Elastic instability condition of the raft structure during creep deformation in nickel-base superalloys

A condition for destabilizing the raft structure has been deduced from elastic energy calculations with the concept of “effective eigenstrain”, where the effect of creep deformation is included in addition to the lattice mismatch. The calculations indicate that the 0 0 1 raft structure is stabilized by a small amount of creep deformation but becomes unstable when the creep strain in the γ phase exceeds the magnitude required to fully relax the lattice mismatch. The excess creep strain is required to produce an internal elastic field that suppresses further creep deformation, and has to be introduced in the primary creep stage. Via the instability of the 0 0 1 raft structure, the raft structure gradually turns into a wavy one in the second creep stage before its collapse.