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

Abstract

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 6wt% 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/4h. 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/4h+1250°C/2h+1260°C/2h+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.