A comprehensive microstructural study of a high entropy superalloy: As-cast and heat-treated states

Abstract

In this study, a detailed microstructural analysis was conducted on a high entropy superalloy (HESA) with a composition of 49.66 Ni, 16.96 Co, 6.49 Cr, 8.99 Fe, 10.28 Al, 5.79 Ti, 0.79 Ta, 0.6 Mo, 0.39 W at% in both as-cast and heat-treated conditions. The as-cast microstructure of the HESA consists of dendritic arms containing fine and spherical gamma ' precipitates with a mean size of 60 +/- 9 nm, embedded in a gamma matrix, along with gamma/gamma ' eutectic islands. After heat treatment, the gamma/gamma' eutectic islands dissolved, and the precipitate morphology evolved into a near-cuboidal shape with a mean size of 412 nm and a volume fraction of approximately 54.7 +/- 2.74 %. Transmission electron microscope (TEM) equipped with energy-dispersive spectroscopy (EDS) analysis of the heat-treated alloy revealed that Co, Cr, Fe, and Mo preferentially partitioned into the gamma matrix, while Ni, Al, Ti, W and Ta were enriched in the gamma ' phase. The diffraction spots in the TEM-selected area electron diffraction (SAED) pattern confirmed that the matrix has a face-centered cubic (FCC) structure, while the precipitates correspond to the L12-gamma ' phase. The heat-treated alloy exhibited a positive lattice misfit of 0.36 %. The gamma/gamma ' microstructure of the heat-treated HESA remained stable after isothermal aging at 900 degrees C for 200 h. The higher hardness of the solution heat-treated state, compared to the primary and two-step aged conditions, is attributed to its greater volume fraction of L12-gamma ' precipitates.