The role of boron in modifying the solidification and microstructure of nickel-base alloy U720Li

This study is focused on the effect of boron addition, in the range of 0.011 wt% to 0.048 wt%, on the solidification behavior and microstructure of U720Li alloy. It has been found that boron increased the eutectic (γ + γ′) precipitation significantly, while reduces the formation of η phase and Zr-rich particles precipitated in front of the eutectic (γ + γ′) obviously. Boron greatly retards the γ matrix solidification which is verified by the fact that it markedly reduces the 'segregated area' around the final liquid pool or eutectic (γ + γ′). Based on this phenomenon, boron is assumed to form a layer at the solid/liquid interface and inhibits the attachment of atoms to attach on or jumping off the solid front, and hence lowers the γ matrix solidification rate. Boron increases the eutectic (γ + γ′) precipitation mainly by retarding the γ matrix solidification, which resulting in deeper undercooling and more eutectic forming elements such as Ti left in the residual liquids. Subsequently, boron reduces the η-Ni3Ti and Zr-rich phase formation because the increasing eutectic (γ + γ′) precipitation favors the consumption of Ti and dissolving Zr in γ′ phase.