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 degrees 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 degrees C. Further increase of annealing temperature to 1300 degrees 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 degrees C, and their shapes changed to very coarse hexagonal prisms at 1250 degrees C, which was followed by the formation of eutectic M2C carbides at grain boundaries at 1300 degrees C. Tensile tests of the as-extruded ODS Alloy 617 showed that the yield strength decreased steeply at a transition temperature of around 600 degrees C, which can be attributed to diffusional creep along the grain boundaries.