MICROSTRUCTURE-BASED 3D FINITE-ELEMENT MODELING OF LATTICE MISFIT ANDLONG-RANGE INTERNAL-STRESSES IN CREEP-DEFORMED NICKEL-BASE SUPERALLOYSINGLE-CRYSTALS

In the present paper, residual stresses and direction-dependent lattic e parameters and lattice misfits in the gamma'-hardened, tensile creep -deformed monocrystalline nickel-base superalloy SRR 99 have been stud ied by three-dimensional finite element modelling based on a ''simplif ied creep process''. The main results are as follows. (i) After creep and cooling to room temperature, the internal strains and stresses in both phases gamma and gamma' in the directions [001] (parallel to the stress axis) and [100]/[010] are different. From these, differences be tween the lattice parameters and lattice misfits in the [001] and the [100]/[010] directions follow. During cooling to room temperature afte r creep, the change in thermal stresses can cause plastic back flow of the gamma-phase. The internal stress state is relaxed accordingly. (i i) During the subsequent heating (after creep and cooling to room temp erature), the differences of the lattice parameters in the [100] direc tions decrease with increasing temperature. (iii) During final cooling from a stress-free state at 105 degrees C, the lattice misfit and the internal stresses between the [001] and the [100]/[010] directions in crease with decreasing temperature. The influences of the creep-induce d dislocation networks at the gamma/gamma' interfaces, of the coherenc y stresses and of thermal stresses on the lattice misfit and internal stresses are discussed in detail. The differences between the values o f the lattice misfit measured by TEM and by X-ray diffraction are reco nciled. The differences between two- and three-dimensional finite elem ent modelling are also discussed. The results are in reasonable agreem ent with the experimentally measured data.