Cavity nucleation in creep

One point which has not been emphasized by previous workers is that cavity nucleation by vacancy aggregation requires vacancy supersaturation. It has been shown that this prerequisite is satisfied in creep. The vacancy supersaturation is due to creep strain, and this induces a chemical stress which facilitates cavity nucleation. The magnitude of this chemical stress is strongly dependent on the strain rate. A high strain rate is always reached during tertiary creep, and this probably explains why some nickel-based superalloys only cavitate late in the creep life. It has also been shown that if the chemical stress induced by creep strain is taken into account, cavity nucleation in creep in the temperature-stress regime of practical interest can be better explained. However, the chemical stress decays very quickly with increasing temperature. A kinetic barrier exists in creep under stresses of 5X10-4E when temperature exceeds 0.55 T /SUB m/ . Neither the ''non-wetting'' concept nor the ''stress concentration'' hypothesis convincingly explains the apparent discrepancy between experimental observations and theoretical prediction. Finally, it must be mentioned that all grain boundaries are not perfect sinks for vacancies, and this aspect of cavity nucleation has now been addressed by the authors