.2. Deformed microstructures during creep of TiAl alloys: Role of mechanical twinning

The deformation mechanisms responsible for the creep behaviour of two TiAl alloys with duplex and lamellar structures have been analysed after carrying out microstructural observations as a function of creep strain and applied stress at 700 degrees C, The duplex alloy exhibits extensive mechanical twinning and the new twin interfaces subdivide the gamma grains throughout the primary stage of creep. At the onset of the minimum creep rate, the twin interfaces in the duplex alloy behave in the same way as the gamma/gamma or the alpha(2)/gamma interfaces in the lamellar alloy. Thus, dislocation accumulation and emission from the interfaces occur under the action of the high local stresses. The high stress exponent, n=19, measured in both alloys at the minimum strain rate has been interpreted in terms of a threshold stress representing the high local stresses required to liberate dislocations from the interfaces and initiate their propagation within the gamma matrix. (C) 1997 Elsevier Science Limited.