Estimation of the lifetime of nickel base turbine blades by using elastic strain measurement

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Development of elastic strain in nickel base super alloy during creep deformation has been investigated. The results show that the change in elastic strain is owing to stack creep dislocations at gamma gamma-prime interfaces. Theoretical calculations about the stability of microstructure indicate that the raft structure is stabilized by a small amount of creep dislocations, however, the raft structure is destabilized by a large amount of creep dislocations. The critical amount of creep dislocations agrees with that for perfectly accommodate the lattice misfit between gamma and gamma-prime phases. In the destabilized raft structure, the most stable lamellar plane normal is not the [001] direction but that tilted toward the <111> direction. This is the reason why the raft structure collapses during the creep deformation. The measurement of elastic strain is useful to detect an early stage of creep deformation, however, the measurement is less sensitive to the change occurred in a later stage of its lifetime.