Inhomogeneous nano-mechanical properties in the multi-phase microstructure of long-term aged type 316 stainless steel

Microstructure-related local deformation behavior was evaluated using nanoindentation techniques for a type 316 austenitic stainless steel used in energy generation plants. The sample was aged for 39,332 h (4.5 years) at 700 °C. The microstructure included the σ phase precipitated at grain boundaries and in grain interiors. The nanohardnesses of the σ phase and the matrix in the aged and virgin samples were evaluated quantitatively. The hardness of the σ phase was found to be extremely high in the order of 17 GPa, which is much higher than the value of about 3.4 GPa for the matrix in the aged sample. The hardnesses of the σ phase at the grain boundary and the adjoining matrix were the same as those in the grain interior. Moreover, the hardness of the matrix of the aged sample was about 30% lower than that of the virgin sample while the Vickers hardness as a macroscopic strength of the aged sample was about 45% higher than that of the virgin one. The deformation and fracture behavior in a local region was discussed in terms of the inhomogeneous mechanical properties in the multi-phase microstructure.