Prior to testing, C-ring samples were cleaned in an ultrasonic bath with IPA (isopropyl alcohol). Corrosion exposures were carried out in a horizontal, controlled atmosphere furnace. The corrosion environment, deposit composition and deposit flux were controlled using the well-established deposit recoat methodology (e.g. Sumner et al. [3]). Specimens were coated with an 80/20 M mixture of Na2 SO4 /K2 SO4 . The mass of depos ited salt was measured per unit area and specimens were recoated every 100 h in order to control the deposition flux. A gaseous environment of air – 300 vppm SO2 was used, and all testing was conducted at 550 °C. C-rings were stressed to 800, 700 and 500 MPa, and exposed for 100, 300 or 500 h exposure times with a target dep osition flux of 5 μg/cm2 /h. In addition, one C-ring at each target stress level was exposed for 300 h without any deposit.
Microscopy and analytical methods
Samples were mounted in a 50:50 mixture of MetPrep’s epoxy set resin and ballotini (40–70 μm diameter glass spheres). Samples were then sectioned, using oil lubricant to prevent dissolution of corrosion products and deposits, before being ground and then polished to a 1 μm diamond paste finish (again using oil lubricant).
Both optical and SEM examinations of the sam ples were carried out. Optical microscopy was used to determine if cracking was present in the C-rings after each exposure period. SEM was used to characterise the results of the degradation mechanism’s interaction with the alloy microstructure. FEI XL-30 and a JEOL 7800F field electron gun (FEG) SEMs equipped with backscat ter energy-dispersive X-ray (EDX) detectors were used for characterisation and SEM imaging. SEM images were post-processed using Image J software to enable accurate measurement of features.
Figure 1. C-ring specimen geometry as established from ISO 7539-5 (a) frontal cross section (b) top view cross section (c) side view cross section (d) isometric view. (Units in mm.)
Figure 2. Example of a C-ring mesh constrained between two plates.
FEA analytical methods
FEA modelling was conducted using ANSYS Workbench 15 [18]. The material model used for this analysis was an isotropic model generated using monotonic surrogate material data for CMSX-4 from Siebörger et al. [17]. The C-ring was constrained between two plates (Figure 2). A frictionless sliding contact was utilised between one of the two blocks and the C-ring in order to allow a small amount of relative movement. The boundary conditions
were applied through a displacement equivalent to those calculated using Equation (1) (and listed in Table 3).
The C-ring was modelled as three separate sec tions to allow more accurate refinement of the mesh in the central section of the C-ring. This was impor tant as it was in this central region that it was antic ipated that cracking could occur. A hex dominant mesh is used wherever possible; however meshing around the crack tips required the use of a tetrahedral
mesh due to the size and complexity of the crack tip geometry.
When clamped, a multiaxial stress condition was pre dicted in the C-ring. As such the von Mises criterion was used to obtain local stress values. However it was also useful to consider the principal or normal stresses in relation to a mode I crack opening, as shown in Figure 3,
as these stresses could be higher for the C-ring geometry in the principal x-axis plane when compared with the von Mises stress.
Linear elastic fracture mechanics (Equation (3)) were used to assess the local stress intensity range (Δk) for micro semi-elliptical cracks within the C-ring. The stress intensity was assessed using the ANSYS R15 [18] stress intensity solver code. ANSYS uses T-stress eval
uation [19] in order calculate the local crack tip stress intensity. Equation (3): Linear Elastic Fracture Mechanics Equation [20].
Figure 3. Crack opening modes (a) mode I (b) mode II and (c) mode III.
FEA generated local crack tip stress intensity predictions were then used to calculate the
geometry factor (Υ) for finite surface cracks in the C-ring geometry. Δk can then be compared to kth to assess the likelihood of cracking. The kth of CMSX-4 has been reported to be 15 MPa.m1/2 in air at 750 °C [21]. Stress intensities _calculated through FEA modelling can be compared to this in order to determine the likelihood of cracking and the effect of hot corrosion on kth.
Results and discussion C-ring results & discussion
Unstressed sections of CMSX-4 C-rings were cor roded at 550 °C with a target deposition flux of 5 μg/ cm2 /h and a gaseous environment of air – 300 vppm SO2 . Samples were removed after exposure times of 100, 300 and 500 h. Inspection showed formation of
an oxide scale containing Co, Ni, S and O (Figure 4 and 5). Sulphidation had occurred beneath the oxide scale, consistent with type II hot corrosion [7,22].
For stressed C-rings, cracking of corroded sam ples was observed at 800 and 700 MPa after expo sures for 100 h; with visible cracking still occurring at 500 MPa for exposure times longer than 100 h (Table 2). By contrast C-ring tests without deposit showed no signs of cracking after 500 h of exposure to the test conditions.
C-rings normally experienced cracking within the most highly stressed central region. However when cracks initiated off centre, cracking would occur either side of the centre line due to the shifted stress distribu tion around the C-Ring (Figure 6).
The corrosion mechanism varied from attack ing the gamma-prime (γ′ ) to attacking the gam ma-matrix (γ) This is visible in SEM backscattered imaging as a shift in the contrast between the two microstructural features (Figure 7). This reduction in back scattered electrons is attributable to the lower atomic number of the S and O present in the corrosion products.