The effect of Mo and Ti elements on long-term microstructure stability of 617B nickel-base superalloy

Abstract

In present work the effect of Mo and Ti elements on long-term microstructure stability of alloy 617B is investigated in temperature range of 650–720 °C for up to 4000 h. An alloy system of Ni–22Cr–12Co-1.28Al-0.043C-0.0024B-xMo-yTi (x = 10, 12 wt%, y = 0.9, 1.5 wt%) is prepared, and field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and thermodynamic calculation are used. The precipitation and coarsening of γ′ phase in this alloy system are accelerated by increasing Ti concentration. The addition of Mo can increase the magnitude of γ′/γ mismatch, and mismatch manifests itself by facilitating the morphology transformation from spherical to cubical. However, the coarsening of γ′ is retarded by addition of Mo, since diffusion of elements is a rate-controlling step in coarsening. Increasing Mo concentration can also promote the precipitation of μ phase. Hence, the addition of Mo can retard coarsening of γ′ phase on one hand, and promote precipitation of topologically close-packed (TCP) phase on the other. Both efforts of Mo should be taken into consideration in thermal stability design.