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OXIDE-DISPERSION-STRENGTHENED (ODS) nickelbase superalloys are candidate materials for enhanced hightemperature applications like gas turbines in aircrafts or stationary power plants, because of their favorable combination of high strength and good oxidation resistance. In addition to hot corrosion and fatigue, the monotonic irreversible elongation of turbine blades during service is a severe damage mechanism that can finally lead to creep rupture. Therefore, materials design for long-term creeprupture response has been carried out so far mainly by the application of parametric methods, which, however, lack a physically based theory for creep and creep rupture. Recently, it was shown[1,2] that not only minimum or steadystate creep, but also transient behavior of complex particle– strengthened alloy systems can be quantitatively described by the Haasen–Alexander–Ilschner (HAI) model[3,4] of internal and effective stress combined with the modified threshold stress (sp2) concept.[5] The present article will extend this micromechanical model (Section II) in order to predict creeprupture behavior of the ODS nickel-base superalloys MA 754 and MA 6000 (Section III). Finally, a comparison with the aforementioned parametric methods is given in Section IV. II.

MICROSTRUCTURAL CONCEPT

The microstructurally oriented approach to describing high-temperature creep-rupture behavior of particle-strengthened alloys was originally designed in 1982.[6] It has been

M. HEILMAIER, Head of the Research Group ‘‘Strength and Plasticity of Metallic Materials,’’ is with the Institute for Solid State and Materials Research, D-01171 Dresden, Germany. B. REPPICH, Professor, is with the Institute for Materials Science, University of Erlangen-Nu¨rnberg, D91058 Erlangen, Germany. Manuscript submitted November 1, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A

exemplified with g '-precipitating superalloys[7] as well as dispersion-strengthened platinum-based alloys.[8] The concept is based on the essential hypothesis of transferring the controlling mechanisms of minimum or steady-state creep—in particular the operating dislocation-particle interaction—directly into the creep-rupture response. Application of the theory of internal and effective stress according to the modified HAI model[3,4] yields the following constitutive equation for the creep rate of particlestrengthened materials:[1,2]

rbC εz 5 sinh (b (s 2 sr 2 sp )) M

[1]

The term in the inner parentheses represents the effective stress seff, which controls the thermally activated motion of dislocations. According to Taylor,[9] the long-range stress component sr of dislocations can be expressed as

sr 5 a G b M =r

[2]

where r is the mean total dislocation density and a the elastic interaction coefficient. Taylor factor M, Burgers vector b, and shear modulus G are material constants. The parameter C is determined by the climb rate of dislocations and therefore directly proportional to the lattice diffusion coefficient; b is related to the activation area of creep.[10] Both pa