EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The versatility of the alloy 718 justifies its use as high-temperature components, gas turbine parts, rocket motors, storage tanks and spacer grids of nuclear reactor fuel elements, for example. However, the improvement in the performance of nickel superalloys in various application areas is a motivation for the search for continuous innovation of these materials. In this sense, the use of rare earth as modifying elements has been considered in recent years for the development of superalloys. Thus, the present work aims to achieve a better understanding of the effect of yttrium on the microstructure and, consequently, on the mechanical properties of this alloy. The analysis was conducted on alloy 718 with different Y contents, which was compared to a reference alloy without Y addition. The yield of Y after melted in a vacuum induction furnace ranged between 50% and 60% over the nominal value added during the melting process. The content of impurities as O, N and S was significantly reduced. The microstructural and mechanical characterization were conducted by scanning and transmission electron microscopy, energy dispersive spectroscopy, and tensile tests. Microstructural analysis revealed the (Nb, Ti)C carbide fragmentation. Also, it was observed intense precipitation of Y-rich particles on alloy with a higher Y content, which contributed to the refinement of grain size. On the other hand, the low-Y alloy presented a larger grain size, due to the great affinity of Y with impurities such as O and S which tends to concentrate at the grain boundaries; improving the grain boundary mobility that resulted in larger grain size. The low-Y alloy showed an increase in ductility and no relevant variation in its mechanical strength. However, the high-Y alloy showed the embrittlement effect caused by the intense precipitation of Y-rich phases, such as Ni17Y2 and Y2O3.