Microstructural evolution during creep of single-phase gamma TiAl

Mechanical experiments and transmission electron microscope (TEM) observations indicate that single-phase gamma-TiAl does exhibit primary, secondary, and inverse creep, but not steady-state creep. Constant stress creep tests of gamma-Ti-51Al-2Mn conducted at 550 degrees C, 597 degrees C, and 703 degrees C have been interrupted at different stages in the creep process. The TEM observations of these specimens were used to document the microstructural evolution that occurs during creep. Superdislocation motion was activated and subsequently exhausted during primary creep. Ordinary dislocations were observed to be pinned during primary creep, but with time, these dislocations began to bow past their pinning points. The extended region of inverse creep has been related to the bowing and multiplication of these ordinary dislocations. Quantitative measurements of dislocation density were performed, and while the density of superdislocations remained constant, the density of ordinary dislocations increased by an order of magnitude during the life of a creep test. The acceleration in the creep rate has been related to this increase in the density of ordinary dislocations, but the change in dislocation density was not high enough to account for the increase in the creep rate. This suggests that both the mobility and density of ordinary dislocations increase as creep progresses.