Time-Dependent Crack Growth Thresholds of Ni-Base Superalloys

Abstract

A micromechanical model has been developed for predicting the time-dependent crack growth threshold and its variability by considering oxide formation or cavity formation ahead of an elastic crack subjected to a sustained load at a stress intensity factor, K, at elevated temperatures in air. It is demonstrated that stress relaxation associated with a volume-expansion process such as the formation of creep cavities or oxides with a positive transformation strain can induce residual stresses at the tip of the elastic crack. The near-tip residual stresses must be overcome by the external load, thereby instigating a growth threshold, K th , 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 several damage processes, including (1) oxidation-assisted intergranular crack growth, (2) K-controlled creep crack growth along an intergranular path, and (3) stress corrosion cracking. The micromechanical threshold models have been utilized to predict the time-dependent crack growth thresholds of a variety of Ni-base superalloys. The material parameters that contribute to the variability of the time-dependent crack growth thresholds have been identified and related to variations of mixed oxides or creep cavities formed near the crack tip. A size scale effect is also predicted for the transformation toughening phenomenon, which is largest at or below K th but diminishes at increasing K levels above the threshold. Finally, the micromechanical models are utilized to identify means for suppressing time-dependent crack growth in Ni-base alloys.

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

“…The anisotropic nature of deformation between neighbouring grains could also impose strain concentrations on the intergranular oxide intrusion. It is also now thought that such intrusions will develop stress fields during their formation because of the associated volume expansion on oxide formation and that enhanced tensile stresses will exist over part of the intrusion length[17,18].Above 800 MPa, an increase in fatigue life is found in the pre-oxidised specimens, with specimens at 825 and 850 MPa exhibiting longer lives and specimens at 900 MPa running out. This occurred in both of the pre-oxidised conditions (100 and 2000 hours at 700 o C).…”

Mechanical properties of thermally grown submicron oxide layers on a nickel-based superalloy

“…For simplicity, it was assumed in the FE model that the oxide layer is both homogeneous and smooth, as shown in Figure 5c. In comparison, Young's modulus is reported to be 234-273 GPa for Cr2O3 [37,39,45], 270.7 GPa for TiO2 [22], 362-393 GPa for Al2O3 [22,46] and 230-260 GPa for NiO [22]. It should also be noted that the exact oxide stoichiometry has a large influence on these values [37,47].…”

“…In the case of surface oxidation, these compressive stresses can become high enough to cause buckling and spallation [20]. In the case of crack tip oxidation, it has been proposed that these compressive residual stresses lead to crack-tip shielding [21,22]. In contrast, a self-consistent mathematical model developed by Cimbaro et al suggests that a wedge-shaped oxide intrusion is always anti-shielding and leads to an increased local tensile stress at the crack tip, the magnitude of which depends on the length of both the crack and the intrusion [11].…”

Embrittlement of an elasto-plastic medium by an inclusion

“…4 and 7 it is seen that the normal stresses within the intrusion differ significantly, even when there is no applied stress, from a linear superposition of the stress fields of the isolated intrusion (7), and the unloaded crack. It highlights the point made in the Introduction, with reference to the work of Chan (2014), of the necessity of satisfying the boundary conditions of traction free surfaces in the crack and a constant normal stress in the plastic zone, when the normal stress within the intrusion is calculated. Figures 7 and 8 show the variation of the normal stress within the intrusion subjected to different values of ω and to different intrusion semi-angles θ , respectively.…”

“…In addition, we find the gradient of the hydrostatic stress within the intrusion is consistent with experimental observations (Kitaguchi et al 2013) of the growth of oxide intrusions under different loading conditions. Chan (2014) argued that an oxide intrusion, with a larger volume per Ni atom than the Ni-matrix, induces a compressive stress in the matrix and thus screens the crack tip. We find the opposite.…”