| Co-Investigator(Kenkyū-buntansha) |
ITO Kazuhiro Kyoto University, Materials Science and Engineering, Research Associate, 工学研究科, 助手 (60303856)
INUI Haruyuki Kyoto University, Materials Science and Engineering, Associate Professor, 工学研究科, 助教授 (30213135)
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| Research Abstract |
Ingots of Ti-47Al alloy with an aligned lamellar microstructure can be grown using a seed of the Ti-43Al-3Si seed alloy. Similarly to the Ti-47Al alloy, seeding should be possible for multi-component alloys based on this alloy if alloying may not cause any significant change in the solidification path of the base alloy. When the alloying elements and the amount of each element to be added to the base Ti-47Al alloy are determined, the Al content of the base alloy is modified to let the relative position of the α/β primary region boundary remain as that is in the binary system. The Al content of the new alloy, C_<AI> can be calculated using the following equation. c_<Al>=47 + aX_<eq> + bY_<eq> + cZ_<eq> +..., where a, b, c and X_<eq>, Y_<eq>, Z_<eq> are the amount and the Al-equivalent of alloying elements X, Y, Z, respectively. When approximately 46 < c_<Al> < 48, the lamellar microstructure can be aligned along the growth direction using a seed of the Ti-43Al-3Si seed alloy at a growth
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rate of 10mm/h.
Creep tests were performed in air at 750℃ and 240MPa using specimens with a gauge size of 30 mm in length and 6 mm in diameter cut from directionally solidified ingots. It was found that not only the secondary creep rate but also primary creep strain were reduced by aligning the lamellar boundaries along the loading axis. The secondary creep rate of the binary Ti-47Al alloy under the creep condition is 2.07 x 10^<-9>s^<-1>, which is still comparable to or lower than that reported for the highly alloyed advanced TiAl-base polycrystalline alloys, is much higher than those for the quaternary and quinary alloys. It is of interest to note that the addition of a small amount of Si and transition metals such as Mo, W and Re dramatically increases creep resistance of directionally solidified ingots of TiAl-base alloys. The oxidation resistance of the directionally solidified ingots of Ti-46Al-0.5Si-0.5W and Ti-46Al-0.5Si-0.5Mo alloys is comparable to that of Ti-48Al-2Cr-2Nb, which is one of the most common TiAl-base alloys. However, adding 3% Nb significantly improves oxidation resistance without giving rise to any significant change in the mechanical properties. Less
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