PSI - Issue 39

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

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fracture; this transition is located at the slow crack propagation zone. No evaluation of this critical stress intensity factor was obtained in this work.

Fig. 5. Fracture surface at high applied load (746 MPa), with crack initiation site.

4. Discussion The in a row heat treatment described in this study allows to improve the ultrasonic fatigue endurance of Inconel 718, by a factor of 30 approximately, when compared to references and particularly for the high applied stress. The origin of this behavior is related to dissolution of precipitates in the matrix  , including the original delta phase δ (Ni 3 Nb with orthorhombic structure), during the first heat treatment. The last effect is combined with the formation of twins and austenitic microstructure, after the double aging heat treatment [30-32]. In addition, since no important variation on grain size was observed during the heat treatments, the origin of twins would be related to recrystallisation twins [33, 34], rather than the grain growth twins. The decrease on ultrasonic fatigue endurance of specimens tested in this work at low applied stress (all specimen broken at 511 MPa), may be related to a combination of two factors: heat treatments and the chemical composition of this tested material. In addition, most of tested specimen showed crack initiation near the surface, considered as subsurface crack initiation. Inconel 718 undergo progressive cyclic softening during ultrasonic fatigue tests; this mechanical behavior is attributed to degradation of precipitates related to γ'’ phase, inducing dislocations in deformation bands and leading eventually to deformation bands free of precipitates. On the other hand, planar defects as staking faults are generated when the delta phase δ is formed from the γ'’ phase [35, 36], growing near the last phase; nevertheless, no crystallographic analysis was carried out to identify this defect.

Conclusions The following conclusions can be drawn from the present work:

Ultrasonic fatigue endurance has been improved with a factor of thirty approximately, for the high loading levels in comparison with the results of others authors; this result was obtained using the combination of the described two heat treatments. The Inconel 718 can be fractured under ultrasonic fatigue testing at the lower stress levels (around 511 MPa), as has been observed in this work under the described heat treatments. The ultrasonic and conventional fatigue tests in this alloy consulted in bibliography present no fracture for the lower applying load (around 520 MPa), with null mean stress rate (R=-1) and fatigue life close to 10 9 cycles. A possible explanation of the last behavior may be related to the chemical composition of the tested I-718 alloy; since there are differences in the chemical composition