CREEP LIFETIME PREDICTION OF OXIDE-DISPERSION-STRENGTHENED NICKEL-BASE SUPERALLOYS - A MICROMECHANICALLY BASED APPROACH

The high-temperature creep behavior of the oxide-dispersion-strengthen ed (ODS) nickel-base superalloys MA 754 and MA 6000 has been investiga ted at temperatures up to 1273 K and lifetimes of approximately 4000 h ours using monotonic creep tests at constant true stress sigma, as wel l as true constant extension rate tests (CERTs) at epsilon. The deriva tion of creep rupture-lifetime diagrams is usually performed with conv entional engineering parametric methods, according to Sherby and Dorn or Larson and Miller. In contrast, an alternative method is presented that is based on a more microstructural approach. In order to describe creep, the effective stress model takes into account the hardening co ntribution sigma(p) caused by the presence of second-phase particles, as well as the classical Taylor back-stress sigma(rho) caused by dislo cations. The modeled strain rate-stress dependence can be transferred directly into creep-rupture stress-lifetime diagrams using a modified Monkman-Grant (MG) relationship, which adequately describes the interr elation between epsilon representing dislocation motion, and lifetime t(f) representing creep failure. The comparison with measured creep-ru pture data proves the validity of the proposed micromechanical concept .