High-temperature low-cycle fatigue behavior and microstructural evolution of an ODS steel based on conventional T91

The aim of this work is to investigate the low-cycle fatigue (LCF) behavior of a ferritic-martensitic (F/M) oxide dispersion strengthened (ODS) steel based on commercial T91 (mod. 9Cr-1Mo steel) and to correlate it to the microstructural evolution after cyclic straining at different temperatures. Therefore, LCF tests were performed at temperature between 500 and 650 °C with strain amplitudes ranging from ±0.4 to ±0.9%. Cyclic stress response and cyclic strain-life resistance were analyzed in detail and compared with the matrix material. The F/M ODS steel showed complex continuous cyclic softening but to a lesser extent than the matrix material and a slight initial cyclic hardening at 650 °C. Due to their high strength at elevated temperatures, strain-life resistance was superior at total strain amplitudes below approx. 0.6%. Minor microstructural changes were observed after testing at 500 and 600 °C. With increasing temperature, the initially tangled dislocation structure was partially replaced by straight and parallel dislocations showing double-slip cross over. At 650 °C, the F/M subgrain structure changed into more equiaxed grains. However, the formation of three-dimensional (3D) dislocation structures, e.g. dislocation cells or slip band formation, was not observed.