms in excess. In single crystals with compositions ranging from Ti-54.7at.%Al to Ti-62.5at.%Al annealed at 1200°C, the Al5Ti3 long-period superstructure embedded in the L10 matrix develops with increasing Al concentration to finally transform fully into h-Al2Ti for Ti-62.5at.%Al. On the other hand, Al5Ti3 precipitates grow with annealing time at 500°C in Ti-58.0at.%Al.
The effects of the Al5Ti3 and h-Al2Ti superstructures on slip properties of 1/2<110] ordinary dislocations are examined both at a macroscopic and a microscopic level. The CRSS for 1/2<110] ordinary slip increases with Al5Ti3 ordering depending on Al composition, or of annealing time in the case of Ti-58.0at.%Al. Dislocations with 1/2<110] Burgers vector group into fourfold configurations to avoid the trailing of extended APBs in Al5Ti3. The CRSS for slip in the <110] direction further increases with the formation of h-Al2Ti particles within the L10 matrix in Ti-62.5at.%Al. By contrast, Ti-62.5at.%Al fully transformed into Al5Ti3 exhibits a CRSS significantly lower than that of the two-phase alloy.
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This work was supported by a Grant-in-Aid for Scientific Research Development and the 21st COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Japanese Ministry of Education, Sports, Culture, Science and Technology. This work has partly been carried out at the Strategic Research Base “Handai Frontier Research Center” supported by the Japanese Government’s Special Coordination Fund for Promoting Science and Technology. T. Nakano would like to thank Iketani Science and Technology Foundation for a financial support. P. Veyssière wishes to express his thanks to the COE program for support, to the Osaka group for providing excellent working facilities. The work was part of a PICS program sponsored by CNRS (France).
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Nakano, T., Hayashi, K., Umakoshi, Y. et al. Effects of long-period superstructures on plastic properties in Al-rich TiAl single crystals. MRS Online Proceedings Library 842, 501–506 (2004). https://doi.org/10.1557/PROC-842-S7.4