Creep behavior and microstructural evolution of a near-gamma-TiAl alloy with duplex microstructure

Uniaxial creep of a duplex near-gamma-TiAl alloy with a nominal composition of Ti-47A1-2Mn-2Nb + 0.8 vol.% TiB2 XDTM was investigated in constant load creep tests between 650 and 750 degrees C and stress levels ranging betwee n 150 MPa to 350 MPa. The stress exponent, n, was found to increase from ab out 3 at low stresses to 8 at high stresses. The apparent activation energy for creep, Q(c), also exhibits a stress dependence and values range betwee n 288 and 398 kJ/mol. A high on-load strain was observed which exceeds the instantaneous elastic response. Three elementary microstructural processes were identified to contribute to creep: During primary creep movement of or dinary dislocations (first process) and mechanical twinning (second process ) control plastic deformation while only a few superdislocations were obser ved. At the creep rate minimum a third process, dynamic recrystallization, was observed. It results in a decrease of local back stresses, which build up as a consequence of the movement of ordinary dislocations, which does no t produce any deformation in c direction. The overall creep rate is a resul t of the superposition and the interaction of these three elementary proces ses.