PSI - Issue 39

Gonzalo M. Domínguez Almaraz et al. / Procedia Structural Integrity 39 (2022) 281–289 Author name / Structur l Integrity Procedia 00 (2019) 000–000

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Fig. 3. (a) SEM images of the as-received Inconel 718, showing NbC carbides and delta phase, (b) SEM images of Inconel 718 after the solution annealing heat treatment, and (c) After double aging treatment with formation of twins.

The formation of twins was detected, as shown in Figure 3(c), and no residues of the δ phase were observed in the grain or in its boundaries. Twins are responsible for the hardness of material: the mechanism of hardening of the twins reduce the dislocations motions, as well as limits the movement at the grain boundaries [29]. 3.3 Ultrasonic fatigue results on Inconel 718 and analysis of the fracture surfaces. In Figure 4(a) are plotted the experimental results of ultrasonic fatigue endurance of the I-718 alloy; the vertical axis represents the level of stress (MPa), which include the range of applied load between 60 to 90 % the elastic limit of this material (511-746 MPa, respectively); whereas the horizontal axis represents the number of cycles to failure. In the same graph are plotted the results obtained by other authors [1, 2, 28]. An increase on fatigue endurance with a factor of 30 approximately, is observed comparing the results of this paper against the referred results, for the case of higher level of applied stress (746 MPa). Two principal observation can be drawn from this graph: higher difference on fatigue endurance between the present work and referred results is observed at high stress loading; this difference decreases with the decrease of applied load. The second observation is as follows: at low loading (close to 511 MPa), specimens have been broken in the present work, whereas no broken specimens have been reported in the referred studies for a near level of stress (550 MPa).