Abstract
This study investigates the very-high-cycle fatigue (VHCF) behavior at elevated temperature (650 °C) of the Inconel 718 alloy fabricated by selective laser melting (SLM). The results are compared with those of the wrought alloy. Large columnar grain with a cellular structure in the grain interior and Laves/δ phases precipitated along the grain boundaries were exhibited in the SLM alloy, while fine equiaxed grains were present in the wrought alloy. The elevated temperature had a minor effect on the fatigue resistance in the regime below 108 cycles for the SLM alloy but significantly reduced the fatigue strength in the VHCF regime above 108 cycles. Both the SLM and wrought specimens exhibited similar fatigue resistance in the fatigue life regime of fewer than 107–108 cycles at elevated temperature, and the surface initiation mechanism was dominant in both alloys. In a VHCF regime above 107–108 cycles at elevated temperature, the wrought material exhibited slightly better fatigue resistance than the SLM alloy. All fatigue cracks are initiated from the internal defects or the microstructure discontinuities. The precipitation of Laves and δ phases is examined after fatigue tests at high temperatures, and the effect of microstructure on the formation and the propagation of the microstructural small cracks is also discussed.
Highlights
Published: 20 February 2021Inconel 718 (IN718) alloy is one of the most widely used nickel-iron-based superalloys in the high temperature structure applications due to its high strength, as well as the high creep, oxidation and corrosion resistance over a wide temperature range [1].The main strengthening phases are the coherent γ00 (Ni3 Nb) and γ0 (Ni3 Al) precipitates in the γ matrix
The sample was designed based on the present in the selective laser melting (SLM) specimen (Figure 2a)
The sample was subjected to the symmetrical tension-compression drites
Summary
Inconel 718 (IN718) alloy is one of the most widely used nickel-iron-based superalloys in the high temperature (up to 650 ◦ C) structure applications due to its high strength, as well as the high creep, oxidation and corrosion resistance over a wide temperature range [1]. Limited studies exist regarding the effect of microstructure and defects on the fatigue behavior at elevated temperature, especially in the very-high-cycle fatigue (VHCF) regime. Nezhadfar et al investigated the low cycle fatigue resistance of SLM IN718 at RT and 650 ◦ C and found that SLM specimens possessed slightly lower fatigue resistance at RT but exhibited a comparable fatigue strength at elevated temperature [12]. They ascribed the similar fatigue behavior at 650 ◦ C to the same initiation sites of surface oxide films formed during the fatigue testing, which could retard the growth of the microstructurally short cracks. The results of this work provide a better understanding of the VHCF behavior of SLM IN718 alloy used in high-temperature applications
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