THE EVOLUTION OF DEFORMATION SUBSTRUCTURE IN A CREEP DEFORMED FULLY-LAMELLAR TIAL ALLOY

The evolution of deformation substructure in a powder metallurgy (P/M) TIAl alloy creep deformed at 760 degrees C has been examined using tr ansmission electron microscopy. Since the multiplication of lattice di slocations within gamma lamellae becomes limited as a result of a refi ned fully-lamellar (FL) microstructure (i.e. refined lamellar spacing) within the PIM TiAl alloy, the deformation of the alloy at the primar y creep stage is mainly accommodated by soft lamellar grains through t he movement of interfacial dislocations in the gamma-alpha(2) and gamm a-gamma interfaces. The mobility of interfacial dislocations is primar ily impeded by grain boundaries, interface ledges and dislocation barr iers formed by the impingement of lattice dislocations to the lamellar interfaces. When the alloy is deformed into the secondary creep stage , the density of interfacial dislocations increases and deformation tw ins nucleate and grow from the lamellar interfaces. It is suggested th at deformation twinning in FL-TiAl results from a stress-relief proces s due to the pile-up of interfacial dislocations during deformation. T he alloy becomes more resistant to creep deformation as a result of th e deformation twinning because the deformation twins further restrict the dislocation motion within the gamma lamellae, and the formation of sessile dislocations at the intersections between the deformation twi ns and the gamma or alpha(2) lamellae provides effective barriers for the movement of interfacial dislocations. (C) 1997 Elsevier Science S. A.