Structural superplasticity at high strain rates of super duplex stainless steel Fe-25Cr-7Ni-3Mo-0.3N

The fine-grained super duplex stainless steel Fe-25Cr-7Ni-3Mo-0.3N consisting of two phases (b-ferrite/austenite) exhibits structural superplasticity at higher strain rates of ? 10 -2 s -1 in the temperature range between 975 and 1100°C. The equiaxed microstructure with an average grain size of d δ/γ 8 μm was produced by thermomechanical processing. Maximum strain-rate-sensitivity exponents of m 0.5 and elongations to failure of more than 500% were achieved. From thermal activation analysis an activation energy for superplastic flow of Q = 310 ± 20 kJ/mole was derived. The superplastic behaviour at higher strain rates is quantitatively described by a deformation model where grain or interphase boundary sliding is accommodated by sequential steps of dislocation glide and climb. The high strain-rate-sensitivity exponent and the observed dislocation density indicate that dislocation climb in the slightly solid solution strengthened austenite is the rate controlling step for superplastic flow. The deformation mechanism reveals that the investigated super duplex stainless steel exhibits superplastic behaviour that is typical for class II solid solution alloys.