This work investigates two austenitic alloys, Sanicro 25 and Esshete 1250, which are candidates for high temperature components of future power plants. The alloys were subjected to isothermal dwell-fatigue testing at 700 °C and thermomechanical fatigue (TMF) in the temperature range of 100 °C up to 700 °C. To understand the mechanical behaviour microstructural investigations were conducted. 

During dwell-fatigue coherent Cu-precipitates and incoherent Nb-carbides were found to impede dislocation motion, increase hardening and improving the high temperature properties of Sanicro 25. Sparsely placed intergranular Cr- and Nb-carbides made Esshete 1250 susceptible to creep damage and intergranular crack propagation, mainly from interaction of the carbides and fatigue induced slip bands. Dynamic recrystallization of the plastic zone at the crack tip appeared to affect crack propagation of Sanicro 25 by providing an energetically privileged path.

From the TMF testing it was found that the pre-aged Sanicro 25 specimens suffered increased plastic straining and shorter TMF-life compared to the unaged specimens. This was attributed by an accelerated microstructural evolution that provided decreased impeding of dislocation motion. There was not a similar clear trend when applying ageing on the reference material, Esshete 1250. However, the structural stability was greatly reduced due to coarsening and cracking of niobium carbides.