Dependence of Stress Corrosion Cracking of Alloy 690 on Temperature, Cold Work, and Carbide Precipitation—Role of Diffusion of Vacancies at Crack Tips

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

“…On this basis, Eq. [11] is directly applicable to stress corrosion cracking, providing that the damage process is controlled by the fracture of the oxide film and not by the kinetics of passive film formation. The threshold, K ISCC , for the onset of stress corrosion cracking is then given by…”

“…In Eq. [11], sgn(x) is the sign function with x as the dummy variable; sgn(x) = 1 when x is positive, and sgn(x) = À1 when x is negative. Equation [11] indicates that K th depends on material parameters such as the oxide modulus, oxide thickness, transformation strain, and the yield stress of the alloy.…”

“…[11], sgn(x) is the sign function with x as the dummy variable; sgn(x) = 1 when x is positive, and sgn(x) = À1 when x is negative. Equation [11] indicates that K th depends on material parameters such as the oxide modulus, oxide thickness, transformation strain, and the yield stress of the alloy. The dependence of K th on the yield stress of the Ni alloy can, at least conceptually, be utilized to consider the effects of grain size and cooling rate on time-dependent crack growth threshold.…”

“…To derive the K th for creep crack growth, cavity formation at a grain boundary is considered as a transformation process that involves coalescence of n v vacancies to produce a cavity [11,40] in the form an oblate ellipsoid. The volume of a void formed by the coalescence of n v vacancies is given by…”

“…This micromechanical framework provides the basis for developing appropriate predictive models for the time-dependent crack growth thresholds associated with damage mechanisms such as (1) oxidation-assisted intergranular crack growth, [1][2][3][4][5][6][7][8][9] (2) creep crack growth along an intergranular path, [12][13][14][15][16][17] and (3) stress corrosion cracking. [11,30,31] The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of Ni-base superalloys used in turbo-propulsion systems. The sources of material variability in values of time-dependent crack growth threshold have been identified, and they can be related to mixed oxides or creep cavities formed near the crack tip.…”