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High strain rate superplasticity in the fine‐grained duplex stainless steel Fe‐22Cr‐5Ni‐3Mo‐0.3N
Author(s) -
Hernandez David,
Frommeyer Georg,
Hofmann Harald
Publication year - 1996
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.199605517
Subject(s) - superplasticity , metallurgy , materials science , equiaxed crystals , grain boundary sliding , strain rate , dislocation , climb , microstructure , deformation (meteorology) , solid solution , grain boundary , composite material , thermodynamics , physics
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 ≈ 3 μm was produced by thermomechanical processing. This steel shows also superior superplastic properties at high strain rates up to ε ≈ 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.