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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inductive proximity sensor to measure the displacements at the free end of specimen, with  2 m  of precision. Induced stress at the neck section of specimens was evaluated by numerical simulation, applying the corresponding measured displacement at its free end. Testing temperature at the neck section of specimen may be controlled by cooling system using air, water or oil; nevertheless, no cooling system was used for the ultrasonic fatigue tests in Inconel 718. The failure criteria for tested specimens was determined with the stopping of the ultrasonic fatigue machine: the stiffness of testing specimen is enough modified to leave the resonance condition when failure occurs.

Fig. 1. (a) Modal analysis and dimensions (mm) of the I-718 ultrasonic fatigue specimen, (b) Ultrasonic fatigue machine,

(c)

Thermographic images of ultrasonic fatigue tests on I-718.

3. Results and discussion 3.1 Heat treatments

The solubilization process was carried out at 1078º C in this work, Figure 2(a), since the melting temperature of the γ' phase is 1050º C, as reported by some authors [16]; at 1078º C the δ phase (Ni 3 Nb with orthorhombic structure), is dissolved into the matrix. The solubilization process destined to fatigue testing is frequently obtained at temperature between 950-980º C [17-20]; nevertheless, at these temperatures there is not transformation of the phase γ'. Carbides and laves phases cannot be dissolved at temperatures below 1032° C for the I-718 alloy [21-24]. Temperature of solubilization heat treatment must be high enough to dissolve the alloying elements in the metal matrix in order to improve the mechanical properties in creep and tensile strength.