Superplastic deformation and cavitation in a microduplex stainless steel

Abstract

The superplastic deformation characteristics of a microduplex stainless steel were examined over the temperature range 700–1020°C. The strain rate sensitivity index, m, was observed to increase with increasing temperature, decreasing grain size, and increasing strain at lower strain rates. For higher strain rates m increased to a maximum with increasing strain and then decreased. Maximum tensile elongations were obtained at 960°C for an initial strain rate of 0.2 min⁻¹, while appreciable elongations were obtainable at this temperature for strain rates up to 5 min⁻¹. During deformation cavities formed at Ti(C, N) particles and at α/γ boundaries. Density measurements showed that the total volume of cavities increased with increasing strain, decreasing strain rate, increasing temperature, and increasing grain-size, although the latter effect was relatively small. A timedependence of void volume of 1.6–2.0 was observed. Interpretation of this in terms of the Avrami equation suggests that many voids are nucleating at pre-existing nuclei, and these are probably provided by Ti(C, N)/matrix interfaces.