The effect of temperature and stress on mechanical twinning during creep deformation in TiAl

Tensile creep specimens deformed at different temperatures and stresses were investigated using transmission electron microscopy for the purpose of understanding mechanical twinning behavior during creep deformation in TiAl. Three types of mechanical twinning configurations, fine mechanical twins at grain triple points, parallel twinning (or lamellar refining) and cross twinning, are compared in the deformed specimens. More mechanical twins are observed in specimens deformed at 730 C than in the specimen deformed at 800 C, indicating that less mechanical twinning occurs during creep deformation at the higher temperature in TiAl. The respondence of mechanical twinning to the creep stress is contradictory to that to the temperature: the larger the applied creep stress, the more mechanical twinning occurs during creep deformation. A formation mechanism for fine mechanical twins and the competition between mechanical twinning and dislocation activation during creep deformation in TiAl are proposed based on the experimental observations.