γ+γ′ microstructures in the Co-Ta-V and Co-Nb-V ternary systems

Abstract The Co-Ta-V and Co-Nb-V ternary systems are investigated in a search for L12-ordered γ′ precipitation, based on recent computational predictions of stable γ+γ′ microstructures. Four alloys with nominal (at. %) composition Co-6Ta-6V, Co-5.4Ta-6.6V-xNi (x = 0 and 10), and Co-6Nb-6V are arc-melted, homogenized at 1250 °C, and aged at 900 °C for 2, 16 and 64 h. Nanometric, cuboidal γ′ precipitates within a fcc-γ matrix are discovered in the Co-Ta-V system after aging for 2 h, and in the Co-Nb-V system after cooling from homogenization. The compositions of these two new γ′-phases, as measured via atom probe tomography, are Co3(Ta0.76V0.24) and Co3(Nb0.65V0.35), respectively. Upon aging at 900 °C, the γ′ precipitates coarsen, dissolve and transform to lamellar C36-Co3(Ta,V) and needle-shape D019-Co3(Nb,V), measured as Co3(Nb0.81V0.19) by APT, respectively. This shows that these ternary γ′ phases are metastable and points to the need for their stabilization via additional alloying elements.