Precipitation Behavior Of W-Rich Phases In A High W-Containing Ni-Based Superalloys K416b

The high temperature strength of Ni-based cast superalloys can be significantly improved by adding tungsten (W), a solid solution strengthening element. Hence, superalloys with high W content have been developed as key materials for the preparation of aircraft engine blades. However, the high segregation coefficient of W results in inconsistent composition and microstructure during the solidification process, which can be difficult to eliminate via heat treatment leading to deteriorated mechanical properties. The Ni-based superalloy K416B contains approximately 16.5%W (mass fraction) and exhibits a high tendency to precipitate W-rich phases, such as the alpha-W and M6C phases, which not only consume a large amount of W in the matrix but also reduce the solid solution strengthening ability of the alloy. W-rich precipitates also become the origin and propagation paths of cracks during stress-rupture testing. Much research on high W-containing, Ni-based superalloys has focused on the effects of W content on W-rich phase formation and mechanical properties. However, the roles of casting temperature and cooling rate on the formation of the W-rich phase are still unclear. In this work, five groups of K416B alloy test bars with the same composition were prepared with different processes. Three casting temperatures were chosen, and the cooling rate was controlled by burying sand in the thick shell and single shell, respectively. The relationship between the precipitation behavior of the W-rich phase in the K416B alloy and casting temperature, and solidification rate under different casting processes were analyzed using SEM and EDS. When the casting temperature is lowered from 1500 degrees C to 1450 degrees C, the grain size is significantly reduced. Results show massive alpha-W phases in the residual eutectic of the alloy at different casting temperatures, and the morphology of the alpha-W phase show few differences. The larger M6C phase in the alloy exists with residual eutectic, and the small M6C phase is embedded at the edge of the residual eutectic. At a high solidification rate, the precipitation of the W-rich phase is inhibited, which is primarily manifested by the decreased number and size of the W-rich phase in the alloy. When casting high W-containing Ni-based superalloys, choosing an appropriate casting temperature and adopting an appropriate heat preservation system to accelerate the cooling rate during solidification will affect the precipitation and transformation of W-rich phases, and optimize the properties of the alloy.