Design of cuboidal<i> γ</i>/<i>γ′</i> coherent microstructure and its stability in low-density Co-Ni-Al-Mo-Cr-Ti/ Nb/Ta superalloys

Abstract

Co-base superalloys generally have high strengths, good oxidation- and corrosion-resistances, as well as excellent creep-resistant properties at high temperatures (HTs), which are ascribed to the coherent precipitation of cuboidal<i> γ′</i> phase into face-centered-cubic (FCC) <i>γ</i> matrix induced by co-alloying of multiple elements. However, the cuboidal <i>γ</i>/<i>γ′</i> coherent microstructure is liable to be destabilized after a long-time aging at HTs in Co-base superalloys. In the present work, the cluster formula is used to design a series of low-density Co-base superalloys with the composition of [Al-(Co₈Ni₄)]((Al_0.5(Ti/Nb/Ta)_0.5Mo_0.5)(Mo_0.5Cr_0.5Co_0.5)) (=Co_8.5Ni₄Al_1.5Mo_1.0Cr_0.5(Ti/Nb/Ta)_0.5). Alloy ingots are prepared by arc melting under an argon atmosphere, and are solid-solutionized at 1300 ℃ for 15 h and then aged at 900 ℃ for up to 500 h. Microstructural characterizations and mechanical properties of these alloys in different aged states are obtained by using XRD, SEM, EPMA, TEM, and HV. It is found that all these alloys with Ti/Nb/Ta, Ti/Nb, and Ti/Ta in an equi-molar mixing have a special coherent microstructure with cuboidal <i>γ′</i> phase uniformly-precipitated into the γ matrix, which is contributed to the moderate lattice misfit of <i>γ</i>/<i>γ′</i> (0.27%–0.34%). Moreover, these cuboidal<i> γ′</i> phase are coarsened slowly during aging, in which the microhardness does not vary obviously with aging time (275 HV–296 HV). Especially, the alloy with (Ti/Ta)_0.5 exhibits the highest <i>γ</i>/<i>γ′</i> microstructural stability with a slow coarsening rate after aging 500 h, and no other second phases appear near the grain boundaries. While needle and bulk particles would precipitate on grain boundaries in other alloys after 500 h-aging.