High strain rate superplasticity in the fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N

The fine-grained duplex stainless steel Fe-22Cr-5Ni-3Mo-0.3N consisting of α- and γ-Fe(Cr,Ni,Mo) solid solutions exhibits structural superplasticity at deformation temperatures of 900 to 1050°C. The equiaxed microstructure with an average grain size of d_α,γ ≈ μm was produced by thermomechanical processing. This steel shows also superior superplastic properties at high strain rates up to ε̇ ≈ 5· 10^-2 s^-1. Maximum strain rate exponents of m ≈ 0.5 and elongations to failure of more than 800% were achieved. The superplastic deformability (m > 0.3) of this steel in a wide strain rate range enables near net shape deep drawing or blow forming of parts with complex shape applying low flow stresses. A deformation model is presented to describe the superplastic behaviour at high strain rates. Grain and interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The maximum m -value of about 0.5 and an activation energy of 260 kJ/mol, which is comparable to that of self diffusion of iron in γ-Fe (270 kJ/mol), and high dislocation densities indicate that dislocation climb in the slightly solid solution hardened γ-Fe phase (solid solution class II type of material) is the rate controlling step for superplastic flow.