Abstract
The effect of elastic driving force on the microstructural change of superalloys in the secondary creep stage is evaluated by elastic energy calculations with the concept of effective eigen strain where both lattice mismatch and creep strain are taken into account The elastic energy calculations indicates that the elastic state in the secondary creep stage is totally different to that in the initial one where the lattice misfit between γ and γ' phases is over accommodated along the [100] and [010] directions by creep deformation in the γ phase. The excess creep dislocations for the over accommodation are required so as to develop an internal stress field to prevent further creep deformations. The planer raft structure with the plane normal oriented to the [001] direction is unstable in the over accommodated state. The γ/γ' lamellar interfaces will be inclined to make a wavy raft structure of which elastic energy is lower than the ideal 001 planer raft structure.
Highlights
The efficiency of aircraft engines and gas turbine generators is improved by increase in the gas temperature and the operating temperature of the turbine blades equipped
We have introduced the “effective eigenstrain” of the γ phase, ε*eff, which is the sum of the lattice misfit between γ and γ' phases, ε0, and the amount of plastic strain caused by the introduction of creep deformations, εp
This agrees with the previous works, that is the 001 raft structure being elastically stabilized by introductions of creep dislocations [14]
Summary
The efficiency of aircraft engines and gas turbine generators is improved by increase in the gas temperature and the operating temperature of the turbine blades equipped. Since nickel-base single crystal superalloys exhibit superior high-temperature creep strength and oxidation resistance, they have been applied to turbine blades in industrial gas turbines and aero-engines. Creep strength of the blades is gradually degraded in service. Under the influence of applied stress, the lamellar interfaces in the raft structure gradually deviate from the (001) plane so as to form a wavy raft structure in the secondary creep stage. Since the inclination is considered to be closely related to the degradation of creep strength, the origin of the local incline of γ/γ' interfaces observed in the secondary creep stage is an important factor for understanding the collapse mechanism of the raft structure
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