Fig. 5 Fracture elongation of specimen to the endurance life.
3.2 SEM observations
Typical SEM observations of the fractured surfaces after static creep and various cyclic creep tests are given in Fig. 6. All the fractured surfaces present a typical ductile failure feature, with a similar cup-cone shape of necking. Two different areas including (I) inner fibrous zone and (II) outer shearing lip can be found in Fig. 6 (a). Creep dimples and cavities located at the centre of the fibrous zone are shown in Fig. 6 (b). Second phase particles can be found in some cavities, as shown in Fig. 6 (c), and analysis results from energy dispersive spectrometer (EDS) indicate that these second phase particles tend to be M23C6 (Fe-Cr-C) carbides, which are precipitated from matrix \(\alpha\)-Fe.36,37Comparing figures from Fig. 6 (d) to (g), the fibrous zone area under cyclic creep is found to suffer an obvious shrinkage compared with that under static creep, which may be resulted from the decrease of ductility. Besides, the distinct fatigue-creep cracks that are reported to be generated under cyclic loading in 9-12% Cr-Mo steels38,39 are hardly detected in the examined 2.25Cr-1Mo steel, and the test environment in air and the relatively low frequency adopted in this work are likely to be the reason.