Microstructural evolution and tensile properties of oxide dispersion strengthened Alloy 617 at elevated temperatures

This study investigated evolution of the microstructure of oxide dispersion strengthened Alloy 617 with annealing temperature. A mixture of prealloyed Alloy 617 and Y2O3 powders was mechanically alloyed and consolidated by hot-extrusion at 1100 °C. Hot extrusion developed a submicron-sized grain structure with M23C6 carbides and finely dispersed Al2O3 and Y2Ti2O7 oxides. The fine-grained structure was stable during subsequent annealing at temperatures up to 1250 °C. Further increase of annealing temperature to 1300 °C resulted in a significantly coarsened grain structure, which was coincident with the abrupt coarsening of oxides. M23C6 carbides in the as-extruded conditions were transformed to M7C3 carbides with complex shapes when annealed at 1200 °C, and their shapes changed to very coarse hexagonal prisms at 1250 °C, which was followed by the formation of eutectic M2C carbides at grain boundaries at 1300 °C. Tensile tests of the as-extruded ODS Alloy 617 showed that the yield strength decreased steeply at a transition temperature of around 600 °C, which can be attributed to diffusional creep along the grain boundaries.