PSI - Issue 58

Juraj Belan et al. / Procedia Structural Integrity 58 (2024) 109–114 Juraj Belan et al. / Structural Integrity Procedia 00 (2023) 000–000

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Increased volume fraction of the δ-phase, Figs. 1c and d, is caused by the dissolution of the  -phase, which is metastable and at temperatures above 700°C releases Nb, which then passes into the δ-phase. The morphology of the δ-phase is acicular along the grain boundaries and inside the grain, it is excluded in the form of Widmanstätten needles (Belan et al. 2018). The results of the assessment of the volume share of the δ-phase using quantitative metallography procedures (Belan 2012) are 39.5% in the initial state and 52.5% in the state after annealing, which represents an approximately 33% increase in the share of the δ-phase, Table 2.

Table 2. Quantitative analysis of δ-phase results, impact energy and HV hardness results for IN 718 alloy IN 718 δ-phase volume fraction V [%] Charpy’s impact energy K [J] HV 10/10 [-] Starting stage 39.5 133 403 Annealing 800°C/72h 52.5 43 494

By comparing the results for the absorbed energy K (J) (dynamic test) and the hardness values (HV), the adverse effect of the increased volume fraction of the δ-phase on the dynamic characteristics is evident. In static tests, the increased volume fraction of the δ-phase under certain circumstances increases the hardness and strength of the IN 718 alloy due to the strengthening of the grain boundaries of the  solid solution by increased dislocation density, which are anchored to the acicular δ-phase inside the grain and thus reduce their mobility. The increase in HV hardness in the annealed state compared to the starting stage represents an increase of approximately 18%. However, on the other hand, a significant decrease in the impact resistance of the alloy, represented by the value of absorbed energy in the Charpy’s test, was noted. This decrease represents a decrease in impact resistance (that is, resistance to dynamic load) by almost 68%. Although in both cases the fracture surface was characterized by a ductile failure mechanism with a pronounced dimple morphology, the proportion of tough fracture was significantly higher in the samples in the starting stage than in the state after annealing. In this case, the increased volume fraction of the δ-phase has a negative effect; the needle (or Widmanstätten) morphology serves as a crack initiator and reduces the activation energy of the propagation of these cracks through the material. This fact also confirm the fatigue test results. The S-N curves obtained by the fatigue tests are shown in Fig. 2. Basquin Eqs. (1, 2) approximated results. R  1, IN 718-starting stage: � �������� � ������ (1) R  1, IN 718-after annealing: � �������� � ������ (2)

Fig. 2. The S-N curves for IN 718 with R  1.