PSI - Issue 23

M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 251–256 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results

3.1. Fractographic study of the samples

The investigation of specimen No. 1 by SEM demonstrated that crack started from the defects in the alloy (Fig. 1 a), which were formed near HfO 2 based precipitates [4]. We found that the crack development occurred in three stages with the formation of a specific surface morphology. The rugged and rough surface covered by round-shaped particles with sizes in the range between 50 nm and 150 nm was formed at the first stage (Fig. 1). It should be noted that the morphology of the particles is determined by their microstructure, not associated with oxidation process. At the second stage, a zone with fatigue striations is formed. The formation of a transition region with the fatigue striations and rounded submicron particles was associated with the change of the crack propagation mechanism. In addition, a third zone was identified and it corresponds to the transition from stable crack growth to the rupture area. In the presented study, the second and third zones were not studied. Unlike sample No. 1, four zones were observed in sample No. 2, and each of them corresponds to four stages of crack development. The first and second zones are indicated on the SEM images shown in Fig. 6a and b. The first zone is similar to the one, found in sample No1. The second zone exhibited quasi-faceted morphology (Fig. 2). Highly likely, it was formed exactly it the moment when the crack had reached the surface of the sample and the admittance of air started with the surface oxidation. The third zone has a distinct appearance with fatigue striations. The fourth zone is similar to the third zone of specimen No. 1 - the transition from stability crack growth to the rupture area. SEM image (Fig. 2) clearly illustrate the moment when the crack reaches the surface. This image was obtained in backscattered electrons (zone 1, Fig. 2 a) and part of it has brighter contrast. That pointed to thinner oxide layer on the crack surface. 3.2. The study of cross-sections. The cross-sections for microstructural analysis were cut out at a distance of 30 μm from the crack origin of sample No. 1 (Fig. 2). The direction of the cut was chosen towards the development of fatigue crack. The HAADF STEM images of the specimens are presented in Fig. 3 a,b. The layer beneath the protective Pt layer exhibits darker contrast and consists of nanoparticles (Fig. 3 a). Several areas with different contrast can be revealed in the enlarged image (Fig. 3 b). The EDX microanalysis indicated that the darker areas between the grains (Fig. 3 b) contains more than 50% C and O. Below this layer, there are large particles, which consist of highly textured sub-grains misoriented relatively each other on 1-2 °. The electron diffraction study of the sub-grains unambiguously demonstrate that the crystal structure correspond to face centered cubic Fm ͞ 3m space group with the unit cell parameter a = 0.34 – 0.36 nm and that is completely consistent with γ -phase [5]. The overall orientation of the subgrains in this area is close to [101] cubic unit cell perpendicular to the specimen surface. Apparently, this corresponds to the orientation of the γ ’ -phase grain before the tests, in which the development of the fatigue crack occurred. It should be noted that, in the volume of sample No. 1, low density of NiCo particles were also detected. The unit cell parameters of this phase with a high content of Ni and Co are close to γ phase [6]. Analysis of high-resolution TEM images (Fig. 4) showed that particles of different phases were formed in the near-surface area of the crack: Ni 3 C (Space Group R ͞ 3c , a = 0.45 nm, c = 12.9 nm [7]) and NiO (Space Group Fm ͞ 3m , a = 0.418 nm [8]). Between Ni, NiO and Ni 3 C particles thick amorphous layers (up to 20 nm) were found (Fig. 3b and Fig. 4c).