Effects of Hf and B on high temperature low stress creep behavior of a second generation Ni-based single crystal superalloy DD11

The as-cast and heat-treated microstructures and high temperature creep properties have been investigated in four experimental Ni-based single crystal superalloys containing various levels of Hf addition (0-0.4 wt%) and B addition (0-0.02 wt%). The experimental results indicated that the creep rupture life showed an improvement with individual addition of Hf, but it was decreased with individual addition of B. The elemental partitioning ratio and interfacial dislocation spacing of γ/γ′ were obviously changed with individual Hf or B additions. Meanwhile, the formation of secondary phases, such as the blocky MC carbide, script-like shape M3B2 phases, was observed in the creep samples, which was also closely related to the high temperature creep behaviors. The high volume fraction of residual (γ+γ′) eutectics was mainly attributed to the significant decrease of creep rupture life for the present experimental alloy containing both Hf and B additions. This study is helpful to better understand Hf and B's role of strengthening mechanism and to optimize Hf and B additions in single crystal superalloys.