Since the last decade, TiAl alloy has been regarded as one of the strongest candidate materials for light and high temperature structural materials. Because of its low density and high creep resistance in this alloy, TiAl alloy is considered suitable for engine valve and turbine blade. Lamellar TiAl alloy that is composed of $D0_{19}-α_2$ phase and $L1_0-γ$ phase has the best creep resistance than other TiAl alloys that has another microstructure.
In this study, the role of edge lamellar dislocations at the lamellar interface in terms of the phase transformation and the change of lamellar interface shape of TiAl alloy of K5CBS, of Ti-45.5Al-2Cr-2.6Nb-0.17W-0.1B-0.2C-0.15Si(at.%), in lamellar interface under primary creep condition has been investigaed. Creep tests were conducted in the condition of 800℃/200MPa and 0.05%∼0.4% crept TiAl samples were observed by using TEM.
In the creep conditions of lamellar TiAl, the results of TEM observations, some interesting microstructural changes are observed. First, the density of dislocation is decreased in primary region. Second, in the as-received state lamellar interfaces are changed from curved shape to flat and the thickness of $α_2$-lath are decreased as increasing creep deformation in primary stage. Up to about 0.05% creep strain, the shape of lamellar interfaces changed from curved to a saw blade like, that is due to the atomic scale ledge interaction of interface and dislocations gliding from γ matrix. The crystallographic plane of lamellar interfaces is {111} plane and the relationship of $α_2/γ$ interfaces is coherency. Disloctions can glide on the lamellar interface and this gliding dislocations can interact the ledge. Extra-half plane of edge dislocation can supply atoms to ledge, which is unstable stage, so atomic stacking sequence and composition are changed. As the results of this interaction, $α_2$ → γ phase transformation is progressed in lamellar interface and during this transformation intermediate phase, $Ti_2Al$, is formed and the thickness of $α_2$-lath is decreased because $α_2$-lath is changed to γ phase.
Considering the activation energy for creep deformation in primary regime, which is higher than that for climb, this phase transformation process may be the controlling mechanism of lamellar TiAl creep on primary stage. If the kinetics of this phase transformation can be controlled, creep resistance of lamellar TiAl alloy will be improved.
