Formation of Serrated Grain Boundaries and Its Effect on Tensile Properties at 950 ℃ in Alloy 617 for Very High Temperature Reactor

The effects of initial microstructures controlled by thermo-mechanical treatment on tensile properties at 950 ℃ were investigated in Alloy 617. Proprietary direct aging combined with a controlled cooling rate led successfully to the transition of serrated grain boundaries (GBs) with a proportion of over 80%. It was newly found that at an initial stage of the transition, the serrated GBs formed without the presence of carbide or a γ' phase in the solid-solution-strengthened Alloy 617. The serrated GBs with stable planar M₂3C₆ carbides enhanced the uniform elongation while the strength was maintained due to higher resistance to GB cavitation al cracking. The fine intragranular carbides, which precipitated substantially through preliminary 5% cold work treatment, were responsible for the strong hardening effect by inhibiting dislocation movement. Based on the results, we propose the initial microstructure for better tensile properties at 950 ℃.