Effects of lamellar spacing, volume fraction and grain size on creep strength of fully lamellar TiAl alloys

Lamellar spacing, volume fraction of constituent phases and grain size are the major structural factors to be controlled in alloy design of fully lamellar TiAl alloys. Creep tests were carried out on binary TiAl alloys with fully lamellar structure, to elucidate effects of the three structural factors on their creep strength. Lamellar alloys give better creep strength than the constituent single phases at higher stresses, due to their higher yield stress. Creep rate of lamellar alloys decreases with decreasing lamellar spacing, suggesting that a fine lamellar structure is effective in improving creep strength. The decrease is remarkable at high stresses. At low stresses however, the difference in creep rate diminishes, due to the increasing contribution of dynamic recrystallization and interface sliding. Creep rate is insensitive to volume fraction of the constituent phases and aluminum concentration is free in alloy design of TiAl alloys as far as creep strength is concerned. Creep rate is also independent of grain size d when d>100 μm. Because of easy dynamic recrystallization in the lamellar alloys, creep rate may increase when d<100 μm. If grain size <100 μm is necessary for adequate ductility, dynamic recrystallization should be prevented by some means.