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

The creep deformation behaviour and creep microstructures of a dual phase TiAl/Ti3Al alloy Ti-48Al-2Cr (at.%) with a fully transformed lamellar structure have been investigated at 800 degrees C in tension creep. The creep curves show no steady state creep rate, rather a minimum creep 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 microstructures show no deformation twins and essentially consist of 1/2<110] dislocations; recovered structures were observed in the fine gamma-grains at colony boundaries whereas long and curved 1/2<110] segments were found in the gamma-lamellae. These microstructural observations suggest a dislocation controlled creep; it ensues that the stress exponent obtained using power-law creep does not reflect the operating creep mechanism. This is attributed to early nucleation and growth of voids and cavities at colony boundaries which do not allow steady-state creep to be established, an underlying condition for the applicability of the power-law equation above. The nucleation and growth of Void and cavities are thought to arise as a result of stress buildup at colony boundaries which in turn results from the dependence of deformation behaviour on the orientation of the lamellar colonies with respect to the tensile axis. The loss of creep strength is found to be primarily due to growth and coalescence of these voids and cavities.