Mechanism of plastic deformation of a Ni-based superalloy for VHTR applications

Alloy 617 is considered a leading structural material for next generation nuclear power plants due to its good corrosion resistance and exceptional strength at high-temperatures. In the present work, a complementary study of deformation mechanisms in Alloy 617 was performed via tensile testing at temperatures up to 1000 °C. Dynamic strain aging characterized by serrated flow leading to temperature independent yield strength was observed at intermediate temperatures. At temperatures higher than 700 °C, the material’s strength reduced dramatically as a result of the predominance of dislocation creep and dynamic recrystallization. The changes in deformation mechanisms were established by direct observations of Electron Backscatter Diffraction mappings and by the analysis of texture development based on the pole figure mappings. Recrystallized grains were found to preferentially grow in particle-rich areas and on high-angle grain boundaries. Finally, fracture was initiated from particle cracks at temperatures up to 700 °C and by triple junction cracks from 800 to 1000 °C.