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

We investigated the relationship between the microstructure and mechanical properties of ternary Co-Fe-Ni alloys. The solution-treated Co ₅₀Fe₄₀Ni₁₀ 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 (11̄0)α′//(11̄1̄) γand [001]α′//[01̄1]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.