SATO Hiroyuki Tohoku University, Graduate School of Engineering, Lecturer, 大学院・工学研究科, 講師 (10225998)
KOIKE Jun-ichi Tohoku University, Graduate School of Engineering, Associate Professor, 大学院・工学研究科, 助教授 (10261588)
OIKAWA Hiroshi Tohoku University, Graduate School of Engineering, Professor, 大学院・工学研究科, 教授 (30005243)
TiAl alloys consists of gammaTiAl and alpha_2Ti_3Al phases, and are considered to be a candidate for high temperature structural applications. A further improvement of their high temperature mechanical properties is required for their engineering applications, For the improvement, it is important to know how to optimize their microstructure. In this research, the correlation between the microstructures and high temperature mechanical properties were studied, and the following results were obtained.
1. Creep strength of TiAl alloys is independent of grain size for grain sizes greater then 100mum. In the grain size range smaller than 100mum, their creep strength may decrease with decreasing grain size.
2. Creep strength of TiAl alloys is insensitive to the volume fraction of constituent phases, in other words, aluminum concentration.
3. Creep strength of fully lamellar TiAl alloys is improved by the refinement of their lamellar spacing. At low stresses, however, the high creep strength of f
ine lamellar microstructure disappears due to the significant degradation of the fine lamellar microstructure. Dynamic recrystallization and discontinuous coarsening of lamellar microstructure during creep are responsible for the degradation, and the consequent weakening of the fine lamellar microstructure.
4. The lamellar microstructure can be stabilized by annealing at a high temperature. The stabilization treatment prevent the microstructural degradation during creep, and the strengthening by lamellar refinement becomes effective even at low stresses.
5. Creep strength of PST crystals, having only one lamellar colony, depends strongly on the angle between the stress axis and the lamellar plates. The hard oriented crystal, whose lamellar plates are aligned parallel or perpendicular to the stress axis, provides substantially higher creep strength than randomly oriented polycrystalline TiAl alloys. The soft oriented crystal, having an intermediate angle of stress axis to the lamellar plates, shows similar creep strength to the polycrystals. This result suggests that creep strength of polycrystalline TiAl alloys can be improved by the control of their texture. Less