APPLICATION OF THE HAI-MODEL TO THE CREEP-BEHAVIOR OF PARTICLE HARDENED NICKEL-BASE SUPERALLOYS

The Haasen-Alexander-Ilschner (HAI-) theory of internal and effective stress is combined with the modified threshold stress (sigma(p)-) conc ept in order to describe steady-state as well as transient creep defor mation of oxide dispersion strengthened (ODS) nickel-base superalloys under constant strain rate (epsilon) conditions. The microstructurally based model correctly reflects the steady-state relationship of epsil on and sigma at large plastic strains, For any given constant epsilon- value the fine dispersed incoherent Y2O3 particles in MA 754 rise the stress level considerably compared to the single-phase solid solution, Nimonic 75, of similar chemical composition, The higher contents of A l and Ti in MA 6000 lead to additional precipitation hardening of cohe rent gamma'-particles and a further increase in stress, However, trans ient creep can only be modelled by splitting sigma(p) into two additiv e terms: firstly a constant sigma(p) value representing the contribut ion for the mere particle overcoming process and secondly an increasin g sigma(rho,p) term reflecting the building up of geometrically necess ary dislocations during plastic deformation.