Study of the deformation and damage mechanisms of a 9Cr-ODS steel: Microstructure evolution and fracture characteristics

In this study, the tensile properties of a tempered martensitic 9Cr oxide dispersion strengthened (ODS) steel are investigated. The tensile tests were performed in the temperature range of 25°C to 800°C at a nominal strain rate of 10−3s−1. At room temperature, the material exhibited high yield strength and ultimate tensile strength of 929 and 1052MPa, respectively. A decrease in strength was observed with increase in temperature down up to 156MPa of ultimate tensile strength at 800°C. The total elongation-to-failure that was 7.6% at 25°C increased sharply with increase in temperature and reached a maximum of 38.4% at 700°C. In order to compare the influence of heat treatment the tensile tests were also performed on specimens when in a ferritic state. Interestingly, at elevated temperatures both states presented a similar trend of strength and elongation. Transmission electron microscopy after deformation revealed a modification of the deformation mechanism with the temperature. The dislocation activity that was homogeneously distributed at room temperature was localized close to grain boundaries at elevated temperatures. A strong particle–dislocation interaction was observed at all testing temperatures. Orowan mechanism is supposed to govern particle–dislocation interaction at moderate temperatures. At elevated temperatures, an attractive particle–dislocation interaction phenomenon called interfacial pinning was identified. The additional microstructural evolution that includes reduced dislocation density, the transformation of lath structure into coarse equiaxed grains and the M23C6 carbide coarsening, resulted in a loss of strength at elevated temperatures. Fracture surface investigation at room temperature revealed intragranular fracture with dimples. As the temperature increased, the fracture surface formed with a shear-lip zone at the outer periphery and dimples that were larger and deeper in comparison to the ones formed at lower temperatures. These observations are associated with the increased ductility. A change in fracture mechanism from intragranular to intergranular fracture was observed at 650°C. This change became fully apparent at 800°C where it is associated with a reduced ductility. The change in damage mechanism is due to the modification of the deformation mechanism. In comparison to other commercial, as well as experimental, ODS steels, the material offers an excellent compromise between strength and ductility.