Influence of yttrium addition on the microstructural evolution and mechanical properties of superalloy 718

The influence of yttrium (Y) addition on the microstructure and mechanical properties of alloy 718 was examined by scanning and transmission electron microscopy, energy dispersive spectroscopy and tensile tests. Two alloys with 0.1% and 1.0% weight Y contents and a reference alloy without Y addition were melted, forged and solution heat treated. The yield of Y after melting ranged between 50% and 60% over the nominal value added. Y addition decreased the oxygen, nitrogen and sulfur contents of the alloy. The low-Y alloy presented a larger grain size, and the high-Y alloy presented a refined grain size owing to the intense precipitation of Y-rich phases. Microstructural analysis also revealed that the Y promoted (Nb,Ti)C carbide fragmentation. Considering the influence of this rare earth on the mechanical properties, there is an optimal yttrium content that can be added. The low-Y alloy showed an increase in ductility and no relevant variation in its mechanical strength. However, the high-Y alloy induced a sharp decrease in ductility, when compared to the low-Y alloy, owing to the embrittlement effect caused by the intense precipitation of Y-rich phases, such as Ni17Y2 and Y2O3.