Creep Mechanisms and Interface-Enhanced Deformation Twinning in a Two-Phase _ Lamellar TiAl Alloy

Deformation mechanisms and the role of interfaces in deformation twinning of a two-phase [TiAI (y)GAl (ad] 1 amellar allo\t creep deformed at elevated temperatures have been investigated. Since the multiplication of lattice dislocations \vithin both y and a2 lamellae is verl\’ limited at a lo\v stress level due to a refined lamellar microstructure, the glide of interfacial dislocations on both y/a? and y/y interfaces (i.e interface sliding) becomes an important deformation mode Obstacles such as impinged lattice dislocations can impede the movement of interfacial dislocations, which glide in a cooperative fashion along the lamellar interfaces. The impediment of dislocation motion subsequently causes a dislocation pile-up in front of obstacles as creep strain accumulates. When the cr)istals deform at high stress level, deformation t\vinning becomes a predominant deformation mode Deformation twins are found to nucleate from the interfaces as a result of a local stress concentration generated from dislocation pile-ups. It is suggested that the deformation twinning in lamellar TiAI/TijAI cr):stals can be vieived as a stress relaxation process for the concentration of stress at the head of each dislocation pile-up. An interface-assisted twinning mechanism is accordingly proposed and discussed