.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 aft er carrying out microstructural observations as a function of creep st rain and applied stress at 700 degrees C, The duplex alloy exhibits ex tensive mechanical twinning and the new twin interfaces subdivide the gamma grains throughout the primary stage of creep. At the onset of th e minimum creep rate, the twin interfaces in the duplex alloy behave i n the same way as the gamma/gamma or the alpha(2)/gamma interfaces in the lamellar alloy. Thus, dislocation accumulation and emission from t he interfaces occur under the action of the high local stresses. The h igh stress exponent, n=19, measured in both alloys at the minimum stra in rate has been interpreted in terms of a threshold stress representi ng the high local stresses required to liberate dislocations from the interfaces and initiate their propagation within the gamma matrix. (C) 1997 Elsevier Science Limited.