Microstructure and dislocation arrangements in Sanicro 25 steel fatigued at ambient and elevated temperatures

Microstructure and dislocation structures in high alloyed heat resistant Sanicro 25 austenitic steel cyclically strained in a wide interval of constant strain amplitudes both at room temperature and at temperature of 700 °C were studied by means of transmission electron microscopy (TEM). The spatial arrangement of dislocations was determined using the technique of oriented foils. The character and the Burgers vectors of dislocations were determined. It was found that at room temperature strong planarity of dislocation slip prevails. Cyclic plastic deformation is localized into thin bands of high dislocation density which have different structure than ladder-like arrangement. No distinctive wall and channel dislocation configurations were observed but bands have character of alternating dislocation rich and dislocation poor areas. Pronounced plastic strain localization leads to cyclic softening. In high temperature cyclic straining enhanced cross-slip leads to substantial increase of dislocation density. Objects of nanometer size coherent with the matrix were found simultaneously with the evidence indicating pinning of dislocations. The high dislocation density, its homogeneous distribution and interaction of nanoclusters and gliding dislocations are the cause of exceptional cyclic hardening of this material at high temperatures. Due to this hardening, the saturated cyclic stress at 700 °C is remarkably high.