PSI - Issue 23

Ivo Šulák et al. / Procedia Structural Integrity 23 (2019) 161–166 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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intrusions inside the material (Fig. 3b). The intrusion approximately 100 nm in depth developed parallel to already existing extrusion. Incipient fatigue cracks are formed from intrusions in progressed fatigue life. At only 10% of fatigue life, PSMs occurred in more than 80% of grains.

Fig. 3 SEM micrographs documenting characteristic features of surface relief evolution in grain of superalloy MAR-M247 cyclically strained with the total strain amplitude ε a = 0.5% for 75 cycles (a) PSMs (b) FIB cut revealing 100nm deep intrusion. In several grains, a secondary slip system was activated at 20% of fatigue life (Fig. 4a) with a typical quadrilateral crack at the intersection point of two PSMs. This is also the point where the rapid initial extrusion growth rate (1.2 nm/cycle) is followed by a nearly ten times slower extrusion growth rate regime (0.13nm/cycle) as it is evident from Fig 2. The stable extrusion growth rate was reported 0.2 to 0.4 nm/cycle for copper (Mughrabi et al., 1991) and 0.29nm/cycle for Inconel 738LC (Obrtlík et al., 2010) at only 5% of fatigue life. The final extrusion height is (47 3 ± 134 ) nm. The observed evolution of extrusion height can be compared with the physically based models which consider localized cyclic plastic straining and point defect production, i.e. EGM-model (Essmann et al., 1981) in which the static extrusion height is expected to be h ≈ 3 ×10 – 4 D , where D is the grain size measured in a direction parallel to the active Burgers vector b . The change in growth rate can be attributed to the formation of dislocations in PSBs and continuous migration of point defects from PSBs to the surrounding matrix resulting in mass redistribution in agreement with Polák’s model (Polák, 1987) . This results in the progressive formation of intrusions accompanying extrusions which represent an effective crack initiation sites. The average intrusion growth rate was measured to be 1.93 nm/cycle. Man with co-workers (Man et al., 2009) made an attempt to evaluate the intrusion growth rate on 316L steel using plastic replicas and reported intrusion growth rate to be 1.52 nm/cycle. Polák (Polák et al., 2016) applied FIB sectioning on superaustenitic steel Sanicro 25 to observe the evolution of intrusions and intrusion growth rate was assessed between (0.3 and 0.71) nm/cycle. However, since it is very difficult to distinguish between intrusions and just initiated fatigue crack, those values can be affected by the crack initiation at the intrusion tip. First microcracks (Fig. 4b) were detected at 40% of N f . In all cases, they started as intrusions. The initiated microcracks propagated in the crystallographic mode along one of the active slip planes. Three dimensional CPEM micrograph of the PSMs with a clearly visible short fatigue crack originating in one of the PSMs is shown in Fig. 4c. The subsequent interlinking of individual short cracks finally resulted in the main crack formation and its propagation in stage I. The typical surface topography at the end of fatigue life (N f = 1486 cycles) is apparent from Fig. 4d which shows the surface densely covered with pronounced PSMs and fatigue cracks growing parallel to the active slip planes. Fig. 5a depicts the dense system of PSMs in a different grain where PSMs have developed later. The average spacing of PSMs is reduced considerably in comparison with that observed in Fig. 3. The characteristic peak-to-valley topography is apparent from three-dimensional CPEM micrograph shown in Fig. 5b. The surface profile in a section across PSMs proved convincingly that numerous extrusions are accompanied by a pair of parallel intrusions (see Fig. 5c). The kinetics of extrusion growth in this grain is similar to the trend shown in Fig. 2. The short initial period of the rapid growth of static extrusion with a rate of 1.23 nm/cycle is replaced by the period of continuous extrusion growth with a much slower rate of 0.14 nm/cycle. The results of TEM observations of dislocation arrangements in the volume of superalloy MAR-M247 cyclically strained up to the end of fatigue life supports the SEM and AFM surface observations. Plastic strain localization into PSBs in the form of slabs of high