Microstructural design criteria of fully lamellar TiAl alloys for improved creep strength

Grain size, volume fraction of constituent phases, lamellar spacing and lamellar orientation are the major microstructural variables to be considered in alloy design of fully lamellar TiAl alloys. Effects of the four variables on creep strength of TiAl alloys were studied by using polycrystalline specimens and PST (single grain) crystals. Creep rate of TiAl alloys is independent of grain size, and insensitive to the phase volume fraction (i.e, aluminum concentration). Grain size can be reduced down to 100 mu m without loosing creep strength. Aluminum concentration is essentially free in the alloy design as far as creep strength is concerned. Creep rate of the alloys decreases with decreasing lamellar spacing, suggesting that refinement of lamellar microstructure is effective in improving creep strength. However, the strengthening by lamellar refinement disappears at low stress due to the increasing contribution of microstructural instability of fine lamellar microstructure. Stabilization of lamellar microstructures is necessary to prevent this loss of creep strength. Creep rate of the hard-orientated PST crystals, in which lamellar plates are aligned parallel or perpendicular to the stress axis, is reduced by two orders of magnitude with respect to the soft orientation. Creep strength of polycrystalline TiAl alloys can be improved significantly by the control of their texture.