Ductility enhancement in Co–Fe–Ni alloys by microstructural control

• Co50Fe40Ni10 alloy consisted of a B2-matrix and a γ-phase with the fcc structure.
• Orientation relationship between the dual phase is determined to be N–W relationship.
• Co50Fe40Ni10 alloy had a high total elongation of 12.4%.
• The considerably ductile γ-phase suppresses intergranular and cleavage fracture.
• Control of the γ-phase plays an important role in high strength and ductility.

We investigated the relationship between the microstructure and mechanical properties of ternary Co–Fe–Ni alloys. The solution-treated Co50Fe40Ni10 alloy was composed of only the α′-phase having a B2 structure with a low degree of order, whereas the Co50Fe40Ni10 alloy annealed at 873 K consisted of a dual phase, i.e., an α′-matrix and a γ-phase with a face-centered cubic structure that primarily covered the grain boundaries of the α′-matrix and precipitated finely in the grain interior. The orientation relationship between the α′-matrix and the γ  -phase is determined to be (11¯0)α′//(11¯1¯)γ and [001]α′//[01¯1]γ, i.e., N–W relationship. The volume fraction of the γ-phase was estimated to be about 26%. Substituting Ni at 10 at% increased the yield, tensile strength, and total elongation. It is noteworthy that the Co50Fe40Ni10 alloy annealed at 873 K had a high tensile strength of 1005 MPa and high total elongation of over 12%. The origin of the dramatic improvement in the ductility is attributed to the considerably ductile γ-phase, which suppresses intergranular and cleavage fracture. High strengthening is also originated from both the solid-solution strengthening of Ni and the grain refinement of the α′-matrix.