Creep crack growth behavior of several structural alloys

Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

“…Creep crack growth involves the formation, growth, and coalescence of creep cavities along grain boundaries located ahead of the crack tip, [3,17,38,39] as illustrated in Figure 1(b). 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.…”

“…Cyclic crack growth generally follows a transgranular path, while time-dependent crack growth due to stress-assisted grain boundary oxidation typically follows an intergranular path. [1][2][3][4][5][6][7][8][9][13][14][15][16][17][18] The transition from transgranular fracture to intergranular fracture depends on the temperature, load frequency, and hold time. In general, time-dependent crack growth under K-controlled conditions can be expressed as [21,23] da dt ¼ B o exp À Q RT K m with m>0 and K>K th ; ½4…”

“…Creep crack growth by g.b. cavitation is observed in Ni-based alloys tested in vacuum or in an inert atmosphere, [1,17,19] but it is also observed in c¢-strengthened Ni-base alloys such as IN 738 and IN 939 tested in air. [24,25] For oxidation-induced crack growth and stress corrosion cracking, the degradation processes are the formation and growth of oxides along grain boundaries, and the corresponding crack paths are also predominantly intergranular.…”

“…[24,25] For oxidation-induced crack growth and stress corrosion cracking, the degradation processes are the formation and growth of oxides along grain boundaries, and the corresponding crack paths are also predominantly intergranular. [1][2][3][4][5][6][7][8][9][13][14][15][16][17][18] For IN 718, the crack-tip oxidation processes during sustained load crack growth in air involved the formation of NiO and Cr 2 O 3 along grain boundaries. [4,5,7] The oxidation of grain boundary particles such as NbC during sustained load crack growth in air and in oxygen environment [26] or fatigue in air [27] was also reported by some investigators.…”

“…It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or the formation of oxides with a positive transformation strain can induce near-tip residual stresses that must be overcome by the external load, thereby instigating a growth threshold for the onset of time-dependent crack growth. 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.…”