Microstructural stability of low-cost Ni-base superalloys with a high volume fraction of cuboidal γ′ nanoprecipitates

The present work designed a series of low-cost Ni-base superalloys with a high volume fraction (f > 70%) of γ′ precipitates via the composition formula of [Al-(Ni,Co)12](Al1(Ti,Nb,Ta)0.5(Cr,Mo,W)1.5) and then investigated the microstructural evolution of coherent γ/γ′ in these alloys during aging at 1173 K. It is found that the basic Co-free alloy with the addition of (Ti,Nb,Ta)0.5 and (Cr,Mo,W)1.5 in an equi-molar mixing has a special coherent microstructure of cuboidal γ′ nanoparticles precipitated into the γ matrix, showing a higher microstructural stability with a slow coarsening rate of γ′ precipitates after 500 h-aging. The addition of Co or the increase of Cr content in the basic alloy can affect the thermal stability of γ/γ′ coherent microstructure, which is closely related to the lattice misfit of γ/γ′ and a moderate lattice misfit (ε = 0.44–0.53%) would contribute to the high stability of cuboidal γ/γ′ microstructure. Also, the alloy with (Cr1.0Mo0.25W0.25) and (Ni8Co4) shows a comparable microstructural stability to the basic alloy. These alloys exhibit prominent mechanical properties, as evidenced by the fact that the yield strength of the basic alloy is 892 MPa at room temperature and 835 MPa at 1123 K, respectively. Moreover, the microhardness keeps almost constant with the aging time, indicating the better microstructural stability.