Deformation, damage and microstructure development of alloy 800H under fatigue stress at high temperatures

The rising demand for computer-aided calculations either of whole systems or single components increases the need for new deformation models. Those models provide the basis for reliable lifetime assessments and thus should be able to handle complex load scenarios in conjunction with changes of the material behavior due to the evolution of the microstructure. The present work deals with the deformation and damage behavior of Alloy 800H under fatigue loadings in the temperature regime up to 1000 C. Besides investigations under low-cycle fatigue, the material performance under creep-fatigue as well as under thermo mechanical fatigue is examined. The strong dependency of the microstructural initial state is analyzed for three different material conditions: solution annealed, heat treated and long term aged at 650 C. Scanning electron and transmission electron microscopy investigations support the characterization of the microstructural evolution during aging at 650 C. This includes either the pure thermal exposure or combined mechanical and thermal load. From those experiments the relation between deformation and the hardening behavior, the occurring damage mechanisms and the related microstructural state is derived. Afterwards, this knowledge is transferred to other aging temperatures like 600 C and 850 C. Finally, these results are used for the development of a predominantly physical-, thermodynamic- and kinetic-based model to describe the microstructural evolution and hardening of Alloy 800H. Parameters of developed evolution equations for the precipitation radius and the volume fraction of the carbides are determined by the adjustment of MatCalc-calculations. Those microstructural evolution equations are linked to an extended Chaboche deformation model. Finally, this model is able to describe the essential phenomena of time and temperature dependent material plasticity under fatigue loadings up to 1000 C. In combination with the mechanism based D_TMF lifetime model the connection to several finite element software packages is feasible to perform computeraided component assessments.