SEM imaging suggests that the initial combined presence of stress and hot corrosion results in the reaction of the γ’′ precipitates. Cracks then initiate from features similar to corrosion pit features (Figure 7) and propagate through the γ’′ where corrosion is present (Figure 8). Using the results of EDX analysis (Figure 5) it is hypothesised that this is because of the lower Cr and Co content of the γ’′ precipitates.
Figure 4. Unstressed corrosion product at 550 °C and exposed to 5 μg/cm2 /h with a test gas of air – 300 vppm SO2 (a) 500 h resulting in 7·7 μm oxide scale (b) 100 h exposure resulting in 2·43 μm oxide scale.
The corrosion attack shifts to the γ, and it is hypoth esised that this happens when the protective NiO/ CoO rich oxide scale is formed, as this depletes Co from the alloy which is mainly concentrated in the matrix.
FEA principal and von Mises stress state modelling in C-rings
FEA modelling predicted that maximum stress occurred in the central region of the C-ring as shown in Figure 9. FEA also predicted the presence of a multi-axial stress state within the C-ring, where the largest resolved principal stress plane, referred to as maximum principal, occurs along the x-axis, and the second largest resolved stress plane, referred to as middle principal, occurs in along the z-axis.
This stress state would suggest cracks would firstly initiate and then propagate in the z-axis where the maximum principal is acting at a normal in mode I crack opening. However as cracks propagate and Δk exceeds kth then secondary cracks could propagate in all three principle directions. A summary of the stress conditions for various ΔD values is given in Table 3.
FEA was further used to predict the stress inten sity and concentration around a crack tip (Figure 10) within the C-ring geometry. These micro-cracks were modelled in the central region of the C-ring using a refined tetrahedral mesh; the results are presented in Table 4.
FEA stress intensity modelling suggests that cracks or pits need to be greater than 100 μm for cracking to occur given a kth of 15 MPa.m1/2 as the reported fatigue threshold for CMSX-4 [21]. Therefore the presence of hot corrosion may have a significant effect on reducing the material's kth as well as concentrating stress through corrosion pitting.
Analysis of a corrosion pit’s size in cracked C-ring specimen implies that a 10 μm diameter pit has initiated cracking during these exposures (Figure 7). Using the FEA calculated geometry factor Υ of 0·836, this gives a theoretical reduced kth of 3·748 MPa.m1/2 when hot corrosion is simultaneously acting with a stress of 800 MPa; a 75% reduction. This means that cracking can occur at considerably lower applied stresses.
Figure 5. Surface corrosion fatigue crack and back scattered EDX characterisation at 800 MPa after 300 h with a 5 μg/cm2 /h deposition flux and a test gas of air – 300 vppm SO2.
Figure 6. Cracking of C-rings at 800 MPa with a 5 μg/cm2 /h deposition flux and a test gas of air – 300 vppm SO2 (a) 100 h exposure cross section (b) 300 h exposure cross section (c) 300 h central cracking (d) 500 h symmetrical cracking.
Figure 7. Secondary electron images of 800 MPa C-ring with a 5 μg/cm2 /h deposition flux and a test gas of air – 300 vppm SO2 (a) 100 h fracture face showing signs of beaching marks (b) 100 h fracture face, crack tip showing attack of γ’′ (c) 100 h specimen surface showing attack of γ’ (d) 100 h high mag fracture surface at crack tip (e) 300 h corrosion attack of γ’′ precipitate (f) 500 h corrosion attack of γ’ precipitate (f) 500 h corrosion attack of γ matrix.
Figure 8. SEM images near crack tips from CMSX-4 C-ring samples stressed to 800 MPa and exposed to a corrosion environment with a deposit flux of 5 μg/cm2 /h and test gas of air – 300 vppm SO2 (a) 300 h exposure (b) 300 h exposure (c) 300 h exposure (d) 100 h exposure.
Figure 9. Axis orientation for C-ring modelling, showing normal stress distribution within a C-ring in the principal x-axis, for a boundary condition of ΔD = 0·612 mm.
A Kitagawa diagram [23] has been plotted to demon strate the stress and crack or defect size needed to exceed the material's kth (Figure 11). This is done both for the calculated theoretical kth in hot corrosive conditions, and using the reported kth for air.
Figure 10. Stress distribution around a centrally located 100 μm crack tip in a C-ring at 890 MPa.
Figure 11. Kitagawa diagram produced from FEA corrosion crack stress intensity analysis, showing the crack defect size required to initiate cracking both with and without the presence of hot corrosion.
Conclusions
SEM/EDX characterisation of the corrosion product produced by stress corrosion in CMSX-4 C-rings at 550 °C is consistent with type II hot corrosion.
Hot corrosion conditions at 550 °C combined with static stresses of greater than 500 MPa can cause a sig nificant hot corrosion stress cracking mechanism. A lower limit seems to exist around 500 MPa. However at exposures greater than 100 h with a flux of 5 μg/cm2 /h cracking is still visibly present.
FEA modelling predicts the multiaxial nature of the stress state within a clamped C-ring and the observed cracking in experimental testing supports the modelling results. By determining the effective stress through the von Mises criterion, FEA calculated equivalent stress concurs with that from ISO 7539-5.
FEA stress intensity modelling around crack tips estimates that fatigue/fracture (kth) can be reduced by up to 75% with the combined effect of hot corrosion in CMSX-4.
SEM imaging suggests the combined hot corrosion stress mechanism initially attacks γ’
′ precipitates, cracks then propagate through precipitates as the mechanism attacks features ahead of its propagation path. A switch to attack of the γ matrix is observed. It is hypothesised that this occurs in order to form the NiO/CoO oxide scale which depletes Co from the γ matrix.