The influence of microstructure and environment on the crack growth behavior of powder metallurgy nickel superalloy RR1000 |
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Authors: | D M Knowles D W Hunt |
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Affiliation: | (1) Materials Performance Technologies, PO Box 31-310, Lower Hutt, NZ;(2) Siemens Westinghouse Power Corporation, 32826 Orlando, FL |
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Abstract: | 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. |
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