Highly alloyed austenitic stainless steels are promising candidates to replace more expansive nickel-based alloys within the energy-producing industry. The present study investigates the deformation mechanisms by microstructural characterisation, mechanical properties and stress-strain response of three commercial austenitic stainless steels and two commercial nickel-based alloys using uniaxial tensile tests at elevated temperatures from 400 C up to 700 C. The materials showed different influence of temperature on ductility, where the ductility at elevated temperatures increased with increasing nickel and solid solution hardening element content. The investigated materials showed planar dislocation driven deformation at elevated temperature. Scanning electron microscopy showed that deformation twins were an active deformation mechanism in austenitic stainless steels during tensile deformation at elevated temperatures up to 700 C.

Fatigue damage in a metallic material during very high cycle fatigue can strongly be correlated to the microstructure. This paper provides a review and a discussion on the micro damage behaviours in a nickel-base alloy during very high cycle fatigue using microplasticity and material mechanics. The results show that cyclic plastic deformation in this material can occur very locally even with an applied stress that is much lower than the yield strength. The fatigue damage occurs mainly at grain or twin boundaries because of local impingement and interaction of slip bands and these boundaries. The crystallographic properties, Schmid factors and orientations of grain and boundaries play very important roles to the fatigue damage. Subsurface fatigue crack initiation in the matrix is one of very high cycle fatigue mechanisms. Twinning and detwinning can also occur during the very high cycle fatigue process.

Sanicro 25 is a newly developed advanced high strength heat resistant austenitic stainless steel. The material shows good resistance to steam oxidation and flue gas corrosion, and has higher creep rupture strength than other austenitic stainless steels available today. It is thus an excellent candidate for superheaters and reheaters for advanced ultra-super critical power plants with efficiency higher than 50 %. This paper provides a study on the creep–fatigue interaction behavior of Sanicro 25 at 700 °C. Two strain ranges, 1 and 2 %, and two dwell times, 10 and 30 min, were used. The influences of dwell time on the cyclic deformation behavior and life has been evaluated. Due to stress relaxation the dwell time causes a larger plastic strain range compared to the tests without dwell time. The results also show that the dwell time leads to a shorter fatigue life for the lower strain range, but has no or small effect on the life for the higher strain range. Fracture investigations show that dwell times result in more intergranular cracking. With the use of the electron channeling contrast imaging technique, the influences of dwell time on the cyclic plastic deformation, precipitation behavior, recovery phenomena and local plasticity exhaustion have also been studied.

Cyclic plastic response of the austenitic heat resistant steel Sanicro 25 has been studied during strain controlled low cycle fatigue tests performed at ambient and at elevated temperature. Simultaneously with the cyclic hardening/softening curves hysteresis loops during cyclic loading were analyzed using generalized statistical theory of the hysteresis loop. The probability density distribution function of the internal critical stresses, the effective saturated stress and their evolution during cycling were derived for various strain amplitudes. The internal dislocation structure and the surface relief at room and at elevated temperature were studied and correlated with the cyclic stress–strain response and the evolution of the probability density function of the internal critical stresses.