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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In some areas nickel carbonates in the form of NiCO 3 (Space Group R ͞ 3c , a = 0.4617 nm, c = 1.4735 nm) [9]) particles embedded in the amorphous layers were found. The formation of these particles could occur after air access. The rounded particles observed in the cross-sections specimens are the ones, which had been found by SEM (Fig. 1b, Fig. 2b).

Fig. 4. (a) Bright field TEM image of nanograins near the fracture surface and the amorphous areas between them; (b)Fast Fourier Transform (FFT) spectrum corresponded to the Ni 3 C phase; (c) FFT spectrum obtained from the area in the square; (d) FFT corresponding to the NiO phase.

4. Discussion

Based on the experimental data the mechanism of an internal fatigue crack development without air access was proposed. At the initial stage (samples No. 1 and No. 2) a plastic deformation zone is formed in front of the crack tip [10]. The formation of nanocrystals observed by SEM (Fig. 1b, 2b), and by TEM (Fig. 3, 4) occurred in this zone. Apparently, the microstructure with the nanocrystals is formed in several loading cycles. The TEM and electron diffraction data pointed to the high degree of misorientation of nanocrystals. The misoriented nanocrystals lead to the appearance of relatively "weak" links at nanocrystal boundaries. As a result, the propagation of a crack occurs through the boundaries of nanocrystals. The formation of "weak" links was also confirmed by the observations of the thick amorphous layers at the boundaries of nanocrystals in specimen No. 1 (Fig. 3b) after high-temperature oxidation of the fracture surface. Thus, the mechanism of failure, discussed above, can be defined as inter nanocrystalline. It is known that the formation of nanocrystals with large misorientation requires an excess of dislocations of the same sign [11]. However, the formation of dislocations ahead of the crack tip is energetically unlikely (Fig. 5a) [11]. To explain the large misorientation of nanocrystals, we proposed the following model. During loading cycles, there are periodic opening and squeezing of the crack. When loading is released and the crack is squeezed, the so-called "cold welding" process can occur [12]. Opposite surfaces of the crack, when they are oxide-free and there is no loading joined and chemical connection forms. After subsequent loading and crack opening dislocations of one sign (see Fig. 5b) ahead of the crack tip are formed due to the stress. A multiple loading cycle arises the misorientation of nanocrystallites (Fig. 5c) followed by further crack propagation along the weak grain boundaries of nanocrystals.