Effect of tantalum on the microstructure stability of PM Ni-base superalloys

Abstract The effect of Ta on the microstructure stability of powder metallurgy (PM) Ni-base superalloy was examined at standard heat treatment and after long-term thermal exposure. The results reveal that the previous particle boundary structure is eliminated with the addition of Ta. As Ta content increases, the γ' morphology changes from spheres to cuboids and begins to split, which is caused by the increase in the γ/γ' mismatch. Reversal coarsening of γ' particles in Ta-containing alloys are observed during exposure at 750 °C. The main reasons are related to γ/γ' lattice mismatch, elements partitioning and soft impingement. Besides, the increase of Ta content promotes the precipitation of σ phase at grain boundaries and in grains.