CREEP-BEHAVIOR OF A FULLY TRANSFORMED NEAR GAMMA-TIAL ALLOY TI-48AL-2CR

The creep deformation behaviour and creep microstructures of a dual ph ase TiAl/Ti3Al alloy Ti-48Al-2Cr (at.%) with a fully transformed lamel lar structure have been investigated at 800 degrees C in tension creep . The creep curves show no steady state creep rate, rather a minimum c reep rate was obtained followed by a steep increase in the creep rate. The application of a power-law creep equation of the type epsilon = A sigma'' leads to a stress exponent n = 7.6. The deformation microstru ctures show no deformation twins and essentially consist of 1/2<110] d islocations; recovered structures were observed in the fine gamma-grai ns at colony boundaries whereas long and curved 1/2<110] segments were found in the gamma-lamellae. These microstructural observations sugge st a dislocation controlled creep; it ensues that the stress exponent obtained using power-law creep does not reflect the operating creep me chanism. This is attributed to early nucleation and growth of voids an d cavities at colony boundaries which do not allow steady-state creep to be established, an underlying condition for the applicability of th e power-law equation above. The nucleation and growth of Void and cavi ties are thought to arise as a result of stress buildup at colony boun daries which in turn results from the dependence of deformation behavi our on the orientation of the lamellar colonies with respect to the te nsile axis. The loss of creep strength is found to be primarily due to growth and coalescence of these voids and cavities.