Formation mechanism and thermal stability of gamma ' and gamma '' compact co-precipitated nanostructure in directed energy deposited Ni-based alloy

In order to understand the formation mechanism of a special nanostructure of gamma' and gamma '' co-precipitated phase which present an important potential in the improvement of high thermal stability, strength and ductility in the superalloy, the influence of the atomic ratio of (A+Ti)/Nb on the microstructures of the Ni-Cr-Al-Ti-Nb alloys has been investigated using laser directed energy deposition (DED). As the ratio of (Al+Ti)/Nb increases in the as-deposited and heat-treated alloy, the precipitation ability of gamma' phase in the alloy is improved, accompanied with a reduction in the precipitation of gamma '' phase. When this ratio reaches 1.5 with the alloy composition of Ni-13.5Cr-5.26Al-l.95Ti-4.8Nb (at.%), the gamma' and gamma '' co-precipitated nanostructure with the pattern of gamma '' phase attaching to six surfaces of the cubic gamma' phase is generally produced in the as-deposited and heat-treated alloy. Furthermore, it was revealed that Ti element has the critical effect on the formation of this compact co-precipitated nanostructure.