The influence of Hf additions on the microstructural changes has been studied in Ni-base superalloy Rene 80, which were conventionally cast and directionally solidified.
The morphologies of MC carbides (M stands for metal) by Hf additions are changed from script to blocky shapes. It has also been found that these carbide morphologies are influenced by solidification rate. These effect of Hf content and solidification rate could be explained by carbide formation period and site during solidification process. And the amount of γ-γ' eutectic has been increased with Hf additions.
Creep test showed that rupture lives and ductilities has been increased with Hf additions and/or directional solidification process. This improvement is believed to be due to the facts that addition of Hf changes carbide morphologies and grain boundary structures, and that directional solidification removes transverse grain boundaries normal to the stress axis. As the creep testing temperature increases from 760℃ to 982℃, this improvement was found to decrease gradually, which is thought to be due to change of creep mechanism.
