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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heating and cooling rates of 20 °C/s. For TMF tests, the most challenging part of the development was to reach the same cycle time as in the ISO tests to provide comparable results for the LCF behavior of the tested materials. The difficulty was that this temperature range was relatively close to room temperature therefore the cooling rate was limited. For this reason an air nozzle was mounted in the test chamber and directed at the gauge surface of the specimen, as shown in Fig. 1. The air flow was interrupted by a magnetic switch during the heating period of the TMF test. 2.3. Microstructural and fractographic investigations After their deliberate rupture, samples were longitudinally sectioned, polished mechanically and etched using appropriate etchants. For the 15Ch2MFA the etchant was Nital, and for the 08Ch18N10T it was a solution of HF, glycerine and HNO 3 in a volume ratio of 2:2:1. Evaluation of the dislocation substructure of the samples at different stages of fatigue life, namely at UF = 0%; 5%; 25%; 50% and 70%, was carried out using TEM. For the fatigued samples of 15Ch2MFA, thin foils were taken from the middle of the gauge length in a direction parallel to the applied stress, and thinned by mechanical grinding to a thickness of 50 µm. Final thinning was performed using 10 keV Ar ion milling at grazing incidence. Low energy (0.5-3 keV) milling was used as a final step after perforation. Thin foils were prepared by thinning the plate cut in the central cross-section of the fatigued specimen gauge. Final thinning to electron transparency was performed using jet electrolytic polishing in 6% solution of HClO 4 in methanol at a voltage of 22 V and a temperature in the range of -65 °C to -55 °C. X-ray diffraction experiments were performed on the samples to provide quantitative results on dislocation evolution during the fatigue process. The X-ray diffraction experiments were carried out in a special double-crystal diffractometer dedicated to line profile analysis with negligible instrumental peak broadening. The measured diffraction patterns were evaluated by using the Convolutional Multiple Whole Profile (CMWP) program [Ribárik and Ungár (2010)]. A fractographic analysis of the tested samples of 15Ch2MFA was carried out. First, an observation of the surface of the test bars was made using SEM with the aim of finding out the number and sizes of cracks, and their positions and distribution. Then the specimens were loaded by tension in a tensile testing machine to break them and open up surface(s) of fatigue crack(s) for a direct observation with SEM. Micrographs of different parts of fracture surfaces were taken and the average distances between striation lines were determined for the initial, middle and final parts of the cracks. 3. Results and analysis The numbers of cycles to failure with respect to strain amplitude are shown in Fig.2.

Fig. 2. Results on number of cycles to failure with total mechanical strain for (a) 15Ch2MFA (b) 08Ch18N10T

As it can be seen from Fig. 6, both materials investigated show lower life in the TMF condition, but 15Ch2MFA is more sensitive to thermal cycling than the 08Ch18N10T cladding material.