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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crack nucleation (Polák and Man, 2016) . At this very early stage of cycling, PSBs were not active enough to produce PSMs on the top surface and thus no evidence of surface roughening was found. The first distinctive PSMs were detected on the surface with continuing cyclic straining as apparent from Fig. 1a showing the specimen surface after application of 25 cycles corresponding to 2% of N f . Four PSMs emerging on the surface of MAR-M247 witness the starting cyclic plastic strain localization. These PSMs have hilly topography and they consist of only feebly developed parallel extrusions within an individual grain, cutting both γ matrix and γ  precipitates. Fig. 1b depicts a three dimensional reconstruction of the surface relief obtained using AFM in contact mode in the area indicated by a blue square in Fig. 1a. The average height of extrusion at this early stage of cycling was ( 81 ± 39 ) nm. It was measured as a difference between the height profile of the original surface in the section indicated by a black line in Fig. 1b and the height profile of extrusion in the parallel section copying the top of an extrusion (see the section indicated by a red line in Fig. 1b).

Fig. 1 SEM and AFM 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 25 cycles (a) SEM, (b) AFM – Accurex IIL.

Fig. 2 Evolution of average extrusion height during cyclic straining of superalloy MAR-M247 as obtained by AFM. For the quantitative assessment of average extrusion height, 150 points were taken on each 12 μm long section and the experimental data were statistically treated. The overall results of quantitative analysis of surface relief evolution are presented in Fig. 2 which shows the dependence of average extrusion height vs. the number of elapsed cycles. Continuing cyclic straining up to 5% of N f resulted not only in the further development of the existing PSMs on the top surface of grain but also in an increase in their number (Fig. 3a). The average PSM spacing in the investigated grain remained during cyclic straining approximately constant. Since CPEM technique was not able to detect the presence of intrusions and capture the true shape of PSMs precisely in early stages of fatigue life (due to limitations arising from both the specific geometry of PSMs and quality of the AFM tip) the FIB cutting was used to expose