The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000

A study of crack growth in vacuum and air at 725 °C (T/T _m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.