The microstructural changes during heat treatment and their effects on the high temperature mechanical properties have been studied in a directionally solidified Ni-base superalloy CM247LC. Special attention has been given to the changes of γ' and MC carbide morphologies which may have great influence on the creep properties.
Two different types of MC carbide morphologies were observed ; i.e. script and blocky. These different morphologies are believed to be originated from different formation temperature during solidification preiod.
Changes of MC carbide morphologies occurs during heat treatment, which is thought to be due to following reaction ;
MC + γ → MC'(Hf) + γ'
This was confirmed by the observation which γ' films around MC carbide are formed during heat treatment.
These undissolved Hf rich MC carbides after heat treatment cause the cavitation under 871℃-45 kgf/㎠, 760℃-78 kgf/㎠ creep test due to dislocation pile-up formed during creep deformation. On the other hand, cavities have been observed in the γ-γ' eutectic region under 982℃-20 kgf/㎠.
