Effect of Ni addition on the microstructure and mechanical properties of Co-Cr-Al oxide dispersion strengthened superalloys

In order to increase the ductility of alumina-forming Co-Cr-Al oxide dispersion strengthened (ODS) superalloys, Ni alloying was introduced into the superalloys. Four grades of Co-20Cr-15Al-xNi (x = 0, 10, 20 and 30) at.% ODS superalloys were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The superalloys were composed of a Co solid solution matrix with face-centered cubic (fcc) structure and an Al-rich (Co, Ni)Al phase with body-centered cubic (B2) structure. Ni addition did not change the phase components, but inhibited the formation of the brittle B2 phase by expanding the solid solution limit of Al in both the fcc matrix and the B2 phase. Compared to the completely brittle fracture of Ni-free superalloy, Ni-added superalloys exhibited ductile fracture and a linear increase in ductility with increasing Ni content, facilitated by the reduction of B2 phase. Preferential fracture starting from B2 phase was confirmed during the tensile test at room temperature. Deformation-induced nanotwins in the fcc phase was observed, which is expected to improve the strength and ductility of the Ni-added Co-Cr-Al ODS superalloys.