gamma plus gamma ' microstructures in the Co-Ta-V and Co-Nb-V ternary systems

The Co-Ta-V and Co-Nb-V ternary systems are investigated in a search for L1(2)-ordered gamma' precipitation, based on recent computational predictions of stable gamma+gamma' 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 degrees C, and aged at 900 degrees C for 2,16 and 64 h. Nanometric, cuboidal gamma' precipitates within a fcc-gamma 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 gamma'-phases, as measured via atom probe tomography, are Co-3(Ta0.76V0.24) and Co-3(Nb0.65V0.35), respectively. Upon aging at 900 degrees C, the gamma' precipitates coarsen, dissolve and transform to lamellar C36-Co-3(Ta,V) and needle-shape DO19-CO3(Nb,V), measured as Co-3(Nb0.81V0.19) by APT, respectively. This shows that these ternary gamma' phases are metastable and points to the need for their stabilization via additional alloying elements. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.