Solutioning and solidification process control in Ta-modified CM247 LC superalloy

Various stepwise solutionizing treatments were designed and performed to reach the highest secondary γ′ volume fraction and prevent incipient melting in a high Ta content Ni-base superalloy designed based on CM247 LC superalloy. Based on DSC thermal analysis data four different solutionizing cycles were selected and tried to eliminate the γ/γ′ eutectics and dissolve the primary γ′. Increasing Ta content from 3.2 to 6 wt% in CM247 LC superalloy, raised the size and volume fraction of γ/γ′ eutectics to about 4-fold associated with severe microsegregation in interdendritic spaces. Ta addition elevated the γ′ solvus and solidus temperatures around 41 °C and 25 °C respectively and minimized the solidification range from 114 °C to 52 °C. Massive elimination of γ/γ′ eutectic and primary γ′ occurred in alloys solution annealed at 1230 °C/4 h. With increasing the solidification rate from 0.4 to 1.2 °C/s remained γ/γ′ eutectic size was reduced about 85% in CM247 LC and 60% in Ta-modified CM247 LC after solutioning treatment. Secondary γ′ volume fraction raised from 36 to 49% in CM247 LC and from 52 to 60% in Ta-modified CM247 LC after aging. A critical solidification rate followed by a stepwise solutionizing treatment as 1230 °C/4 h + 1250 °C/2 h + 1260 °C/2 h + AC was proposed to minimize incipient melting areas and remained γ/γ′ eutectic size after solutionizing treatment combined with a maximum γ′ volume fraction for Ta-modified CM247 LC superalloy.