PSI - Issue 2_B

Balázs Fekete et al. / Procedia Structural Integrity 2 (2016) 2164–2172 Author name / Structural Integrity Procedia 00 (2016) 000–000

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3.1. Investigation of the kinetics of the fatigue process To relate the investigated mechanical behavior to the dislocation substructure, the TEM results together with the X-ray diffraction analysis of the samples interrupted at stages of UF = 0%, 5%, 25% 50% and 70% were taken into consideration. The 15Ch2MFA steel had a fine grain (0.3-1 µm) homogeneous bainitic structure and contained  0.1-0.2 µm particles of Cr-Mo-V carbides Fig. 3(a)). The initial dislocation concentration was 0.7  10 14 m -2 due to the bainitic transformation [Mayer et al. (2012)]. After 5% UF (total strain amplitude ε=0.3) the cell size remained unchanged but the dislocation density increased to 1.2  10 14 m -2 . This tendency was preserved with further loading to 25% UF when the dislocation density reached its maximum (3.4  10 14 m -2 ), while only a minor reduction in the cell size was detectable. The most significant change could be observed after 1180 cycles at 50% UF. In this sample the size of cells increased, the cell walls bent and the dislocation concentration slightly decreased (3  10 14 m -2 , Fig. 3(b)). At 70% UF further decrease in dislocation density (1.2  10 14 m -2 ) and increase in cell size could be observed. The structural changes above 25% UF increased the mobility and mean free path of dislocations i.e. increased the number and average spacing between the obstacles to dislocation glide explaining cyclic softening in 15Ch2MFA steel in the 25 % to 70 % UF range. We found that of the dislocation density increasing in early fatigue life for 15Ch2MFA steel even though this material showed a cyclic softening behavior (Fig. 3(c)). This contradiction can be resolved by mean free path analysis of the dislocations in the system.

Fig. 3. Investigation of the kinetics of the fatigue process for 15Ch2MFA (a) TEM micrograph in the initial state (b) TEM micrograph in the 50% fatigued state (c) evolution of the dislocation density with fatigue

The substructure of 08Ch18N10T steel in as-received condition consisted of a disordered dislocation net with a low dislocation density of 0.7x10 14 m -2 . Quite different substructures occur in the steel after cyclic loading. The visibility of the dislocation network depends on the crystallographic orientation of the individual austenite grains. The applied stress was always normal to the plane of the foil and the foil was tilted up to ±7° in order to adjust the two-beam conditions necessary for optimum contrast. Dislocation-poor and dislocation-rich areas were formed during cyclic deformation, similar to those observed in fatigued monocrystals of fcc metals [Kettunen and Kuokkala (2003)]. Dislocations began to concentrate in the early fatigue life into bands, the so-called veins, separated by dislocation-free areas indicated as channels. According to the model published by Mughrabi the vein substructure consists of bundles of edge dislocations and channels that contain some screw dislocations [Mughrabi et al. (1979)]. Such a structure precedes the formation of well-known persistent slip bands (PSB) and dislocation cells [Kettunen and Kuokkala (2003)]. The width of veins decreased with increasing number of cycles and PSBs formed. A well-developed cell substructure and wavy PSBs separated by dislocation-poor channels occurred in the specimen with UF = 50% (Fig. 4(b)). The size of dislocation cells decreased slightly with increasing number of cycles.